ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello from a new member and long time happy lurker (-:
Our Beomaster 1700 (type 1701) has been in various lofts for 20years. Spurned on by the CV19 lockdown here in the UK I’ve been trying to repair the amplifier board. The loudspeaker cable on the left channel was shorted and fried a few components.
I’ve read as much as I can find on the web (manuals etc) yet found your forum the best place going forward. There are a few issues that I hope members can help me with:
As suggested in previous posts i replaced all burnt-out parts (resistors, a variable resistor, a diode and all transistors) and electrolytic capacitors all with original tolerances. nb, the 0.39ohm 1W wire-wound resistors were only available as ceramic 5W at the time.
The Heat-Sinked TE 1088 and TE 1089 are now obsolete (only available as ‘used’ on ebay) so I looked up BDX33a/34a as replacements only to find that they have been replaced by BDX33c/34c which seem to have slightly different electrical characteristics but purchased them anyway as there seemed no other option.
In the manual, the VR1k resistor sets the 'no load' current. I set this resistors' trim as close to the original one that i took off the board. Disconnected Speakers and input connector from pre-amp stage and turned the mains supply on….
Within two seconds the replacement 0.39 Ohm 5W ceramic wire-wound resistors glowed (located between the emitters on BDX 33c/34c. There was a burning smell from those and the 27 Ohm 1/8W resistor connected to MPS A05 and MPS A55.
Maybe a few things caused this
1. The variable resistor 1K value needs to be changed
2. The Ceramic 0.39 ohm 5w resistors were the wrong type.
3. The replacement BDX33c/34c are so different that a few other resistors values need to be changed.
4. Having no audio signal load to the board may have also caused the ‘thermal run-away’
I have now ordered lots of 0.39 ohm 2w wire-wound resistors (original spec), a supply of various fixed resistors and variable resistors - further £20 spent (-:
Q; How can I check the 12mV (maybe it’s different now?) across one of the 0.39 Ohm resistors without blowing everything again?
Q; Is there a formula to work out alternative values of resistors and mV value in this amplifier stage - due to change in output transistors?
Q; How can i test transistors without an oscilloscope or wave generator, i only have a multimeter with hFE pnp/npn holes?
Q: Is there anything above that I’ve overlooked?
The B&O was a gift from my uncle in the late 80’s - My uncle died last year, he loved his music and various B&O through the years and it would be lovely to get this up and running as a tribute and for it to take pride of place in the home office.
I haven’t done much in electronics since the 90’s so If I can’t repair this amplifier board I’ll consider trying to find a power amp stage that can run on a single 50 volt supply and hide it in the case! I really hope this isn’t my only option.
I hope to attach images:
Output stage diagram for one channel - The ‘Red’ squares indicate parts that burnt-out when speaker cable was shorted many many years ago
Photo of board as it is now) while waiting for more replacement parts to arrive after 1st attempt - R212(vR) - R214 - R221 - R222 - also showing some of the old components removed from original shorting mishap circa 1990's
Electrical Characteristics of BDX 33a/34a (B&O’s suggestion for replacement to TE1088/9)
Electrical Characteristics of BDX 33c/34c (interwebs suggestion for replacement to BDX 33a/34a)
Any help would be warmly received - Thank you in advancee.
Beocord 1200 and Beomaster 1700 type 1701
Received an assortment of "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.
Is that minimum resistance towards R211 or R213 ?
Apologies if this seems obvious but I haven't a clue
Well well well it looks like B&O printed incorrect capacitor polarity markings on this PCB. (c108/208 '5000uf')
I took the originals off (4700uf) to replace them and foolishly trusted the markings on the PCB ! I've only just discovered that I replaced those two biggies on the board the wrong way round! - The evidence is in the old leads on the 4700uf caps which i removed and in the circuit diagram, maybe thats why everything got a little hot ?
I'll find out later when I turn this thing on... probably for the last time before I ceremonially smash it to pieces with a large rock
Pete: Well well well it looks like B&O printed incorrect polarity markings on this PCB.
Well well well it looks like B&O printed incorrect polarity markings on this PCB.
