REPAIRING FUNCTION GEN - Damaged my IEC F34 Function Gen - Really dumb mistake

[Kleinstein]

90 C for the outside of a TO92 is a little to hot. A TO18 or TO39 with a heat sink can dissipate more heat even though the silicon inside may be the same. To get the maximum speed out these old transistors they might need to run at rather high current and thus get relatively hot in many older RF circuits.

You can likely get away with the 2N2907 instead of the more exotic and only slightly faster 2N3251. But I would at least use the TO18 version with a heat sink or maybe the TO39 version.

[FlyingHacker]

Thanks for the reply, Kleinstein, and thanks again for your help.

I do have 2n3251As on the way. So I might as well use one.

For Q17 and q18 the plan was to use the 2N3251a BJTs.

For Q19 I was going to use a 2n2907A. It  is one of the ones with a manufacturer part number. That is the compliment to Q20 the 2n2905A. Sound OK?

Grrr. I just realized my 2n2905As are TO-39, which is what I want (the heatsinks for Q19 and Q20 are for TO-39) The 2n2907A is a TO-18, though. OK. I found some TO-39 2907A on ebay...

[FlyingHacker]

I got my transistors and put them in. I ended up going with:

Q17 -  2n3251A
Q18 -  2n3251A
Q19 - 2n2219A
Q20 - 2n2905A

I am pleased to report Q19 and Q20 (TO-39) are now around 47 deg C at full load, with the nice round heat sinks in place. After thermal compound 39C

Q18 and Q19 which are TO-18 are around 63C with their smaller heat sinks. After thermal compound 58C

Tomorrow I will replace the main filter caps. I had replaced all the other caps when I got the unit over a year ago, but I have new main filter caps coming tomorrow. I saw a little too much ripple for my liking.

Thanks everybody for your help, especially Kleinstein.

[FlyingHacker]

Well crap! 

It failed again after a few hours continuous use. I had a 50ohm terminator on there. Perhaps it was heat related. Maybe my transistor choices were poor. I need to see what failed this time.

[FlyingHacker]

I am pleased to report that the new issue was caused by two heat sinks touching. After some careful bending of pins to get the heatsinks separated things are work great!

[scopeman]

You might try these the next time you have an issue.

https://vetco.net/products/2n3053-2n4037-npn-pnp-matched-pair-transistors-nte129mcp

Sam
W3OHM

[william_b_noble]

if you are still looking for pieces, I have an F34 that is missing some of the semiconductors, it probably has the pieces you need.  drop me a message if you still need something for parts.