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Beomaster 1900 no power, not even standby light

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FredFredson
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FredFredson Posted: Mon, Nov 29 2021 11:49 PM

Hello all

 

My beomaster 1900 is not powering on. No power LED.

 

I took out TR27 and it tests fine. Confirmed it is insulated from the metal housing / screw.

 

When plugged the TR27 measures

B = 24V

C = 12V

E = 24V

 

Is that expected?

 

I also replaced D50 as I had a spare one but no change.

 

Measured voltages in the attached image

RED = 0v

BLUE = 12V

GREEN = 24V

 

 

 

Also measured TR28 and TR29:

 

TR28

B = 24v

C = 0

E = 24v

 

TR29

B = 0v

C = 24v

E  = 0v

Dillen
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Dillen replied on Tue, Nov 30 2021 8:46 AM

First guesses:

Bad D52
Bad TR29

Martin

FredFredson
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Hi Martin,

Thank you so much for the guidance.

 

I removed transistor TR29 and it passed the multimeter tests for NPN transistors

D52 was removed and tested fine too.

 

I also removed TR28 as it was close by and that passed transistor PNP tests.

 

It seems the standby light runs on 15V but I don't seem to have 15v anywhere. Would you know where the source of the 15V comes from and I can test back to that component? thanks again for the help!

 

 

FredFredson
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I found the schematic. 15V comes from the centre C pin on on TR27. Yet this is 12V.

I'm also getting 0V at Test Point's 15 and 16 which should be the same 12V.

 

Powered the unit down and continuity tested. It turns out TP15 and TP16 are shorted to ground somewhere...

 

I removed TR27 again but the short to ground is still there.

 

Followed the schematic on the 15v line from TR27 C. The next component was TR31. Removed this and it tested fine. Put it back in and now the unit is working....... A little odd but I'll take it!

 

 

manfy
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manfy replied on Wed, Dec 1 2021 5:09 AM

FredFredson:

....... A little odd but I'll take it!

"A little" odd?? Isn't that a little bit of an understatement? Hmm

I'd say you messed up the measurements somehow. For starters, it's impossible to get 24Vdc out of a 15Vac supply
Where did you connect the black probe of your multimeter in these measurements?

Assuming that the deck really didn't work when you started out, I'd guess you have a grounding problem, ie. broken wire somwhere, a cold solder joint, broken GND trace or similar. It's not the desoldering/resoldering of TR31 that fixed the problem, but the handling of the board while doing so.
Unfortunately that's not a permanent fix. As soon as you touch the wrong wire, move the board a bit, etc etc, it will happen again. So, it's worth investigating the issue while the unit is still dismantled.

FredFredson
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LOL I have to laugh (or cry....). you are spot on manfy - I put it all back together and...... back to square one. No standby light.

 

manfy:

For starters, it's impossible to get 24Vdc out of a 15Vac supply

I've numbered the measurements below with multimeter in DC mode. Which one should I not be getting 24v from? I've also marked where I had the ground clip (on C92)

RED = 0v

BLUE = 12V

GREEN = 24V

Not marked in the photo above butTR28 and TR29 measured:

TR28

B = 24v

C = 0

E = 24v

 

TR29

B = 0v

C = 24v

E  = 0v

 

 

Here is another photo where I get continuity on the multimeter which shows a short to ground from TP15 and TP16:

 

 

Interestingly I'm not getting any beep measuring from the TR27 Collector pin to TP15 or TP16 so perhaps that's the issue

 

Thank you!

 

Edit: It's working again simply by flipping the unit around..

In standby mode the measurements in DC are:

1 =not measured this - is this AC?

2 =22v

3 =23v

4 =23v

5 =23v

6 =22v

7 =0v

8 =15v

9 =15v

10 =15v

 

Edit 2:

I powered it off and on a few times. Then powered it off, tested TP15 to metal case ground and got continuity (still a short to ground?)... and now the unit will not power on any more.

