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
Before the bottom cover went on the front had to come off and the volume pot looked into......sure enough there was a couple of quite large blobs of the conducting graphite grease which had been applied far to liberally...….not the best picture but one can just about see the blob of grease in effect shorting between the carbon slide and the return track, cleaned it off and when all was reassembled I gave it a try....and all is now well, lesson learnt!
Moving on it was the turn of the quiet current trimmers next, remover R385 (right channel) and measured it up, I like to set them as close to the original value as possible to start with.....oddly enough it was reading well high across the resistor legs at 153ohms....
And as the wiper and one leg are connected together in the circuit the important value is the wiper and the other leg.....this read 155ohms (don't ask how this happens! )
This is a worry...particularly as the pot is clearly marked as 100ohms!!…..it has been set at some point at its maximum value.
Soldered the replacement 25 turn pot into the board, after adjusting the pin lengths to suit and plugged in the variac once again.....hooked up my multimeter across R469 and set it to millivolts.....gingerly wound up the variac and the meter reading crept up to 4.0mV. Good I thought....however, when I tried to adjust it up to the prescribed 5.0mV it wont move....up or down!
As a quick check I measured the Left channel quiet current setting to and discovered that I'm getting the same 4.0mV across R460 and the trimmer R373 has no effect on the reading up or down...….tested the unit by connecting the BG4000 back up with a couple of speakers and it's working fine!
I know I'm measuring in the correct place and adjusting the correct pots to set these values, and as both are showing the same symptoms I'm suspecting a common fault......I have checked with my working BL5000 and have confirmed the above......so I'm going to turn to my constant companion at times like this Sauvignon Blanc. As always any assistance will be greatly appreciated in the meantime
Check the 330R and 470R resistors nearby as well as the BC115, (- and of course the NTC, though it rarely fails).
Martin
Cheers Martin.....I have carried out a visual inspection of the above components and nothing looks to be in any trouble, at least no loose connections or obvious burn outs. So before the soldering iron puts in an appearance I decided to have a look at the voltage levels as prescribed in the manual and started with the voltage at the base of BC115, should be sitting at 31.6v
Measured the voltage level at the same point on the left channel...….
I did think the lamps on the front panel where a little dull, very dull in fact but wasn't overly concerned.....now that I put a meter to them I can see why!
Finally measured from the BP point to ground and got this.....I'm suspecting the suppression component I have fitted as its the only thing I have done around the power supply, though its hard to imagine why......but I will start by removing it from across the transformer and see what I measure then...
Check the ceramic resistors at the regulator, and check that the regulator is running on both 2N3055.
BTW nice backdrop on that last photo.
Thanks Martin....I will measure around there next, looks like the transformer is good after all.....should be getting 80vdc out of the rectifier, getting this so looks to be ok
Craig,
The snubber you added should not affect anything, the 76 volts you're getting is the proof.
My suggestion would be to put efforts into that power supply circuit starting by possibly shorted or leaking capacitors, next, possibly the corroded trimmer and check those zeners / diodes too! Power resistors have to be checked too but they rarely fail.
Very detailed tutorial!
Ok...brief interlude yesterday as it was the first day back at work I felt I needed some therapy in the form of an evening out to dinner, took several measurements this evening...….
The obvious looking villain of the piece is the Zener diode ZF18...….pulled him out and sure enough he's shorted out in both directions!
Accurate prediction Charles! well done...……."borrowed" a diode from my working machine, replaced the old capacitors, wasn't sure why the diode had shorted so thought it wise to replace the caps as I had them waiting to go in anyway, and via the variac powered up very slowly monitoring the voltage at ZF18. It slowly rose to 18v.....the lamps illuminated to the correct intensity and I now have the 60v power supply back...…..thanks again to Charles and Martin, another quality joint effort
Replaced the trimmer on the power supply too, the checked the quiet current setting once again......its now responding to the trimmers position, however it only goes up to around 4.9mV before the trimmer bottoms out.....this explains why the old skeleton trimmer was also wound all one way, I'm thinking maybe a dried out cap which is way off spec maybe responsible? non the less set it to 4.7mV and moved on......will investigate further following the recap.
Dillen: Check the ceramic resistors at the regulator, and check that the regulator is running on both 2N3055. BTW nice backdrop on that last photo. Martin
I have a suspicion that the back drop is become some thoughtless so and so hasn't been to pick things up! Sorry Craig - on holiday next week and will try to come down! I have something you might like mending and then keep for amusement - I don't think the other collectors will have one!
Peter
Craig: Replaced the trimmer on the power supply too, the checked the quiet current setting once again......its now responding to the trimmers position, however it only goes up to around 4.9mV before the trimmer bottoms out.....this explains why the old skeleton trimmer was also wound all one way, I'm thinking maybe a dried out cap which is way off spec maybe responsible? non the less set it to 4.7mV and moved on......will investigate further following the recap.
You bet Craig, you will have to go thru all the adjustments again as you change parts. For now, I think it is the best you can do with the current parts. I would also check the driver transistors and match them (hfe) as close as possible by channel, that might help too...
