Transformerless Vari Mu Compressor build thread

[living sounds]

I've got a problem with my build. I've got +224V going into the STP10nk60ZFP, but only +95V going out. I couldn't find a fault with any of the surrounding parts. Bad MOSFET?

 

[Heikki]

Pull the tubes and check if the voltage stays at 95V even without tubes. Then check that on main board R49, R50, R51, R52, R54 are correct value and D3 is 200V zener and not accidentally 100V zener. Also check that C35 and C40 are oriented correctly.

 

[living sounds]

So it turns out I got a 100V zener after all. Mouser sent me two bags labelled identically to contain 200V zener each, but one of them had a 100V zener inside. And since there's a "2" written even on the 100V zener I assumed it to be the correct 200V one...

After a few minutes of warming up the voltage has crept up to +180V - enough I assume?

 

[living sounds]

Heater voltage is slightly lower at 23.75V.

 

[Heikki]

180V is just as it should be. Your heater voltage is a little bit low but not low enough where I would worry about it.

 

[living sounds]

OK, it all checks out beautifully now (after 10 minutes or so I am getting +182V), except for the slighly lower V+.

There's only one problem: I couldn't get the Omron relay, so I bought another one (datasheet attached). However, even with R1 bridged it won't switch. It is a 24V DC type with 2880 ohm coil resistance, same pinout. Voltage at the relay entrance checks out as expected with the bypass switch (dis)engaged. Any idea what I can do?

 

[Heikki]

Looks to be almost identical to the Omron relay so it should work. Are D1 & D2 oriented correctly?

 

[living sounds]

Looks to be almost identical to the Omron relay so it should work. Are D1 & D2 oriented correctly?

Yes, and the almost +24V can be measured at the cathode with bypass engaged. But the relay doesn't click and there is no continuity between the respective input and output pins...

 

[living sounds]

Guess I could try and use the "raw" 36V DC feed for the relays... But according to the datasheet it should switch at +16.8V DC, so + 23.75V should be plenty....

 

[Heikki]

When the unit is powered off can you get some resistance measurement from the relay coils. Measure resistance across D1 or D2.

 

[living sounds]

I am getting 125 ohm...

I've got more of those relays, and the ones not soldered in measure correctly (+/- 10% of 2880 ohm).

For some reason there is also a ca. 5 ohm resistance between the A- ground and the main PCB ground.

 

[paolomunarini]

I am getting 125 ohm...

I've got more of those relays, and the ones not soldered in measure correctly (+/- 10% of 2880 ohm).

For some reason there is also a ca. 5 ohm resistance between the A- ground and the main PCB ground.

Low resistance from A- to GND is normal. R1 on PSU board sets that. I think it's like a filter/current limiter

The relay is probably busted...

 

[living sounds]

Low resistance from A- to GND is normal. R1 on PSU board sets that. I think it's like a filter/current limiter

The relay is probably busted...

Both relays... but they are brand new and cannot have received an overvoltage...

 

[paolomunarini]

Both relays... but they are brand new and cannot have received an overvoltage...

With the relay removed, what is the resistance between the pads on the PCB?

Is there any short?

Can you test them on an external PSU?

 

[living sounds]

There's 25 ohm between v+ and main PCB ground...

And 20 ohm between v+ and v0... Why doesn't it blow up? That's probably the heater's resistance, right?

 

[Heikki]

Like paolomunarini said the 5 ohm between A- and main board GND is supposed to be there if the wiring is done according to the layout pictures.

 

[Heikki]

There's 25 ohm between v+ and main PCB ground...

If you have tubes in the sockets then that‘s the resistance of the cold heaters.

Your 125 ohm measurement is exactly what it should be when the tubes are plugged in the sockets. Could you do the measurement after taking tubes out of the sockets.

 

[paolomunarini]

Also:
Try to measure the relays unmounted from the board.

Try to feed 24V with an external PSU to the coil and see if it switches.

 

[living sounds]

Yes, with a tube removed it is >1600 ohm.

 

[Heikki]

Yes, with a tube removed it is >1600 ohm.

That’s what it should be because we’re measuring the relay coils in parallel plus 100 + 100 ohms. So I don’t think the relays are faulty.