Author Topic: troubleshooting my new to me Mingda (Read 11265 times)

donberry · « **on:** April 10, 2011, 12:02:46 PM »

I apologize if I should not have started a new thread, just thought since this was a troubleshooting problem....
Was enjoying my new to me Mingda MC34AB, using it in Power amp mode. First I was in ultralinear mode. In between songs, I noticed a popping sound. Turned the music off and a Rhythmic popping sound was coming out of the left speaker. I isolated the problem to the new Mingda of course. Checking out the problem, when it was first turned on, it was real feint, but after 3-5 minutes, it became louder and rhythmic. Looked at the tubes and one side of the EL34's was arcing -
Hoping it was just a bad tube, I swapped the tubes to the other side. The popping sound does not get as loud as it did before, but with no music on or between songs, quite passages etc, you can hear it about every 4 seconds or so ,- but now it is on both speakers ?
After swapping the tubes, I noticed when I turned it on, one of the 12AT7's got really bright, like a lightbulb, then dimmed down to normal. It is a Telefunkin tube and the guy included the original tubes with it, so I switched the 12AT7's with the Chinese ones that came with the amp. The one 12AT7 still did the same thing - on power on it lights up like a light bulb ? I do not know if that tube lit up like that before I swapped the tubes around. I did not notice it and I think I would have.
But now, tho I still hear the pop, faintly, I no longer see any of the EL34's arcing - tho I am not playing the music as loud (grandkids are over)...sooooo, I apologize for being new and needing help so soon.
One thing I did NOT do was bias the tubes. To be honest, I have zero idea how to do that. Looking at the pics, is fairly obvious where you adjust them, but I don't know where the test points are - and I have no idea if that is the problem anyway.

I am guessing the one 12AT7 should not be lighting up like a light bulb.....all of the tubes look "normal", even the 12AT7 after it gets thru flashing so bright.
Any ideas would be greatly appreciated.
Thanks

kajak12 · « **Reply #1 on:** April 10, 2011, 12:16:15 PM »

you could have bad valves causing problems
have you got a multimeter?

donberry · « **Reply #2 on:** April 10, 2011, 02:21:30 PM »

yes, I have a multimeter. I have repaired/restored quite a few solid state pieces of gear. just not familiar with tube gear.

kajak12 · « **Reply #3 on:** April 10, 2011, 03:25:35 PM »

Quote from: donberry on April 10, 2011, 02:21:30 PM

yes, I have a multimeter. I have repaired/restored quite a few solid state pieces of gear. just not familiar with tube gear.

you should find a 10r resistor that connects to the trim pot the voltage across that resistor is your bias.

donberry · « **Reply #4 on:** April 11, 2011, 12:21:20 AM »

have not biased tubes yet - the arcing is back. Arcs on the same side that the 12AT7 lights up.....
bummer, not how I wanted to be introduced to my new amp

kajak12 · « **Reply #5 on:** April 12, 2011, 12:02:36 AM »

Quote from: donberry on April 11, 2011, 12:21:20 AM

have not biased tubes yet - the arcing is back. Arcs on the same side that the 12AT7 lights up.....
bummer, not how I wanted to be introduced to my new amp

do you know anybody with a valve tester?
i have a feeling that a tube i gone

donberry · « **Reply #6 on:** April 12, 2011, 06:10:37 AM »

no, no one that I know of with a tube tester. I live in a rural area, I will check around though.
BUT, as usual for me , there is a twist.

Just out of curiosity, I hooked it up again as an integrated amp. I figured it would do the same thing as I did hear a feint popping sound, out of both speakers. I have been listening now for well over 15 minutes and tho the feint popping sound is there (can't hear it when the music is playing), the tubes are not arcing on that side as they did when I used it as a power amp.......
I could be wrong, and often am, but I am thinking this is telling me that the problem is in the circuit somewhere ?
I still need to swap out the 6SN7's tho, have not done that yet.

So basically - always has a slight popping sound, about once every 4 seconds. In power amp mode, after about 5 minutes, one side of EL34's start arcing at the base of the tubes.
In integrated mode, after at least 15 minutes - no arcing.

Sure wish i was in the land down under

zenelectro · « **Reply #7 on:** April 12, 2011, 12:57:02 PM »

Quote from: donberry on April 10, 2011, 12:02:46 PM

I apologize if I should not have started a new thread, just thought since this was a troubleshooting problem....
Was enjoying my new to me Mingda MC34AB, using it in Power amp mode. First I was in ultralinear mode. In between songs, I noticed a popping sound. Turned the music off and a Rhythmic popping sound was coming out of the left speaker. I isolated the problem to the new Mingda of course. Checking out the problem, when it was first turned on, it was real feint, but after 3-5 minutes, it became louder and rhythmic. Looked at the tubes and one side of the EL34's was arcing -

Arcing where? Tube sockets? Inside tube? Please be specific.

