Perplexing Plexi Problem...Solved...It Lives!

[Blindog]

Hey guys,

I've rebuilt my '68 Traynor YBA-1 a couple of years ago to Marshall 1987 specs, more or less. I've added a split/shared cathode switch, post PI master volume, and a passive effects loop to use my Dumbleator. This amp has been one of my favorite builds, and my main gigging amp. Last year, just playing around at the house, I heard a crackle and my volume dropped. Now, where the amp would start to go into crunch territory with volume between 4-5, (and loud as hell, lol) it starts to break up at around 3 or so and then just gets extremely fuzzy and compressed from there on up, and with low output.

Things I've tried:

Swapped out the OT
Replaced original output tube sockets
Swapped out all tubes
Removed the PPIMV
Reflowed solder joints
Chopsticked everywhere!

I've attached my voltages and pics. Keep in mind, the way the Traynor was wired from the factory, V2A and V2B are reversed from the typical Marshall circuit.
I'm also running KT88's, so that's why my bias voltage is different.

I'm at my wits end with this thing...I actually haven't touched it for about 6 months now, but I can't give up on her! I have an oscilloscope, but I really don't have any experience troubleshooting with one.

Any ideas would be greatly appreciated!!

Mark

[NickC]

Is there a broken ground connection on the right-hand chassis mounted filter/power supply eCap near the lower left hand side of the image?

[Blindog]

Is there a broken ground connection on the right-hand chassis mounted filter/power supply eCap near the lower left hand side of the image?

Hey Nick, I went back in to check it out, it checks ok. The only thing that looked strange to me was the plate voltage on V1, about 30V higher than what I've seen on voltage charts...don't know if that would affect things...

Thanks for the input!

Mark

[n6fxl]

Hey guys,

I've rebuilt my '68 Traynor YBA-1 a couple of years ago to Marshall 1987 specs, more or less. I've added a split/shared cathode switch, post PI master volume, and a passive effects loop to use my Dumbleator. This amp has been one of my favorite builds, and my main gigging amp. Last year, just playing around at the house, I heard a crackle and my volume dropped. Now, where the amp would start to go into crunch territory with volume between 4-5, (and loud as hell, lol) it starts to break up at around 3 or so and then just gets extremely fuzzy and compressed from there on up, and with low output.

Things I've tried:

Swapped out the OT
Replaced original output tube sockets
Swapped out all tubes
Removed the PPIMV
Reflowed solder joints
Chopsticked everywhere!

I've attached my voltages and pics. Keep in mind, the way the Traynor was wired from the factory, V2A and V2B are reversed from the typical Marshall circuit.
I'm also running KT88's, so that's why my bias voltage is different.

I'm at my wits end with this thing...I actually haven't touched it for about 6 months now, but I can't give up on her! I have an oscilloscope, but I really don't have any experience troubleshooting with one.

Any ideas would be greatly appreciated!!

Mark

I recently got a Traynor YBA-1 which had a similar volume problem.
Produced some output but nowhere close to where it should. After
checking voltages and replacing parts and getting nowhere, one
day I accidentally brushed against the treble pot, and the amp
got loud again. The treble pot is directly in the signal path, must have
been damaged just enough to let some signal through, but not all of it.
I replaced that pot, and the amp is as good as new.

Fred

[Blindog]

Fred, I'll try swapping out the treble pot tomorrow. Thanks for the input! I'm hoping for a good report, this has been frustrating

Mark

[randalp3000]

My AC50 had very, very low output and it turned out to be the coupling caps going to the power tubes were leaking DC.

[Blindog]

Swapped out the treble pot, no change. Swapped out the caps to the power tubes, and a strange thing happened. I had my full volume back for about a second, then reverted back to the same condition as before. The old coupling caps checked ok on my Fluke dmm. They were new Sozo's from about 2 years ago. I swapped in some new Mallory 150's. Now I am really scratchin' my head. Anyone live in the ATL area care to come help troubleshoot? I'll bring it to ya! This amp sounded *so* good before, I miss it!!

Mark

[groovtubin]

Swapped out the treble pot, no change. Swapped out the caps to the power tubes, and a strange thing happened. I had my full volume back for about a second, then reverted back to the same condition as before. The old coupling caps checked ok on my Fluke dmm. They were new Sozo's from about 2 years ago. I swapped in some new Mallory 150's. Now I am really scratchin' my head. Anyone live in the ATL area care to come help troubleshoot? I'll bring it to ya! This amp sounded *so* good before, I miss it!!

Mark

give reeltarded here a e-mail, he`s in ATL area!

[Reeltarded]

Savannah.

Still..

[Blindog]

Savannah.

Still..

Hey there "neighbor"

I actually live in Suwanee, 'bout 4 hours from you...man it sure would be nice to get a second set of eyes on this thing!

Mark

[Blindog]

With some help from Miles (Reeltarted) I was probing around with a disconnected lead, and I thought I might have a bad CF tube socket. It was snap crackle popping when probing around the socket, and when wiggling the tube. The sockets are original so I went ahead and replaced it, fingers crossed...well, I was very hopeful but back to square one. Still have the same problem. I did notice something odd when I was probing around with a non attached lead. When I touch the junction of the 68k resistors on the bright Channel I hear an oscillation like a ring modulator...and I can vary it with the volume control and the treble control..this doesn't happen on the normal channel, just a good old 60 Hertz hum. Hmmm....

[Reeltarded]

That is just the lead becoming an input/antenna..

Still thinking. I still think you have a weak connection or maybe a.. thinking..

[Richie]

Snap crackle pop is usually a bad connection to B+. If you probe that area it happens, turn off the lights and look for a small spark. You could have that or some small piece of wire or solder blob object around there. Push around and tap the board and chassis.

Check the MV hookups, or bypass the MV to rule it out. if you have a loop jumper it if its possible. May have to get a magnifying glass to look at the connections. Could be its under the edge of the board. If possible take it loose and lift it up some.

[Blindog]

Snap crackle pop is usually a bad connection to B+. If you probe that area it happens, turn off the lights and look for a small spark. You could have that or some small piece of wire or solder blob object around there. Push around and tap the board and chassis.

Check the MV hookups, or bypass the MV to rule it out. if you have a loop jumper it if its possible. May have to get a magnifying glass to look at the connections. Could be its under the edge of the board. If possible take it loose and lift it up some.

Thanks for the input Richie! I haven't had any other snap crackle pop incedents since I replaced the CF tube socket. I've had the MV out of the circuit for a while now, it was one of the first things I did when this whole thing started. I've also lifted the board up and looked for solder overflows and such. Also I've tried jumping the loop. This thing has me frustrated for sure. The only other thing I haven't tried is removing the split/shared cathode switch on V1. It seems to be functioning ok. That's the kicker...everything *seems* to be working ok, with the exception of the low output and super fuzz tone from about 4-5 on the volume control....Argh!

Mark

[Blindog]

Yep, I'm *still* trying to figure this one out...

I wonder if a faulty volume pot could cause something like this? I replaced the bright channel pot with a Bournes 1M when I first got the amp a while back...I haven't been very impressed with those pots in other projects.

Frustrated!

Mark