Yes, good catch. The screenprint is wrong. Negative must be connected to the coils L100/L200. The layout drawing in the service manual does show the correct polarity.
Pete: Received an assortment of "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.
I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.
If I were you, I'd measure the setting of the old trimmers and use that as the base setting for the new trimmers at first power-up.
manfy
Thank you!
Of the two 1k VR's removed; one is fried and the other is reading 1.1k with 0.6K towards R113 0.5k towards R111 so I'll set the values and see what happens, one channel powered up at a time.
Below is what the service manual says regarding components near IC100
Thanks again
manfy: I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.
Turning the trimmer towards R113 would take the idle to max as it would drive IC100 off.To start with minimum idle current IC100 must be on = trimmer slider moved towards R111.IC100 is clamping the two bases of the output transistors, and it's an NPN type.
Martin
Thanks for this, but naah, I still can't see intuitively how IC100 sets the idle current of the output transistors. It balances the base voltage of the output transistors and I guess by doing so it indirectly sets their working points and idle currents.Anyway, never mind. The circuitry does work in principle or else it wouldn't have been realized that way.
The function of TR102 is very clear with this description and thinking of your initial description of the problem, I'd carefully look at that transistor. If this one is shot, i.e. shorted from C to E it certainly would explain why your output resistors R121/122 go up in smoke immediately after power-on.Please note that the description in the service manual is just a conceptual explanation! If you look at the circuit diagram you see that 21R1 is fed by +50V and not +34V!! And that in turn would nicely explain why IC100 got shot and why all those resistors got fried.
In your first post you asked about TE1088 vs BDX33C. Don't worry about it. The service manual shows that B&O used 3 alternatives for that darlington: BDX33A, TEO1088 and FJ3001.So, BDX33C is perfectly fine. The only difference between A, B, C, D type is max Vce of 60, 80, 100, 120V respectively.
Do you have a DC power supply that goes up to 50V?? If so, the output amp can be tested nicely on its own: Leave input and output disconnected and slowly increase the supply voltage while watching the current drawn. This way you can measure various points on your circuit at a lower voltage that is non-lethal to the components. This allows you to assess what's wrong without sending additional components into nirvana.
--------------
Cross-posted with Dillen. He's a professional with hands-on experience in B&O equipment, so his word can be trusted!
Do you have a DC power supply that goes up to 50V?? ...
No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet..
Not sure what to try next
Pete:No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet..
Believe it or not, that's actually a good thing to see such a high current at such a low voltage: It shows that you have a major problem with one amp and that is usually quite easy to find - as opposed to intermittend problems for instance.Your 3-15V supply is sufficient for that.
First to your caps C108/208: You said the speakers were disconnected during first powerup. That's good. The caps were "almost free floating" except for R106 and R107 that connected them to GND. That makes good 5 kiloohm, so with a bit of luck you didn't seriously damage those caps. (I trust you measured their capacitance ok when you reassembled them the right way, right?!).Since the speakers were not connected, those caps mounted wrongly were certainly not the reason for the excess current.
So, it appears that one channel is working fine and the other has this overcurrent problem. That makes things simple because you can compare measurements from one channel to the other and if any value is substantially off, you can focus on the components in that path.If the current is sufficiently low at 3V then you can measure and compare voltages. Usually you'd start around the transistors because they are most likely to fail.Alternatively you could just compare resistance values at various test points without any need for a power supply. Such a high current suggests that something is shorted somewhere in the faulty amp.
thanks for your reply manfy
I'm somewhat confused at the high current at such a low voltage given that the B&O power supply throws out 50v and I also think both channels are now affected after previous tinkering. Originally one channel worked but then I replaced everything in the hope of a simple balanced result - with the capacitors the wrong way round and perhaps the board shorting somewhere on the case as I didn't nip off the component leads enough below the board - so stupid me!
I'll be removing one leg of most of the components to test their values over the next few days (when it's /raining outside!) and I have spent a further £12 for all Transistors/IC's on eBay (arriving Friday) - not a bad price compared to my first attempt via 'mouser' in the USA (-: So will write a result of resistances/uf's before attempting to replace anything out of kilter.
