FG504 not producing signals other than flat line, where to start with repair? - Page 1
Forum section: electronics / repair

author: buzf
author_rank: Newbie
timestamp: February 16, 2021, 10:30:07 pm
content: My Tektronix FG504 stopped producing any output signal other than flat. If measured at the output, attenuator seems to be the only control that affects the output. Need any suggestions where to start researching to fix it.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 16, 2021, 10:48:23 pm
content: IIRC there are a number of bi pin fuses mounted on the PCB's , check those are okay and of course the power rails are good if not done already. Also the front panel switches /Pots are all quite ancient now and could be well oxidized, so try operating them a number of times while observing the output for any notable response. A clean/lube of the switches would be in order at least.

author: bob91343
author_rank: Super Contributor
timestamp: February 16, 2021, 11:14:08 pm
content: You say no output and then you say the attenuator affects the output.Did you check the power supply?

author: buzf
author_rank: Newbie
timestamp: February 16, 2021, 11:30:33 pm
content: It normally turns on but if I measure voltage at BNC cable it shows that it does not react to any controls but attenuator. Three different oscilloscopes show just a flat line, being basically just a step function. Something similar had happened once a while ago, when I could not get any signal but then it got back up again. It was working fine for around 10 minutes yesterday but the signal suddenly just became flat upon rotating frequency control. Since then it never got back again.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 17, 2021, 12:18:43 am
content: Sounds suspiciously like intermittent connections , either switches or pots and maybe PCB interconnects that would be where I would be looking. Particularly switches and pots as I have seen similar types of issues in both TEK and old EXACT vcf generators. Contact oxidation can easily stop the waveform generation.

author: srb1954
author_rank: Super Contributor
timestamp: February 17, 2021, 02:31:45 am
content: Quote from: buzf on February 16, 2021, 10:30:07 pmFirst check that the Trigger/Free Run button is depressed and the Hold/Run is not depressed, the Frequency vernier is set somewhere on the calibrated portion of the scale and the Frequency Multiplier it set to 1 or greater.

author: buzf
author_rank: Newbie
timestamp: February 17, 2021, 02:51:48 am
content: Just some noise at the smallest volts division on oscilloscope. Nothing seems to affect it. It is possible to change only gain via attenuator, output function is still just step. I had followed "first time operation" procedure from the manual - same.

author: bob91343
author_rank: Super Contributor
timestamp: February 17, 2021, 05:19:09 am
content: As I said originally, check the power supply voltages before anything else.

author: srb1954
author_rank: Super Contributor
timestamp: February 17, 2021, 11:33:46 am
content: Quote from: bob91343 on February 17, 2021, 05:19:09 amNot so easy to do with an FG504. It is nigh on impossible to check the FG504 circuitry under operation unless you have two of the rare TM500 extender cards to be able to operate the FG504 outside of the main frame.Since the fault is highly likely to be a shorted tantalum capacitor it is much easier to initially do a thorough visual inspection of the PCBs followed up by checking for shorts on the supply rails with an ohmmeter. The series pass transistors on the back of the main frame should also be checked as these may have failed if there has been a short on any of the internal supply rails.

author: MarkL
author_rank: Unknown
timestamp: February 21, 2021, 09:50:18 pm
content: Quote from: srb1954 on February 17, 2021, 11:33:46 amI've done troubleshooting on several occasions on my FG504 without the extenders. The top/side cover and bottom cover of the mainframe can be removed to get access to the module under power.Line voltage is exposed in the back section of the mainframe with the cover off, so proceed with caution.

author: buzf
author_rank: Newbie
timestamp: August 07, 2021, 09:47:34 pm
content: After a while I finally figured out what the problems was. One of the tantalum capacitors on loop board got shorted. Removed it for now and ordered the replacements parts, that I had wasted in the process of searching. Thanks to everyone!

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: August 10, 2021, 06:36:52 pm
content: Quote from: buzf on August 07, 2021, 09:47:34 pmI have a dozen or so TM5 modules and with the exception of my current probe amplifier, each has had at least one shorted tant cap. It's just something to expect when you get one of those modules. Glad you found it.

author: CaptDon
author_rank: Super Contributor
timestamp: August 13, 2021, 02:09:14 am
content: The FG504 has some higher than normal voltage rails and the filter caps are probably needingreplaced soon. You will see lots of 120hz A.C. ripple on the output waveform. Also the filtercaps in the mainframe are probably soon due for replacement. Tektronix has sure had itsissues with bad/poor capacitors!!! Every failed Tek module I own was due to capacitors andalso the really crappy 'drift-o-matic' trimmer pots. They are constantly shifting and breakeasily!!! I love my old Tek stuff, but I think it is time to take most of it to the hamfests.I am beginning to test my test equipment more often than the things I am actually tryingto fix!!!