Crate CA120DG Durango power amp problem

[Enzo]

So the amp works OK other than the DSP FX, at least without Q13? I assume so from earleir post.

You suspect the DSP card is bad because you get no DFX. Fair enough, it may well be bad, but we need to be sure first.


DSP ON refers to the LED immediately left of there. The panel LED indicated DSP is on.

OK, look top center between TP12, TP13. Signal comes through C55, then branches. One branch is the direct dry signal, which goes through Q15 and IC8a to reach TP13. When Q15 is ON - no gate voltage - it is like a switch on. Since we get signal, we can assume it works. But a test would be while it is running, measure resistance drain-source on Q15. if it is a low resistance, the part is ON. And verify no voltage at the gate. If we had voltage at the gate to turn the thing OFF, then we ought to measure a high resistance between S and D.

That other branch went down through R105 and to the DSP.

So far this is really much like any old Fender where the output of the preamp comes up to a 3.3M resistor, which passes the dry signal along to the power amp, but also branches before that resistor down to the reverb drive.

Now the output of the DFX leaves the DSP card, through R107 and Q14 and on into IC8a. Again Q14 acts like a switch. Put voltage on its gate and it turns off, so no returning DFX would get through. Remove that gate voltage and the JFET reverts to ON and it conducts the signal. As before, measure resistance source to drain on Q14 while the amp is running, and see that it foes high or low in value as the gate is controlled.

So if we turn Q15 on and Q14 off, we get the dry signal and no FX. And if we turn Q15 off and Q14 on, we get the DFX in there. SO we need to have those control voltages appear at those gates one or the other. When one is on the other is off, and vice versa. If we find that the two JFETs stay with their gates always the same, them that is at least part of the no DFX. If we find Q14 reliably going on and off, then we can assume no FX is leaving the DSP card.

By the way, that connector J12 is poorly labelled. There are send and return for the DSP itself, but there are also send and return for the loop jacks on the chassis. If you turn the FX off and the amp plays through the speaker OK, then TP12, TP13 are working.

The gate signals for Q15,14 are DSP_A and DSP_B, and they come from Q1 and Q16 down by your "DSP ON" area. Note the collector of Q1 links to the base of Q16, and that means whenever Q1 is on, Q16 will be off, and vice versa. Normally Q17, through SW2, grounds the base of Q1, turning it off, so the collector rises, putting voltage on DSP_A and darkening the DSP ON LED. Also it raises the base of Q16, turning it on so DSP_B goes to zero. So DSP_B off allows dry signal through, and DSP_A on blocks the DSP.


But I am not sure we are all on the same page. On J12, pins 2 and 1 are the preamp send TO and the return FROM the DFX reverb. That is the "left" channel through the DSP. Pins 5 and 9 are the send and return to and from the DSP for the feedback stopper. As far as I know the DSP A and B switching we have been discussing is the feedback filter. If your lack of DFX is a missing reverb, that will not involve Q14.15. SO if I have led you astray, I apologize.

If you are missing reverb, and have signal to pin 2 of J12, OK. No signal at pin 1 then?

[Tone Meister]

We are on the same page now. When I referred to DSP, I meant no reverb, so that one's on me.

Yes, I've learned that DSP_A and DSP_B are indeed the two feedback filters (FF) and SW2 turns on the FFs and in turn lights up D17 LED. One thing I noticed was the amp mutes when I engage SW2 to turn on the feedback filters. Even though these measurements aren't important now, Q14 D-->S = 140 ohms, 13.8v and Q15 D-->S = 109 ohms, 0.62V, which is the hi/lo condition you described.

When I put a signal into the FX RET on the jack board, I can pick up weak signal at pin 5 of J22 and nothing on pin 1 and nothing on pin 2. When I put the signal into the channel, I pick up nothing on pin 1, weak signal at pin 5 and strong signal on pin 2.

[Enzo]

If you get good +10 into the DSP card, and I believe you did, then go on the card and look for th voltage regulators. What are there, three of them I think? (I closed the schematic file, not handy) See if they are making th 5v 3v and whatever else it needs on the schematic. Beyond that, I am not usually enough of a hero to actually fix the DSp card.

[Tone Meister]

Yeah, I measured 10v at the DSP card and traced those voltages and signal across the card a day or two ago. Seems like I measured ~3.3 volts in places and picked up signal on several places on the card. I would never attempt to screw around trying to fix the card because of the SMT components, I just wanted to pronounce it dead without fear of it sitting up on the gurney on the way to the morgue.