'Lockdown' here in the UK is good for these sort of things
Pete: I'll be removing one leg of most of the components to test their values over the next few days ...
I'll be removing one leg of most of the components to test their values over the next few days ...
Yes, that's one option, I guess....but I'd strongly advise against that!! Lots of things can go wrong with all that soldering, not to mention the mechanical stress on the components...
If you must, remove all the transistors and test them out of circuit with a multimeter -- or a transistor tester if available. Carefully compare their values between left and right channel.Once all the transistors are out, you can easily test resistance values on the board without danger of interference by other components.The only other components on that board that are likely to fail are the e-caps. Did you recap the board before first power-up? There's no point in messing with the resistors - they never fail except for overheating or mechanical failure.
PS: If you're unfamiliar with transistor testing with a multimeter, research it on the internet. You should find plenty of tutorials or uTubes on that!
The enduring learning curve to enthusiastically repair one channel of a shorted Beomaster 1700 amp continues..
Step:
1.Google (other search engines are available) to find similar issues/solutions
2. Find a service manual/circuit diagram/schematics et ‘al
2.Forums are your new friend
3.Try to explain everything
4.Ask the right questions
5.Don’t expect all the answers
6.Hope for questions…
7.Photograph everything before, during and after ‘tinkering’
8.Remove components from a PCB with a sucker or wick to avoid damage to PCB - see photos
9.Research any discontinued component electrical characteristics to find valid alternatives
10.Test component values / replace if necessary
11.Learn the difference between PNP / NPN and understand that not everything is the same in component pin layout ie BC556b is PNP yet has a different Base/Collector/Emitter configuration/orientation to others!
12.Don’t trust PCB screen print layouts or schematics, compare every component to other circuits and industry standards. eg Capacitor polarity
13.Buy new components based on above
14.Send out white flag
15.Wait
16.Turn on…
= Success / Failure
This weekend all the defective components will be replaced to the PCB and every one checked for connection to their correct place on the PCB according to the circuit diagram. Check over many times and check again then to power up - (power supply checked!)
Wish me luck - see you on the other side
The ‘Life is too short’ option is:
HiFi 2.0 Channel Power Amplifier Board HiFi Stereo Audio Amp DIY KIT MJ15024 and hide it in the B&O case (-:
Photos illustrate chaos, frustration and incompetence, mildly mixed with enthusiasm and hope (-:
board without TR/IC's
PCB connections drawn based on circuit diagram due to soldering train wreck!
what a messHope this helps someone out there...
Maybe its me but it looks a bit like some of the traces are missing. and lifted.
Honestly don't know what to make of it. Is that supposed to be the desoldered or soldered state? I can't see how any of that could be working.
Pete:PCB connections drawn based on circuit diagram due to soldering train wreck!
lots of tracking had lifted so had to work out what goes where before putting transistors back. What u see is the board without transistors. Work in progress - sort of 😇
If you ask me, then what you see is a board beyond rescue.Your soldering iron is way to hot.
You are so correct!
..but i will not be beaten (-:
I've noted where all the tracks are supposed to be (see diagram with my red line markings) and plan to link all areas where I've previously destroyed them with a too hot iron and too much tugging!
The problem of removing components came quite late to me after I realised I had a solder sucker from years ago which would have made it so much easier and tidier Lesson learnt
All new components have arrived by mail/post so plan to instal them on the PCB and carefully check (twice/three times) all the links i make where there are no tracks.
But first i will check all new transistors with multimeter for irregularities, inc hfe, pin layout, dudd components - a 'kind of' quality control
This really is my last go at this. 'Lockdown' with CV brought me here to learn with time at hand
Please don't think I'm ragging on you...
Poor technique has made a simple (ish) job rather more involved than it needed to be. Take some time to watch a few soldering videos on YouTube. There are also a few on desoldering. MrCarlsonsLab https://m.youtube.com/channel/UCU9SoQxJewrWb_3GxeteQPA might be a good place for you to start.
