Musicman bias issue, i think.

[catstrat]

Greetings everyone. This my 1st time posting on this site so please try to be gentle. I have a Musicman 2475-65 that came to me for repair. This has the transistor drivers and el 34 o/p tubes.. With a guitar signal at the input it has a weak distorted sound like something isn't biased right. You have to pick the strings really hard to get any sound. I pulled all the LM307 opamps out and checked all the supply voltages at pins 4 and 7 and got around --- 16.75 volts at pin4 and +16.75 volts at pin 7. With the opamps installed except the one on the far right "with amp upside down and the controls facing you" and using a generator set at 400cps i get a good sine wave to that last opamp. With that opamp installed the signal goes to crap. Coming out of that opamp the scope shows a wave that is ,well,a very thick horizontal line. I don't know what that type of pattern means. I thought a bad coupling cap. I changed that opamp,replaced what i think are the coupling caps , .1 and .22 along with the 100 pf disk cap in the feedback circuit. I also isolated and checked all the resistors in that area and found all ok. None of it worked. The voltages at the dip 8 chip also apppears to be good. I think it's something in the o/p stage because both channels sound crappy. Now, the plate voltages are good, both hi and low power settings. But i'm only getting about 6 to 7 volts at pin 5 of the o/p tubes. i've already changed the driver transistors and tried to set th bias with tr 1 and not much happened. I also resoldered many of the solder joints and,of course, that didn't help either. Questions. Do the opamps have a bias voltage at pins 2 and 3? What should the voltage be at the driver tansistors? What about voltage at pin 5 at the o/p tubes? So if any of you have any suggestions please let know. This amp is a confidence killer. Thanks, catstrat

[Amp Kat]

http://www.pacair.com/discus/message...tml?1048166315

Try that as these amps do not pull much current so your only going to get about 10 to 15 ma's. The 1692 transistors rarely fail as they don't really ever get hot. What I would do in this case is to make sure you are getting a good clean signal at the grids to verify that your problem is indeed either back from there or forward to the outputs.Even though a positive voltage, make sure the control grid is less negative than the Cathode from ground and if I remember correctly it should be by about 20 volts.

[catstrat]

musicman bias problem,

Thanks amp kat. As i remember i checked the signal at the grids of the tubes and found the wave pattern unbalanced in that either the bottom or the top of the wave, i can't remember, was just barely off the baseline while the other half of the wave was almost normal in appearence. The dip 8 chip is getting an unbalanced signal as well. That leads me to think that it is still something between, what i call the pi section and the dip 8. I have no layout drawing so i'm just assuming these parts are what i think they are. I do have a schematic though. The other thing ,the last lm307 opamp, the one on the right side of the board away from the inputs seems to be getting hotter than the other lm307. Is that chip the pi section? I guess a layout drawing would be a big help since i can only speculate what parts do what at this point. I pulled the dip 8 and substituted a 4558 in its place. I hope a 4558 will work. Anyway the results were the same. Is circuit loading causing this to make me think 1 componant is bad when actually it's something closer to the output? I've never ran into this problem before. Of course i work mostly on tube amps but i'm getting more solid state stuff in. Sometimes i think SS stuff is a curse. Thanks for your ear.

[Amp Kat]

http://www.datasheetarchive.com/sear...q=LM307&page=3
While the LM307 is probably less gainier than the 4558 it should work. At this point it's going to be hard to troubleshoot it without a scope but if you have one I'd start with the first 307 and check the output on pin 6 and see how your signal looks then move on. Go to the volume pot and treble control and see if you get a good PP signal there then move down the line. It's possible a bad cap or fried resistor could be causing the problem. Since you have a goo +/- 16 volt rail that eliminates the most basic problem with these as the source is the 16 volt zeners in the power supply. It is possible something in the output is dragging that one side down and you'd have to find a point in the circuit to detach it to find out. Good luck and let us know what you find.