2100-130

Hi,
I took in a -130 bass head for repair from a friend, and want to post what I've found, just to be sure I'm heading in the right direction. The complaint by the owner was that it was blowing fuses. This is the SS version of the phase inverter.
I put the amp on my power analyzer which I run through a variac, and slowly brought up the power. Sure enough, by the time I get up to 30-40 volts, it's sucking down major amps. I shut down the power and pulled the output tubes. On visual inspection I found a tube that had arced and shorted between the plate and the heater pins (I think that's correct - pins 2 & 3). I couldn't see any other components that looked bad, and I stuck my meter on almost every thing I could see, and it all seemed okay.
I brought it up again with the tubes pulled and there was no short. I plugged in two of the remaining three good tubes, one in each half of the socket pairs (hope that makes sense) and again it came up with no short. So...
I plugged in a new set of four tubes, brought it up again while running a sine wave through the input section and things are starting to look dicey again - there is almost no volume coming through the output, and the signal looks ugly after the final op amp (which is a TL072). So I start probing again and figure out that the TL072 is sending voltage to ground via pin 5 and pulling down the dual supply to +/- 7 volts or so. I removed the op-amp and the supplies came back up to normal.
whew - so my questions are: based on the experience of people here, are there other points I should check before dropping in a replacement op-amp? And (having read the section on replacement op-amps) are NTE replacements worth trying, or should I order some NE5532's? (I'll have to explain to the owner that the sound is probably going to change some no matter what...).
Lastly, just to be sure - regarding the 'death cap', the only cap I see that goes from the power section to the chassis is on the ground lift switch - is that the one?
Thanks,
Scott

Most of the time, the op amps themselves are just junk. They're cheap, plug it in and see if it cures it. I assume that you DID measure the cathode driver resistors BEFORE you started passing the signal through it. Did THAT correct your problem with the current draw? Yup-thatt's the cap.

Regarding the cathode driver resistors - nope, I dint. While I've got an edumacation in electronics, I don't, as they say, have the street smarts. So on the schematic it calls for 470 ohms each - two measured approximately 390 ohms, and two measured around 480 ohms - that seems like it would imbalance the bias pretty well, yes (tho they're still within 10%)? I've got an NE 5532 coming (no local source) and will give it a go.

By the way - thanks for the reply!

Oops - I wrote that last post at the end of a long day - 390 is not within 10%. I guess I'll be replacing those. When I was checking components, I was looking for obvious shorts or opens, and didn't pay too much attention to how close they were to correct value. Now that I'm checking the values, quite a few of those resistors are drifting.

Yeah ut remember, MSOT of the resistors back then were 20% resistors(yaeh, I'm old). That isn't that far off. Id be more concerned about the shorts and the low gain first. I would get too anal about thteresistors until you find one 10x it's value or something like that.

Jeez, I shouldn't type before coffee.

Thanks for the tips, you're doing better than me - I can't even make sense before coffee.
I double checked things again, and there was nothing too far out of whack. I plugged in the PI chip and got a good strong signal out, so I plugged in the tubes (new matched quad) and brought it up slow again on standby and noticed that the tube in the socket where the original short occured was not glowing. I took it out and very carefully filed the edges of the burnt socket connectors to get through to fresh metal, and squeezed them together... and then ran out of time. I'll have to try to bring it up again when I get back in town. I think I'm getting pretty close now though. I can't wait to hear this thing playing!

Okay - I set up to measure the bias voltages, and just about had a coronary when I measured about 150mV across the first 390 ohm resistor. I shut it down and double checked my setup, and double checked the value of the resistors - all seemed okay, so I cranked down the trim pot and tried it again. After fooling around with it, here's what I figured out: I started out with the trim pot all the way down, and an imbalanced bias of ~24/7 mV. I creeped the trim pot up, measuring as I went, and got as far as 47 / 19 mV. When I tried to shoot for a high reading of 53 or so, the bias on the high side shot up to about 57mV and the bias on the LOW side shot up to 80mV. At that point I cranked the pot back down, and after 2-3 minutes the bias was back to where I started at ~24 / 7 mV. Does this seem like the bias pot is old and not trustworthy anymore? Or maybe there's a squirly component in there somewhere? Any thoughts are appreciated!

POt bad is one thing. I would look at the caps on that driver board. There are electrolytic caps on that board that could have dried out over the years. Also, the SS driver could be thresholding so you MIGHT have to replace the driver. Don't have a schem here at hand but the SS components are pretty obvious on that board. Try replacing them if all else fails. Cjeck all resistors leading up to and away from the drivers also.

Well, after a lot of drama around my house, I finally have time to work on this amp again. Thankfully, the owner is a friend who hasn't needed it back.
I've replaced all the 470 ohm resistors, and all the electrolytics on the driver board, double checked the adjustment pot. I tried to set the bias again and this time I can't get it any lower than 60/80 mV with the pot all the way down. My gut feeling (for what thats worth) is that the driver transistors are okay, but I really don't have the experience to back that up...
Are there any other components that sound likely?
Also, how far up can I turn the quiescent current while I'm testing (with no signal) without worrying about blowing things up?
Thanks for all the input so far - its really helped!!
Scott

Ha!
I think I found the problem. When I checked out the datasheets for the components, I looked at what the schematic called for, or whatever accepted replacements I could find. Stupidly, I didn't look at what driver transistors were actually installed assuming they were accepted replacements - They are MJE15032's - rated for just about twice the voltage and current handling as what is called for in the schematic. I've orded two 2n6488's and hope that cures my bias problems.

Okay, now I'm lost again!! I replaced the two driver transistors with 2N6488's, fired it up and checked the bias...now I'm sitting at 95 / 105 mV with the adjustment pot all the way down. I'm not sure where to go now. All the other components like grid resistors, caps, diodes associated with the tubes test okay. Help!

WARNING: words of an uninformed hack follow-
Maybe I'm misunderstanding something here, or am confused about just what the amp in question is, but in both of my mm's with solid state pi (hd65 & hd130) the driver ic is a 1458 dual op amp. Could be the trouble???

The LM1458 op amp is the phase inverter or signal splitter (as I understand it) which sends one phase of the signal to one cathode driver transistor (the ones I'm taking about) and a 180 degree inverted signal to the other cathode driver transistor for a push-pull type amp circuit. For what it's worth, with help of another amp guy I found, I think I'm closing in on the problem, but not quite there yet.

I was indeed a bit confused, after re-reading the OP I can now see you mean the signal first looks rough after the last preamp opamp, not the PI. When you mentioned plugging in the NE* and TL* replacements there, I thought you might be headed for trouble. But glad to hear you're getting closer. All the best!