Properly desoldered parts more often than not just fall out, no tugging or force required.
A hot iron is not bad on its own, but dont dwell too long, otherwise you'll end up melting the board and resinous material, separating the the traces and pads - which is the mess you've got now. IIt might be recoverable, but you need to be patient and not continue to make the same mistakes.
Seriously, spend half an hour to an hour watching some vids on soldering technique and you'll be in a better position to complete this and will probably save yourself at least tht amount of time by not making things worse.
Thanks for the advice on soldering (-:
I’ve replaced all Electrolytic Caps/Transistors(+ic’s) and checked all joints and values of all components. Overal the values of each component on left and right channel seem symmetrical. Traced all tracks according to circuit diagram, checked continuity.
Turned on power without pre-amp input plugged in. No heat anywhere on the PCB, tried to measure the 12mv across R221/R121 and didn’t get a value at all no matter what i set the variable resistors to. Tried to measure 30ma collector of IC101/201 to ground, equally no result. !
Just out of curiosity measured voltage across left and right channel speaker output - both at 45v dc !?
Then I plugged in Preamp output to the board, powered up and immediately R214 (27ohms) is hot and smoking !! so quickly turned everything off.
Arrrggghh what am I missing?
Please... this is driving me nuts
It would take several defective components to make R214 start smoking.Shorted TR201, Shorted IC200, IC201..Or some connection on your board is now wrong.But wasn't this the working channel?
When I said this board was beyond repair, it's partly because of all the damage it now shows, but mainlybecause the board material (pertinax, which is basically made from pressed layers of cloth and lacquer) tend tobecome slightly conductive if scorched.This cannot be cleaned away. Only way is to remove the scorched material (cut holes).
I admire you persistence, but this board will never be good again.Even if you manage to get it working, sort of, it would be a neither stabile nor presentable repair.
Thanks Martin
The channel that died when the speaker was shorted many years ago is the channel that smokes!
I daren't wire up the speakers for the other channel, esp now that i have replaced all its parts too!
I didn't know the board could become conductive, there are quite a few scorched areas ( a few more than when I started to fiddle).. I'm guessing it's very likely I've made it worse and/or maybe missed the obvious referring to the circuit diagram (and the other channel on PCB) and have killed all the new parts when soldering them in + <deleted further excuses>
There's enough parts here now to build one of these a few times over from scratch. I despair So very sad for me to have failed to repair my dear departed uncles hifi. Looks like I'll have to buy an off the shelf Chinese Quad clone amp on pcb than will run on 50 ish volts and hide it inside and probably mess that up too
So very depressing - yet I want to thank you and fellow BeoWorld members for their help along the way.
I daren't continue the Reel-to-Reel Beocord 1200 repair despite re-fitting all the belts successfully. Maybe when this 'lockdown' is all over I'll be able to buy ingredients for baking a cake or basket weaving
Pete: Just out of curiosity measured voltage across left and right channel speaker output - both at 45v dc !?
For what it's worth, I think you cleaned up your soldering or desoldering mishap(s) quite nicely!
What you did forget in your reactivation attempt was measuring, measuring, measuring! Once you noticed that you couldn't adjust the no-load current, the ONLY logical thing to do would have been to compare the nominal voltages that are given in the circuit diagram with the actual voltages of all those points! The results would have given a clear indication for what to do next - if not for you then certainly for some members on this forum.
Based on your description so far, I'm suspecting that you have a ground problem on that board. You measured 45V from speaker output (before the output cap C108, I hope!?) to GND and that suggests that some tracks that should be GND are floating (ie. not really connected to power GND). The fact that the board behaves so differently when the preamp is connected supports this, because now some parts of the board are suddenly connected to GND via the preamp.
All of this is fixable, but it needs a systematic approach that starts with measuring and then making sense of that measurement. Trial and error component swapping alone doesn't cut it. A power supply with adjustable output current would be a great help, but well, life is not always fair. There are other ways to limit the current.
