[GetCool]

I recently sold my used MG Head OTL to a forum member, and he has just informed me that it did not work correctly upon delivery. We have tried to diagnose the problem but nothing has worked. I have never experienced the problems he has described in the 2-3 months I had this amp, so I don't know what else to suggest. I will post his e-mail to me:

Quote:

Colin, Well first off I am not placing any blame at this point, but I know I didn't break it. Nor does it look like it was mistreated in shipping, but maybe it got banged around & something came loose inside during shipping? I carefully took the amp out the box, put the tubes in, connected to my source, plugged my cans in, turned it on, & the amp only works on one channel at a time on both jacks. Push the headphone plug all the way in correctly & I get sound out of the right channel only. Pull the plug half way out & I get sound on the left channel only. I tried this in gradual increments as well, but can not get sound out of both channels at the same time. This is the same for both jacks. I already ran all the trouble shooting test you mention before e-mailing you. Tried 3 different pair of cans, tube swap etc. Does this sound like it could be the tubes? I would love to have those tubes, & really hope we can get this thing working. I am thinking of pulling the bottom plate off just to see what I can find. It would be nice to find some pics of the circuitry to compare. If you have any other ideas I am all ears. I'm really bumming right now, but if there is a easy fix I'm open to it. Thanks Todd

I gave him the stock tubes, which I tested before I shipped the amp. I am wondering if it is a tube issue. I am going to send him a new set of tubes (a pair of JJ power tubes and a GE 5751 signal) to try, but I don't know if this will help. Does anyone have any ideas on what may be causing this problem?

Thanks!

 

[Born2bwire]

Mine sometimes will have one of the channels cut out on startup. But I've had the pot replaced on mine. To correct this, with the headphones on, I sweep the pot and eventually both channels will kick in. Why? I don't know. Unfortunately this could be a number of things from a bad tube to one of the wires being unconnected. Trying different tubes could work, ask him to switch the power tubes (EL84's) and see if the channel that works switches. If it does than one of the EL84's is bad. Unfortunately the 12AX7 could be bad as well but you'll need another one to swap out since the 12AX7 is a stereo tube.

 

[Sean H]

Something is definitely wrong with the left channel. When the plug is pulled halfway out and you get sound in the left channel what is happening is the portion of the plug that carries the left channel is touching the right channel pad inside the output jack.

First thing is to swap the power tubes to see if it is one of them but it sounds like you guys already did that. Next up is the center tube. Does he have more than one center tube? If so, has he swapped this tube out? The center tube can very likely lose one channel but not the other, it's not necessarily an all or nothing thing with that tube. If I were a betting man I would say it's that tube.

Beyond that you'd be looking at a broken solder joint or a failed part, but likely a broken solder joint somewhere in the left channel (which could be VERY easy to fix).

 

[GetCool]

Thanks for the suggestions, guys... I am going to send him another set of tubes. But in the meantime, take a look at these pics he just snapped:

Pic1
Pic2
Pic3
Pic4
*Pics removed as of July 13, 2003. If anyone should have a need for these pics let me know and I can send them to you.

It looks like something is not connected. Could this be it?

 

[Sean H]

There seems to be a problem with the links you posted to the pics.

EDIT: I see, the problem is you have some backward slashes in your URL, change them to forward slashes and it will work.

EDIT: There you go.

 

[GetCool]

Quote:

Originally posted by Sean H There seems to be a problem with the links you posted to the pics.

Now?

 

[Orpheus]

i think i've had some intermittent problems too (one channel went out, just like yours.) maybe once or twice. but after i shook the unit a bit and "rebooted" it worked fine. but as i have said before, ASL units are just bad quality. period. though, sometimes they are too good deals to not take a chance on....

anyway, you can always open it up, as it's pretty easy to do. and look for broken solder joints where the wires meet the RCA jacks... or where the pot is, or anywhere else there is some external pressure. beyond that, probably let a pro or someone knowledgable look at it. also, be careful.... as i understand it, caps in tube amps are dangerous.

EDIT: oh... sorry, posted after you guys figured it out. cool.

 

[Born2bwire]

Definitely looks like it could be the culprit. I'm assuming that the wire should be connected to the pot and then pass the signal to the transformer. I'll need to go home and open mine up and take a look to see. One thing nice about the MG Head is the color coding of the cables helps to trace everything.

Now the only thing that stands out at this point is that if this does hook up to the transformer than I would think that the amp should retain both channels in OTL mode. But then again I don't remember the operation of the amplifier well enough to be certain. I'll take a look at the schematic and see. In the meantime I'm sure if Joe saw this I'm sure he could tell you in an instant what is wrong. That guy knows his amps like nobody else.

EDIT: BAH! Divergent only has the schematics for the original MG Head.

 

[Sean H]

Aha! Broken connection to that coupling cap. Looks like it to me anyway. Easy fix. Strip the wires down and re-solder. Providing he can solder.

EDIT: Is that a coupling cap or an electrolytic? Looks to me like a coupling cap, they use paper in oil/metal can caps now. See the broken lead coming off that cap? And then the white wire from that tranny?

 

[GetCool]

Quote:

Originally posted by Born2bwire Definitely looks like it could be the culprit. I'm assuming that the wire should be connected to the pot and then pass the signal to the transformer. I'll need to go home and open mine up and take a look to see. One thing nice about the MG Head is the color coding of the cables helps to trace everything. Now the only thing that stands out at this point is that if this does hook up to the transformer than I would think that the amp should retain both channels in OTL mode. But then again I don't remember the operation of the amplifier well enough to be certain. I'll take a look at the schematic and see. In the meantime I'm sure if Joe saw this I'm sure he could tell you in an instant what is wrong. That guy knows his amps like nobody else.

Thanks, I'd really appreciate that.

 

[Orpheus]

hey, i see that your power chord is STILL connected to the unit when you opened it up! DISCONNECT it dude!

 

[GetCool]

Quote:

Originally posted by Orpheus hey, i see that your power chord is STILL connected to the unit when you opened it up! DISCONNECT it dude!

I just e-mailed him... remember I am not the one with the amp in my hands, I am a middle-man right now. Thanks for the warning!

 

[Sean H]

Tell that guy to be careful, electrolytic caps in tube amps can kill you!

Is that a coupling cap or an electrolytic that has the broken lead? Looks to me like a coupling cap, they use paper in oil/metal can caps now. See the broken lead coming off that cap? And then the white wire from that tranny?

 

[Born2bwire]

I looked at that wrong, I think Sean is right in that it's from the cap. That Cap is the capacitor for the OTL output I believe. I had the MG Head DT and then got an OTL and Joe did not modify the PCB to accomodate the cap for the OTL mode so it sits off like that. Again if that's so it makes me wonder why both modes do not work. Meh! It's 3 o'clock, think I'll go home otherwise this is going to bother me all day.

 

[GetCool]

Quote:

Originally posted by Born2bwire I looked at that wrong, I think Sean is right in that it's from the cap. That Cap is the capacitor for the OTL output I believe. I had the MG Head DT and then got an OTL and Joe did not modify the PCB to accomodate the cap for the OTL mode so it sits off like that. Again if that's so it makes me wonder why both modes do not work. Meh! It's 3 o'clock, think I'll go home otherwise this is going to bother me all day.

Thanks a lot! I'd sure like to know what yours looks like inside.

That definitely looks like the problem to me, but your right in that if that is the OTL output cap, I would think only the OTL output would be messed up.