Brig 001 amp: Left audio channel allways strangely quits working after some minutes

[martin0507]

Hey guys,

I've found an old Brig 001 hi-fi amp that sounds great, but the left channel quits its job after several minutes in use (between 5 and 30 minutes, I feel like it also varies depending on the volume). When I shut down the amp, the channel recovers after a while. If I switch to mono mode, I can listen to the right channel on both speakers and the problem occures independently of the input channel, so I would estimate, that the error lies between the channel selection dial and the stereo/mono switch.
Another interesting observation was that the left channel once came back when I gave the amp a slight hit, but I couldn't reproduce that later.

I really have no explanation, but I think it might have something to do with components that heat up or charge.
Because it is old russian technologie, the components are rather big and I decided to replace the components in the area of the circuit that might cause the failure.

Before I start, I wanted to ask you guys, if you have a better tactic to find the error or if you have any assumptions which components might be the problem and how to solve it.

Anyhow, I think it is a quite unusual behavior and I'm curious to find out more about it.


PS: I am not a nativ english speaker nor do I have a degree in electrical engineering so please don't judge to hard

 

[shrtrnd]

Two possibilities come to mind for me:
1) The switch controlling the speaker selection needs to be cleaned or replaced.
That's the most consistent problem I've found with the symptom you describe (especially the 'hit' that brought
the speaker back into service temporarily). You may be able to verify that by selecting different switch selections
repeatedly and seeing if the problem corrects itself. Dirty or worn switches make intermittent contact.
2) If you determine the switches are ok, I'd invest in a can of compressed 'freeze spray'. After the components
warm up, and the speaker goes out, spraying each semiconductor, one at a time should isolate the problem, as
when you cool the faulty component, the speaker will immediately come back on-line (until it heats-up again and
fails again).
Good luck, but it sounds to me like it's a worn or dirty switch contact.

 

[duke37]

It is apparent that you have a stereo amplifier so you can compare one amplifier with the other. If both speakers can have output in some conditions, then both amplifiers are working and the problem is the input. I would agree with a switch or volume control problem.

 

[martin0507]

Thanks for your thoughts on it!

I am not to sure, if it really has something to do with the input or switch, because as I mentioned, It has several inputs, from which you can choose via a knob/dial and the problem occures in the same way everytime. Also, if the left channel is gone, I can't bring it back by toggeling the switch or changing the input settings. The only way I found is to turn it of and let it settle for a while. I am sure, that both amp units are working fine, but it must be an error in the preamp part, or whatever that is.

http://www.audiovintage.fr/leforum/viewtopic.php?f=74&t=18663

here, someone posted some pictures, maybe it helps to imagine I think that the bad part is the circuit on the left circuit board

Thanks for the trick with the freeze spray, I'll test it, if I can't find the weak spot.

 

[martin0507]

I also soldered an input directly to the mono stereo switch, and it worked fine, but while I turned the bass and treble knobs, it resulted in loud plopp sounds. Maybe I can fix that and I would be happy.

 

[duke37]

The problem could be a temperature effect as previously stated.
It could also be due to a leaky electrolytic capacitor which changes DC conditions. Measure voltages before and after the problem occurs.

 

[73's de Edd]

I think that I see a headphone jack on the front of the unit, why don't you wait until the unit goes off and see if both channels are present on the headphone or not.
That would then let us know how early the stage is being, that the trouble is occurring in .
Also if the top cover is off the unit, why don't you try some tappa -tappa - tappa with the insulated plastic handle of a screwdriver to see if having a loose connection , or opened solder connection might be involved on one of the units PCB's.

73's de Edd

 

[martin0507]

Thanks for your replies. I've already tapped the components and it had no effect. The components do not look damaged neither. To me, it doesn't seem to have a mechanical cause.
I can't work on it until weekend, but I'll prepare everything and try out your suggestions. The headphone jack is an old 5 pole DIN connector type, but I'll see, if I can find a suitable plug. It should work.

Can I use some sine wave as input and measure with a normal ac voltmeter?

I'll give an update as soon as I find out something new. Thank you a lot so far!

Some schematics, if you are interested and able to interpret the Cyrillic script
http://s44.radikal.ru/i104/1010/73/77a8ceaa51d0.png

 

[73's de Edd]

Sir martin0507 . . . . . . . .

Soooooooo . . . . . it looks that this is going to be a multi national effort, with the products internal views
and layout, initially being provided by a French link.
Then, with our Russkie schematic being provided by compliments of a Russian ham . . . . along with a
massive dose of repetitive malware / spam pick up enroute.