Anyways, it may be a good idea to take a rest from this project for a while. You'll probably not be able to find the problem quickly, but now you're too eager to get it working, it seems - and that leads to "shortcuts" and mistakes in the repair process.But don't throw the board out just yet! Putting in another different output amp is not gonna be plug-and-play either.
rgds,
Many thanks for your help..
manfy - 'You measured 45V from speaker output (before the output cap C108, I hope!?) to GND'
I measured it from the end of L100/200 on the 4pin central white connector to the other terminals on the white connector. At this time the board was out of the case so I'm guessing not connected to earth (via heatsink?) and then as you say got earthed as soon as i plugged the preamp output to the board - which was when the frying started!
as for voltage measurement at other place i wasn't sure what voltages I'm looking for as it was noted that some given in the circuit diagram are incorrect. Could the circuit diagram be wrong so as to stop people copying it? naa I'm just going bonkers!
Giving this whole episode a rest is a good idea, a very very good, big, great idea (as trump would say) tho I wish I could find a photo of a working undamaged board taken from below so i could double check my track repair... but I haven't seen any pigs flying past my window these last few weeks
Pete: Could the circuit diagram be wrong so as to stop people copying it? naa I'm just going bonkers!
Could the circuit diagram be wrong so as to stop people copying it? naa I'm just going bonkers!
Haha, naah I got a better explanation: The BM1700 was made some time in the seventies and back then, Cannabis was probably not yet a controlled substance in Denmark...and the designer of the docu and circuit diagram consumed particularly much of it....
I even have proof....aaand an explanation why your offline tests didn't work out:According to the circuit diagram in the service manual, there's only one type of GND on that board. But when you look at the layout drawing, you can see that you actually have 4 (in letters: FOUR) separate ground planes: power ground left, power ground right, signal ground left, signal ground right.
Granted, all those ground planes are galvanically connected at a star point in the power supply, but they are physically separated on the amplifier board! No wonder your offline test didn't work!
In any case, my recommendation of taking a break from this project still stands. Get some mental distance from it for a few days. You'll still have to build a current limiter before you can start actual tests.Can you still get incandescent light bulbs in London? If so, try to get a hold of a 25, 40, and 60W bulb! ...and an old bedside table lamp!Wire it up so that you can use it in series with the BM1700. This way you can limit the maximum current that can flow through the transformer of the Beomaster. 25W = max 0.1A at 230V, 40W = max. 0.18A, 60W = 0.26A
If you only can get one, aim for 40W. That should be able to handle the normal no-load current of the amp and limit any excess current while you're searching for the actual fault.
Thanks manfy,
After more than a few days off and a few night dreams about the B&O amp pcb/circuit I've dug out a 40W bulb to wire up in (series?) on the live 240v, yet know not what voltages to test for on the power amp stage on the pcb.
The earth configuration is indeed a crazy, specially since it takes the placement of the tiny pre-amp output plug to this board to earth despite there being two earths from the power supply going via this board to the headphone and speaker outputs! Those earths literally come to this board only to be routed to the speakers via the white four pin connector without touching anything else on the board!! (purple and brown wires)
So in my further quest to find an accurate pcb track diagram after my solder digging/mining disaster and perhaps for more voltage info, a printed service manual has been ordered in the hope the tracks are more visible than on the photocopied one downloaded some time ago + a variety of small capacitors (non electrolytic) and a better transistor tester.
Rebuilding the amp on euro vero is a final option. Yet due to likely B&O cannabis influence in the past, confidence in the manuals' circuit diagram is V-low... for example ceramic C102/202 is 1nf on circuit diagram yet a ceramic marked 3n3 is placed on the board. Obviously everything worked originally but if I want to replace components with better alternatives what values do I choose?
This begs my final question... has anyone on this planet ever repaired this board with 21st century equivalent components (especially those pesky Te1088/89's) after a speaker shorting of one channel ? If anyone has, evidence of said repair is thin on the ground, including here!
eg This thread ends abruptly with no solution: https://archivedforum2.beoworld.org/forums/p/18778/162645.aspx#162645 and even illustrates electrolytic capacitors placed the wrong way round on the board - a fully understandable mistake considering the circuit digram and screen print on the pcb!