 

 

 

Dillen
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Dillen replied on Wed, Dec 1 2021 12:05 PM

manfy:

I'd say you messed up the measurements somehow. For starters, it's impossible to get 24Vdc out of a 15Vac supply

15 x 1,4 = 21
Unloaded the transformer would give a few volts more than loaded, so 24V is not at all impossible.

Martin

FredFredson
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Hi Martin, should I be getting a path to ground from TP15 and TP16 on the multimeter or is that likely the cause of the problem (a short to ground somewhere in the 15v circuit)?

 

I recall having this issue before (no standby) when I didn't fully insulate the TR27 but TR27 is 100% insulated. I still had a path to ground from TP15 and TP16 with TR27 removed.

manfy
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manfy replied on Wed, Dec 1 2021 1:38 PM

Dillen:

15 x 1,4 = 21
Unloaded the transformer would give a few volts more than loaded, so 24V is not at all impossible.

Martin

Granted, my statement "it's impossible" doesn't hold. Even in the seventies the grid voltage was specified with 220V +/-10%, ie. 242Vrms maximum.

I suspect that the voltage selector switch is still set to 220V even though the actual mains voltage is somewhere near 240V and that gives you an extra 10% on the trafo secondary. But that's not a good idea. If the actual mains voltage is 235V or above, it's safer to switch the selector to 240V. Semiconductors are not very forgiving when it comes to over-voltage and you need some headroom for the occasional power surge from the grid!

FredFredson:

Hi Martin, should I be getting a path to ground from TP15 and TP16 on the multimeter or is that likely the cause of the problem (a short to ground somewhere in the 15v circuit)?

Absolutely not! TP16 and TP15 IS the 15V rail and if that's shorted to ground the fuse will blow.
TP16 should show 15V as soon as the unit is plugged in, TP15 will show 15V when you bring it out of standby.

Are you buzzing the circuit through or are you really measuring 0.00x ohms?? Depending on your multimeter the continuity buzzer can come on from 0 to 20-30 ohms... but 20ohms is NOT a short to ground!!
In fact, if your light bulbs 6IL1 and 6IL2 are not blown, you should read some 25ohms between TP15 and GND.

------------

Here's a thread from last year that might be helpful <BM1900 dead after recap>
Accordingly you should see following resistances to ground from various testpoints:

TP14 - 2-8k ohm

TP15 - 14.7 ohm      (!!!)

TP16 - 1.6-10k ohm

FredFredson
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Thanks for that link.

 

Here is what I'm measuring.

 

 

TP14 - no photo, but it was measuring higher and higher the more I held the probe. starting at 2k ohm climbing to 7 k ohm when I removed the probe.

 

 

TP16 as marked with an arrow on the base of the PCB:

0.3 ohms

 

 

TP15 as per PCB arrow marking:

15.9 ohms

 

 

 

 

If this confirms TP16 is short to ground then why is the fuse not blowing. Can you see from the photos am I using the correct Test Points?

 

 

manfy
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manfy replied on Wed, Dec 1 2021 6:14 PM

FredFredson:

TP14 - no photo, but it was measuring higher and higher the more I held the probe. starting at 2k ohm climbing to 7 k ohm when I removed the probe.

That's ok. The measuring voltage charges up capacitors on the 12V rail and that's why the value keeps increasing.
TP15 looks good too.

FredFredson:

TP16 as marked with an arrow on the base of the PCB:

0.3 ohms

[...]

If this confirms TP16 is short to ground then why is the fuse not blowing. Can you see from the photos am I using the correct Test Points?

I can't explain that value. I don't know whether the PCB print is correct or not; the service manual doesn't show layout drawings. Don't blindly trust the print on the board. Check on the component side to make sure that the test point matches TP16 in the circuit diagram.

What happens when you reverse the probe polarity on the multimeter? I can see no reason, why you would read 0.3 ohm between TP16 and ground.
correction: I see there's a wire (+15V Stby) going from TP 16 to the program selection panel. If you cannot find any reason for the short circuit on PCB2 then disconnect this wire and see what resistance you read then.
And yes, the negative lead of C92 is a good GND reference point for that measurement.

