Crate GX-40C weirdness

[yldouright]

This one stumped me for weeks so I'd like to know what it could've been. The amp came in for scratchy pots and cutting out. It was dirty and abused so I gave it a good once over but then I discovered the following weirdness.

Symptoms: the amp was fully functional on the headphone output but the chorus circuit didn't output to the speakers. I thought it might be a diode or logic gate that might be keeping the chorus effect from mixing properly but why would it work properly on the headphone/line out? I have the schematic for reference if anyone wants to provide some hints.

[The Dude]

That unit has 2 power amps and runs a true stereo chorus. If the speakers are wired out of phase, the chorus effect is minimal. I would check to see that the speakers are wired correctly. If the headphone out works as it should, I can't think of another cause. Or, one speaker isn't working at all.

[yldouright]

@The Dude
Happy Memorial Day and thanks for response. What you describe would be more like a muted effect and not one completely missing but still, it's worth a try. I've informed the current owner of the suggestion and will report when I get his result. Anyone else have any ideas?

[Jazz P Bass]

Which GX 40C do you have?

There are two versions.

The older one uses the speaker outputs for the headphone.

The newer DSP model uses a standalone opamp circuit to drive the headphones.
The power amps are separate & distinct circuits.

G40C.zip

[yldouright]

@Jazz P Bass
I have been notified it has separate speaker terminal outputs in the rear above the 8" drivers, the headphone out is about 6'' away from them and does double duty as the line level output (unamplified). Another curiousity is the missing "bright" switch on the front panel. In its place is a tone shaper circuit in the dirty channel that has both a switch and a pot which accentuates/cuts the mids from its 12 o'clock position when you turn it clockwise/counterclockwise. Switching that circuit off didn't disable the shaper but it reduced its effect. If I recall correctly, this switch was also only fully working on the line out (headphone) but not out the two 8" speakers. The switch off position still allowed the tone shaping but there was no difference in the speaker output when the switch went on. Again, what stumped me is why both reverb channels worked perfectly well even with these anomalies.

[Enzo]

Please do not confuse GX40 and G40 and other variations.

[Jazz P Bass]

Please do not confuse GX40 and G40 and other variations.

Oopsi!

Is there more than one GX 40C?
If the OP has the DSP circuit, then what I said is true.
The headphone is opamp driven.
So there may be a chorus output circuit issue.

[yldouright]

@Jazz P Bass
I can confirm the amp is a GX-40C and not a G40 but I can't say if it's a DSP circuit.

@The Dude
It looks like you were right all along. Owner confirms only one speaker output and switching the speaker wires moves the output to the other speaker so it looks like both speakers work and at least one set of speaker leads are good. If I get the amp back, I'll do a continuity check on the J5 leads (red/black) and if they're good, the amp channel is probably blown. It looks like this amp uses the TDA 2040 op-amp, would I better off changing both op-amps or searching out an original replacement? What's a suitable drop in?

[g1]

op-amps are voltage amps, TDA2040 is a power amp IC.
I would just go with the original type, if you can find one from a reputable source.

[The Dude]

^^^^^^Agree. These guys have 5 in stock.

Audio lab of Ga

Edit: If one amp works, no reason to change it's output IC. They are independent amps. Don't fix it if it ain't broke.

[Enzo]

DSP is digital signal processing - the FX unit. Look inside. If the amp has a busy little circuit board with 100-leg ICs on it, that would be the DSP card. . Lacking that you'd have one large board with regular stuff scattered about.

google Crate DSP board, and there are images of one.

[yldouright]

@Enzo
I couldn't find a photo of the DSP but my model appears to be a GX-40C+ which doesn't have the DSP controls on the front panel like the GX-40D. I seem to recall two long boards inside that amp; one for the pre-amp connected with two flat teflon multi-conduit cables to the power amp section. I don't recall seeing a big multileg chip and I'm pretty sure the boards had a fairly unsophisticated layout/mask. I noted Ebay has these TDA2040 chips for about $0.60 each in lots of 5. The link above provided by @The Dude doesn't appear to be different from those but I know that chips get produced with different (shrinking silicon wafer) masks and therefore have different parameters. I'd hate to introduce oscillation by having chips with largely mismatched parasitics/parameters or speed. Also, the physical layout has one conduit pair (J5) very close to the chip mounted on the right and the other conduit pair a full 18" from the other chip. I presume these correspond and are distanced in the layout to create the chorus effect. Is this a plausible presumption?

[Enzo]

OK then, GX40C+ is specific enough to get the right drawings. If we don't have it here, you can get it from Loud technologies (Crate's parent) If you have the proper schematic, please post it here.

Here is a DSP board for future reference.


Basic older chorus amps used the MN3xxx series ICs, little 8-leg guys.

Chorusing amps have two channels - stereo amp, each speaker individually driven. In dry mode the preamp signal goes to both for a dual mono. In chorus mode, one side remains dry signal, now in just one speaker. The dry signal is also sent through the delay/phase circuit to make chorus and then fed to the second speaker. Now we have the dry in one and chorus in the other to phase back and forth with each other. That is much more satisfying than mixing it all together into one like a chorus pedal in front of the amp.

If one power amp dies or its speaker, you lose something. If you lose the chorusing side, you get dry all the time. If you lose the dry side, you get either dry or the unmixed chorus. But problems can arise in the signal routing back in the preamp.

SO what we need to do is very specifically determine what is happening. Diagnose the two power amps first, then see what is actually missing in the preamp. If it all works in the phones, that does point to the power amp circuits.

[yldouright]

Yes, it had an 8 leg DIP and the MN3xxx series rings a bell for me. The chorus switch and pots had absolutely no effect on the speaker output if I recall correctly so it seems I lost the wet channel. I've been told it was the red and black leads that didn't output. Now, just one clarification; we are excluding the reverb tank when describing this wet/dry side. I say this because the reverb continued to work for both the clean and dirty channel. I'll be getting the amp back this weekend and I'll know more then. Thanks for the explanation. A Loudtech search for GX-40C+ yields this page.

[The Dude]

......I noted Ebay has these TDA2040 chips for about $0.60 each in lots of 5. The link above provided by @The Dude doesn't appear to be different from those......

The thing you have to look out for on eBay is that there are many counterfeit semiconductors sold there. I wouldn't mess with it unless you can verify that it's a reputable seller and they are "real" parts.