Those pre-drug-control era engineers were certainly having a good time
If anyone one this forum has one of these Beomaster 1700 (type 1701's) please can they open it up and photograph the underside of this board + transistors in situ on top so i can triple check again and get some sleep
muchios apreciatum
Just to set the record straight: Keeping signal GND and power GND separated really IS a good design practice. It avoids noise in the signal path and it's still a common thing to do in modern electronics. But when this happens you normally use different GND symbols in the circuit diagram.So, basically it was my mistake to tell you that you could test the board by disconnecting input and output and supplying power with a DC power supply and then start measuring.
We will still do an offline test of the amplifier board, but you will have to prep the board first. So please do not try to power up the amp before we've done that.
Regarding the light bulb, yes, you have to connect it in series into the mains lead. The light bulb acts as a cheap power resistor. If everything is fine with the board, it will only draw a small current, the voltage drop will be small across the bulb and it will be dim. If there's a short circuit or similar problem on the board, it will try do draw a high current. Now also the current in the mains line will increase, i.e. the light will be brighter and there will be a bigger voltage drop across it, which means the trafo voltage will go down which also lowers the current drawn.It's quite simple actually but if you want, you can search the net for "dim bulb tester" for additional details.
Generally you can assume that the circuit diagram in the service manual is correct. But don't take that as a guarantee. Mistakes can happen. Especially for vintage products made before computers were mainstream, such typos or mistakes may have been intentionally overlooked because scrapping and reprinting of 10,000 service manuals was jsut not a viable option. They were probably just reported internally with separate change notices. Just keep in mind the legal disclaimer: "Subject to change without notice." and if something looks really starnge, just double-check the circuit diagram with the actual board, layout and components and if you find a discrepancy, you have to figure out what is right and what is wrong.
There's one more thing I want you to check and confirm: The power transistors on the heat sink need to be isolated from the heat sink! Normally there's a mica sheet between the metal part of the transistor and the aluminum heat sink. Check the resistance between the collector of every power darlngton and the heat sink. If there's a short, fix it.
Last thing: terminology. Earth (or PE [potential earth]) and ground are 2 distinct concepts in electronics. To avoid confusion, forget about "earth". Let's call it GND from now on.
More later!
Thanks for this manfy, I've checked heatsink isolation after putting fresh compound between all the mica/transistors - isolation is good - from everything (-: I also understand your points about GND and separation.
All suspected newly blown components will be removed later today and hope to replace in the next few days when the latest deliveries arrive; polyester film capacitors, transistor checker and a few 39 pf ceramics - In the meantime, a 'dim bulb tester' will be created
Fingers crossed that the manual showing a clearer view of the PCB track layout will arrive soon also.
Cheers
Pete
Hi Pete,
Here's how I would approach this fix now:
If everything's good on the board, you'll see something like this in the first 6sec after power on:
The idle current through R121 is below 1mA, supply voltage close to 50V, the voltage at TP101 (speaker output) is about 24V and you will be able to adjust the idle current with R112.
If there's an open at IC100, you'd see good 350mA at R121 (which is not a problem because of the dim bulb tester)
But without the dim bulb tester and the same fault on the board, you'd see 2.5A, i.e. 2.5W on a 1W resistor, ergo R121/122 would have every right to smoke!
Brilliant ! Thanks manfy.
I'll set everything up on Monday as I've promised the wife I'll spend time with her on her weekend off, then I'll be able to take each step you've provided at a dedicated slow pace.
PS Monday is going to be a long day !
Photos below are not for the faint hearted - LOL
attempted work-around ruined pcb tracking !!!!
After everything...starting from scratch to make one channel of the amp onto double sided vero is the final option feared!
Once all is in place there'll be testing and measurement as advised by manfy and if it works out another channel will be built exactly the same
There was never any intention to restore and sell this B&O, I just wanted it as a living (working) reminder of my uncle. Everything works apart from the output stage - bit like life really (-:
Yes, I'm quite mad!
Pete:Yes, I'm quite mad!
...as long as you know
You are pretty far in now, and it is too late to scan that original PCB...
