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Advice & information sought: Repairing a Feedback FG601 function generator

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tomizett

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Hi All,
I wonder if anybody might be able to help me repair my Feedback FG601 function generator? Chiefly I'm looking for schamatics or a service manual, but google hasn't found any for me. Does anyone know where I might find this information, or does anyone have experience working on these machines themselves?

For the sake of completeness I'll give a rundown of the fault, which has been intermittent for a while but has now become a perminent failure. It produces no AC signal on either the main output or the TTL. You do, however, get DC offset control and an "additional" DC signal which varies with the output level control. The polarity of this "additional" DC changes depending upon the selected waveform. When the fault was intermittent, the instrument could sometimes be coaxed into working by switching between frequency ranges or waveforms.
Given these symptoms, I'm pretty confident that it's the main oscillator at the heart of the instrument that isn't oscillating - rather it's just staturating against one rail and sitting there.

I've had the thing apart previously and, suspecting a machanical problem, cleaned all the switches, reworked a lot of solder etc. I wasn't able to track it down to any kind of mechnical factor, but that would still seem a feasable cause (maybe a hairline crack in a trace etc).

There is a surprising amount of stuff inside theis little green box, so any information you may have would be greatly appreciated!
I'm going to take the lid off it again now...
 
How old is it? Older stuff never had electronic copies of documentation so you will not find any online.
Have you tried the manufacturer?
 
Looks like it's probably early 90's... hand-drawn 2-sided PCB, tidily made. It's virtually all discreet, save for a few 741's - the things that appear at first glance to be ICs turn out to be transistor and diode arrays, not opamps!
I've not tried the manufacturer (not sure if they're even still going) - I'll see if I have any luck.
 
Update: The manufacturer does still exist, in name at least, but have no knowledge of any stuff they used to make - it seems to be a very different business nowadays.
Looks like no-one knows much about this then, but I'll be back on it in the next few days so I'll post any info I find here.
 
Could draw the schematic?
 
I'm getting some of it worked out... if I get a reasonable section of schematic drawn up I'll post it up here.
Thanks for the interest.
 
:oops: Is that an embarrassed face?

The dry joint that was causing the problem has been located and fixed...

I'm glad to have it working again, although almost slightly dissapointed it wasn't a more taxing problem to get stuck into. It's a curious circuit - probably not the kind of thing you see in more modern gear. I may take a closer look and get to grips with how it works anyway.

Cheers guys!
 
We always like to hear a success story :).
 
Well... as promissed, here's a schematic of the oscillator section. The complete instrument contains quite a lot more circuitary, so I stopped short of tracing it all.
I've put together an LT spice simulation of it, which works at least to a first approximation - the simulated waveforms look to the naked eye pretty much like what I've seen on the 'scope. Note that I've not used the correct component models, and I've left a couple of the trimmable caps as zero value (their purpose is not at all clear to me). The component names all correspond to the silkscreen on the board.

Hope this might be of some interest or help to someone!
 

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  • FG601-osc.png
    FG601-osc.png
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Hello, Basic question here:
how did you remove the cover of this thing?? I took all screws off but still struggling with plastic handle; looked up for service manual but there is none available neither web nor factory website. thank you
 
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