Crate problem PART 2

[John_H]

..

[Valvehead]

Im a "tinkerer" , not a pro tech. something i do for fun and cash on the side. Trying to learn all the time.
Ive repaired at least 25 of the small, newer Marshall amps. Usually the input jack literally explodes into pieces, or the output chips burn up , solder joints as well.

This is the first Crate ive worked on in 3 years at least.


I got this one for $100 and it did sound very,very good. Probably the best solid state distortion ive ever heard. No kidding! And its not even a "modeler" . This is their "Flexwave" preamp design.


Anyway, Ill dig into the amp today--thanks as usual Enzo

[Valvehead]

ok, did some looking today. I re-flowed all the joints for the ribbon cables.
I have good signal out of the High pot. Pot operates fine.

C76 tests good, and signal both ends.

I have plenty of signal out of IC4A and IC4B and all the pots work fine on the Ryth. and Solo channels AFTER the amp "warms up"--i mean after it sits on for about a minute.


so i powered it down for 20 minutes, then powered it up while i had my headphone probe on IC4. At first NO signal (in Ryth mode), Then as the amp "warmed up" over a minute at least, the signal crept up higher and higher to normal volume--for the RYTH. and Solo channels.

So does this mean IC4 could be the culprit ? Could the nightmare be over ??

[Valvehead]

well, i had a R4558 sitting around so i swapped it in there for IC4.

The amp fired right up at full volume on all channels!

im going to let it sit overnight and fire it up tomorrow to be sure. Im not getting my hopes up yet.



will the 4558 be fine in there instead of the TLO72 ?

[PerroGrande]

I don't have the schematic for your amp in front of me, but there are a few differences between the 4558 and a TL072. They are certainly pin compatible, so no worries there, of course.

The TL072 has a *much* higher input resistance due to the JFET front end. It also has a notably superior slew rate and a lower distortion rating. The 4558 will swing a bit closer to the rails.

So, it will really depend on where in the overall circuit this particular dual op-amp is situated. The concern that comes to mind is the lower impedance of the 4558 loading down an input or a previous stage (unlikely, but I suppose possible). Certainly for testing, though, it shouldn't cause any problems.

[mykey]

a bad IC will usually get really hot in these amps,
turn on the amp for about 10 minutes, then turn
off. feel each IC with your finger to see which
one is getting hot. replace the overheating ones.
but watch it, it might be so hot that it burns you finger,
so feel it quickly or use a temperature probe.
when a IC shorts it will kill the power supply
voltage which is regulated, in this case from
the zeener diode.
also the IC might be soldered to the PC
board which is often the case in crate amps.
it takes some skill to remove it. but if you
can, install an IC socket first when you replace
the bad IC.

[Valvehead]

I fired it up today and the amp is working ! So it was just a bad chip.

THANKS ENZO

the only other problem it has is this random "staticy" noise at idle . Im not too worried about it--might just sell it like that.