Quote

Hoping it was just a bad tube, I swapped the tubes to the other side. The popping sound does not get as loud as it did before, but with no music on or between songs, quite passages etc, you can hear it about every 4 seconds or so ,- but now it is on both speakers ?
After swapping the tubes, I noticed when I turned it on, one of the 12AT7's got really bright, like a lightbulb, then dimmed down to normal. It is a Telefunkin tube and the guy included the original tubes with it, so I switched the 12AT7's with the Chinese ones that came with the amp. The one 12AT7 still did the same thing - on power on it lights up like a light bulb ? I do not know if that tube lit up like that before I swapped the tubes around. I did not notice it and I think I would have.

This initial bright heater glow is normal for some tubes.

Quote

But now, tho I still hear the pop, faintly, I no longer see any of the EL34's arcing - tho I am not playing the music as loud (grandkids are over)...sooooo, I apologize for being new and needing help so soon.
One thing I did NOT do was bias the tubes. To be honest, I have zero idea how to do that. Looking at the pics, is fairly obvious where you adjust them, but I don't know where the test points are - and I have no idea if that is the problem anyway.

I am guessing the one 12AT7 should not be lighting up like a light bulb.....all of the tubes look "normal", even the 12AT7 after it gets thru flashing so bright.
Any ideas would be greatly appreciated.
Thanks

Probably best to get your amp to a service Tech who knows tubes.
There are any number of things that could be wrong with it but I don't think the pre amp tubes are the issue here.

cheers

Terry

stevenvalve · « **Reply #8 on:** April 12, 2011, 02:52:05 PM »

The 12at7 getting Bright on turn on, is normal, some do, some don't, 12ax7s and other valves can do it as well.

donberry · « **Reply #9 on:** April 12, 2011, 11:04:24 PM »

Quote

Arcing where? Tube sockets? Inside tube? Please be specific.

it is arcing inside the tube, at the base. Does it on all 4 tubes. Other side is fine. I swapped the 4 tubes to the other side, still does it

Quote

This initial bright heater glow is normal for some tubes.

I just thought this was strange as it really isn't tube specific. I swapped out that tube with the original 12AT7 and it did the same thing. I put the Telefunkin's back in, but swapped sides - the tube in the one side is still the only tube that flashes. Tube that was on that side no longer flashed in the other socket

Quote

Probably best to get your amp to a service Tech who knows tubes.

While I agree with that, I live in a rural area and only way to do that is to drive a few hours or mail it to get to someone qualified. Is why I learned to repair/restore my ss gear as that could get expensive real fast. A long, long time ago, I used to repair inertial guidance systems for the minuteman missiles and F-16 aircraft - so I knew my way around schematics and can solder pretty good (tho we did use microscopes to solder and my hands are a little shakier now). I only say this as due to my location - unless I want to start spending hundreds of dollars, I am pretty much on my own. Just shipping this to and fro at 80 pounds would not be cheap.

I am watching my grandkids today so am going to have time to open it up and check the bias, and do a cursory check of things while I am in there.
I emailed Pacific Valve and asked if they had any ideas, and they seemed to think it was a bad EL34 - tho not sure why it would still do that after swapping them around. If the circuitry checks out, I will have to wait until the 1st of the month to order some new tubes....

I do want to seriously thank everyone for any and all info u have given and can give........amp does sound sweet . Don't really care for the preamp section, like my CJ much better.

donberry · « **Reply #10 on:** April 13, 2011, 06:11:40 AM »

1st - don't ask

2nd - apparently I am dumber then I thought, and I think I am pretty dumb - where is the fuse on this amp ?
I have even opened up the power humpty and do not see anything. I must be blind because surely, and I really hope so, they put a fuse on this.

mcb · « **Reply #11 on:** April 13, 2011, 02:30:02 PM »

Don

Mine is located underneath the IEC power connector on the power humpty. Disconnect the power cord (the one from the wall and not either of the other two), then you should see the outline. Use a small screwdriver to flick out the fuse carrier. With a bit of luck, there will also be a spare fuse in the carrier.

donberry · « **Reply #12 on:** April 13, 2011, 11:17:20 PM »

thank you so much......I looked everywhere for that thing. Actually pretty neat where they put it

donberry · « **Reply #13 on:** April 14, 2011, 06:17:30 AM »

thought I had it today. I adjusted the bias ( what voltage should the bias be set at ?) and turned it all on - not even the feint popping sound. After a good 15 minutes, just silence........I cranked it up to really check it out, going thru various songs just to check out different things music wise.......so happy - and then I heard it out of the left speaker only, loud, rhythmic popping, knocking sound.
Checked the tubes and none were arcing this time......

edit -
turned off the amp and waited about a half an hour. I just listened to 2 full albums at normal listening levels and it did not do it - so it seems it will just do it if I crank her up for a bit.