Any idea why the amp would mute when SW2 (FFs) was engaged?

[Tone Meister]

OK, for the sake of those who will stumble upon this thread, the original problem was fixed simply by replacing the Q9 (TIP120) transistor. Also, Q13 (J175) was pulled and not replaced, mainly because I didn't have a P channel on hand that would work in its place.

The DSP is another issue altogether, and I believe the DSP card is bad. The feedback filters are part of the DSP, so that could explain why the amp mutes when the feedback filters are turned on with SW2, but I can't prove that.

Since the 06B885-22 DSP card is no longer available, I am going to screw it back together and lick my wounds. Thanks JPB and Enzo for your expertise.

[Enzo]

The feedback filter is part of the DSP, so those JFETs I had you testing are switching the signal path either around or through the DSP. If teh DSP card is dead, then when we switch the signal through it, we will lose it. One way to test would be a jumper wire in J12 shorting the send and return together. If that brings the signal back, then we know the card is not working.


Q9 is a power supply regulator. You found it defective and killing your 10v, so the amp was muted. But in the process of dying, who knows what voltage it may have let into that DSP card?

[Tone Meister]

With the feedback filter (SW2) engaged, the amp mutes - and jumping pins 5 & 9 on J12 brings back the sound, but jumping pins 1 & 2 does not. Also, jumping 2 & 9 brings back the sound, but jumping pins 1 & 5 does not. I think it's safe to conclude that the DSP RETURN from the board through pin 1 is not working.

[Tone Meister]

OK, not so fast. I can pull the 10-wire ribbon cable from the DSP board and leave it connected to J12 and get the same results when jumpering those pins. That implies there is something aloof in the return circuit in the preamp. Maybe the DSP card isn't bad after all.

EDITED TO ADD: R98 and R99 both test good and the non-polarized C53 (2.2/50) measures right at 2.2 uF and has an ESR of 2.5 ohms using my ESR meter; however, it measures 3.8uF on my Fluke 87. Replaced C53 and no change.

[Enzo]

The DSP has two channels, one is used for reverb, the other as a feedback filter.

Look at the schematic, pins 5 and 9 are the I/O for the feedback channel, pins 1 and 2 for the reverb.

Pin 1 is the return reverb signal. Apply some test signal there or just jumper it to a signal, it ought to come through if the reverb is up.

With feedback on, the amp goes quiet. The signal path through the DSP is broken. Now look at drawing. Pin 5 is the send from the main signal path. Pin 9 returns to it through Q14. When feedback is ON, Q14 is on and Q15 is off. So pin 9 acts as an input to the power amp. So if you connect the signal on pin 5 to the return on pin 9, you get sound. If the DSP channel were working, you;d get sound. Now pins 1 and 2. Each channel has an effects send level control. If that control is up, whatever input to that channel is will also be on the FX send bus. That is what is sent through pin 2. SO if you jump pin 2 to pin 9, sure enough you get the sound from teh FX bus.

To make pin 1 happen, we need to have pins 5 and 9 jumpered, because all that is after the reverb. With 5-9 connected, now a signal applied to pin 1 ought to come out the speaker as long as the DSP return control is up. See drawing, pin 1 then through DSP return pot, then onto main bus, which sends us upper left on page.


If you ONLY jump 1-2 then no sound results because 5-9 is open. If you only jump 5-9 you get sound, but no reverb. If you jump both sets, you get sound, but the reverb path won;t be heard, because it is just the dry signal without the DSP.

Jumpering 5-9 tells us the rest of the path works, and the DSP card cannot pass signal.

[Tone Meister]

• With DSP OFF (SW2) and 5 & 9 NOT jumpered introducing a signal at pin 1 = sound through speaker, controllable with DSP RETURN control (P17)
  
• With DSP OFF (SW2) and 5 & 9 NOT jumpered introducing a signal at pin 9 = NO sound through speaker
  
• With DSP ON (SW2) and 5 & 9 NOT jumpered, introducing signal at pin 9 = sound through speaker, NOT controllable with DSP RETURN control
  
• With DSP ON (SW2) and 5 & 9 NOT jumpered, signal at pin 1 = NO sound through speaker
  
• With DSP ON (SW2) and 5 & 9 jumpered, introducing signal at pin 1 = sound through speaker, controllable with DSP RETURN (P17)