FG502 no output [SOLVED]

[Warsp]

Hello.
I recently go a FG502 function generator but it only produces a negative voltage on the output. I believe the cause for there being no waveform may be the Q230A, B comparator in the triangle generator, but I am unsure since it appears to work fine after testing it in circuit with my component tester. As for the negative voltage on the output I am assuming its probably something to do with the output amplifier, but I have no idea whats wrong with it. So far what I have done is replacing the out of spec carbon comp resistors and replacing a burnt resistor and neighboring transistors in the power section. Any help is appreciated.

[m k]

There is a switch for different modes.
If all modes are missing then the problem is more likely later than that switch.

[Kleinstein]

I would suspect the output amplifier, as this part is exposed to external events, like extra voltage or ESD. Some of the old designs also ran relatively hot.
It should be possible to probe the signal before the final amplfier, e.g. at the function selection.

The old output amplifiers used rather fast medium power transistors (e.g. TO39 case) which by now may be hard to get (and no easy alternative). So one should be a bit careful with just replacing burnt resistors as this may cause more damage. Most damages transistors fail short - so one can find them relatively easy with a incircuit test. One the downside this also often means that they can cause more damage to other parts. I would test all the transistors in the output section for a short.  If possible the test under power would be with a limited current supply and than frist looking at the DC levels.

[David Hess]

The old output amplifiers used rather fast medium power transistors (e.g. TO39 case) which by now may be hard to get (and no easy alternative).

Replacements were a problem even 30 years ago, although they may still be available through Central Semiconductor for a premium price.  The closest modern parts might be audio VAS transistors which do have the advantage of low capacitance, like the KSA1381/KSC3503 and KDA1142/KSC2682.  If I was designing a modern replacement, I might use a pile of fast but lower power transistors in parallel.

[Warsp]

Found the problem. Turns out Q230 wasn't the culprit but it was actually its socket, which I failed to notice when I initially trouble shooted it, which explains why I couldn't find any shorted transistors. As for the output amplifier, it seems that it is now behaving itself now that the triangle generator works. I bought some replacement transistors anyway just in case it breaks in the future since they weren't too pricey. I am going to replace the filter caps and do a calibration on the function generator and after that it should work fine. Thanks for your help.