I guess I will just wait until next month when I get paid and buy a new set of EL34 and see what happens

zenelectro · « **Reply #14 on:** April 14, 2011, 10:30:08 AM »

Quote from: donberry on April 12, 2011, 11:04:24 PM

Quote
Arcing where? Tube sockets? Inside tube? Please be specific.
it is arcing inside the tube, at the base. Does it on all 4 tubes. Other side is fine. I swapped the 4 tubes to the other side, still does it

If you indeed have arcing -inside- the tube and it is not a tube fault but being caused by the amp itself,
you have something that is causing huge voltage transients, large enough to cause arcing.

Q1, Does it happen only when music is played or does it happen even when the amp is idle.

Q2, Between what pins is the arcing occurring? The tube should have numbers under the base.

At a guess, it sounds to me like the OP transformer secondary, or something in that part of the CCT is going 'open'
intermittently causing the voltage transients on the primary (tube) side.

Turn the amp off, wait till the HT has discharged completely and then check -all- the solder / wire connections around
the OP tubes -> transformer -> speaker. When I say check, I mean grab the wire with a pair of pliers and gently
wiggle it enough to make sure the solder joint is not dry (loose) but not so hard as to damage the joint + wire.
This includes all components.

Beyond this get it serviced. Where do you live?

Terry

donberry · « **Reply #15 on:** April 14, 2011, 12:13:16 PM »

it isn't arcing anymore since I adjusted the bias. I listened for about 1 1/2 hours at normal volume and it sounded great, no problems.
I turned it up loud, and after about 15-20 minutes, the rhythmic popping was back, but no tubes were arcing this time.
I would like to know what voltage the bias is supposed to be ?
thanks

kajak12 · « **Reply #16 on:** April 14, 2011, 09:05:32 PM »

Quote from: donberry on April 14, 2011, 12:13:16 PM

it isn't arcing anymore since I adjusted the bias. I listened for about 1 1/2 hours at normal volume and it sounded great, no problems.
I turned it up loud, and after about 15-20 minutes, the rhythmic popping was back, but no tubes were arcing this time.
I would like to know what voltage the bias is supposed to be ?
thanks

0.30 to 0.40 any where in between (could be a faulty el34)

donberry · « **Reply #17 on:** April 15, 2011, 01:59:22 AM »

thanks - I was in the ballpark then.

Question about triode/ultralinear mode. I am taking for granted you should shut the amp off to switch modes ?

mcb · « **Reply #18 on:** April 15, 2011, 02:32:30 PM »

Quote from: donberry on April 15, 2011, 01:59:22 AM

Question about triode/ultralinear mode. I am taking for granted you should shut the amp off to switch modes ?

Hi Don,

Yes it should be turned off. I had switched mine back and forward on a couple of occasions, whilst on. So it does not automatically let the magic smoke out.

However when subsequently chatting to the tech my amp, he was very clear that it should not be switched, whilst on.

I am assuming switching with the amp on, it stresses the system.

donberry · « **Reply #19 on:** April 15, 2011, 03:41:58 PM »

was taking a music break today while it was getting dark outside. Just kicked back and enjoyed some music. The amp does sound pretty darn good to me with the Jirvana tubes, am anxious to round up some Winged C tubes.
I kept checking the tubes, pretty cool since it was dark out (i leave the top on my CJ pre).
One thing I noticed was 4 tubes on one side all had a blue glow, swirling around from the bottom of the tubes. New to tubes, I quickly googled it. Apparently not a bad thing. Was curious tho that it was the tubes all on one side, none on the other.
Did not play the music loud, was actually my 1st "soft" listening session. Sounded pretty darn good to me on the ML speakers. Happy it sounds so good at low volumes. Still had the light popping sound in the panels of the speakers. Could only hear it with no music playing.
Really hoping it is a tube as I really hate going thru all of the components checking them out.

I know that flare up on a 12AT7 is supposedly common, but just seems strange to me it only does it on the one side. It isn't the tube. If I switch the tubes around, it still only does it on the one socket. That tube also glows brighter then the other side. When I get some time with it, I may go in with my trusty meter and see what the difference is between that side and the other.
Does anyone else's do this ? Facing the amp, it is the 12AT7 on the left side. I also tried the chinese tubes and it did the same thing, swapped sides and still only does it on that one side. It isn't the tube, it is the socket. Not having the schematics and spending much time looking at it, I would have thought both sides were wired the same and would act the same. I am obviously not that experienced with tubes tho.