I wonder if you wouldn't have been better off going for something like an LM1875 (in a pair) solution if the goal is just to get it to make sound again? Fewer components needed and probably an easier solution overall.
fkatze: You are pretty far in now
Oh yes way in deep over my head <gasp>
I've scanned the PCB layout from the manual received via snail-mail so as to check all tracks against circuit diagram - all seems ok - (notes on diagram)
Checked a few resistances on the connectors to the Power amp inside the case (Power Amp PCB not connected)
Signal GND to Chasis = 90 k Ohms
Signal GND to GND on Power Supply = 0.5 Ohms
I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board?
I'll construct the new board with that 'option' over the next week - just makes it a little harder with two separate sources for GND
all good fun
Pete: Signal GND to Chasis = 90 k Ohms Signal GND to GND on Power Supply = 0.5 Ohms I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board?
0.5 ohms between both ground planes at the power supply is normal because that's where they are centrally connected. 90k at the board is possible because of various components that link the power path and signal path.
Those 2 holes between signal GND and power GND look like a precautionary design measure. At the time of board design they were not sure if power GND/signal GND separation was "worth it". But ultimately they found that the separation was benefitial in terms of noise rejection, so they kept the planes separated, which forced them to run the extra wires from power supply to amplifier.
When you construct the new boards, you should retain that separation, or else you'd create a ground loop when connecting the preamp and that might cause a hum or other noise. For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.
In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!
manfy: <snip> For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed. In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!
<snip> For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.
Very valid points, will be taken on board - literally - Thank you (-:
Nearly there - in principle!
Latest draft of many before soldering components onto vero board:
...might just check the circuit against the manual a few more times tho
(Click photos to enlarge).Tested this afternoon prior to dismounting. Plays fine.All original apart from my fingermarks in the dust.
Interested?
I also have the power supply.
Behold a thing of beauty ! - brings back happy memories of what my board looked like a few months ago before i started 'repair'
Very interested... but have I gotten too deep into the replacement project to afford such a solution?
You'll know my first question..
please PM me a cost inc UK delivery
Thanks for thinking of me and posting such fab photos
a few more snaps to amuse you allOk...
One channel completely rebuilt / circuit checked and wired up with untested original board containing new parts.
Now to start from scratch (again) and follow Manfy's kind instructions on how to unwire and rewire power supply and various GND points to test voltages whilst having a 40w bulb in serial with live 240v. A 40w bulb in serial with mains power is tested (without amp board connected) and hey-presto bulb lit briefly and then went very dim - just need to understand wiring (signal and power GND conundrum) for each channel test as I've incorporated the 'startup circuit R1/C1' on the new board! - before power on to everything
Hopefully next week we'll have sound
Today marks a very special occasion for me. The B&O my dear uncle gave me many years ago sings again!
Since lock down here in the UK, I have for the first time ever, built a circuit board from a diagram, tested it and it works.
The voltages manfy illustrated tested symmetrical on both left and right channels (all be it slightly lower by a few volts in places due to power supply on throwing out 45v) and the only issue had was when the board was clamped into place with the spring clips onto the heatsink. It seems a 50 year old board does not like my soldering technique (or lack of it) as the new BDX’s strained at their feet and lifted welds to the remaining tracking on the pcb… but after a quick repair with a bridging wire… hey presto! the amp produced its warm tones once more.
Having restored all the other components to function correctly, all potentiometers cleaned, belts and clutches cleaned and tightened, and no load current adjusted to spec it’s time to close the lid (hide the train wreck) and clean the outside. The only thing about this amp/receiver that isn’t working is one LED light on the first FM slider.. I will leave that off in memory of my uncle. He would have found that amusing (-:
Thanks go out to everyone who helped along the way. It’s been an enjoyable (sometimes frustrating) learning curve... and a special thank you to Dillen who sold me an original amp PCB which arrived the day I repaired to old one. Ironic
Removal of components and soldering new into place will never be the same and 40 watt tungsten bulbs are a godsend
I'm glad you've got it up and running again. Hopefully it holds up and doesn't cause you any more troubles.