JCM 2000 DSL 401 Troubleshooting reverb problem

[DrGonz78]

I have this DSL 401 that has a reverb problem. The main problem is that the reverb is not working at all. I have checked all the usual suspects: tank, cable, voltage to the chips, and plugged other tanks into the amp & plugged the 401's tank into another amp. When I plug the 401's tank into another amp it works, so it is not the reverb tank or the cables.

Anyway, jumping ahead I checked ever resistor and transistor in the circuit to no failures. Now I last decided to check all the zener diodes on the entire board just to make sure and I ran into >>> ZD5 (BZC558). It is open and was reading .009 on both sides. Looking at the schematic it makes sense that this could be my culprit. My question now is, if I have other zener diodes laying around that rate higher than this one, can they be used in this case or do I have to make it match up completely? Also, if this part is bad is there anything else I should be worried about after replacing the diode.

Thanks for being here to help and it is always much appreciated.

[Enzo]

Let's be more precise. A diode is a part with two leads. "It is open and reading .009 on both sides" doesn;t make sense to me. .009 WHAT? Ohms? VOlts? And at each end? From each end to WHERE?

We test a diode several ways. You can measure resistance from one end to the other. SHouldn;t measure shorted. We can measure junction voltage with the diode test function of our meter. We measure one direction, then the other. We should read open one way - reverse - and one junction drop forward. For a zener, that might be in the area of .7v.

Did you mean You measured .009 something in both directions across the part? Ohms? VOltage drop? Actually either one of those units wouold mean a shorted diode.

Not trying to be a prick here, but I nitpick because we need to know what one another is talking about. When I say some voltage is at both ends of a part, I mean with respect to ground unless I specify otherwise.


ZD5 is a 12v zener. That means just sitting there, amp running, it ought to have 12vDC across it. 100k R135 connects it to the +24v power supply. If there is no 12v across it, then either it is shorted, thus grounding out the resistor, or transistor T15 - which is parallel to the zener if you look closely - is shorted. Or maybe R135 is open.

If you measured ZD5 as shorted, you must then remove it from the board and retest it alone.

And what does T15 do? Appears to be a reverb mute transistor. No 12v, no reverb.

[DrGonz78]

Okay sorry about that one... Yeah I was a bit vague in my posting. I was actually referring to continuity of the diode. As I test it for continuity it sounds the beep and reads .009 on both sides. I was measuring all the others throughout the board and had no similar readings per continuity. When I put the ohms setting to 200 it reads 6ohms of resistance and infinite on the other direction. I soldered off the board and have the same readings too. So maybe it is ok? I will try to be more specific as I go...

I already have removed a leg of R135 and it is good. T15 and T16 I have tested as good transistors. I will next solder ZD5 back on and check the voltage... Which side of the diode should I measure voltage and from where? From ground or another spot?

[DrGonz78]

When I put the ohms setting to 200 it reads 6ohms of resistance and infinite on the other direction.

I take this part back after looking at it again I get 6ohms in both directions. It was reading it different at times for some reason but this is more accurate.

Edit: I also have another zener diode here and it tests on the continuity setting(which I think is what it said in the manual to test diodes) as .615 one way. Reversed it reads (I) infinite. Still when I test this other diode from the amp it tests .009 in both directions.

[Amp Kat]

As Enzo pointed out that it really depends on what the diode is tied to as there are things in the circuit that could make it read .009. Not saying it's not bad but when I get erroneous readings on a zener I always revert back to the voltage test. If it's say a 12 volt zener and I read 12 volts on the cathode saying it's positive voltage then it's good and if it reads anything else it's either bad or something is dragging it down. Another thing Enzo said which btw I couldn't have witten that post he did any better as usual with him that lifting one end is huge taking it out of the circuit eliminating other erroneous readings.

IME these problems usually end up being some type of connection problems when everything seems good except the circuit.

[Jazz P Bass]

The BZX558 is a 12 volt zener diode.
It's power rating is 1/2 watt.
You can install a 12 volt zener with a higher wattage rating (if the leads will fit in the PCB holes).
I would not go lower.

[DrGonz78]

I soldered off the board and have the same readings too.

The first thing I did when I got strange readings from the diode in circuit was to take it off the board. It reads the same in circuit as it does off the board.

[g1]

Then it is shorted and you must replace it.