More work hours spent 🙄.....

1st  swapped the uP board only from the donor unit (uP test passed) - vol issue unchanged.

2nd  swapped interconnecting cables - vol issue unchanged.

3rd  swapped unrestored preamp board from donor unit - vol issue unchanged.

as said before, unless both tone/filter boards have faulty vol attenuator ICs and both preamp boards are faulty 

I almost dare to say the problem could/should be hiding somewhere else in the unit ?!!

but where 😳🤬😳 !!

yes, we discussed the unlikely fact of having two faulty sets of vol attenuator chips I still want to dismiss that option.

both uP boards seem to do their job...is it unreasonable to conclude a problem in the output boards ? But,

I was always under the impression the unit would not switch on if there are problems on the output stages (fault switch !)

Seems we (I) need more brain-power here ...... just got that feeling I am almost there.......

if you guys have any further information call “crime stoppers”

ALF

If the problem isn't on the input to the volume control then it must be coming out of the volume control.  Are you sure the problem isn't your output amplifier assembly?

John

Thanks John for caring so much.....

well, the symptoms are similar to my last “rescue mission” - I crank up the volume to 6 on the scale and can hear a very low signal

something you could perhaps experience in a “cross talk situation” when the PH input is selected but idling  and a tuner crosstalk signal might

be audible at very high vol setting ?!

so far I was under the impression that the fault switch would engage if the output amp is faulty or wouldn’t it ?!

last time I discovered a wrongly connected cable which did the rescue after discovery - so I checked again and made sure it is not again the trouble source.

coming back to the output amp assembly: I can check whether those wire tabs are incorrectly plugged in, but assume this would be the case 

surely the fault switch would kick in ?!

any signal from the preamp would be fed through 4C128/228 to the vol attenuator and via 4P24 to the output stages - both those caps are connected correctly.

I shall get back to investigate the output assemblies........

ALF

Are the amps powered up at all?

Martin

Nothing visual obviously wrong on the output boards, however I measured the signal (VDC) at 4P24 pin1 and pin6:

both were about 170mV unchanged when the volume was turned up or down !

which tells me the output boards basically get nothing worth putting out?!

again, is this pointing to faulty vol attenuator ICs ?? 

Just getting sick of getting fakes from china - wish someone would sell NOS or working used ones ....

ALF

It doesn't sound like volume selection or source selection if you have tried more than one of those boards (preamp, filter & tone control, microcomputer).
On the output amplifiers though, did you say neither channel does anything?  What is the voltage across the emitter resistors (the no-load current measurement)?  What is the DC Offset measurement (across the speaker H/L). 

Did you also say in the past that you disconnected the audio input to the output amplifier assemblies and checked that cable with your scope?  I have done that before to measure the THD and frequency response of just the preamp section. The measurement point amplitude will (should) go up and down with the volume control.

If that test also fails then I would return to the Beomaster wiring. With multiple boards failing it seems most likely the problem is something common to them all which is the cabling of the Beomaster.

John

Back to work....

the no-load adjustment is at about 17mV and the DC offset is about 3.1mV which should not pose any danger !

I also swapped over the connecting cable 4P27 to 3P16 without any impact.

again, I looked at the signal at 4P24 pin1 & 6 which remained unchanged during vol increase at around 180mV depending on chassis point.

how much more head-scratching can one endure 🙄😬😬 ?

as things stand I would pass on the output boards as suspects - it is quite clear to me that these boards actually get nothing to work with !

I wish I could get my hands on a pair of reliable attenuator chips in order to either fix the issue and/or exclude those from the suspects list.

to be continued .........

ALF

Hi ALF,

I'd go back to basics to see where you lose the signal.

Starting at the obvious, do you have the external filter links in place?

Using the FM as a source do you have an AC signal at R250 on the preamp board? You can used the leg of R250 that is common to R249 as the measurement point and the other leg of R250 as signal ground.

If so, do you have an AC signal at R258? Use the leg that is common with connector P16(5) and the collector of TR203/204.

If so, do you have an AC signal at R236 on the Filter board? You can leave the signal ground at R250 and measure the R236 leg that is common with pin 15 of AD7110.

Regards

Lonnie

Hi Lonnie,

BINGO 🙏🤙🤗

the bloody famous “obvious”.......I can’t believe I missed that one !

yes, volume has been restored - thank you so much.

to be honest, i have never looked at those jumpers as they were on all BM8000 I have had so far.......

now I learnt something new again and get started with the next project......!

thank you to everyone who has tried to help......well and truly a team effort.

good to know one is in great company.

ALF

Hi ALF,

Well done, mystery solved. I agree with you, the people on this group are a fantastic resource and I've learnt a lot from their words of wisdom.

So... please don't keep us in suspense..... what is the next project???

Regards

Lonnie