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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all,
I have a BM4000 type 2406, ser 286958 in for restoration. This unit is mint in rosewood, and has been with the original owner since new. It had a problem, which he took to a local tech, who wasn't able to fix (not surprising, as over 30 capacitors I just measured have ESR through the roof).
Anyway, it found its way to me, and I am still doing research on it before I start soldering. It has the later 800295 PCB (Output Amp) with the 4 extra 5K trimmer potentiometers. I have not worked on this PCB version before, so I would like some advice from those of you who know it.
I need to change all 6 of the potis on this PCB, as they are totally black and burnt. I'll use either Bourns or Piher cermet units.
A service bulletin states that these 4 5K potis (585, 586, 587, 588) are factory set, and should not be touched. All very well and nice, as long as the previous tech didn't touch them. I can't be sure that he didn't, because the Bias Emitter potis are both miraculously set to full scale 250 ohms (obvious why he left them like that), and if he fiddled with those, he probably fiddled with the 4 "Don't-touch" potis as well.
One of the output MJ3001's is shot as well.
My guess, and it is only a guess, would be to calibrate the Emitter voltage to 12mV, and then adjust those 4 potis for minimum distortion on the Audio Analyzer. Would that be the correct direction?
Menahem
That sounds correct to me. When you go through that process please document it with some pictures. It will be helpful to reference if/when I have to change those trim pots on mine.
I think from memory,these pots are part of a dc stabilisation circuit for the output transistors,.I think all you need to do is set the base voltages of the transistors tr44/45(e.g.lh chan) to 33v,using preset 585 for tr44,and 586 for tr45.
Nick
Nick,
That's great info, thanks!
Now why couldn't B&O put that simple description in the Service Manual?
This is a Beomaster 4000 I intend to refurbish,and you can see it needs the pots doing too!No work has ben done yet,so the pots are still in the factory set positions.If you set your new pots to these settings,you won't be far out,and can "fine tune" with a meter,after setting the quiescant current.
I sent a request to B&O for clarification, and this is what they sent.
It's definitely a factor of distortion.
I have now received feedback from one of our techncians, who informs that the problem is to be found in the electronic fuse. Please see the enclosed description (in Danish). I have tried to make a translation into English:
Change of the electronic fuse in the RF output
In order to compensate for dispersions in the components use din the electronic fuse, a modification has been made: diodes pos 485 and 486 have been changed to type AA143 (or OA47) No 8300142.
Resistors pos. 487 and 494 15Kohm, have been split into a 10Kohm resistor, pos. 487 and 494 and a 5Kohm trimming potentionmeter pos. 585 and 586 no 5370058.
Adjustment of potentiometer pos. 585 and 586
Connect oscilloscope to the speaker output
Amplifier is fed 1 kHz sine, until clipping starts
Capacitor 4.7uF is mounted across C-E of TR43
Resistor 190ohm 23Wa is mounted across C-E of TR46
Potentiometer 5Kohm pos 585 is adjusted until distortion on negative peak
Resistor is moved to C-E of TR47
Potentiometer 5Kohm pos 586 is adjusted until distortion on positive peak
Left channel is adjusted as above
Looks simple enough to do it, but where does one find a 190ohm 23W resistor?
Electronic fuse?The plot thickens indeed.
Remarkable that you got a reply from B&O tech,I never have,must be something I said?
I'll stay with my original advice though.It's about biasing the output pair,and they are driving it into distortion as a datum point for setting.
Interesting though.I think I'll get on and sort my own Beomaster 4000,and try out their method.
Regards,
Menahem Yachad:where does one find a 190ohm 23W resistor?
probably paralleling two of these will get close enough (195R)...
Gunther (7th try to post this )
solderon29: Electronic fuse?The plot thickens indeed. Remarkable that you got a reply from B&O tech,I never have,must be something I said? I'll stay with my original advice though.It's about biasing the output pair,and they are driving it into distortion as a datum point for setting. Interesting though.I think I'll get on and sort my own Beomaster 4000,and try out their method. Regards, Nick
Electronic fuse they are referring to the output protection.
Olly
Thank you for that Menahem, best inform Peter so he can integrate this to the archives?Nice to see B&O positively respond to requests for technical information. Would love to see the archives at Struer!
