TSL602 Low gain

[PJ McGrath]

My TSL602 has very low gain on OD1 and OD2. It just doesn't have the crunch that is should. Clean seems to work OK. I changed the tubes in the pre-amp and power amp sections and biased them but it didn't help. I've got the amp apart and checked for loose or burned components. Everything looks OK. I've got the schematic and a multimeter but don't really know where to go from here. I was going to completely remove the PCB from the chassis and go over all the solder joints. Anyone have any other ideas? Thanks...P.J.

[oc disorder]

Hi PJ and welcome to the forum.
Have a look at this thread.
http://music-electronics-forum.com/s...ead.php?t=8548
I put some photos with voltage measurements that may be helpful.
Often its the DC heater supply that plays up in these but maybe not in yours.
Also use the search function in this forum and look for other threads
related to the TSL series.

[tim]

some of these amps (I can't remember whether it was the TSL or DSL series) had a problem with U8 (I think it was U8) on the preamp board and when that IC blew, it would exhibit that exact problem...there was also on the same ECO from Marshall a procedure to put back to back zeners on the input jack to protect the circuitry from it getting zapped by a static discharge when plug from guitar was plugged in. I just went on Korg's service center site and couldn't find it but I know it used to be there...I'll have to look through some of my Marshall notes when I get a chance in the next couple of days and see if I can locate it.

[Enzo]

Those are the AVT series amps, with the solid state preamp panel on th front of the amp. Not too many op amps in these amps.

I'd be thinking a broken or troubled control along the panel, or solder to one, or maybe a stuck or leaky mute/switching JFET.

[PJ McGrath]

Voltage readings

Thanks for the quick response! Here are the readings I got. Most of my high voltage readings are significantly higher. Everything else looks close except for pin 9 on all three valves. I'm getting negative rather than positive voltage. Thanks....P.J.

V1 V2 V3 Resistors
Pin Pin Pin R60 341
1 242 1 277 1 230 327
2 -.001 2 ---- 2 ---- R61 364
3 1.66 3 1.05 3 1.32 353
4 3.07 4 3.05 4 3.07 R62 364
5 3.07 5 3.05 5 3.07 341
6 233 6 252 6 365 R64 365
7 -.001 7 ---- 7 230
8 1.66 8 1.92 8 232
9 -3.08 9 -3.05 9 -3.09

[oc disorder]

Well all those figures look in the ball park !
Pins 4,5 and 9 are the DC heater connection ..add up to approx 6.3v so thats ok.
Enzo's suggestion "I'd be thinking a broken or troubled control along the panel, or solder to one, or maybe a stuck or leaky mute/switching JFET." seems to be path to investigate so nows the time to do what you said -
" I was going to completely remove the PCB from the chassis and go over all the solder joints." .
I would after touching up all the solder joints use a scope to pinpoint where the signal is "dissapearing".
Without one I suppose its a matter of sitting down with the circuit ,a coffee and some blank sheets of paper and figuring out what fets are common to both od 1 and od 2.
Also I'm not sure what the M5201 is actually switching.
The nearby pots look like eq values rather than gain. VR 14, 15 ,16 perhaps you can see the " VR ** " printed onto the board near the actual control to identify them.
These also can break down. See the attached picture basically pin 1 selects A or B to go to the output at pin 5.
Guessing I would replace F11 ( J174 ) F14 ( J174 ) and depending on what VR4 was maybe F4 ( J111 ).
Lets hope you find a dodgy solder joint .. I just thought it would be eaiser to replace the fets while the board was out.
Hopefully someone will correct me if I am leading you up the wrong path !

[PJ McGrath]

Thanks again. Sorry about the messy chart! It looked much more organized prior to posting it. I went over the solder joints for the tubes, the rectifier, plus a couple of others that looked suspicious today. No change. I can borrow a O-scope and go over the rest of the solder joints this weekend. I'll let you know what I find....P.J.

[tim]

Those are the AVT series amps, with the solid state preamp panel on th front of the amp. Not too many op amps in these amps.
.

Yup, you're right Enzo...for the life of me, I couldn't remember what series amp that was in. I had forgotten about the AVT series...been a while since I've worked on one.

[bnwitt]

I can borrow a O-scope and go over the rest of the solder joints this weekend. I'll let you know what I find....P.J.

If you can't get a scope a listening amp will work just fine. You can make a simple one out of any amp you have lying around. Look at the bottom of this webpage:


http://www.el34world.com/Hoffman/tools.htm