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Beomaster8000 repair/resoration

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ALF
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ALF posted on Thu, Mar 21 2019 8:53 AM

greetings ALL 😁

I decided to start a new thread with my new donor-unit BM8000 - this time in the correct forum !

this donor unit was purchased as a parts-reservoir but turned out in very nice cosmetic condition.

so I took a close look and noticed all boards were recapped, the displac is fine....well, so far so good.

further testing reveiled there are issues:

Plugged in it shows the StBy dot.

turning on, no relay clicking while the display comes on.....hmmm ?!

checked the power supply board....all associated transistors have tested fine off-board, all caps are in the correct way, all connector joints have been reflown.....?!

next thing I noticed the small heatsink with the 3 voltage regulators is getting VERY warm....?

next thing : with no relays energized I can measure the 15V and -15V regulator which should only be possible

when the start-up circuit gives the “green light”.

5V should be present at Pin14 / 9IC4 during STBY mode and going down to ‘O’ when ON is achieved....no, it remains at about 25mV

meaning: the BM according to this is actually out of ST BY !!

5V are only present at the 5V regulator when I disconnect P76 of the processor board ?

a faulty processor 9IC4 ??

I did change the two 2MHz crystals (as suggested as a precautional measure by Beolover) after removing 

both uPs for safety reasons......! No change !

I disconnected the keyboard in case the on/off key is stuck and used the remote to turn off/on  the BM....

no luck here either 😡

disconnected P51 and 50....no change.

well, what am I missing here ?

and this is only unexpected work at the donor-unit while waiting for those speaker switch replacement to

continue work/repair/restore on my original BM8000 project...😳

guess help is badly needed here 🙄

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ALF
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last picture is the component side Signal switch "fly wireing" to D18/R25

can someone shed some light why this would be necessary ?

as always - many thanks

ALF


ALF
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ALF replied on Sat, May 11 2019 3:14 AM

No comments on those rather strange tone/filter mods ???

anyhow, another donor unit arrived and I was keen to swap over its control board to see whether the volume issue has gone away....

NO - it was still the same 🤬

I began to wonder if it would have been too much coincidence that both boards had mal-functioning AD7110 attenuators?

yes, possible but likely ???

question: who would be the next suspect on line here ? 

I will move on to the preamp board, however still trying to make sense out this !

as usual, any input would be most appreciated - it feels like I am almost there......😳

ALF

sonavor
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No comment on the board someone modified. I couldn't tell you why they did what they did so I would use an original as a guide to put it back right.

It is pretty unlikely that the latest unit you received also has faulty volume control ICs. I would try swapping the microcomputer boards (before altering anything).  Same goes for the preamplifier board. 

John

ALF
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ALF replied on Wed, May 15 2019 12:46 PM

Hi John,

thanks for your comment - swapped over the uP board as suggested first.....same outcome - that is with the tone/filter board from the “new” donor unit.

will move on to swap the preamp board but want to recap it first - no point running into way out if spec caps (the red Roe ones) and its consequences.

report back as soon as....

ALF

sonavor
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So are you saying that in the Beomaster 8000 that has the volume problem you are currently using a different filter & tone control board and a different microcomputer board?  If you switch the preamplifier board next I would first try it as is...before changing anything. If the problem persists then start changing the cables between the boards, one at a time, checking the problem after each swap. 

John

ALF
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ALF replied on Sat, May 18 2019 4:33 AM

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

sonavor
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sonavor replied on Sat, May 18 2019 11:28 PM

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

ALF
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ALF replied on Sun, May 19 2019 3:50 AM

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

 

Dillen
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Dillen replied on Sun, May 19 2019 8:16 AM

Are the amps powered up at all?

Martin

ALF
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ALF replied on Sun, May 19 2019 10:14 AM

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

sonavor
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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

ALF
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ALF replied on Mon, May 20 2019 9:47 AM

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

 

Lonnie
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Lonnie replied on Wed, May 22 2019 3:58 AM

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

ALF
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ALF replied on Wed, May 22 2019 6:15 AM

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

Lonnie
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Lonnie replied on Wed, May 22 2019 7:43 AM

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

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