Yaqin MC-10L losing volume

[JayDCason]

Hello All,

I am having a problem with this amp....the Amp runs fine for four hours then begins to degrade to fuzzy and the bias voltage goes to half. Instead of .35vdc when cold after four hours of play it ends up at .15vdc with the sound getting really fuzzy etc. Any thoughts or advise on where to begin would be appreciated. After looking at the schematics posted here I believe it's a cap to ground problem in the bias circuit ? Or worst is the mains transformer? Just blew V3 several weeks ago and replaced all the EL34 with a fresh quad set last week. Only other mod was replacing the stock fuse with slow blow since my unit died after a couple of hours of play when I first received it two years ago. I love the sound of this amp and really want to repair it. I have worked on troubleshooting cellphones and UHF low wattage two way radios so I have a little experience but nothing high voltage so I am a little concerned with the HT supply etc on this unit.

[SY]

First step: CAREFULLY measure the B+ when the amp is running OK, then again when the amp is fading. If it's dropping dramatically, that will have a different set of possible causes than if it's holding steady.

Can't emphasize enough- be careful. These are deadly voltages.

[JayDCason]

Thanks Sy,

When I powered on the amp this morning it operated sub standard right off the bat making the audio sound garbled. So all I can do is as you requested and take the B+ operating voltage which started off at 400vdc and after an hour warming up it read 386vdc. My system has a white wire for B+ versus a yellow that I noticed in this reference picture:

http://homepage.ntlworld.com/lez/YAQ...1_voltages.JPG

thanks again for your interest and help.

[SY]

OK, so play it hard for a while, then when you hear the distortion, measure the B+ again. Is there a schematic for this amp posted anywhere on the 'Net?

[JayDCason]

This is the only one I know of .. found it on this site.

http://homepage.ntlworld.com/lez/YAQIN%20MC10/

[Bas Horneman]

fter looking at the schematics posted here I believe it's a cap to ground problem in the bias circuit ? Or worst is the mains transformer?

If it is in both channels. Then I would suspect the bias circuit. What are the bias voltages for each of the EL34's? (Where the -21.2 and -19.0) is measured?

Ps... Don't know why the bias voltage should be different for each tube? Or is that just an example of what it sometimes take to balance the output tubes against each other?

[JayDCason]

Bias voltages were set to .35vdc for all four EL34s. Now it reads .140vdc. B+ is about 386vdc.

[SY]

Originally Posted by Bas Horneman

If it is in both channels. Then I would suspect the bias circuit.

Could be, but why would you suspect that rather than B+? We still don't know if that's OK after the amp gets to its Dark Place.

Jay, what you're measuring is not the bias voltage. Bias voltage is a negative voltage applied to the output tube grids to control the idle (no-signal) current. What you're measuring is the voltage at the output tube cathodes- there's a small resistor there which is used to convert the idle current into a voltage that's easy to measure (no real audio function for those resistors). So, yes, you measure that voltage to set the bias, but you're measuring an effect twice-removed.

[JayDCason]

What should B+ be? 440vdc? Cause my amp started out in its "Dark Place" with no warm up and only reads 386vdc on B+ and the bias measement twice removed is less than half of what I had set it last week.

[Bas Horneman]

Found a different schematic.