Oh, and while you're at it, try to fiigure out why the collector of TR27 is not connected to this test point - cold solder joint? damaged PCB track? or whatnot??

 

FredFredson
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Found a great website with pics

https://beolover.blogspot.com/2019/02/beomaster-1900-type-2904-initial-power.html

This shows TP16 on top and it's different to the one I was using on the base. .....

 

Real TP16 to ground is measuring OL

Real TP16 to TR27 C has connection 0 ohm.

FredFredson
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Followed the schematic tested all links.

 

Soon found that the input to R146 was not connected. It should be connected back to the output from the rectifier/c92+/r147.

If I pressed down really hard on the back with the meter it connects so it looks like a bad connection. Re-flowed the solder and still nothing. Soldered in small piece of wire to link it back to the C92 positive side.

Device is now turning on again. Another lucky fluke that will not last?

 

 

 

manfy
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manfy replied on Thu, Dec 2 2021 6:30 AM

FredFredson:

If I pressed down really hard on the back with the meter it connects so it looks like a bad connection. Re-flowed the solder and still nothing. Soldered in small piece of wire to link it back to the C92 positive side.

Device is now turning on again. Another lucky fluke that will not last?

Congratz! Now you found something that can easily explain the missing 15V. But yes, I'm afraid, it might not be a permanent fix... You haven't found a root cause for the missing connection between R146 and the incoming 19++ volts. I see that R146 is a standard carbon film resistor that is mounted vertically. A failure of the solder joint is possible but that's normally fixed by reflowing/resoldering the joint. Your description sounds more as if there were an internal micro crack in the resistor. If that's the case, the failure will show up again as soon as thermal or mechanical stress is applied to that resistor and that region of the board.
But anyway, that's nothing to worry about right now. Just keep it in mind and if the failure reappears, replace R146 with a new one.

You definitely should measure the voltages in that circuitry to see how well those values compare to the nominal voltages given in the circuit diagram. This should give you some clues as to whether you should do some extra work before putting the deck back together.

FredFredson
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So far it seems to be holding up well. If it goes again I'll replace that resistor.

I have to say the measured the voltages do not all match the schematic. C92+ is measuring 22V but schematic says 19v.. Then again 22v is what the beolover blog is measuring too so maybe the schematic is wrong. https://beolover.blogspot.com/2019/02/beomaster-1900-type-2904-initial-power.html

 

Thanks for the help Manfly and Martin!

 

 

manfy
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manfy replied on Sun, Dec 5 2021 11:19 AM

FredFredson:

I have to say the measured the voltages do not all match the schematic. C92+ is measuring 22V but schematic says 19v.. Then again 22v is what the beolover blog is measuring too so maybe the schematic is wrong. https://beolover.blogspot.com/2019/02/beomaster-1900-type-2904-initial-power.html

No, the values in the schematic are not wrong per se, you just have to put them in perspective. The manual was written in the seventies and analog multimeters were the standard back then. You didn't really get the precision and the accuracy you get from cheap digital meters today.
I believe, in one of the older service manuals they specified the multimeter accuracy with +/-5% or better and they specified that the actual values should be within +/-20% of the values shown in the schematics - that's quite a lot. Smile
Additionally, the mains voltage in the seventies was 220V in central Europe and Germany, so probably the same in Denmark. Today it's formally 230V +/-10%, but in most places it's closer to 240 than 230V.
Nevertheless, those values in the manual can be immensely helpful for troubleshooting and rapid fault finding.

And yes, the 22V from the rectifier sound fine to me, much better than the initial 24V.

 

Dillen
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Dillen replied on Sun, Dec 5 2021 12:17 PM

manfy:

No, the values in the schematic are not wrong per se, you just have to put them in perspective. The manual was written in the seventies and analog multimeters were the standard back then. You didn't really get the precision and the accuracy you get from cheap digital meters today.

And input resistance/impedance. Though in this case the mains voltage is probably playing the greatest role.
I agree that 22V sounds reaonsable.

Martin

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