Peter: Dillen: Check the ceramic resistors at the regulator, and check that the regulator is running on both 2N3055. BTW nice backdrop on that last photo. Martin I have a suspicion that the back drop is become some thoughtless so and so hasn't been to pick things up! Sorry Craig - on holiday next week and will try to come down! I have something you might like mending and then keep for amusement - I don't think the other collectors will have one!
I will be around all next week, but working through the day...finish 13:00 Friday, tou have piqued my curiosity ;¬)
Craig
got finished on the drivers this morning...…..all caps and trimmers replaced. I don't know if I should replace the transistors as Charles suggests, I haven't done this in the past unless one is faulty....I know Frede recommends replacing the black cap units as they are prone to failure.
Had this delivered yesterday, after struggling with the idea of spending £15 on a screwdriver I finally decided if I can spend it on a bottle of wine I can spend it on one of these...….don't know why I didn't do it years ago, so usefull
I'm not recommending you replace the drivers; I recommend you check the HFE and match them as close as possible to each other, per channel.
So how does one do that?......isn't the hfe a fixed value that varies quite a bit from one device to another? so if if two had different hfe how would one match them without replacing one or the other.....you can see I haven't done this before
You have the right tool for that purpose Craig, your DCA75! This tool will measure and show the HFE if you scroll down the display.
The HFE is the amplification gain of a transistor and when used as complementary pairs, ideally they should have similar characteristics.
So first, for a given transistor, make sure it meets the minimum recommend manufacturer HFE. When used in pairs, ideally, the HFE difference between the two should be around 10-20%. Above 50% , the weaker will work much harder and may create distortion.
Before removing the transistors, mark their position and part number. Since a pair is a combination of an NPN and a PNP transistor, you don't want to end up with the same kind as a pair...
Let me know if you need further clarifications.
Charles
As this is new to me im happy for any advice, is there a thread detailing this process? pics etc......which transistors should be checked as a matter of course, if they do turn out to be above 50% apart would one need to replace a pair?
I don't recall seeing a thread discussing the subject. As a standard procedure, i'm checking all transistors and checking mostly the HFE and leakage but also, the transistor has to be properly identified the DCA75. I've seen transistors identified as diodes so this is clearly not good.
In a complementary configuration (Pairs), we are looking at a 10-20% HFE difference... if greater than 50%, look if one is below the minimum or close. This is the one to replace, you don't have to replace both. If you decide to buy transistors, go and buy the two kinds (PNP and NPN) in quantities of ten (approx), this will give you flexibility in your matching.
Ok....I have done some searching through the threads and Sorens BL5000 saga provided a lot of information, I have since contacted Frede for some additional advice regarding replacements, he advises as follows:
BC 550B for npn
BC560B for pnp
When the Beolab was designed (before 1967) only a few transistors were available is really low noise versions – therefore B&O have chosen a number of different types of coarse also being able to be used and meeting the actual voltage level. But I have for many years used the above type in the pre-amplifiers.
And I am using the transistors type B because higher gain in classic electronics design is not a good idea (this is my opinion 😊)
B U T --- for the power amplifier and the power supply – I only use the original RCA types !!!
For the output power transistors and psu transistors --- I am only using the original Motorola or RCA (there are to many “fake” 2N3055 out there).
When I am writing the original it is actually the original because I re-use these transistors from Beolab 5000 which is scrapped.
However when I look around I'm having trouble finding them, can anyone recommend a suitable equivelent for them?
Hold that last request.....found some here
reichelt elektronik GmbH & Co. KG Elektronikring 1 26452 Sande
Wait before ordering, i'll get back toy you...
When looking for replacement transistors, it is important to stay close the the original characteristics like the voltages, forward current, heat dissipation expressed in watts, pinout, better but not less.
Regarding the BC550B/560B, they will be good replacement for the black bullet ones found on the pre-amp section but I don't think they will be suitable for the output stage.
For the output stage, you may find modern equivalents into the TO-220 package instead of the TO-92.
Here's my best suggestion for now.
Take the output board schematic, write down the HFE you are measuring for each corresponding transistor and post the picture here. Same as you did previously for the voltages.
I will give you the best answer I can and maybe you don't need to change transistors at all, we never know!
What do you think?
Sounds like a plan......I have ordered the BC550B & BC560B for the pre amp black tops and will measure up the driver stage devices and see where we stand...........
I have some good used original vintage 2N3055 in the drawer.
Well.....this is an interesting exercise....left stage up first.
Now the right stage......
I will get to the remainder tomorrow...….however 453 looks in trouble
The one you spotted is lower than the minimum spec (HFE 30).
The modern equivalents are:
- 37741 = 2N5320
- 37740 = 2N5322
Ok....so the next question is should I replace all 8 off 37741 and the 2 off 37740 devices? bearing in mind Frede's comment regarding his preference to re-use old RCA parts salvaged from scrapped BL5000 sets over purchasing modern day devices? or should I only replace the one out of spec device