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Messages - substatica

#1
Here's a video of the current behavior,

https://youtu.be/9_6zlbHQyQo

And here's a shot of the board right now, after replacing all but one of the caps and one transistor,
#2
Quote from: phatt on September 02, 2019, 08:23:09 AM
Maybe check the trim and pot wipers are actually in contact with tracks.
That can stop it from working.
Phil.

Yeah they are, I took both of the off, checked/cleaned them and remounted them.

While I'm replying, here's the layout diagram for this board spruced up with a bit of color.
#3
Quote from: mandu on September 01, 2019, 10:08:04 PM
Did you check the 470 mfd emitter bypass capacitor? If low value, then less gain at the very low frequency setting and the oscillator will start and stop at power up. You can temporally place a similar capacitor to test. Regards

Yeah I've replaced all the caps except for the 2.2 mfd, though I can re-check the 470 mfd tomorrow.

I swapped out the 2SC828 with a 2N3904, which still doesn't solve the issue but I can get some variation in the effect messing with the 500K trimpot and the 270k resistor beside it. At this point the only items I can't fully test are the Transistors and JFET. They pass the basic tests. I don't have drop in replacements for the 2SC644 or the JFET on hand.
#4
Also, the schematic calls for 24 VDC onto that board and it's getting 26 VDC, would that present a big issue for the circuit?
#5
Replaced all but one, I don't have a 2.2uf kicking around. No change.

Though I have noticed that both when the amp is powering up, and when it's powering down I hear the effect change as if it starts to oscillate as components drain.

It's got me stumped, because it's not just the missing oscillation, the intensity control has no effect either -- but I can hear the wah effect when it's turned on, it just doesn't change in intensity and it doesn't oscillate. It's like this whole control board is being ignored.
#6
Hrm, I've replaced all the electrolytics, and all the tantalums read fine on a capacitance meter out of circuit.
#7
How might I measure the gain in circuit? I believe I've tested all the components on this board now and found no faults.  The collector of the left-most transistor fluctuates between 2.675 VDC and 2.685 VDC.

Not sure where to look next.

#8
Anyone have any insight into what part of this circuit, or component, is mainly responsible for the effect oscillation?
#9
Working on a Yamaha YTA-95. I've got everything functioning except that the Speed/Intensity controls for the Wah/Tremolo have no effect. I can hear the Wah filter, but it doesn't oscillate or vary in intensity with controls.

I've re-capped the effect control board and the wah board. Tested the two transistors and JFET on the control board. Any ideas?
#10
Quote from: g1 on August 09, 2019, 01:50:34 PM
Pin3 of TDA2040 is connected to chassis via AGND1.  Now if we insulate the tab of the TDA2040, and run an extra wire from pin3 to chassis, the chip will overheat and shut down.   ???

As far as I can tell AGND1 should not ground to chassis. AGND is the chassis ground. It could have been a combination of the missing thermal compound (which I cleaned off when I disassembled) which dropped the resistance between AGND and AGND1 and a cold joint destabilizing the circuit in another way.
#11
Quote from: g1 on August 09, 2019, 01:50:34 PM
This is so bizarre to me that I have to make an analogous statement just to show how illogical this is.
Pin3 of TDA2040 is connected to chassis via AGND1.  Now if we insulate the tab of the TDA2040, and run an extra wire from pin3 to chassis, the chip will overheat and shut down.   ???

Has R63 been verified ok with at least one end lifted?

I don't think I lifted R63, but I did check it, if a resistor reads its proper value and doesn't show any damage I generally test them in circuit. I did pull most of the electrolytics, a bunch of the other caps and diodes and everything has tested fine, and showed no signs of damage.
#12
Quote from: phatt on August 08, 2019, 10:37:48 PM
:dbtu:

BTW, Thermal Paste although not conductive does not Isolate.
Never rely on it as an insulator, you need the washer.

Re the pic,,The tab of U1 is still grounded because the screw is not insulated.
But don't try to insulate it as that will break the link with C26 and D10.
Phil.

Yeah, I did realize that, however, as best I can tell, this is the original setup of the amp, which is what I was trying to get back to. I don't have it open right now, but the final resistance from the U1 tab to ground was like 700k.

At best, if we consider the paste (or my silicon pads) non-conductive and the parts of the circuit board that the heat sink screws are through are AGND and not AGND1 (I didn't check) then it's the screw through U1 and the screws through the heat sink that are the conductive path and perhaps coated to increase resistance. I emailed Peavey to see what the intention was here.

If someone else stops by with a Rage 158 that is stock they could let us know what the deal is here.
#13
I think I've got it licked. I'm pretty sure there's no missing hardware. They just put non-conductive thermal compound between the TDA2040 and the heat sink and between the heat sink and the chassis. I added silicon pads between them and that seems sorted out.

The last issue (hopefully) was that flexing the chassis at all, which happened when the strap bolts were tight, would cause the signal to cut out. I poked around with a chop stick, poking and flexing the board and C23 seemed to react quite sensitively. C23 showed no visible damage or bad joint, but I pulled it, tested it (tested good) and resoldered it and now I can't seem to trigger the issue. Must have been a cold joint at C23 that wasn't visible (or at Pin 5 of the TDA2040 which is on the same trace).

Currently I'm 3d-printing a couple of pillar-type supports to ease the pressure of the strap bolts on the chassis and then I'll re-assemble and test again before calling this solved.

Update: Reassembled and all seems good. Thanks everyone who contributed.
#14
Ug, I put a silicon pad between the TDA2040 and the heat sink and another between the heat sink and the chassis and all seemed well.

Until I put it in the cabinet and tightened the strap bolts that go through the chassis, when they're all the way tightened something else must be grounding out as the signal drops.

If I turn it on with the bolts tighten the sound starts very low and then fades out. If I loosen the bolts the signal comes back. The only thing it seems the bolts could do is warp the board slightly -- I didn't see any damage to the board that would suggest a cracked trace or whatnot.

Out she comes.
#15
Quote from: phatt on August 07, 2019, 03:40:18 AM
*Chas* would obviously mean Chassis/Case.

I can only assume that input socket is meant to be grounded to Chassis and therefore the Power chip needs to be isolated with a mica washer or silicon pad.

The break between the 2 grounds can also be seen at R49 / C32 of the power supply.
Phil.

I worded that wrong, I realized CHAS meant Chassis, by "what does that indicate" I was asking about two grounds in general. It does seem like this is the issue, one I suspected early on, but I found no evidence of a mica washer or silicon pad, perhaps it had been lost/removed before I got this amp -- but I don't see one in any other photos of this amp online, though they're not the clearest shots.

Attached photo of another Rage 158 with TDA2040 in place as I found this one, with only some sort of thermal compound isolating.