Crate problem PART 2

[Valvehead]

The amp worked fine for a day and has developed a new problem . It still switches channels fine . After it sits overnight , it wont put out any volume on the rythm ch. and the solo ch. The Clean channel is fine.

What is strange is , after the amp is on for 5 minutes or so , the volume on these 2 channels will Slowly and gradually build back up --to normal output levels.

could it be that zener i replaced? wrong value or something ?

Im sick of taking this thing apart !

[Valvehead]

i think enzo is shying away from this post..........

anyone ?

any ideas?

i do have the proper Zener on the way for it..., although i doubt thats the problem..

[Valvehead]

i replaced that zener. Didnt help.

[Enzo]

I'm not shy, but I did enjoy a few days off. Refresh me, what amp model was this?

[Valvehead]

GLX65 . Its the one you helped me on last week--i got the channel switching to work by replacing D25 (zener) , but now the ryth and solo channels have very,very low volume. The clean channel is acting normal. The amp was fine until it sat overnight, now it developed this new issue when i fired it back up.

[Enzo]

If it switches channels, then the zener is fine. The zener doesn't know if the channels sound right or not, it just limits voltage to the switch circuits. But it is easy enough to monitor the voltage across it in the two states.

The channel in question are turned off and on by the switching circuits though so it might pay off to watch the control voltages that sit on the gates of those JFETs. Does it drift in as the signal returns? Or do they stay crisp and change with the switch settings?

Apply a steady test signal to the input jack. The whole dirt channel is just four op amps with some other stuff. Check for signal at the output pin of each and see where the signal is diminished and where it slowlu comes back up.

[Valvehead]

ok, the volume on the amp isnt creeping up like it was. (i havent changed anything) . the ryth. channel and the solo channel just have super low volume. clean ch. is fine.

I ran a cd through the input jack, and got the same level of output through pin7 of the op-amps. (Into a pair of headphones)


as far as the Jfets, which exact ones do i need to check?
i checked the Gates of the following :

Q1 7.5V CLEAN
6.6V RYTH
0.7V SOLO

Q2 7.0V CL
6.6V RYTH.
0.7V SOLO

Q3 13V CL
1.4V RYTH.
13V SOLO

Q4 .060MV CL
10V RYTH.
10V SOLO

Q10 .084MV CLEAN
10V RYTH.
10V SOLO

Q21 -.065MV CLEAN
11.2V RYTH
11.2V SOLO

Q25 13V CL
1.6V RYTH
11.7V SOLO

Q28 8V CL
7V RYTH
0.63V SOLO

Q29 12.5V CL
0.71V RYTH
12V SOLO


I hope this makes sense ?

[Enzo]

Headphones? If you have no scope, set your meter to AC volts and use that.

Pin 7 is only one side of those op amps. DOn't forget pin1.

Look at the JFETs as switches, the gate turns them off and on. The source and drain legs are interchangeable. They either are open between them, or they drop to a couple hundred ohms.

Look at the drawing, Q25, Q28. The rhythm and solo level controls have signal coming out their wipers. Turn on one or the other of those JFETs and it completes the circuit to the rest of the amp for the associated control. They send it to IC4 there, just as Q21 serves up the clean channel output.

SInce the clean channel works, we know IC4B and everyting after it works as well. And of course the input stage works.

What do they do? Looking at them as if they were switches, think about what a switch would do in each spot. In no particular order:

Q7 turns the P3 SHAPE control off/on. Clearly if the JFET is off then P3 is just a resistor for the circuit.

Q29 opens the bottom end of the Distortion channel tone stack. That disables it so it has little effect.

Q21, 25, 28 we just mentioned above. They could just as easily be relays.

Q2, Q3 also select either solo or rhythm channels. Note how Q3, Q25 both are controlled by the same line. SO when in the rhythm channel Q3 selects teh appropriate gain control, and Q25 selects the right level control.

Q1, Q4 turn off/on gain and tine elements depending on channel.

All your transistors have a control voltage at the gate that changes in one channel assignm,ent or another, so all are being controlled. DO they work is a separate question.

Apply a signal. Got signal at IC2A? Solo adds in IC1B, but either way we continue through IC2B, then the EQ and through IC4A and from there to the level controls. Follow the signal at each of those outputs, then does it show up at the level controls? Essentially I just described TP2,3,4,5.

[Valvehead]

I dont know how tho check that with an Ac meter--where do the leads go? positive for probing and neg. to ground?

ive been running a cd player into the input and probing with the positive on a set of headphones.


So far i found this:

IC1A PIN1
CLEAN= LOW SIGNAL BUT THERE
RYTH LOUDER

IC2A PIN1
SIGNAL IS THERE--NO CHANGE FROM CLEAN TO RYTH.

IC2B PIN7
CLEAN VERY. VERY LOW SIGNAL
RYTH LOUD SIGNAL --SOLO EVEN LOUDER


IC4A PIN1 NO SIGNAL AT ALL

PIN 7 LOUD ON CLEAN
VERY VERY LOW ON RYTH.

seems i should have some signal at the output of IC4A .

that would narrow it down to the components between IC2B and IC4A , correct ?

