Re: Troubleshooting FG501A


If the output transistors are (were?) hot, I would start looking back at the input of that ckt. The bases of Q2101 and Q2113 should be very close to zero. And the bases of Q2013 and Q2321 should be about 1.4V different, and probably centered around zero (+0.7V and -0.7V).

Make sure to check the zener VR2213 also.

If the output transistors blew, you will also probably have blown the 10 ohm resistors (R2122, R2121, R2225, and R2226) on whichever one is bad. If those resistors look like they haven’t overheated, you’re probably okay.

Note that I am not speaking from experience of fixing one of these, I am just looking at the schematic and telling you where I would look.

Best of luck –




From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Dave
Sent: Monday, January 17, 2011 3:49 PM
To: TekScopes@...
Subject: [TekScopes] Troubleshooting FG501A



Hi Everyone; Today I tried to start where I left off last week.
I verified that everything was OK up to the end of the circuit description and theory. but I noted that I was not able to adjust the current on both sides. I also noted a intermittent waveform at the output. where there was none before. what I found today was the intermittent was due to the probe adapter. While I discovered that last week I was measuring the voltage at R1521 and R1536 with respect to ground. that was incorrect. I should have measured across the resistors mentioned. when I did I found I could balance the two currents there reading .808 each I also noted on the second FG501A this value is nearly double.I don't know why! No waveform present on the second generator. When I went back to the first generator I noticed I lost the waveform. I noted last week that the final output transistors were running very hot, I took this as normal. Now I'm thinking that was a mistake and I ignored a condition too long and may have lost the output transistors, I hope not there are 4 of them. I was so upset this is where I left off today! I'm going to try to upload a picture of the waveform.


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