Repair of Tektronix FG501A Function Generator - Page 1
Forum section: electronics / repair

author: xrunner
author_rank: Super Contributor
timestamp: May 05, 2017, 11:42:58 pm
content: So I got into the Tektronix TM500 series test equipment, and have acquired several items. I recently got an FG501A advertised to have no output, and thought I'd have a go at fixing it.The unit arrived as advertised - no output meaning no sine, square, or triangle waves. It powered on and the output changed DC levels but generated no other signals. I initially thought the output amp was going to be the culprit, so after checking all the power supplies I pulled an internal connector and injected a signal from an external function generator. This caused it to output an amplified replica of the input. After further tests I was satisfied that the entire output stage was working properly. I also carefully examined the circuit board and found no visual sign of failed components, nor any sign of it ever being repaired, as far as I can tell.However, the unit is not generating any internal time-varying signals whatsoever. I have probed many of the components in the timing loop and have not found any signal yet that is varying or clocking. It does not indicate an intermittent output issue when being tapped in various areas. From studying the circuit description it appears to have a failure in the internal trigger loop which all gets going from charging/discharging capacitors. I've only started on it today so we have a long way to go. Fortunately I have an extender cable so I can work on the thing on the bench while it's powered up from the TM500.Any advice is welcome in this thread.

author: Rerouter
author_rank: Super Contributor
timestamp: May 06, 2017, 12:15:34 am
content: I have a number of fg503's and an fg502, at a block diagram level they are pretty similar, try setting to a multiplier like 10^3, then make sure the frequency pot it creating a changing voltage, towards 0v on all ranges it stops oscillating,Next up try using the vcf input as that bypasses the pot somewhat. (From memory they sum)

author: Cyberdragon
author_rank: Super Contributor
timestamp: May 06, 2017, 01:44:48 am
content: Possible red herring alert. The trigger loop does not start the oscillator, the VCF tuner does. The voltage differential output from the tuner is what starts the loop, no output here and it will sit there like a brick.Quote from: Rerouter on May 06, 2017, 12:15:34 amThis bypasses some of the op amps too. Also try feeding a small voltage into pin 3 of U1440 as this would bypass the darlington (Q1445). From there check circuits in the order they are mentioned in the "Loop" section of the circuit description.

author: xrunner
author_rank: Super Contributor
timestamp: May 06, 2017, 01:53:26 am
content: Quote from: Rerouter on May 06, 2017, 12:15:34 amQuote from: Cyberdragon on May 06, 2017, 01:44:48 amExcellent ideas guys, will dive into it in the morning ...

author: xrunner
author_rank: Super Contributor
timestamp: May 06, 2017, 09:35:44 pm
content: Could not find any issues with the VCF voltages, tried going in the VCF input and also checking the output of U1440, looks like good variable voltages.I wanted to see if the triangle buffer, sine wave shaper, and square wave generator worked. What I did was put paper under all the contacts of the time base switch so I could go into Q1725 clean. I used an externally generated triangle waveform. I found that the triangle buffer was OK.What happened when triangle function button was pressed was the triangle made it to the front output BNC. When switched to sine, the output was also a triangle wave, butbarelynoticeable was a small curve to the wave. Since the sine wave shaper is a goesinto-goesoutta circuit, I think that also has an issue, but we'll put that aside for now because it's not causing the main problem.What came out when the square wave function was selected was ... nothing. I found that there was no signal at the output of U1700A into the flip flop. I found that the reference at pin 5 of U1700A was not correct, so that no output of the comparator was possible. In the circuit description it states that at the input of U1400C p12 should be ~400mV (it was), and this is simply buffered out. But what was at the output was -19 mV.I isolated the output p14 by removing R1711 and pulling J1611, but the output did not improve. Since I had R1711 pulled I connected a ~400mV adjustable voltage to p5 of U1700A and lo and behold the output started producing highs and lows in accordance with the triangle wave input and the output responded to the voltage being adjusted. If this part of the circuit does not work no square waves are generated and no loop back goes to Q1531/Q1433.So as of this afternoon I believe a problem lies in the U1400 chip - a CA324 quad op amp. I also believe that the sine-shaper has a serious issue - perhaps the CA3045 NPN transistor arrays.Any comments are welcome.And now it's Miller Time.

author: xrunner
author_rank: Super Contributor
timestamp: May 07, 2017, 10:35:43 pm
content: Good news: Don't want to count too many chickens before they hatch, I think it's going to be fixed with a replacement LM324 quad op amp IC.I went ahead and removed U1400 and in preparation for the new part (which is on order) I installed a 14p socket. I then went in on p14 and p8 with an external adjustable DC which was initially set to +400 mV to see how it would respond. The scope signal looks like hell but it actually means that the unit is trying to operate, you can see a triangle and square wave in the center. It's hampered at the moment since U1400 does not exist. But it's passing signals and they can be changed in frequency too.Not sure whythatIC seems to have failed, but I guess shit happens.

author: xrunner
author_rank: Super Contributor
timestamp: May 10, 2017, 09:15:40 pm
content: Fixed. It was one bad op amp in an LM324 quad op amp. Working up to it's full rated frequency of 2 MHz.Looks great, now I need to do the complete cal. Then I'm going to post series of 4 channel screen shots from my scope. This is going to document valuable test point information for anyone in the future to reference if they need to see waveforms from a working unit.

author: xrunner
author_rank: Super Contributor
timestamp: May 11, 2017, 06:36:55 pm
content: Troubleshooting data - Ch 1 is at the output BNC. Waveform selection is square wave.

author: xrunner
author_rank: Super Contributor
timestamp: May 11, 2017, 11:18:34 pm
content: Troubleshooting data #2 - Ch 1 is at the output BNC. Waveform selection is sine wave.

author: xrunner
author_rank: Super Contributor
timestamp: May 12, 2017, 10:01:34 pm
content: Oh what the heck - one last pic.Alignment done, finished the cosmetics/cleaning. Only issue I had with that was one of the two screws that held the plastic dial on was seized up solid - metal screw into brass never touched since the factory. I had to drill it out from the front but I managed it without damaging the plastic.and I found a replacement too.Now it can assume it's rightful place among it's peers.

author: MarkL
author_rank: Unknown
timestamp: May 13, 2017, 05:57:28 pm
content: Mini-project suggestion:Jumper the trigger output from the FG501A to the DC504 input using the TM504 backplane connections. The TM500 series have pads for jumpering signals between slots.Then you can read the exact output frequency from the FG501A by moving the "Source" switch on the DC504 to "INT" (internal).

author: xrunner
author_rank: Super Contributor
timestamp: May 13, 2017, 11:25:46 pm
content: Quote from: MarkL on May 13, 2017, 05:57:28 pmYea, I had known you could connect signals by wiring them up on the backplane. I might do that one of these days.