Kalamazoo Bass 30 Tone Stack

[Chuck H]

I can't comment on the diode test. I've never even tested a rectifier diode or sought to understand their failure mode. I CAN say that it seems to me that with only one diode per leg in that amp they must be unusual, high current diodes. So I can't guess how they should test. If you have 1n4007's on hand (and I think most of us do) you can simply use two of those in series to replace each one that is in the amp and be done with it. g1 will probably want to comment on your findings though since he's the one that requested the test.

[Helmholtz]

I removed the rectifier from the circuit and tested the two diodes with my Fluke diode setting. Typically when I test a generic diode with the red lead going to the anode and the black lead going to the cathode the DVM beeps and gives me a reading of somewhere in the neighborhood of 0.5V (forward voltage?). In the case of this rectifier the tester is not beeping and the readings are 2.1V on both sides.

A diode should read open (infinite resistance) in one direction and low resistance in the other. Your measurements indicate that the diode is shorted and will pass AC.

[g1]

Yes, the diode that reads the same both ways (2.1) is bad. Replace it with a 1n4007 like Chuck said. And those 2 bias caps, 20uF. Best to go with higher voltage caps there, 35V or 63V. 22uF will be the standard common value to use.
You didn't mention how those other 2 diodes (DI-57) measured, I guess ok? They would be higher voltage diodes and 1n4007 may not be suitable there. Anyway, I think they are probably fine as your B+ voltage seems correct.

[bobloblaws]

Yes, the diode that reads the same both ways (2.1) is bad. Replace it with a 1n4007 like Chuck said. And those 2 bias caps, 20uF. Best to go with higher voltage caps there, 35V or 63V. 22uF will be the standard common value to use.
You didn't mention how those other 2 diodes (DI-57) measured, I guess ok? They would be higher voltage diodes and 1n4007 may not be suitable there. Anyway, I think they are probably fine as your B+ voltage seems correct.

Aww, shoot, I messed up and confused the issue somewhat. The diodes that showed as 2.1V on the Fluke diode tester are the ones in the rectifier (DI-57). I had it confused with the "SCXE" marking, sorry about that.

As you said, the B+ seems OK, and I'm not seeing a lot of AC on that B+ line (about 250 mV), so is it possible those readings (2.1V) are normal for that rectifier, or is it an indication of it possibly being on it's last legs?

So to further clarify, I think the diode in the bias circuit is fine, I tested it yesterday with the Fluke, and the negative bias voltage is back in the -22V--24V range after straightening out the cap that was flipped.

[Helmholtz]

As you said, the B+ seems OK, and I'm not seeing a lot of AC on that B+ line (about 250 mV), so is it possible those readings (2.1V) are normal for that rectifier, or is it an indication of it possibly being on it's last legs?

A diode that drops the same 2.1V in both polarities/directions is defective. Maybe a measuring error? Even 2.1V in only one direction would be bad for a silicon diode.

[bobloblaws]

A diode that drops the same 2.1V in both polarities/directions is defective. Maybe a measuring error? Even 2.1V in only one direction would be bad for a silicon diode.

I'm talking about the rectifier, it has two diodes. What I meant was that on the diode tester in the one direction they both show O.L. as per normal, and in the other direction they both show 2.1V (and no beep) whereas typically when I test diodes on the Fluke DMM it will beep and measure somewhere around 0.5V to 0.7V. Sorry for any confusion I added by mixing up the markings on the schematic.

[Helmholtz]

I'm talking about the rectifier, it has two diodes. What I meant was that on the diode tester in the one direction they both show O.L. as per normal, and in the other direction they both show 2.1V (and no beep) whereas typically when I test diodes on the Fluke DMM it will beep and measure somewhere around 0.5V to 0.7V. Sorry for any confusion I added by mixing up the markings on the schematic.

2.1V forward voltage would be way too high for a (single) silicon diode. But maybe its a selenium rectifier. Or is it actually a chain of 3 silicon diodes in series? Can you post a picture?

[bobloblaws]

2.1V forward voltage would be way too high for a (single) silicon diode. But maybe its a selenium rectifier. Or is it actually a chain of 3 silicon diodes in series? Can you post a picture?

Sure, it's the turquoise colored puppy...

[Helmholtz]

Sure, it's the turquoise colored puppy...

[ATTACH=CONFIG]51809[/ATTACH]

Can't know for sure but it's possible that this part contains 3 or 4 silicon diodes wired in series per side. Maybe you can find a datasheet.
If so its probably ok.

[g1]

I agree those rectifiers for the HV circuit are triples. 3 x .7V gives 2.1V on your meter. So 2 triples in that pack.
If they read OL with meter probes reversed (and 2.1 the other way), they should be fine.

Now you've got your negative bias voltage back to normal by changing the cap, so the bias diode should be ok too.
But if you just flipped it, don't trust it. Caps (electrolytic) that have been subjected to reverse voltage are likely damaged or at least weakened and should be discarded. So both those caps with the + ends grounded should get new replacements.

Now just one question to confound the whole issue and open the discussion for others to speculate wildly.
You lost proper bias before you touched either of those bias caps, right?

[bobloblaws]

Now you've got your negative bias voltage back to normal by changing the cap, so the bias diode should be ok too.
But if you just flipped it, don't trust it. Caps (electrolytic) that have been subjected to reverse voltage are likely damaged or at least weakened and should be discarded. So both those caps with the + ends grounded should get new replacements.

Yes, I threw out any caps that I might have compromised. I have a brand new one in there now.

Now just one question to confound the whole issue and open the discussion for others to speculate wildly.
You lost proper bias before you touched either of those bias caps, right?

Yes, I'm pretty sure, but one of my problems was that at one point I was troubleshooting without being aware that one of the legs of the rectifier had come free (as in the photo), so that might have been a factor. I can't remember the exact sequence of events. I'm just glad I got it sorted out, with the help of everyone that chimed in, much appreciated everybody!

[J M Fahey]

FWIW:

In the SEMTECH modules, the diodes are always 1 kV each, so a 10kV stack would be ten in series per leg.

If it reads 6-7V, for example, there's likely about 10 junctions in series. The SEMTECH ones would read a bit higher, more like 8-10V for 10 junctions in series, because they use a different kind of junction that has slightly higher Vf than a typical double diffused junction.