Beomaster 6000 refurbishment

3 replies
B(
Offline
beonix (archived2)
Bronze
Posted June 22, 2018 at 14:28 GMT
#2253838

I am in the process of refurbishing a BM6000...

Amongst other things (such as 7-seg displays etc), I have recapped the Power Supply PCB, remembering to link the two pins of 'C1', and also the Power Amp PCB along with fitting new trimmers for quiescent current and output offset voltage...

Rather than risk overheating of the Power Amps, I decided to run up with a Bench Power Supply +/- 50V DC.

I have found that the 'no load current' on the left hand amplifier immediately jumps to 21mV at power on, but cannot be adjusted with R234. The left hand output sits at around -45V.

The 'no load current' on the right hand amplifier climbs to 22mV, and is adjustable with R134, but the output sits at around -10V.

I have checked all diodes and transistors with my PEAK tester, and they all show good...

 

In desperation, I have now tried a second Power Amp PCB from another BM6000, and very strangely, it behaves in an identical way...

 

I have double checked the power connections on P34 (1 = -50v, 2 & 3 = 0v, and 4 = +50v). I am pretty confident that there is no need to connect anything to connector P31 & 32 at this stage (although input will be muted)...

 

I can only think that all the semiconductors need replacement, on both PCBs... Would seem unlikely...

 

Does anyone have any suggestions... Am I doing something wrong ?

 

Tony

 

 

DI
Offline
Dillen
BronzeModeratorFounder
Posted June 22, 2018 at 15:18 GMT
#2253839

beonix wrote:

Does anyone have any suggestions... Am I doing something wrong ?

Most likely cause is, that you didn't notice the way ground connections are made.

When you disconnect the amplifier module from the Beomaster to run it on external power,
you break the common ground connection.
From memory I think it's P14 on the tonecontrol board that joins them.

Fit the amplifier back into the Beomaster and do the adjustments there. 
(They won't be correct anyways if set with external power). 

Check also the small signal transistors solder joints.
Because of thin transistor pins in large PCB holes, the transistors are easily pushed through the board, so that their solder joints break. 

Martin

B(
Offline
beonix (archived2)
Bronze
Posted June 22, 2018 at 16:16 GMT
#2253840

Thanks once again Martin... That did indeed fix the problem...

As I am the nervous type, I linked P31/3 to P34/3, and P32/3 to P34/2 temporarily, and this restored normal operation on the bench supply...

You are right that single sided PCB tracks can easily be broken by pressure on component legs... one to watch for...

 

Thanks again..

 

Tony

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