[Valvehead]

bump

[mykey]

the main problem being, you bought a crate. these amps are the worst designed in the entire industry, and if you do fix it, something else will go wrong.
Reynolds pays 15 cents a pound for scrap metal, and its all cash.

[Enzo]

Mykey, that is just juvenile. Take that crap to Harmony Central or someplace. Remarks like that are not helpful to anyone in the least.

You may not like Crate amps, and they may not be as well made as some other brand you prefer, but they do work. This is the man's amp, and he wants to fix it regardless of your approval or lack thereof.

OK.

Look at the path through the amp. You get a hot signal at IC2B and nothing at IC4A. Either the signal is not getting to IC4 or IC4 is bad. Does the signal get as far as the tone controls? Got signal on the highs control? Make sure the tone controls are not turned down.

A cracked highs control P5 will kill the signal path. if it can't come out the wiper of that pot, it won't ever get to IC4. It is also conceivable that C76 there from that wiper to IC4 input is open or broken free. But my money is on a bad IC4 or a broken P5.

[mykey]

Mykey, that is just juvenile. Take that crap to Harmony Central or someplace. Remarks like that are not helpful to anyone in the least.

You may not like Crate amps, and they may not be as well made as some other brand you prefer, but they do work. This is the man's amp, and he wants to fix it regardless of your approval or lack thereof.

OK.

Look at the path through the amp. You get a hot signal at IC2B and nothing at IC4A. Either the signal is not getting to IC4 or IC4 is bad. Does the signal get as far as the tone controls? Got signal on the highs control? Make sure the tone controls are not turned down.

A cracked highs control P5 will kill the signal path. if it can't come out the wiper of that pot, it won't ever get to IC4. It is also conceivable that C76 there from that wiper to IC4 input is open or broken free. But my money is on a bad IC4 or a broken P5.

this is the best advice yet, if you fix a crate, something else will go wrong with it. is it really worth it to keep repairing the same amp over and over, or should you buy a reliable amp to start with? its not my fault that crate amps are designed so poorly. juvenile, hardly...realistic, definitely. I used to run a crate service center and it became obvious due to high failure rates and very pissed customers. i have actually seen these things catch on fire, right after being serviced at the factory, and this is absolutely true! advice is invest in another amp, and buy a reliable brand. incidentally, saint Louis music has dumped their own crate brand label and sold off the line to another company- guess why?

[Enzo]

No, SLM did NOT sell off the Crate line, LOUD technologies BOUGHT the WHOLE company Crate included. I run a service shop authorized for Crate, Marshall, Fender, PV, and many others, and I will say with all honesty, I see a lot more defective out of the box MArshall stuff than I do Crate. Certain Marshall models tend to eat output transformers, while I don't recall ever replacing a Crate OT. How about that little 22pf plate to screen cap on the Marshalls that likes to burn up? Crate has no equivalent problem. Ever repair a MOde 4 Marshall? How many of those burn up repeatedly? APparently more than we would like. Servce department at Korg says, "that is just the way they are. No wonder they quit making them." That was in response to my query as to why they keep blowing up and is there something we can do to make them more reliable. Their head tech says, no there isn't. In fact all the recent Marshalls using the TDA7293 have been less than reliable. The AVT series. And that problem in the preamps where the op amp blows a lot?

If an amp caught fire, one has to wonder what fuse was in it.

What goes wrong with Crates? In my experience, they have more than their share of jack issues, and I write that off to the post wave solder wash. Once i clean the shunt contacts on those jacks, they never come back to me. Their pots tend to be pretty cheap and are easily snapped. I always check the cement resistors for vibrational cracks in their solder. other than that, I see no reliability issues in the Crate line that I don'r see elsewhere.

But the point was that when a man asks for help learning something, I feel it is pretty low to tell him, Oh don't bother to learn that, I think it sucks anyway.

[mykey]

crate is unreliable because of the design, highest failure rate of any amp i can recall in the last 34 years. plagued with dissatisfied customers and dealers.
in the case i mentioned, the crate was equipped with a factory installed circuit breaker, which did nothing to stop the circuit board from turning to charcoal.
customer purchased and after 6 hours smoke came out of amp. I did not repair, returned to factory for replacement. factory repaired and returned amp after 2 weeks, with same charcoaled circuit board. we tested the amp for 1 hour at 1/2 power rating and it went up in smoke for the second time. we returned amp again and it was repaired by the factory by adding several mickey mouse modifications, with same charcoaled circuit board.
this customer was SCREWED by crate. do you think this customer will buy another crate amp? do you think anyone who has experienced this would ever buy a crate amp again? i don't think so.
who owns saint Louis music? they dumped it. guess why? they may sucker a person the first time but after that they learn.
if crate wants people to say good things about crate amps, then they should start building better amps. otherwise if we don't complain things will never get better. what goes wrong with crate amps? just about anything you can imagine, and some things you have never imagined.