Power Designs TP340

[vindoline]

I just picked up a "parts or repair" TP340 triple power supply for a price I couldn't pass up! On initial inspection, it looks like someone smoked the crowbar circuit 

Does anyone have a good legible scan of the schematic? The manual is easily available, but the schematic is only marginally readable. Thanks a lot in advance!

[vindoline]

I think I may have found a problem 
Old school GE SCR - in the crowbar circuit! R233 is also supposed to be a 100R 1% resistor. It's now about 65R!

[james_s]

Yikes, that got hot.

[FlyingHacker]

I supppse the pass transistor(s) could have failed causing overvoltage to appear across the output and get shorted by the SCR.

Check the rating of the installed fuse. If someone had too large of a fuse in there it could have caused that once the SCR fired.

[mtdoc]

I recently struggled with the illegible schematic issue with my recent TP340A acquisition. The TP340A schematic from the available online manual is even worse than the TP340.

The CR211 SCR on my TP340 is attached to a heatsink. ( see pics here ). I’d be surprised if the TP340 wasn’t designed to have one as well. Did yours? Or perhaps it fell off.?  Surely not vaporized?

[vindoline]

I supppse the pass transistor(s) could have failed causing overvoltage to appear across the output and get shorted by the SCR.

Check the rating of the installed fuse. If someone had too large of a fuse in there it could have caused that once the SCR fired.

That's a good point about the fuses - however I've checked now and they're both the indicated 2A. Interestingly, I've disconnected the Anode of the SCR and the supply appears to work correctly without it, so I think the pass transistors are OK. Of course now there's no over-voltage protection. I need to check if anything upstream in the trigger circuitry is faulty as well.

[vindoline]

The CR211 SCR on my TP340 is attached to a heatsink. ( see pics here ). I’d be surprised if the TP340 wasn’t designed to have one as well. Did yours? Or perhaps it fell off.?  Surely not vaporized?

Yes, my TP340 had a heatsink on the SCR. I forgot to take a photo before I took it off! Nice work on your unit, by the way 

I recently struggled with the illegible schematic issue with my recent TP340A acquisition. The TP340A schematic from the available online manual is even worse than the TP340.

It is frustrating that the scan of the schematic is so poor. It's great that someone took the time originally to scan and share the manual, I just wish they did the schematic in higher resolution so you can read the text. My old eyes have a tough time with it! I'm surprised that no-one here has a copy to share. If I ever get my hands on one, I'll share it with the group.

[vindoline]

Any advice on a replacement SCR for this unit? I can get a NOS part off eBay for about $5 shipped, but I'm tempted to go with a more robust, modern part such as https://www.mouser.com/datasheet/2/389/tn1610h-6t-957174.pdf
Thanks in advance!

[james_s]

I doubt it's critical. I would look up the datasheet for the original and find a modern part with the same pin arrangement and similar specs.