EEVblog 1452 - Fluke PM3370B Combiscope REPAIR

[EEVblog]

Repairing the Fluke/Philips PM3370B combiscope.
Or have we?

[Old Fart Analog Engineer]

Its the Over Voltage / Over Current protection SCR circuit in the "mains" power supply. There is a trimpot to adjust it, which works as a voltage divider to set the trip point. It is likely corroded and increases or opens which triggers the SCR to to latch and shut it all down. Clean that with some contact cleaning spray, maybe also "rock" it back and forth a little bit. It is likely to have silver plated contacts, which are tarnished. If it is a "Piher" brand (used throughout European electronics) it is junk. Philips also made some which were marginally better.

[mc172]

I had the very similar PM3382 that did the same thing whilst I was right in the middle of repairing some equipment for a "customer". It would turn itself off and then buzz/whine. I did some looking around inside and couldn't see anything obvious so started poring over schematics and decided I didn't want to be fixing an oscilloscope in order to fix someone elses bit of kit, especially since I'm not actually a repair shop! I got the Rigol out instead.
Later on I put the Fluke back together and tried it on power for shits and giggles. Unfortunately it didn't fix itself like your one appears to have and the glass fuse in the IEC socket at the back exploded in quite dramatic fashion. It was quite loud and there was glass everywhere when I opened the fuse holder. New fuse, glass double everywhere. In the end I binned it as it was the lowest spec model of the bunch anyway. It was 4ch but on two of them you've got either something like 1V and 5V/div vertical scale options and that's it! That wound me up on more than several occasions. It's one or the other, next to pointless having the channels but I'd always forget that it did it, only to be reminded when I needed more than two and get irritated.

edit... 3382, not 3384

[EEVblog]

Its the Over Voltage / Over Current protection SCR circuit in the "mains" power supply. There is a trimpot to adjust it, which works as a voltage divider to set the trip point.

I don't see any trimpot on this board.

[EEVblog]

Follow-up:

[Old Fart Analog Engineer]

Looked like a pot on the original video.  Now that you have put up the schematic and details can be seen, I was thinking that R1018, 1.5K on the schematic page 287 was a pot, but it is listed as an NTC thermistor on page 293.  If this opens or significantly increased in value, it would trigger V1014 thyristor which would latch everything off until power was cycled.  This thermistor is between the two aluminum heatsinks, next to a resistor and the TO-92 Thyristor, in "disguise", looking like a gold tantalum cap, about an inch from the one you've been messing with (inrush current limiter on bridge rectifiers), also shown on page 285 of the manual as a rectangle.

[EHT]

Looked like a pot on the original video.  Now that you have put up the schematic and details can be seen, I was thinking that R1018, 1.5K on the schematic page 287 was a pot, but it is listed as an NTC thermistor on page 293.  If this opens or significantly increased in value, it would trigger V1014 thyristor which would latch everything off until power was cycled.  This thermistor is between the two aluminum heatsinks, next to a resistor and the TO-92 Thyristor, in "disguise", looking like a gold tantalum cap, about an inch from the one you've been messing with (inrush current limiter on bridge rectifiers), also shown on page 285 of the manual as a rectangle.

Yes, I think R1018 thermistor is suspect. Failing that other components around the voltage control - quite a few diodes, Opto-Isolator. Hopefully not too hard to find if it starts failing again. Not a fix the moment though!

I'm not sure why we continued looking into the dry joints when as observed at the start, it doesn't fit the failure mode. If they're suspect then how about giving it a good old whack!? Reflowing the joints is complicating things since the fault hasn't been replicated. It's yanking your chain Dave!

Anyway, great video exploring the PSU circuit.

BTW - how come it is so clean!? Its like new.

[EEVblog]

BTW - how come it is so clean!? Its like new.

Yep, never seen a used bit of gear this clean before.