Hi guys,
I am repairing an old HP 6253A powersupply. I bought two of these half a year ago, with one of them listed as broken.
When investigating I found that in the powersupply There were some clear indicators of damage, with a few of the bias diodes and resistors in the series regulator being turned to ash. On of the resistors literally turned to dust after removing. After removing the components I could also see some serious damage to the board.
Now, when digging in further I found that one of the main power transistors, Q7, measured a short between collector and emitter. This is where the problem lies: All other components I can find the specs of in the service manual. In the service manual, they are just labeled as:
Q6,7 Power NPN Si. HP 1854-0225.
(Quick note: the actual device markings say motorola and 1854-1017)
So my question now is, how do I find a replacement? I tried using some of the transistor I had laying around in one of the other channels, but none really worked very well. I would usually look at the datasheet and try finding a similar part based on that but I can't seem to find any datasheets on this part. And yes, I can buy NOS transistors of this type but at often 10-15 euros a piece (if I include shipping) I want to try and look at other options first.
So how would I go about trying to replace this transistor? The worst case I can imagine is use the fact that I have at least 7 more of these transistors in working order, and I could use one of those to get a rough idea in terms of expected values...