Crate Vintage Club 50 Problems
Ok, got this as a fixer-upper. The clean channel works, but
when you really drive it hard, it starts popping at the audio
peaks. The overdrive channel works, but the output level is
I found the schematic:
It's likely the problem is in the pre-amp stage (but
maybe it's the finals that have a bad connection, that
rattles when you turn up the volume?), so
I swapped J42 and J44 AX-70 tubes, just to see if the clean
channel would still work, and it does, although it was about
3 AM when I tested it, so I couldn't crank it.
But the funny thing about the distorted channel, is that the "level"
control, and the "bass", "mid", and "high" knobs didn't do anything.
Actually, the level made a tiny bit of difference in volume, but nowhere
near what you would expect. It's a shunt pot to ground, so I should be
able to make it totally quiet, which I was not able to.
Perhaps optoisolators OC1 and OC2 are not working, and the clean signal
bleeds through, when it's supposed to only be the distorted signal?
This is my first stab at tube amp repair (I'm an RF engineer, who also repairs on the side), so any advice is greatly appreciated.
Oh, yes, and the reverb doesn't work. I'll tackle that problem
later, but that doesn't look too hard, with my o'scope.....
I'm not allowed to look at that file. The file says pdf but when I try to load it it comes up with a .dwg suffix and never stops loading. It is only a 300k file, should load in a couple of seconds.
Optoisolators can get wimpy and have lower and lower "gain" but they rarely "bleed through".
Do you have a scope? or an analog voltmeter with a 20 VAC and 2 VAC scales? You can't check popping with a dvm, they average over 3-5 seconds. You have to put a .47 uf cap in series with the probe of the AC scale of a VOM to make it not respond to DC.
I would measure the gain of the tubes by measuring the voltage going in and coming out, and using the plate resistor value calculate Gm. However, if the drawing has normal DC plate values, if they are low it would tell you that the tubes are gassy. If the power supply values are low, then a normal 20 year re-e-cap is in order, maybe a rectifier tube replacement. (8000 hour service).
Your level pot could have an open wiper, which could be why it has no effect. Check it with one end removed and an ohmmeter, or if it terminates in a tube grid, just with the power off. Also could be a bad solder joint.
If you have tone controls and a volume control that "do nothing" then I worry that the ground connection under them is broken.
Bad optos might leave a channel always on or always off, but if sound comes THROUGH a tone/volume stack, ther is no way for the optos to cause the controls to lose effect.
The end leg of each of those volume controls should be at ground, check that with your ohm meter.
Ok, woke up this morning and.....
The distorted channel works full volume!! AND the level and
tone controls work now!! I guess last night I must have had it
too low or something, I don't know, but it works!
I would guess the problem could have been a bad tube socket
connection. I sprayed contact cleaner over all the pre-amp
socket holes and pins. It appears to have worked.
Now I will put my o'scope on the input and output of the spring
reverb, to see what is going on there......
Thanks for all the suggestions!
Ok, as you can see in the schematic, there are two FETs
driving the reverb level, which allow separate reverb levels for
both clean and distorted channels. THE DAMN GATES HAD
A $#@*!!% SOLDER BRIDGE BETWEEN THEM!!
I was scratching my head, why would the gates always be
-4.5V, regardless of which gate had the -9 V pinch-off voltage?!
Haha! But I can't imagine it came from the factory like this....someone
else must have tinkered a bit, unsuccessfully.
Ok, and another mystery solved: If you have the gain knob all the
way up, and the level knob all the way down, there is a weird area
where the EQ controls have very little effect. That's what I was likely
seeing before, at 3AM in the morning. I suppose it's normal that the
level knob cannot make the amp completely quiet if the gain knob is
all the way up. It doesn't make much sense to have all that distortion
at a whisper quiet level!
Ok, I won't celebrate too quickly....I'm gonna rock with this thing
for a few days, just to make sure it's all fixed.....stay tuned!
Damn! There's still a problem.
But it's very intermittent, so it's hard to find what is wrong. Sometimes
the gain in the distorted channel will drop dramatically, and then it will kick back in when I turn the level knob up, and blast the **** out of me. Also, it sounds like
the gain comes back if i play something loud.
Yeah, that's a good idea, i will check the wiper and the ground connection on that level pot.
****, back to the drawing board.
Ok, I've re-soldered most of the pads in the gain chain, and I believe there
is still a problem with too low of a level for the distortion channel.
If you look at the schematic I posted in my original post, they have
given o'scope waveforms at each test point, with what I believe
is a 10mV peak to peak input signal.
But the DC voltages are with no signal applied.
I assume I should do the DC measurements first, just to make
sure everything is biased properly, and then (assuming the
low level problem is consistent enough) measure the waveforms,
which should clearly show which stage is bad, right?
Oh yeah, and I checked all the pots, and they seem fine, but I swapped
the level one for the clean treble pot, and the problem is still there.
Shorts to ground on the pots seemed good too.....
Ok, this is strange.
When I first turn the unit on, it will usually be in the defective state
on the distortion channel (low output, but too much noisy signal when
"level" knob turned all the way down). Then if I turn up the level
nearly all the way, and I play something loud on the guitar, the amp will
shreik in feedback, and "pop" back into the regular working state (output
level is healthy, and very quiet when "level" knob turned all the way down,
as it should be).
So it's some strangle toggling that is going on here......
|All times are GMT. The time now is 09:40 PM.|
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio