Mengyue Mini EL34B stereo balance issues..

Zommies · 7th December 2010, 10:45 AM

Hi everyone. I recently purchased this amp on eBay, yet low and behold.. another dud... Of course, the guy I purchased it from claims "it was fine when I used it"... Anyway, it seems to have some sort of a balance issue. The stereo balance is never spot on like with my solid state gear. Everything being played through this thing is either on the left or fades to the right. Both channels work mind you, but one is always much weaker than the other. I purchased a complete set of brand new tubes. That didn't help. My father, who works on old shortwave, A.M., F.M. tube radios looked at it but couldn't find anything physically wrong. He said if he had a schematic for it, he might be able to isolate the problem.. Anyone hear have any suggestions as to what might be wrong? Does anyone happen to have a schematic of this amp? Any help would be greatly appreciated.

tinitus · 7th December 2010, 10:55 AM

attenuator tracking issues ?

maybe this could be used http://www.audioxpress.com/magsdirx/...encowe2996.pdf

THD+N · 7th December 2010, 11:46 AM

A lot of times, accurate schematics are not available. The best schematic is the one you draw based on your amp circuit. It does not take too long and well worth the effort when trying to troubleshoot or modify an amp.

If the left / right signal level is that different in level, it maybe the input pot / volume control. You could disable it (the volume pot) temporarily by putting two 100k input resistors (matched of course) in place for a test.

chops · 9th December 2010, 08:39 PM

What doesn't make sense is the fact that the OP says that the sound will be louder in one channel, but then FADES over to the other channel.

Does it do this after the amp warms up or does it just randomly fade over?

Zommies · 9th December 2010, 10:15 PM

[QUOTE=THD+N;2390041]A lot of times, accurate schematics are not available. The best schematic is the one you draw based on your amp circuit. It does not take too long and well worth the effort when trying to troubleshoot or modify an amp.
Wish I could but this stuff is Greek to me. I was just looking for info that I could relay back to my father so he could possibly fix it.

Zommies · 9th December 2010, 10:33 PM

[QUOTE=chops;2393029]What doesn't make sense is the fact that the OP says that the sound will be louder in one channel, but then FADES over to the other channel.

Sorry, I didn't mean that it would fade back and forth while listening. Just when it started, it would play much louder in the right channel. I was running my laptop through it and didn't realize I had the rca's connected to the amp backwards.. I switched them, so then the left side was obviously louder. Sorry for the confusion...

Zommies · 9th December 2010, 10:36 PM

Does it do this after the amp warms up or does it just randomly fade over?

When I first received the amp, the channels would be centered when it was cold or just turned on. After about ten minutes or so after it had warmed up, is when everything would shift. After a couple of days that this had started, it remains this way as soon as you turn it on. No warm up anymore..

Ian444 · 10th December 2010, 01:06 AM

Quote:

Originally Posted by Zommies

Sorry, I didn't mean that it would fade back and forth while listening. Just when it started, it would play much louder in the right channel. I was running my laptop through it and didn't realize I had the rca's connected to the amp backwards.. I switched them, so then the left side was obviously louder. Sorry for the confusion.

That sounds like a problem with the RCA leads coming from your laptop, or the soundcard settings, or the mini-jack going into your laptop. Have you tried running the amp from a CD player or some other source? While your father has the amp, ask him to check that the mains earth is connected to the chassis, as most Meng Yue's do not come with that fitted standard, and you need it to prevent possible electric shock.

Zommies · 10th December 2010, 03:07 AM

No, it's not the rca's. I tried different ones and it still did it.

chops · 12th December 2010, 01:33 AM

I found out shortly after my last post here that the OP is my brother!! I knew he got a new amp but didn't know the name or anything.

So anyway, he's got it over at my place and we have it connected to my NHT 2.9's. Just for giggles before I took off to work, I put my NOS RCA 5751's in his amp. When he fired it up, the left channel was loud, the right channel quiet. He let it play that way for several hours. He shut it down after about 3 hours, swapped out the 5751's for his new Mullard 12AX7's. When he turned it back on, it was playing normal in both channels, but the sound was a little distorted like something was being overdriven, but nothing other than those two tubes were changed. He let it play that was for another couple hours before shutting it down once more and swapping back in my 5751's. This time around, it did a flip-flop and now the right channel was loud and the left channel quiet. It was this way until I came home.

Since he was using my preamp and old iPhone running Pandora, SomaFM, etc, etc as the source, he set the volume on the amp at around the 3 o'clock position. Later through the day, I asked him to turn the volume all the way up (on the amp) as I was thinking that maybe the cheap volume pot may be unbalanced unless wide open, however, nothing changed.

I'm wondering if there's something wrong with the PC board that the tube sockets are mounted to as when he swapped the driver tubes all three times, the channel balances changed... ALL THREE TIMES!

Do you guys have any ideas?

