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
The fault switch circuit also detects (both positive and negative) DC on the amplifier outputs and it's far more likely thatone or more output stage transistors have died from overheating due to lack of cooling.The output stage darlingtons often fail short, which will almost always result in outputting an amount of DC.
The lack of temp. feedback to the idle current circuitry alone will see the output overheat even when not playing.
Martin
Yes that's what I was thinking, the design principles doc identifies it as protection from DC for the speakers. Do you think replacement of IOC 100 & IOC 102 will be sufficient to resolve the issue as these where the only ones affected by no cooling, are they readily available and do they need to be matched pairs?
How much needs replacing depends on how much have died - and how.You will have to measure around a bit - but my guess would be everything from the idle current trimmer (incl.) forward.
Matched darlington pairs would be ideal but the emitter resistors will easily compensate for small deviations.The good old well-established producers have reached a point where the deviations within each production run are practically neglectable,so fitting pairs from the same production batch will in most cases be just fine..
Thanks for that, looks like 2 off TIP141 (npn) and 2 off TIP146 (pnp) will be required, I will try Radio Spares.
Craig
these are the two TIP's I inadvertently destroyed which turn out to be IOC202/IOC203. I cant see any obvious heat damage to or around the components but will change out all four on Martins advice.
This is the circuit diagram view
I suggest:
Replace:0IC200, 0IC201, 0IC202, 0IC203TR220R277R276C238At least check (ideally replace):0TR200TR215TR216TR217TR219D204D205R271R272R278R279Martin
That's a very comprehensive and detailed recommendation that I will get on with right away, thanks for this...I appreciate you taking the time. pictures will be posted as work progresses.
Regards Craig......
Are these Darlingtons good enough quality to use as replacements?
Ok...
Everything on order with the exception of D205 which the component list has identified as a SFD 184, cant seem to locate one of these...can anyone advise an equivalent I can use? will an IN4148 be suitable?
or maybe an NTE109
Hi Craig,
a 1N4148 is a good replacement. A search for the SFD 184 at google gives a link to a scanned google doc of a german databook from 1968/69 which contains the SFD 184. The data for the diode is very similiar to the 1N4148, which is a bit better.
So go for it
Ralph-Marcus
I agree on the SFD184.And don't buy no-name darlingtons. Buy well-known brands from well-reputed suppliers.Or - even better - old stock remains.
There's simply too many fake semiconductors around.
Thanks for that...I will place an order for another IN4148, meanwhile I placed an order with farnell for the parts I had confirmed and took delivery today.....all looked ok until I got to the darlingtons, TIP146 looked fine (and are Multicomp which I believe are reputable) however inside the TIP141 antistatic packet I found 2 off 32 pin CMOS SRAM IC's!........the packet label correctly identified the part as TIP141 and carried the correct Farnell order number but as can be seen the contents disappoints..
I have discussed this shortcoming with Farnell who apologised and promised to post out the correct items right away, they said I can keep the IC's as they are low value items......I'm still awaiting some more parts so its not a concern, still if anyone needs a CMOS SRAM let me know
Multicomp is Farnells own brand.I suppose they can be trusted to put their logo and label only on devices from known good and approved suppliers.
Ok....all parts received with the exception of the IN4148 high speed diode, will make a start this evening.
Martin.....looking through the manual I noticed a paragraph under Adjustment of No Signal Current which recommends replacing 5C138/238 in the event of a failure resulting in DC on the output.......is this something I should take on board?
Depending on the way the output stage failed, the center rail (= output rail) could have assumed a DC level for a period of time andwith it the capacitor.If an electrolytic capacitor sees reverse DC voltage it will often be damaged and in this case the cap in question is merely a 10V or 16V type, soa high DC level, even with correct polarity, could have killed it .So yes, if you had DC on the output rail, replace the capacitor.The same goes for many other B&O amplifiers though it's not mentioned in the manuals.
Thanks for that.....given the uncertainty around the DC fault level on the output rail, and there must have been some, and minimum amount of effort required to replace this component I will do as you you suggest .
Decided to start with the TIP146 Darlingtons, as they where the first casualties.....and naturally the replacement components where bigger than the ones that came out......we wouldn't have it any other way!
another view.......
OTR200 next up (BD165 NPN Transistor) had to mount this component face toward the heatsink, the component removed was also installed in this orientation.
Replaced all heatsink mounted transistors now, next up will be TR215/TR216/TR217
Still a fair bit of work to do
Struggling to get on with this exercise due to being "Dragooned" into service by my partner, she seems to think decorating the living room has a higher priority over my leisure time than "fiddling around with old radios" this is probably due to there not being an "App" for it! non the less I did get TR215/216/217 replaced......its difficult to pick out these small junctions and even more difficult to tell they have been replaced, I will test the devices for obvious damage, though I understand that a component that measures fine with a multimeter can still fail under operating conditions.
You can just make out TR217
All the transistors Marin has recommended I replace have now been exchanged for new ones, the quiescent current trimmer also. The emitter resistors I bought look a lot clumsier than those I removed and when I measured them against the one's I removed they where not exactly the same value as I was expecting, when measured with a multimeter.....the old one measured approx. 0,5ohm and the new ones 0,5 also. These are supposed to be 0,18ohm.......will this matter (Martin) ?
First the original resistor (both R278 & R279 gave this reading, or almost)
And the replacement resistor reading....
Right! I have another holiday this coming week - Would you like the 4401 for a little holiday? I can drop it down one day if convenient - not Wednesday though! And Monday could be tricky! Probably bring a boring car so will bring 4001 as well! Do you want to compare some cartridges as well?
Peter
Forgot to say - email is mcevedy (at) mac.com
Hi Peter....
Next Friday would be best for me as I finish work early, usually home for 13:30. Will email you Monday with my post code and address if you like ;¬)
Craig: And the replacement resistor reading....
I had different readings with different DMMs, you are measuring in the 400 Ohm range, changing to a lower range would probably give a better reading, My best DMM measured around 0,2 Ohms but fluctuating up and down a little
Collecting Vintage B&O is not a hobby, its a lifestyle.
Soren
Just tried changing the range on my DMM to no avail....I suspect I would need a wheatstone bridge to measure such a low resistor, or maybe an old AVO 8 would see it......however the replacement resistors are now installed for better or worse ;¬)
I am sure I can make that. Look forward to seeing you.
Went all out this morning.....seeing as we have been donated an extra hour daylight! finished up all of Martins recommendations, time to power up and see what happens, this was done with no small amount of trepidation.
Powered up....no explosions or sudden smell of burning, much to my relief.....emmiter voltage set up fine and everything looks good.
Connected to a pair of beovox and plugged in Spotify.......sounds good as new! will keep it "on test" for a couple of days, though at present I cant detect any high temperatures at the heat sink....all seems very cool.
Martin.....thank you for your guidance, much appreciated ;¬)
Good job, Craig !
Very good job Craig, been following since the beginning, I always connect through my Dim Bulb Tester when I power up the first time, even so I am sticking my nose into the amp the first couple of minutes, nearly wetting my pants every time. Now enjoy, My BM 4400 is my main amp/receiver playing every day at least 4 hours