Looks like this units power output stage is a variant of the use of a right and left pair of Russkie
versions of the 2N3055 /2N2955 power output transistors.

They are being WIRE remoted from the audio output amp/driver stage PCB, so there might be a
connection problem possibility in the involved plugs and transistor sockets ?

Since you are able to HEAR music from both left and righ channels, you should be able to see the audio
excursions and peaks on a meter used in its lower AC ranges.
Being MUCH MUCH MUCH more of a better possibility, if you are using an old analog meter, instead of
the snail paced conversions and captures of a modern digital meter .
UNLESS that meter aditionally incorporates calibrated hash marks as bars that respond like an analog
meter scale. These meter styles will respond a bit faster to voltage changes / displays.
Notice the mentioned lower bar scale on the dual scaled meter shown just below.

Lets start with the marked up schematic provided just below and have you do an AC voltage test on some easy to find, multiple shared points, near some identified transistors.

The YELLOW marked up area near the VT13/14 and VT15/16 output , with a proper selection of AC voltmeter ranging and the level that you are playing your music should show a fluctuating of the metering with the changing audio peaks.

The PINK input area near VT3 base will be a bit more challenging , with metering, needed to be shifted on down to millivolt levels sensitivity.

You will initially set up incoming music and monitor YELLOW to confirm audio on both channels.
This testing point is PRIOR to any protection relays, so they will not be at fault for any drop off of audio due to them possibly engaging.
We are now looking for a loss of one of these audio stages or possibly, their supply voltages.
If / after you do loose audio on one of the channels, confirm that there is loss at one of the earlier measured YELLOW test points.

Or go back to the earlier measured PINK test points and measure to see if the loss was due to no audio coming in to that point.

If this YM board checks out, then we move to an earlier pre amp stage for more testing.

Ceste le . . . .MARKED RELEVANT SCHEMATIC :

https://s22.postimg.org/p47xvz4sv/YM_Audiutput_Board.png


73s de Edd

 

[martin0507]

Thanks Mr. 73s de Edd,

by bridging the YC board part, I was able to listen to both channels again, so I doubt that the YM board is the problem, but I'll check for it as you proposed.

As far as I understand, the signal comes from the input, goes through the input channel selection, then through the YC board and I don't really understand, what is happening there. When I connected the three wires from an old headphone jack to the connections 3, 4 and 5 between YC and YT, I was able to listen to music in stereo and would use it like this, if there wouldn't be a loud plopp noise, everytime I turn the bass or treble knob for every step I turn it. The YC part seems to prevent that from happening. So I would screw the whole YC board part and build a new aux input, if I'd know how to fix that noise problem, because that's all I need.

Btw I don't understand Russian, only a little bit French and some English which makes the whole thing slightly harder but somehow it's starting to become fun and I can't wait to go to my workshop this weekend

 

[73's de Edd]

Sir martin0507 . . . . . . . .

REVIEW TIME :

I had marked up the YM board and I could not quite interpret if you STILL loose one channels speaker audio from that board after a bit of run time ?

I was just getting ready to mark up the board which feeds that YM board, which is the YT board.

Looking back at the very top of the main schematic, at top right is the YM board which receives its R and L channel incoming audio on that left boards connectors # 3 and 11.

The YT board receives its incoming audio from that boards left connector through pins #3 and #1 with its pin #2 being their shared ground.

Can I assume that the wiring harness to the just mentioned connector to the left, with the marked 3,4 and 5 is being the same as your reference . . . . and that you never loose one channel when monitoring that input.
If so, that suggests that the loss of one channel is occuring within YT board.

If in agreement on that, do you want me to now continue on a markup path, of the audio flow path inside circuit board YT ?


73s de Edd

 

[martin0507]

Hi, a little update: I have removed the top of the amp but I cannot start to look for the error, because it works just fine since yesterday... If it had a mechanical cause, carrying the amp around might have solved it for now. If it was a thermic issue, it might be due to the cold workshop, though I tried to bring the amp to almost normal room temperature.
Anyway thank you all for your help!
I will keep track of it and come back, when I need your help again.

 

[martin0507]

Hello Guys,

I got a little advice from an electrical engineer. He told me to first check on the DC voltages from power supply to ground at different points in the circuit.
By measuring the DC voltage at different spots on the YC board, I finally found out, that the transistor VT12 temporarily caused a short. After I replaced it with a new similar one it works just fine again.