2n222a 2n2907a amp in CAN-3 case in a HP 8903

[cncjerry]

Dumb question.  I brought the smoke out if my new HP8903a with the processor upgrade.  I was probing, found the blown output fuses, replaced them, pulling the leads off, my Parkinsons tremor caused an issue on the output attenuator board. Smoke, etc.  I guess the ground lead touched a metal transistor can or something and that was all she wrote.  One transistor had a CE short and two 16.2ohm resistors went up in smoke.  No other damage from what I can tell.  The circuit is signal-carrying.  If anyone is interested and has the schematic, they were A6q6,a6q8 and the two, 16.2ohm,  current limiting resistors.

So here`s the question, should I use a CAN-3 package for both transistors, as they were,  or is it safe to switch to plastic?  I'm concerned about noise.  I have many (read that as 1000s) of the plastic. The cases weren`t grounded, no heat sinks or heat spreaders, etc.  This isn`t rf, just the last stage of a high quality, or higher quality, audio oscillator. 

Thanks,

Jerry.T.Hancock

[moffy]

Plastic should be fine: Air Temp: TO18 Pwr@25C = 500mw, TO92 Pwr@25C = 625 mw. Just make sure the pinouts match when you solder the new devices in.

[macboy]

I agree, there is no harm in trying the plastic. If you desolder safely (clip the body off of the leads and remove leads one-by-one) then you have little risk of damaging the board so you can rework it later to install the metal can parts if necessary. Any extra noise caused by the cheaper parts should be evident when you do a loopback test of the instrument after repair.  You are probably familiar with what the THD+N floor was before, so you can compare.

[cncjerry]

The plastic parts would allow me to match the gain better. I have a tracer for small signal devices to +/-15v. The manual doesn't say they have to be matched  but I will anyway. 

I clipped the leads and pulled them out like you suggested. I tried the solder sucker on those pins but I've had problem getting that old HP solder to flow with my power gun.

I ran the loopback test before I blew it up and it was .0018 for THD but I didn't get the other measurements.

Thanks,

Jerry