SE Labs Scope Repair

[Geaney]

I've encountered some problems on the EHT side of things and need to replace what I understand are transient suppression diodes - marked SCM30. From my surfing online the closest equivalent I can find is NTE4940 but even these are not easy to find. The characteristics of the diode are on line 6 in the attached sheet. CORRECTION - wrong sheet attached, see later post - apologies.

Any suggestions on equivalents/replacements is most welcome. I'll try to scan and post the circuit I'm working on.

Essentially the problem came about because of a faulty trim pot on the +12v supply. It went intermittent and created huge noise spikes. This supply feeds an oscillator which drives a step up transformer to create the EHT. When I powered up the HT went mad and before I could find the problem it looks like these diodes went oc - there are two of them in the transformer secondaries.

Cheers,

John
At this stage I'm really hoping I can get back to valves again soon

[Geaney]

Correction - correct data sheet attached - see line item 6.

John

[Refugee]

I would just use 3 1N4007 diodes in series for each one.

[Nick]

I would just use 3 1N4007 diodes in series for each one.

SCM30s are not ordinary diodes like the 1N400x series - they are unidirectional transient suppression devices which have specific breakdown characteristics - replacing with a string of 1N4007s would not work and would be unsafe as they would do nothing.

You should be able to use the cheap & common 1.5KE-series TVS diodes - do you have the schematic of the HV section that we can see so that we can suggest an alternative?

This is also a 30V, NOT a 3KV, device, so the common 1.5KE30A device would be fine - 33p ex-VAT from Farnell - http://uk.farnell.com/stmicroelectronics...dp/9884963

EDIT: Need to qualify this reply a bit - I suspect that these are TVS diodes, not ordinary ones, but until we have the schematic I reserve judgement!

[Geaney]

SCM30s are not ordinary diodes like the 1N400x series - they are unidirectional transient suppression devices which have specific breakdown characteristics - replacing with a string of 1N4007s would not work and would be unsafe as they would do nothing.

You should be able to use the cheap & common 1.5KE-series TVS diodes - do you have the schematic of the HV section that we can see so that we can suggest an alternative?

This is also a 30V, NOT a 3KV, device, so the common 1.5KE30A device would be fine - 33p ex-VAT from Farnell - http://uk.farnell.com/stmicroelectronics...dp/9884963

Thanks Nick, I'll see about scanning the circuit and posting over the weekend. In the diagram they look like they're acting as regular rectifier diodes but I'm not familiar enough with them to know exactly what role they're playing. More to come.....

Cheers, John

[Geaney]

Here's a scan of the circuitry where these diodes are located. I've also included a photo of the circuit board itself. One of the diodes can be seen lower centre of the photo, the other is just underneath where the HT lead is connected.

I'll remove one of the diodes later and test it out of circuit.

Cheers, John

[AlanBeckett]

The Data Sheet that I found says it's a bog standard 3kV PIV 10mA Silicon Diode. That's fits with the Circuit as far as I can tell. Shouldn't be difficult to replace if necessary.

Alan

[Nick]

There are several diodes called scm30 which is where the confusion lies - having looked at the schematic I concur that in this case any decent 3kv 10mA or so diode would be fine....

[Geaney]

Thanks Guys, that makes my life a lot easier. I removed both diodes and they both show completely open circuit on the DVM and the component tester does not detect that a component is hooked up.

I'll get some diodes and report back once I've fitted them. Hopefully that's all that suffered during my little incident!

Cheers,

John

[jimmc101]

Hang on, they may be OK.
The SCM30 is actually a number of series connected diodes in one package.
The datasheet gives the forward voltage drop as up to 15v at 10mA, so a normal DMM diode test/Component tester will show a good device as open circuit.

An AVO on the ohms x100 range or an 18v battery and series resistor (1k8 upwards) would work.

Jim