Crate G120CXL Rebuild

[J M Fahey]

does not look like it. I've got about 30 VAC from both sides of the secondary. I've got about 40vdc after the rectifier though.

Why "though"?
Thatīs what you are supposed to have there.

So my 29.5 supply to the amp is about 40. any ideas on that one?

Yes, that voltage is normal.

I'm assuming that is why my + and - 16V are high. Input for the circuit is supposed to be 29v not 40.

Even IF input were higher than norm zener woukd regulate it for needed value or die trying.
The zener you refuse to test.

[russell072009]

Why "though"?
Thatīs what you are supposed to have there.

Yes, that voltage is normal.


Even IF input were higher than norm zener woukd regulate it for needed value or die trying.
The zener you refuse to test.

Thanks Fahey,

So having 40V on the 29,5 supply is normal. Then even at 40V input to the 16v supply i shouldn't have 25v output from the zeners. I'll change them again but even the working g212 has the same 25v from the zeners. that's where i get confused. At this point all the components on the g120 are new and show 25v on both 16 supplys and the working g212 has the same voltage. You're sure there is no reason for that to be happening? All 4 of the zeners between the 2 amps are bad?

[russell072009]

What are you using for the meter ground point? Did you try the voltage from pin to pin on the IC power pins? The dual op amps cannot stand 50v, more like 35 tops. If you find 30v across the ICs, then we have a measuring issue, rather than a voltage problem per se.

I'm using chassis ground for measuring. IC1 - IC6 use both supplies + and -. IC7 - IC9 look like they only use the -16V line on pin 4. Pin 8 is chassis ground. I'll have to wait until i get the new resistors in to be able to check across pins 4 and 8 on IC1 - IC6. Just to be sure you want me to measure pin 4 to pin 8 and not 4 to GND and then 8 to GND?

[Enzo]

Because of the MN chips, some op amps in the midle of the chorus circuit do run on one side only, but the general op amps in the audio path run on the split supply.

I want you to measure power pin to power pin right at the op amp IC instead of two readings to ground, because I want to be sure the 25v you are seeing instead of 16v is true, and not an artifact of our process. Yes, taking each to ground and figuring SHOULD yield the same result a directly across the IC, but that won't explain our 25v on 16v zeners.

If it turns out something is fooling us, but it is normal, then we can write it off as a mystery to be solved later, while the basic amp is working. We can then go back to concentrating on the chorus.

[russell072009]

Thanks Enzo i'll get that done as soon as i can.

While i am waiting for the resistors to arrive, is it normal to have transformer output creep up like i have? It starts at about 25VAC and creeps up to about 35VAC. I've never seen a transformer do that before.

[Enzo]

What is your mains voltage doing at the same time? Is the battery fresh in your meter? it is very difficult for a spool of wire wrapped around a hunk of iron to change the ratio between primary and secondary.

[russell072009]

What is your mains voltage doing at the same time? Is the battery fresh in your meter? it is very difficult for a spool of wire wrapped around a hunk of iron to change the ratio between primary and secondary.

That's what i was thinking. The only thing i could come up with is heat. Voltage goes up as the transformer warms up. Of course i have no idea what the thing is rated for. The only part number on it is crate's and i can't cross reference it to anything. For all i know the secondary is supposed to be putting out 40vac per side.

I'll check the main line voltage but i doubt that's an issue. I'll also make sure i put a fresh battery in my meter just to be sure.

[Enzo]

Measure the battery with another meter? MY meter has a low battery indicator, but I find that voltages become unstable readings, and resistance readings can get real wacky.

Transformers are like a set of gears, the input and output have a certain ratio. If you change one side, the other side changed proportionately. The actual wire resistance of a winding will cause a voltage drop as current changes through it, but not on the order or 25 to 35 volts. A 40% rise in voltage on the secondary would require a 40% rise on the primary.

40v AC on a side would resctify and filter up to 56vDC, unlikely. By the way, to get 29v DC we'd expect an AC vopltage of about 20vAC to start with.

[g1]

When you find that secondary voltage to be creeping from 25VAC to 35VAC, are you measuring right at the winding, one meter probe to each end? Or are you measuring to some ground ref., which I think may be the problem with some of your readings?

[russell072009]

Ok i want to start out with i'm sorry and i am a regular jack a##. With a fresh battery in my meter (apparently mine does not have a low indicator) the power lines all look good. i have 16V at the zener, 40ac across the secondary. Still no chorus though.

So, I measure about .8vac max on pin 7 of IC8. (plug in guitar and strum it with all controls for channel B at 10) I get the same on pin 3 of IC10. I do not have output beyond .01vac on either pin 7 or 8 of IC10. Now, even if there was an issue with IC11 or IC12 wouldn't i still get something out of IC10? I have -12vdc on IC10 pin 4, and -13vdc on pin 5 so it's got power. Pin 1 has good ground.

The remaining IC's all have proper power as well. IC9, IC11, IC12.

This is the second MN3007 i have installed. Do you think i should try again?

[Enzo]

Why would we keep throwing 3007s at it?

3007 is a bucket brigade, it requires a clock signal to step an input along. It is like a conveyor belt. You can throw input into it all you want, but nothing will come oput unless it is moving. The 555 is an oscillator, and it drives the 4013 flip flop to make clock for the 3007. The schematic shows the oscillator output at 15v p-p at 120-200kHz. Probably way too fast for your meter to read. In fact the signals coming out of the 3007 will be mostly high freqs, not audio, so what appears at the output of the integrator, pin 7 of IC7, just to the left and a bit above the 3007? That should be back to audio, if there.

[russell072009]

I'm not sure if the 3007's i have are good. Anyway, no pin 7 on IC7 has no signal. So without a scope i'm not going to be able to see if the 555 has proper output. I'll buy a scope if i have to. It's been a long time since i used one in high school. Anything else i can try before spending the money for a scope?

[russell072009]

I did just try to see if there is power coming off pin 3 of the 555. i get a very unstable reading between 3 and 7vac. it does seem to change as i play with the rate and depth controls.

[J M Fahey]

Thatīs your meter telling you: "Bwana, sorry, this is beyond me, get proper help"

[russell072009]

So a buddy of mine let me borrow his scope. It's been a while but i think i was using it properly.

Coming out of the 555 to pin 3 of the 4013 is supposed to be square at 15v. I get the voltage but it's not stable. It is always moving and changing frequency. It's more drastic if i play with the chorus controls but with both controls at 0 the signal still has a frequency variation. It's also not a true square shape signal. It has spikes on the leading edges. i can take a picture if you want. not sure if that is normal.

I do have the proper signal from IC8 pin 7 to the 3007 input but i have nothing on IC7 pin 5.

I do have a signal from IC12 pin 5.

I also have the clock signals from the 4013 to pins 6 and 2 on the 3007