Traynor YCS100H2 high gainer, faint sound

[LarBal]

Hey all, I'm stuck on this repair, this a 3 channel head with all the bells and whistles. I'm getting faint, ugly sound out of the speakers when everything is dimed.
Here's what I've determined:
-The preamp out does indeed provide a good sound to an external power amp, so everything before the jack is good.(that jack is on the bottom left of page 5)
-Bridging the pre-out/power in with a 1/4" cable doesnt fix it, same with fx loop jumper
-Voltages on el34s are where they should be, but of note, the heater string reads 6.4VAC on one side , between pins 2 and 7, and 6.2VAC on the other
-LTPI voltages seem good, plate resistors are dropping voltages ok and I switched that tube out to make sure that wasn't the issue

So everything is fine after the preamp out switching jack, that I know.
My first problem is reading that damn schematic. I'm getting lost just following the signal flow after the preamp out jack.
Beyond that, I feel that the problem lies between the pre-out jack, and the PI entrance, but again, I'm not sure what exactly is between those points, and even then, I'm not sure what I'm looking for, or how to troubleshoot what I assume to be a bunch of possible TL072 failures.

ycs100h2__service_manual.pdf

[g1]

Just to verify, when you plug straight into the PA In jack, no sound?

[LarBal]

Just to verify, when you plug straight into the PA In jack, no sound?

Yep that is correct

[g1]

Both the tuner mute and standby switches could give this issue if they fail. That would be a good place to start I guess.

Looks to me like PA In (line in) goes to U11B > U12B > U12A and 'mute out' goes to power amp circuit.

[LarBal]

Both the tuner mute and standby switches could give this issue if they fail. That would be a good place to start I guess.

Looks to me like PA In (line in) goes to U11B > U12B > U12A and 'mute out' goes to power amp circuit.

So taking readings from the middle lug of the switch(non-LED side), with the amp fully on, when the mute is engaged, I get 4.2K .... (but R201 definitely reads 4K7, as it's in parallel with R107 and the DI level pot).

With the mute off, center lug to ground reads 9 ohms... so the switch could be bad OR............following the mute-out to the switching page, it connects to a relayK2, the solo boost pot, and the aforementioned U12:A , pin 1 of the TL072.

So does that mean U12:A is bad? Pin 1 of the TL072 shouldnt be at ground potential right?

[g1]

I doubt the IC output is shorted to ground, but you could desolder pin 1 and check the resistance at that switch again. Solo boost pot looks ok? I guess if it were physically damaged it could short out that leg to chassis?
Also on that line to the switch is a spot called 'PA IN' which stupidly does not go where would make sense, the PA In jack. I saw somewhere before where it went, but can't find it right now , something around DI out?

edit: the DI pot, like you said. Maybe I should pay closer attention.

[g1]

Oh, it's at the DI level pot, but it's isolated by R107. It's possible that 'PA IN' buss may go other places too.

[LarBal]

Ok well, I yanked both DI level pot and the tuner mute switch off of the boards completely. The pot removal didn't change anything, and oddly enough, yanking the mute switch off the board actually diminished what little signal I did have down to nothing.
Also I took R107 out of circuit and confirmed it's good at 47K.
Put the switch back in, and regained what little output I originally had???
Also, the solo pot does indeed boost what little signal I do have...

[g1]

What about your 9 ohms from U12 pin1 to ground, what happened with switch and pot removed?
The switch makes the connection from 'mute out' to R224 at power amp input, so it makes sense that you lost all sound with switch removed.
Are you 100% sure this is the correct version of schematic for your unit? Yorkville makes a lot of revisions.

[LarBal]

Oh duh, that's right(about the switch connections).
Hmmm I'm not sure if I implicitly measured pin 1 with the components out, but at the power amp in point, at the 680R, was dealing with the same think, 4.7K to ground with the mute on, and seems like I had a jumpy 20-30R reading with the mute off.
Everything I've encountered up to this point leads me to believe the scheme is correct, some of the revison notes I've seen are accurate too.

[g1]

Probably a good idea to go back to your U12 suspect. I was wrong to discount it, I hadn't realized that pin 3 is grounded.

[Shamus71]

Probably a dumb question and you most likely have tried, but did you try replacing the phase splitter with a known good tube? Is there signal at W58 when unmuted?

[LarBal]

Yep I subbed out the PI, the W58 question provides a good clue I think. I'm reading 660R to ground from there, at the standby switch, so the issue has got to be on the other side of R224 right?

Ok so I swapped out U12, and while I still have very low output, the proper TONE of the amp is now present, wherein before it sounded crappy/bad. So looks like there are multiple things wrong...

edit: with amp on, pin 1 of U12 still at 10 ohms to ground

[Shamus71]

the 660R at W58 could mean U12 is short, but check the mute switch for proper function 1st. Do you have a scope? Real easy to see if signal at W58.

[LarBal]

the 660R at W58 could mean U12 is short, but check the mute switch for proper function 1st. Do you have a scope? Real easy to see if signal at W58.

I replaced U12 with a new one, and unfortunately not having a scope is biting me in the ass right now. IIRC te switch was functioning as it should, but I had to get something else on my bench so will doublecheck tomorrow