Ibanez TSA15H Low volume

[mikeskory]

I could use some help eliminating sections in this amp so I can focus on the actual broken area. It's a store stock item. My salesman noticed the volume was low so he replaced both 12ax7s then all low voltage bridge diodes blew. I replaced those and a 1000mf cap. That brought it back but I've still got the original low volume problem. Sadly, I have spent time on this but am getting nowhere. I can email the schematic if one of you techs wants to offer troubleshooting suggestions.

Right off the bat I have questions about the pre-amp ICs. There are 2 4558s. I'm getting 4.4v dc on all the pins except pin 4.

[g1]

Schematic here: https://music-electronics-forum.com/...etch?id=929711

The IC's are running on single ended 9V supply, so approx. half of that on all the IC's pins is normal (except pin4 which goes to ground).

[mikeskory]

Ok! That eliminates that. I'll move up the food chain. My next suspect is in the phase-invertor circuit. Thank you

[nosaj]

check the resistors on the phase invertor.
nosaj

[The Dude]

I'd start with the easy. Run the Send to another amp and see if signal out is as expected. Run signal into the Return and see if the amp section is working as it should. Try to narrow down whether it's a preamp or power amp problem.

[mikeskory]

All back together. Yes I tried audio from the effects send. It sounds good. The "tube screamer" section works there but not the main controls. I'm getting 30.4vdc at at pins 2 and 7 of the 12ax7 phase inverter. I'm getting more than 1 volt audio at pin 2 and nothing at pin 7. There is a 2.2M R21 going to pin 7. I get audio on one side of R21 but not the PIN 7 side. I checked the R21 with the tube out...checks ok. (not replaced). I did replaced the .1mf C17 also coming from pin 7 thinking it might be killing the signal.

[Helmholtz]

I'm getting more than 1 volt audio at pin 2 and nothing at pin 7. There is a 2.2M R21 going to pin 7. I get audio on one side of R21 but not the PIN 7 side. I checked the R21 with the tube out...checks ok. (not replaced). I did replaced the .1mf C17 also coming from pin 7 thinking it might be killing the signal.

The purpose of C17 is to AC-ground pin7, so no signal.

What signal ACV do you get at PI outputs (pins 1 and 6)? The power tube grids require about 22Vrms for full output.

[mikeskory]

Yes easily 22rms. Easily 40v p2p depending on the volume control. (Bonus...just realized there is a problem w my scope. Ch 1 input kills signal on the 2v and 5v scales)

Edit: I'm using 800htz in. The treble was all the way up. So about 30vp2p with tones at half and volume up.

[Helmholtz]

22Vrms cooresponds to roughly 60Vpp.

[mikeskory]

Yes thank you Helmholtz. The treble was up and I'm using 800 hrtz in so about 30p2p with tones at half and volume up. I've been around this circle before. At first I was happy that both Pin 1 and 6 had signal but then it dawned on me I was basically reading pin 1's output at pin 6. So we're back to the loss of signal at pin 7 via R21. Is that correct assessment?

[Helmholtz]

The treble was up and I'm using 600 hrtz in. so about 30p2p with tones at half and volume up.

Don't understand what you mean.

I thought we were checking the power amp and I'm saying that the 6V6 grids need 60Vpp for full power (independent of frequency and control settings).
But even 40Vpp should produce a strong speaker output. If not, there is something wrong with the power stage (or speaker).

[mikeskory]

The audio measurements at PI pin 1, PI pin 6, R10 (to 6v6) and R16 (to other 6v6) are the same. About 25 to 30 with treble at mid. This voltage goes up as the treble control is turned up to at least 60v. Is that the measurements you are asking for? .

[Helmholtz]

The audio measurements at PI pin 1, PI pin 6, R10 (to 6v6) and R16 (to other 6v6) are the same. About 25 to 30 with treble at mid. This voltage goes up as the treble control is turned up to at least 60v. Is that the measurements you are asking for? .

Just let me know if you still suspect the power amp.

[mikeskory]

Ok...at the 6v6s I get -29v at pins 4s.357v at both pin 4s. 357v at both pin 3s. The input audio at both pin 5s is good-same as we've been seeing. The 6v6 pin plates into the 5/15 watt selector is ugly but there . At the speaker the sin wave looks "ok" but very low. I just put in new 6v6s. It's only slightly louder. There is not much left...trans?

[Helmholtz]

Ok...at the 6v6s I get -29v at pins 4s.357v at both pin 4s. 357v at both pin 3s. The input audio at both pin 5s is good-same as we've been seeing. The 6v6 pin plates into the 5/15 watt selector is ugly but there . At the speaker the sin wave looks "ok" but very low. I just put in new 6v6s. It's only slightly louder. There is not much left...trans?

In 15W mode and with 40V peak to peak at the 6V6 grids you should be getting around 20V peak to peak or 7V rms at the output into an 8 Ohm load.