WZ68 Copper cap tear down

One of the WZ68 copper caps I use in my M125s finally failed (after about a year). Symptom was blowing fuses. I opened up the unit just to check everything out to be sure.

I took the opportunity to do the yellow sheet mod with Fr207 diodes, and add some Mogami signal cable vs the twisted wire. Playing music again I decided to "fix" the copper cap.

One of the 1N4001 diodes in the unit did in fact fail. Since I have a basement full of diodes... I was able to dis-assemble the copper cap, replace the diodes, and put it all back together. The circuit is simple... 2x14N001 diodes on each leg, and NTC-100D-11 thermistor I was not able to cross reference and 2x18ohm 10W resistors feeding pin 8. NTC gives slow start, resistors give the drop. I put in the same FR207 diodes, for no reason other than insurance.

Now, when I put the copper cap back in service.... I will have three diodes on each leg of the rectifier, insurance, overkill, or no big deal?

PS I did order another copper cap too.

Bob C

Rec wrote:One of the WZ68 copper caps I use in my M125s finally failed (after about a year). Symptom was blowing fuses. I opened up the unit just to check everything out to be sure.

I took the opportunity to do the yellow sheet mod with Fr207 diodes, and add some Mogami signal cable vs the twisted wire. Playing music again I decided to "fix" the copper cap.

One of the 1N4001 diodes in the unit did in fact fail. Since I have a basement full of diodes... I was able to dis-assemble the copper cap, replace the diodes, and put it all back together. The circuit is simple... 2x14N001 diodes on each leg, and NTC-100D-11 thermistor I was not able to cross reference and 2x18ohm 10W resistors feeding pin 8. NTC gives slow start, resistors give the drop. I put in the same FR207 diodes, for no reason other than insurance.

Now, when I put the copper cap back in service.... I will have three diodes on each leg of the rectifier, insurance, overkill, or no big deal?

PS I did order another copper cap too.

Bob C

Nice Rec.
I'm happy to see you doing this. You have all of the parts there. All you need is diodes and you have them too. Yep....Just repair the WZ68 instead of replacing it with a new one that in time will fail to. Also replace the old diodes with diodes that will handle more amperage.

Is 3, potentially different diodes in series an issue here? Eg yellow sheet + Weber cap. My inclination is they are redundant and my cause additional problems due to voltage balancing etc? Just a new way to fail?

Rec wrote:Is 3, potentially different diodes in series an issue here? Eg yellow sheet + Weber cap. My inclination is they are redundant and my cause additional problems due to voltage balancing etc? Just a new way to fail?  

Diodes are very cheap. More or less $ 1.00 each. If it were me I would replace all them with the same type of diode rated at 1000V at 3 to 4 amps each to be on the safe side.

Rec....

I also noticed while looking at your pictures and components there is also a black Thermistor (that looks like a ceramic capacitor).

This component is there to control voltage surges and helps protect your circuitry.

When you first turn on your amplifier and the voltage is applied the thermistor slowly starts to heat up and the resistance of the thermistor starts to decrease and balance out at the specified rating of the component. Any potential voltage that is more than what is specified or applied is buffered or stopped from continuing any further into the circuitry giving the amplifier a safety net.

A thermistor is somewhat similar to Bob's TDA (Time delay relay) only that the thermistor, if pushed too hard, will just simply burn out. On the other hand the TDA will simply trip the relay and stop the current from flowing any further into the circuit.

Normally thermistors are used so that the high-voltage doesn't instantaneously hit the tubes or the circuits all at one time and it slowly raises the voltage as it heats up and then levels off at the specified rating of the thermistor.

Sometimes I have seen thermistors installed directly into the AC line before the current reaches the power transformer.

It seems kind of strange to me that even with this protection circuit or thermistor in place it wasn't able to protect those small diodes from burning out or shorting so to me the weak point in this rectifier circuit is simply those small diode's. Also the thermistor needs to be checked to see if the component is at it's specified rating when cool and functions properly when heated.

I agree. Understand. The only components that were bad were the two series diodes. Running fine with tube rectifiers. The Weber was 1 year old. Anyways now I have 3 per leg when I run the Weber. The thermister is after the diodes in this case. In my life I have never blown a diode in a tube amp application. I feel like the diodes may have gotten stressed during the amps early days and a poor solder joint on initial startup or whatever. Keeping a cautious eye on everything.

3 diodes per side.... Note that the Weber WS 1 has only two (beefy) diodes per side. I’ve opened that one too. This has been rock solid in my st120.

I haven't read the technical specifications on a thermistor in over 25 years so I went and looked for a YouTube video to better explain the function of these components.

Why Should You Use a Thermistor in a Tube Amplifier? https://youtu.be/nRCH0dtyFCk

What is a Thermistor https://youtu.be/DDdlzDFnFu8

Note that the Weber WS 1 has only two (beefy) diodes per side. I’ve opened that one too. This has been rock solid in my st120. wrote:

Yep... Those bigger beefier diodes are perfect for this particular application.