7th December 2010, 10:45 AM	#1
Zommies diyAudio Member Join Date: Dec 2010	Mengyue Mini EL34B stereo balance issues.. Hi everyone. I recently purchased this amp on eBay, yet low and behold.. another dud... Of course, the guy I purchased it from claims "it was fine when I used it"... Anyway, it seems to have some sort of a balance issue. The stereo balance is never spot on like with my solid state gear. Everything being played through this thing is either on the left or fades to the right. Both channels work mind you, but one is always much weaker than the other. I purchased a complete set of brand new tubes. That didn't help. My father, who works on old shortwave, A.M., F.M. tube radios looked at it but couldn't find anything physically wrong. He said if he had a schematic for it, he might be able to isolate the problem.. Anyone hear have any suggestions as to what might be wrong? Does anyone happen to have a schematic of this amp? Any help would be greatly appreciated.

7th December 2010, 11:46 AM	#3
THD+N diyAudio Member Join Date: Jul 2007 Location: Orlando, Florida	A lot of times, accurate schematics are not available. The best schematic is the one you draw based on your amp circuit. It does not take too long and well worth the effort when trying to troubleshoot or modify an amp. If the left / right signal level is that different in level, it maybe the input pot / volume control. You could disable it (the volume pot) temporarily by putting two 100k input resistors (matched of course) in place for a test. __________________ D-500, 700 II, 700B, (5) 400, (3) 400 II, (2) C-500, 4000 Preamp, Audio Precision System One (Dual Domain)

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7th December 2010, 10:55 AM	#2
tinitus diyAudio Moderator Join Date: Nov 2005	attenuator tracking issues ? maybe this could be used http://www.audioxpress.com/magsdirx/...encowe2996.pdf

9th December 2010, 08:39 PM	#4
chops diyAudio Member Join Date: Apr 2003 Location: St Pete, Florida	What doesn't make sense is the fact that the OP says that the sound will be louder in one channel, but then FADES over to the other channel. Does it do this after the amp warms up or does it just randomly fade over?

9th December 2010, 10:15 PM	#5
Zommies diyAudio Member Join Date: Dec 2010	[QUOTE=THD+N;2390041]A lot of times, accurate schematics are not available. The best schematic is the one you draw based on your amp circuit. It does not take too long and well worth the effort when trying to troubleshoot or modify an amp. Wish I could but this stuff is Greek to me. I was just looking for info that I could relay back to my father so he could possibly fix it.

9th December 2010, 10:33 PM	#6
Zommies diyAudio Member Join Date: Dec 2010	[QUOTE=chops;2393029]What doesn't make sense is the fact that the OP says that the sound will be louder in one channel, but then FADES over to the other channel. Sorry, I didn't mean that it would fade back and forth while listening. Just when it started, it would play much louder in the right channel. I was running my laptop through it and didn't realize I had the rca's connected to the amp backwards.. I switched them, so then the left side was obviously louder. Sorry for the confusion...

9th December 2010, 10:36 PM	#7
Zommies diyAudio Member Join Date: Dec 2010	Does it do this after the amp warms up or does it just randomly fade over? When I first received the amp, the channels would be centered when it was cold or just turned on. After about ten minutes or so after it had warmed up, is when everything would shift. After a couple of days that this had started, it remains this way as soon as you turn it on. No warm up anymore..

10th December 2010, 03:07 AM	#9
Zommies diyAudio Member Join Date: Dec 2010	No, it's not the rca's. I tried different ones and it still did it.

12th December 2010, 01:33 AM	#10
chops diyAudio Member Join Date: Apr 2003 Location: St Pete, Florida	I found out shortly after my last post here that the OP is my brother!! I knew he got a new amp but didn't know the name or anything. So anyway, he's got it over at my place and we have it connected to my NHT 2.9's. Just for giggles before I took off to work, I put my NOS RCA 5751's in his amp. When he fired it up, the left channel was loud, the right channel quiet. He let it play that way for several hours. He shut it down after about 3 hours, swapped out the 5751's for his new Mullard 12AX7's. When he turned it back on, it was playing normal in both channels, but the sound was a little distorted like something was being overdriven, but nothing other than those two tubes were changed. He let it play that was for another couple hours before shutting it down once more and swapping back in my 5751's. This time around, it did a flip-flop and now the right channel was loud and the left channel quiet. It was this way until I came home. Since he was using my preamp and old iPhone running Pandora, SomaFM, etc, etc as the source, he set the volume on the amp at around the 3 o'clock position. Later through the day, I asked him to turn the volume all the way up (on the amp) as I was thinking that maybe the cheap volume pot may be unbalanced unless wide open, however, nothing changed. I'm wondering if there's something wrong with the PC board that the tube sockets are mounted to as when he swapped the driver tubes all three times, the channel balances changed... ALL THREE TIMES! Do you guys have any ideas?

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