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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 πŸ™„

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sonavor
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Hi Alf,

Don't try to tackle too many issues at once. Right now it appears your issues are related to the power supply circuits. You need to troubleshoot the relays and see why they are acting the way they are. Determine if the relays are good or if the circuit controlling them is faulty. If you think the voltage regulators are running hotter than expected then check what their incoming voltage levels are. I would go through the power coming in from the transformers through the rectifiers and the power up circuit. Getting the power supplies in order first will make troubleshooting other issues a lot easier. Plus power supply problems will cause other things to fail so there is no sense hunting those things down until the power is fixed. You said the Beomaster has had a lot of restoration done on it already. You don't know if it was done correctly though. Someone might have made a mistake like an incorrect value or polarity. You will need to double-check everything that was done.

John

chartz
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chartz replied on Thu, Mar 21 2019 8:34 PM

Hi Alf,

No time to delve into that tonight but I’ll certainly be around!

I have two working ones.

Jacques

chartz
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chartz replied on Fri, Mar 22 2019 12:49 PM
Try switching it on with both amp modules disconnected. I’d do that first. Then plug back each amp separately and see what happens.

The left side cooler does get quite warm, that’s normal. So to me this is a rather good sign, if anything.

Jacques

Lonnie
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Lonnie replied on Sat, Mar 23 2019 1:24 AM

Hi ALF,

I agree with a previous post of John's about faulty relays, I've also had to replace RL2 on a BM8000 I recapped a couple of years ago.

I'd measure the collector voltage of 6TR18 after you  plug the unit in and it's gone into Standby. It should be around -24v. When you try to power the unit on the collector voltage should increase to around -0.2v. If this happens and if you measure 0v at 7R1 then then I'd replace RL2. If the voltage doesn't change at the collector then I'd look for a fault in the relay circuit (6TR11/12/18).

Once you've resolved the issue with RL2 you can see if there is also an issue with RL1 or not.

Regards

Lonnie

ALF
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ALF replied on Sat, Mar 23 2019 9:33 AM

Thanks John,

indeed, checking someone else’ work is worth doing:

first finding were on the tone/filter board with 2 caps in the wrong way (πŸ™„) and a cracked IC2 chip.

Off to the power supply board 6: all caps are fine and put in the correct way, good news....

with P76 unplugged I have all voltage regulators IC4/5/6 delivering with

IC4 (5V) gets 11.3V supply, IC5(+15V) gets 24.4V supply and IC6(-15V) gets -25.2V and  P48 pin2/3 delivers 5V.

 

so far so good but, I am loosing the 5V the  moment I plug in P76 to the processor board - still scratching my head WHY ??

the processor board should get the 5V supply via P76 pin8/9. Unless I crack that 5V supply voltage issue I won’t even progress to further check RL1/2!

something must shorten out the 5V supply in the processor board ?? Damaged uProcessor ??.

btw, it is the same scenario with unplugged output boards 5, as thing Jacques suggested.

sorry Lonnie, I could not even get to your suggestions - many thanks for getting on board!

ALF

sonavor
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If you probe at P76 before plugging it in and there is 5V then lose it on plugging it in then there must be a short there. You said earlier you changed out the microcomputer IC oscillators. Did you remove the microcomputer ICS when you did that?  If so, did you make sure the oscillators were shorted while doing the procedure?  It is too late to worry about now but in the future one shouldn't really mess with the oscillators unless there are symptoms there is a problem related to it. I do change them out on restorations that involve expensive shipping just as extra insurance the unit won't develop a fault and have to be shipped back.

For now you need to verify the board is causing the short then see what component or trace is doing it. Since someone else previously worked on the board perhaps a bad solder attempt shorted a trace.  


John

ALF
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ALF replied on Sun, Mar 24 2019 1:48 AM

thanks for your getting back John, I guess ‘guilty as charged!’

no, I did not shorten the crystals 😒 but I carefully removed the two uPs before I caried out the operation and reset both on new machined tulip-style sockets. Quite possible that I may have damaged the main processor ??

too late now to worry about - i do remember now when I studied how to do that.

there are definitely 5V present at the loose end of female P76, meaning the connecting cable is fine. 

As I understand the 5V come in to board 9 via P76 pin 8/9 !.....9C86, 9L4, 9D6, 9C85 as first ports on entry ?!

time to re-check the soldering again, ....done that twice, but perhaps I missed a accidental bridge ?

ALF

ALF
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ALF replied on Sun, Mar 24 2019 3:17 AM

Ok,

solder-bridge at 9L7 !!!

the 5V supply has been restored πŸ™„πŸ˜

the uP test came out as ‘TP’ πŸ€™πŸ™

now I get a chance to follow uo on Lonnie’s suggestion about a possibly faulty 7RL1 or 2...

I’ll be back

ALF

ALF
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ALF replied on Sun, Mar 24 2019 4:06 AM

Hi Lonnie,

just measured through your suggestions:

6TR18-C showed

about +430mV in STBY

about 17.imV in ON

and at 7R1 fluctuating voltages somewhere between -100mV to - 1.1V

shouldn’t I see about -25V at the kathode of 7D2 ?? I get basically 0V

ALF

Lonnie
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Lonnie replied on Sun, Mar 24 2019 6:16 AM

Hi ALF,

The measurement on 7R1 should be an AC voltage measurement, can you confirm if that was the case?

The anode of the diode 7D2 should always be at -23v or thereabouts. The cathode should be -23v in STANDBY and -02v (approx) during ON.

Do you still have the -15v while in STANDBY? If you do do you have -23v at P51(2)?

Regards

Lonnie

ALF
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ALF replied on Sun, Mar 24 2019 10:32 AM

Apologies Lonnie,

dived straight into verifying your suggestions - should have looked at the circuit diagram....didn’t realise that 6TR18-C is against ground....

you can’t measure ground against ground !

I have to revisit the measurments and will report back once done.

ALf

ALF
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ALF replied on Sun, Mar 24 2019 11:20 AM

Hi Lonnie,

at 7R1 I get about 27VAC in STBY and about 2VAC in ON mode.

no, I don’t  have the -15V anymore in STBY.

I can not verify the measurments at 6TR18 you suggested ?!

ALF

ALF
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ALF replied on Mon, Mar 25 2019 6:28 AM

Ok,

I checked and trsted off-board 6TR18 once again - no adverse findings.

as per circuit diagram its collector is connected via P51- pin1 to 7RL2.....

here is where the problem may be hiding?

took out RL2, checked the resistance of its coil.....OL !!! Meaning its coil is burnt 😳😒

now the tricky part: 

where do I get a replacement relay ?

any suggestions are most welcome - thank you

ALF

Lonnie
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Lonnie replied on Mon, Mar 25 2019 11:11 AM

Hi ALF,

The replacement relay I used was:

SPST, Coil: 24VDC,  Switching Current: 10A@240VAC,  Coil resistance: 880ohm.

I found some 5A relays with a max current rating of 7A but decided to go with the 10A instead. The 5A is probably ok for the 240V version of the amp given that the fuse F2 is spec'd at 4A.

You can buy it here:

https://www.ebay.com.au/itm/24V-10A-RELAY-SPST-TYCO-EBERLE-SHRACK-AUS-STOCK-/180538893095

 

I've had a look at the power supply ON sequence and realised that the -23v is only going to be available when the the amp is switched out of STANDBY to ON. The C of 6TR18 will be 0v at standby and around -0.2v when ON. It will be easier to see if the relay engages once you replace it rather than measuring the voltages at C of 6TR18 again.

Regards

Lonnie

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