FG504 not producing signals other than flat line, where to start with repair?

[buzf]

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.

[lowimpedance]

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.

[bob91343]

You say no output and then you say the attenuator affects the output.

Did you check the power supply?

[buzf]

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. 

[lowimpedance]

 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.

[srb1954]

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.

First 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.

[buzf]

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.

[bob91343]

As I said originally, check the power supply voltages before anything else.

[srb1954]

As I said originally, check the power supply voltages before anything else.

Not 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.

[MarkL]

As I said originally, check the power supply voltages before anything else.

Not 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.
...

I'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.

[buzf]

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!

[Ground_Loop]

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!

I 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.

[CaptDon]

The FG504 has some higher than normal voltage rails and the filter caps are probably needing
replaced soon. You will see lots of 120hz A.C. ripple on the output waveform. Also the filter
caps in the mainframe are probably soon due for replacement. Tektronix has sure had its
issues with bad/poor capacitors!!! Every failed Tek module I own was due to capacitors and
also the really crappy 'drift-o-matic' trimmer pots. They are constantly shifting and break
easily!!! 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 trying
to fix!!!