Yes indeed,very useful info,thanks
We never had much need to touch these pots back in the day,except in cases where the owner had had a go at repair and "tightened all the screws"!!!We would then set everything up as normal,and set these pots to datum points from experience.In a busy workshop there was allways a risk of "tuning for maximum smoke" too!!
Happy day's!
Here, according to the service bulletin, I have connected the 4.75uF capacitor and 190Ohm resistor.
The 2 red wires go to the Resistor - 200Ohm paralleled with 4.32KOhm - sitting at the top rear.
And now I'm stumped!
The bulletin states to feed in a 1KHz sine wave until clipping starts. So I did, inputting the signal via the TAPE1 preamp input.
Well, here is the scope showing a PERFECT waveform at the speaker output with no distortion at all.
There is no distortion to correct and make it appear on either the upper or lower peaks!
Adjusting the 5KOhm poti makes no change in the waveform. Increasing to max volume does not introduce clipping either.
Have I missed something here, or have I done such a perfect restoration that there is no distortion at all ;-) ????
I've done this a few weeks ago.
First you have put a maximum inputsignal which makes the amplifier almost clipping. That makes proabably the difference.
I've seen the waves being cut off (clipping) in the positive or the negative part depending on the which poti is adjusted.
Menahem Yachad: Here, according to the service bulletin, I have connected the 4.75uF capacitor and 190KOhm resistor. The 2 red wires go to the Resistor - 200Ohm paralleled with 4.32KOhm - sitting at the top rear.
Here, according to the service bulletin, I have connected the 4.75uF capacitor and 190KOhm resistor.
You should have used a 190 ohm /23 Watts resistor instead of 190Kohm.
I've made mine out of 3x 560Ohm/10 Watt resistors parallel. It gave me 187 Ohm.
Typing error - it is a 190 Ohm!
I'll try your recommendation to increase the input signal now!
I increased the input signal to max, and got this waveform. I then adjusted the poti, but no response on the waveform.
On both TR46 and TR47's potis, the clipping is shown on the negative peaks. with no response to adjustment.
Sorry, I am not familiar with these digital scopes. How many volts per division?
I started digging in my papers and found this one, maybe it helps?
As I remember, I fed the amplifier straight at the entrance on the amplifier board itself. Not via the Tape input.
Those small marks at the bottom shows where the audio signal is connected.
OK, tomorrow, I'll try the sine signal connected direct to the amp PCB (instead of via the Preamp).
My Tek 2465 is pure analog, since 1987. I can't work with the new Digital scopes either - they're much too complicated!
The Voltage/division is the number at the left bottom - 1V AC. The speed is 200uS.
If you feed the amplifier directly with maximum signal, then you should measure an amplified signal with around 60 Vtt.
Then you need at least 5 Volts/division!
surely you have it set to x10 on the probe?
Olly - you are very perceptive! But I was using a direct connection (x1), but at low volume, via the preamp.
I still haven't solved the problem, but this is what I did:
I had previously set the Bias Current to 12mV (approximately 60mA), so I decided to set it for 80mA, according to the SM.
I connected the sine signal directly to the Amp inputs, and did the adjustments again - still no response on the waveform when adjusting the potis. I have left the 4 potis at the factory setting, which is about 1.6KOhms, measured in-circuit.
Having followed the B&O procedure to the letter, and not having success, I can only conclude that the level of distortion on this restored unit is below that which causes the potis to be of any effect. If any of you have any ideas contrary to my assumption, I'd be happy to hear them!
I am now running a 48-hour soak test at medium volume to make sure that everything is fine before returning it to the customer. After about 4 hours so far, it is running perfectly through 2 S-45 speakers. The cooling fins are barely warm to the touch.
If any of you see anything which gives you a hint on the next waveforms (x10 probe), please let me know.
The first waveform here is with a HIGH-level sine wave input, and the BM4000 starts to clip at 7.5 on the volume scale. Voltage is more than 45V P-P. (The voltage scale is >5V per division).
This next waveform here is with a HIGH-level sine wave input, and the BM4000 is fully clipping at max volume. I tried adjusting the potis here, but again, no response to the waveform.
This next waveform here is with a "just-above-minimum"-level sine wave input, and the BM4000 is NOT clipping at all at max volume. Voltage is more than 37V P-P.