n00b with broken Tectronix FG503

[gadget73]

New year, new parts, new attempt at this mess.

I set the freq to 1Khz.  I have a triangle wave of 9.3V P2P at the following points that drive the triangle into the shaper:

The three stage amp voltages that feed the shaper are:

Junction of CR130/Cr140 gate of Q201 (double FET): 9.3V P2P;

Base of Q221 taken from R211: 9.3V P2P;

Junction of R223/R224 Emitter of Q221: 9.3V P2P;

Junction of CR130/CR140/Q201 B, 3.5v p-p

Q211/R211, 3.4v p-p

R223/R224/Q221 E, 3.4v p-p

Just thought of something else when I was looking at your frequency issue.  Your frequency problem is probably related to the VCF control voltage.   That would also support the problem you 'fixed' by adding that resistor. It is not unusual for people to drive too much DC or sweep into the VCF connector.  You will notice that those opamps are socketed.  One is a 741 and the other a LM1458.  I now suspect one or the other is bad.  They cost about a nickel.  I would swap the LM1458 first as the 'B' section is closest to the VCF port.  Make sure you note the pin one location on both when replacing them.  At one time you could get both at Radioshack.  I wouldn't even bother checking the other voltages above until you swap them out.

Tried new op-amps, no change.  I realized I screwed up and got a 4558 instead of the 1458, but it seems those are mostly interchangeable.  Absolutely no difference between them though in terms of performance or voltages.  The numbers above were taken with the original chips in place again.


Time to put this back in the pile of misfit electronics and spend some time working on something that doesn't make me want to beat my head against a wall

[cncjerry]

The problem must be around or before that diode bridge with cr127, cr130,cr140, etc.  At this point without having it in front of me, I can't really help.

Maybe Q121a or Q121b?  Q145?  Q141?  If I had it next to me I could quickly figure it out having 3 of them.

You can pickup another on Ebay for $20, so maybe that is the quick solution.

Best of luck in the future.

[gadget73]

Bah, I was really hoping those op-amps would do it.

I'll poke around at it some more if I get bored.  Its not mine, I was attempting to fix it for a friend.

I appreciate the help though.  At least now I have a pretty small area to look at for the problem.  There just isn't much else left in here.  Output from the op-amps hits the dual FETs, so I'll check for signal in and out of there.   Out of that it hits the diode bridge, and those are easy to check too.

[BarryBreaux]

I realize this is a very old post! I had the same symptoms and came across this post after being unable to identify the failed component. In my case it was C226 281-0205-00 CAP.,VAR,PLSTC :4-65PF,100V that was causing the issue.

It was a bit tricky because C226 does not appear in the sine shaper schematic.

I hope this helps someone!