Re: FG501A repair points +TM500 plug ins



Great write up. I’m printing this email and putting it in my copy of the manual.

George KD6NEW

On Jul 13, 2019, at 2:50 PM, walter shawlee <@walter2> wrote:

I had to fix three FG501A's this week, and a few interesting general points really became clear.
These have a lot of the open back pushbutton switches (schadow style), and ALL were intermittent in EVERY unit. This meant levels, and waveforms were wildly erratic. A small hit of Nu-trol (I find this is easier to apply than de-oxit in these particular long switches) fixed every one, a few mechanical cycles, and it is like new. I re-checked after a few days, still 100%.

One also had a sticky "symmetry" concentric pushbutton (part of the symmetry control). these get very sticky, and bind in a phantom position, nether in nor out, and a tiny touch of Nu-trol on the concentric shaft junction and the entry of the pushbutton shaft into the switch assembly will make it work correctly again.

One unit was even more erratic, and smelled like something was burning. I quickly pulled it out, and opened the covers for a look. it had two orange sprague radial aluminum parts, 10uF/50V by the input fingers, bypassing the +/-33V lines, both were bulged at the top and leaking below. I have always been very skeptical of this series from sprague, they were very low quality, and masqueraded as high quality US parts when thy really were sub-standard, meant to compete with low end asian parts. I extracted them, and installed some high quality 100V nichicon replacements, and that did it.

Dial calibration on these is not intuitive, so read the manual, and follow BOTH sections on dial cal (mechanical and electrical), or it will never work at both ends. When running, these are great function generators, one of my faves in the TM500 series, and really worth the effort to get running.

TM500 Plug Ins
I have all the left-over TM500 plug-ins photographed, and listed, and I am emailing those that asked after them in the same order I got your requests. If you haven't heard yet, it's because I am waiting for answers from the person ahead of you. To answer the burning question from everybody, they range from $15 to $30 for very late high end units.

all the best,
sphere research corp.

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