Power Transistor Suitable Replacement

Thread Starter

Lorne Allaire

Joined Apr 17, 2017
26
Hi, I'm trying to find a suitable replacement for a power transistor for a 70's dumb terminal. The only markings on it are "SJ7608", "8213" and the Motorola logo. I think the 8213 is the manufacturing date ... 13th week of 1982. Tried googling the other number but there's very little information on it. It comes from a 24V switching power supply. Any info you can provide would be greatly appreciated. Thanks.
 

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ian field

Joined Oct 27, 2012
6,543
From the schematic, it looks to be an NPN.
There's a guess, and then there's an educated guess.

Plenty of things from that era used flyback SMPSU, and something of that kind might give up a suitable part. The transistor from a TV SMPSU would have adequate voltage rating and probably fast enough - but if your VDU PSU also provides a lot of logic supplies, you might need more power rating.

Many early pre ATX PC PSUs were flyback - the transistor from one of those might do it.

TV PSU transistors evolved to the sort of power ratings - but the metal TO3 gave way to the TO3P, you can still fit them though.
 

Thread Starter

Lorne Allaire

Joined Apr 17, 2017
26
Thanks for the advice guys.

When I first plugged the dumb terminal into a power source, the tantalum cap blew (C30). I have since replaced this tantalum (only one) as well as all the electrolytic caps and diodes (shown in solid green). Did not touch any of the ceramic caps as they all appeared to be fine. I tested all the resistors and transistors (shown in hollow green) and they all checked out fine.

Now, when I plug it in to a power source, I hear a clicking noise about every 1.5 seconds. I tested the voltages and the +15VDC and +12VDC rails work but the +5VDC and -12VDC do not. The voltage readings on these ones are sporadic. The +5VDC rail jumps from 0.5-1.5V after every click. The -12VDC rail does pretty much the same. Short of changing every other component on the board, can anyone suggest the possible failure component?
 

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ian field

Joined Oct 27, 2012
6,543
Thanks for the advice guys.

When I first plugged the dumb terminal into a power source, the tantalum cap blew (C30). I have since replaced this tantalum (only one) as well as all the electrolytic caps and diodes (shown in solid green). Did not touch any of the ceramic caps as they all appeared to be fine. I tested all the resistors and transistors (shown in hollow green) and they all checked out fine.

Now, when I plug it in to a power source, I hear a clicking noise about every 1.5 seconds. I tested the voltages and the +15VDC and +12VDC rails work but the +5VDC and -12VDC do not. The voltage readings on these ones are sporadic. The +5VDC rail jumps from 0.5-1.5V after every click. The -12VDC rail does pretty much the same. Short of changing every other component on the board, can anyone suggest the possible failure component?
Ticking is usually an overload - it tries to start but shuts down, when its down it can't see the fault and tries to start again.

Tantalum capacitors can be tricky, they usually go very leaky or short - in PSUs; the dipped bead type usually go off like match heads. Ceramic capacitors are less likely to fail but can be hard to spot - they often crack or if epoxy coated they sometimes burn through a little snot-hole.

If you've replaced all the rectifiers, the short is probably downstream - the horizontal output transistor is prime suspect.

It may be a bit early for Shottky barrier rectifiers - but that's something you have to check and make sure you got right.

SB rectifiers are unlikely to survive the voltages that need regular silicon type, and regular type where SB was specified would overheat and fail because of excessive reverse recovery losses.
 
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