Popping Fuses. Op-amp replacement question...

[jdparsons1]

I did not. Is it easy to set the bias? I know where the pot is for it but i dont know how to do it.

[52 Bill]

Look at the left side of the schematic. The note gives the instructions on where to set the bias level.

Did you check the output for voltage?

[J M Fahey]

Hi JDPARSONS.
Let's play it very safe here.
1)Plug the amp into the lamp/bulb current limiter.
If you haven't built it already, do now.
It's dirt cheap and much faster than you to limit excessive current.
For plans search in MEF.
2)With all controls on 0, no signal, no speaker attached, measure the voltage from Q12base to Q15 base, that's to say, from one TIP142 base to a TIP147 base. Use the 2VDC scale, only if overflown switch to 20VDC scale.This shows the bias *applied*
Then measure from one TIP142 emitter to a TIP147 emitter, use 200mVDC scale here, it shows the bias *effect* or *results*
NOTE: be very careful, grab your test probes firmly and apply them perpendicular to the surface you are touching, it's very easy to slip and touch 2 pins , shorting them.
One millisecond later you lost your transistors.
Don't be afraid but careful.
1000 times I said "oh sh*t !!!" because of that.

[jdparsons1]

ok. Im going to build the lamp limiter. found some instructions that look pretty nice here... Light Bulb Current Limiter

Funny question, this thing is basically acting as a fuse. So whats the purpose of the built in 2.5amp fuse on the amp?

[J M Fahey]

this thing is basically acting as a fuse. So whats the purpose of the built in 2.5amp fuse on the amp?

The fuse"is not there" , literally it's a piece of wire, it limits nothing, simply melts and cuts power if you use more than 2.5A.
Problem is, with a shorted amp, you can measure nothing in the milliseconds it takes to blow, and to boot, if only one transistor from a pair is shorted, the next time the other half usually blows too, and all this, for nothing.
The bulb, by definition, never burns, because it's made to stand the 120 or 220 V you have at home, so a bad amplifier can be measured usefully and still-alive parts may stay so.
*But* a 100W SS amplifier with a 60W bulb can hardly put out more than , say, 20W RMS, and with unbearable sag.
You can't do a show with a series lamp, but it's very useful on the bench.
Mind you, bulb limiters straight in series with the speaker are used today to protect horns and tweeters (as in SVT 810 HE) and were once used as short circuit protection in Standel SS amps in the late 60's.

[jdparsons1]

Im not blowing fuses anymore though. The amp stays powered, im just getting static noise. So can i set bias level without the lamp limiter?

[jdparsons1]

Ok. I just tested the way you said. Base on TIP142 to Base on TIP147 is 2.5V on both sets of transistors. Testing emitter to emitter was around 230 mV but it took a while to count up to that.

[J M Fahey]

OK, your transistors are way overbiased, that's why you can play for a few minutes , while they heat their heatsinks until they die.
We'll move on slow and safe.
*Still with the limiter on* move your bias preset a little and re-measure the emitter to emitter voltage, where you found excessive 230mV , which mean: 230mV/0.66r (ohms)=350mA per transistor, around 12 times what's needed.
Crate's wording choice is not the best, but they seem to imply that the right bias current is: 7.5mV across *one* 0.33r = 22mA per transistor.
Measure the 142 to 147 emitter voltage (Vee for short), so you find if it increases or decreases.
I can't tell you " move the preset to the right/left" because I don't see the board, but you can easily find out.
Move the preset trying to *lower* the Vee, ideally to 15 mV .
Twice the recommended but we are measuring across two resistors, not one.
If you can do that, fine; if not or it does not vary (or even worse, if it jumps upwards and the lamp shines strong) then your preset is open or scratchy and *must* be replaced.
If all is fine, place a larger bulb there, say, 100W, and recheck your bias.
If all OK, play 10/15 minutes, watch for anything unusual.
If all OK, plug the amp straight into the wall, no limiter, recheck bias, if OK play 3 or 4 minutes, touch the heatsink, it should be warm but not burn your hand.
If all OK, you have a healthy amp.
We took 5 or 6 steps to advance what could be made in 2, but we played it *very* safe all along.
Good luck.

[jdparsons1]

:-) I just read your message this morning. Last night, without adjusting the bias or anything i turned the amp on and played for over an hour. Every once in a while i would get some occasionial (barely noticeable) static. Im going to recheck bias with my meter to verify the readings i gave you. One thing i was wondering is I sprayed the entire board down with electronics spray after i was done soldering everything just to clean off flux residue and im wondering if some of that got in one of the pots and that was causing the large amounts of noise i heard before. Would I be able to play it for that long if the bias was 15 times what was needed?

[Steve Conner]

Maybe, but you're running the risk of burning out the transistors eventually. I agree with JMF, strongly recommend turning it down to 10mV or less.

[jdparsons1]

Ok. Vee on left side is around 18mv and right side is right at 15mv cant seem to get them to run the same but im guessing thats close enough. I put the meter on it and had my wife turn it just a hair both clockwise and counter-clockwise to see which way it went. Ended up being clockwise to turn the voltage down and it only took a very small turn to bring it down.

[J M Fahey]

Good
The combined facts: that you sprayed the board with some kind of cleaner plus "moving it *very* little" solved the problem, means your pot was dirty and the sliding contact was resting over a dirty place or it was slightly rusty or whatever, and the cleaner softened that crud and moving it a little wiped it away.
That bias current is fine, whenever possible I would replace the bias pot with a new one, rechecking bias repeating what you did before, you don't want it playing this number again in the future.
Good work

[jdparsons1]

There is still static. sometimes it is more than other times. it kinda comes and goes. but it doesnt seem to effect playing. as far as i can tell its mostly when im sitting idle. if i start playing, even on the clean channel, it seems to go away. I have wiggled the 1/4 inch jack coming from the guitar and it doesnt seem to be that. I have also moved and wiggled each adjustment pot with no luck either. is there an internal component i could test or could it be anything and i should just deal with it?

[olddawg]

Noise like that is usually caused by a physically or thermally intermitant component, connection, or a failing cap. The first thing you need to do is to find out if the noise is in the preamp or the power amp. If you have an effects loop or main/pre jacks clean those also. You will probably need a scope to track down intermitant pops and crackles if it isn't a resistive jack or a bad connection. Make sure that you tighten all board mounting screws that are ground points. You might want to reflow the solder on every connection. Bench techs do that a lot. The good old "resolder the everything" approach. You can also try to track it down with some freeze mist, but if you are not familiar with the process you could end up chasing your tail.

[jdparsons1]

well. as far as WHERE the noise is. i can tell you that this is a two channel amp, I get the static on both the lead channel as well as the rhythm channel. and i hear the static even if the guitar cable is not plugged in and the levels are all the way down. not sure if that helps.

I can definitely try the "resolder everything approach"