FG504 repair

[ampdoctor]

I've got a Tek FG504 over here that's owned for the better part of 20 years that has developed a bit of a problem this past week. All of a sudden the sine waves are flat topping on me. All the other functions and waveforms seem to be working perfectly, so my initial suspicion is that it's either a cap or a transistor in the wave shaper that's causing the problem.

Here's the problem. How the hell do you get inside this thing to do the repairs? My biggest fear is that if I take it apart to do the work I'm never going to get it back together again. It's got 3 nested boards and all the interconnecting cabling is bundled with old school lacing twine. Moreover, the boards have multiple + and - Vcc rails taking power from the TM500 through a backplane which is going to make performing diagnostics a royal pain in the ass.

So my root questions are, if anybody has worked on these before, and are there any tips or tricks to doing repairs on this thing? I spent the better part of yesterday afternoon staring at it thinking this can't possibly be this hard and that there's got to be a trick to it that I'm not seeing. This is a fantastic generator that still commands a hefty price and I don't want to fleabay the thing off over a 50 cent transistor or tantalum cap.

[simingx]

Could it be a power supply problem? Low voltage on the rails resulting in clipping?

[ampdoctor]

if that were the case I would think that other functions would be affected as well. I've also got my frequency counter and multimeter in the other slots and they work fine without issue so I don't think it's a power supply module problem. Though given the supply's age, replacing all the e-lytics might not be a bad idea just for good measure.

[sdg]

You need two interface extenders. Mark Sims (holrum@hotmail.com) from the volt-nuts forum, has some of his own making for sale, around US$20 apiece plus shipping.
The 500-series modules aren't difficult to work on, they're full of test points, so complete disassembly is probably not needed. The disasembly procedure is described in the service manual.

Edit: you will need a 1/16 hex key to take the knobs off.

[MarkL]

The FG504 takes the triangle wave and shapes it to approximate a sine wave through a number of carefully selected resistor values and matched diodes.  So, if the triangle wave is fine as you've stated, I would start your investigation with the shaper.  See the service manual "Sine Shaper & Buffer", schematic #3.  Expected waveforms for about 8 test points are documented.

As sdg points out, TM500 extenders are the way to go, but in a pinch I've done servicing on my FG504 by taking all four covers off, and sliding it into a TM503 with the top and bottom covers off that too.  It's not as convenient as extenders, but you can access all the test points and internal cal adjustments.

And, yeah, check all the power supply rails first.