• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

YAQIN MC-5881A Channel volume balance.

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irondutchess

irondutchess

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Joined 2018
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#1
Hi,

I have a YAQIN MC-5881A Vacuum Tube Integrated Amplifier that's about 18 months old,

The right channel is noticeably louder than the left. I have tried swapping over the speakers, also swapped the valves side to side, neither resulted in any change. There is no balance control, so I can't compensate that way. Both channels are quiet with no input and volume up, and there is no discernible distortion on either channel.

When I first had the amplifier, there was no noticeable difference in the channel volume, but it has come on over time. Any suggestions as to what might be the cause would be appreciated.

I am able to strip the amplifier and work on it with confidence, so testing and replacing components isn't a problem. The only equipment I have is a digital multimeter and a small portable made in China scope.

The problem I have is not knowing where to start and what the various test readings should be, and where the various readings should be taken from.

Basically as far as Valve radio equipment is concerned, I'm pretty much starting from scratch. All the equipment I have worked on was transistor or IC, and nothing to do with audio amps or radio.
 
#4
PRR said:
Are you safe and competent to check internal DC voltages?

On a Stereo amp, comparing voltages between Good and Bad sides will reveal a large percent of common troubles.

Yaqin MC-5881A amplifier improvements
http://g4cnh.com/public/A_LOOK_AT_THE_YAQIN_MC10L.pdf
Click to expand...

Many thanks for the .pdf link. I had a quick read through this morning. I'll take some measurements and post what I find.

I am okay working on live mains voltage equipment, I did it for a living for many years, just not on this type of equipment.
 
#5
disco said:
Is the difference between L and R constant with the position of the volume knob?
Click to expand...

Many thanks for the fast reply.

I took the volume from zero to full volume by progressively reducing the input signal so as not to turn the speakers inside out. I also swapped the left and right inputs just in case it wasn't the amplifier at fault.

The difference in left and right volume is constant through the whole volume range.
 
#7
Might be worth checking the values of the 47k and 51k in the phase splitter.
Channel imbalance has been traced many times to the volume control but I gather you have ruled that out. This amp uses cathode auto bias, have you tried swapping tubes over from Right to Left and vice versa to see if the miss balance follows.
At least this will rule out any tube deficiency and you can concentrate on other components.
 
#8
Hi_Q said:
Might be worth checking the values of the 47k and 51k in the phase splitter.
Channel imbalance has been traced many times to the volume control but I gather you have ruled that out. This amp uses cathode auto bias, have you tried swapping tubes over from Right to Left and vice versa to see if the miss balance follows.
At least this will rule out any tube deficiency and you can concentrate on other components.
Click to expand...

Tube swapping L to R makes no difference, still louder on the left.
 
#10
Well serves me right for not being more thorough with my testing.

It turned out to be a dry solder joint in the RCA phono plug on the lead from the Dune box. The connection must have moved and connected properly when I swapped out the amp to try another one, so masking the problem.

When the amp was put on test with a 1K sine wave it tested up okay, the penny dropped when I reinstalled it and it worked okay. A bit of lead tugging revealed the problem.
 
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