Racal 9839 display fault

[Vintage_RC]

My faithful Racal 9839 frequency counter has given good service for 20 years - until today. It is a UHF counter with an 8 digit Nixie tube display. When I turned it on today all digits in all the Nixie tubes were illuminated rendering the instrument unusable. Does anyone have a manual with the circuit for the Racal 9839? and does anyone recognize the fault?

Thanks, Alan

[Vintage_RC]

Here is an image of what the display looks like. All digits in each Nixie are on.

[PETERg0rsq]

Sorry do not have a manual for the 9839, but suspect the nixi circuit will be similar to another Racal counter.

What IC's does it use for the divider/display. Is each nixie on a seperate PCB, or do they share a common PCB.

With this information I might be able to match it up to another counter circuit.

Working on old digital counters is quite easy, even without the manual/schematic, as the function of each chip is straight forward.

Initially just check the input to the decade dividers, and the reset lines. This will tell you a lot about what is going on.

[factory]

Are the low voltage power supplies for the counter circuit working?

David

[Vintage_RC]

Hi Peter, thanks for your reply. I have attached a photo of the board with the Nixie's on it. You probably won't be able to read the IC numbers at the limited resolution permitted here but looking at the columns of IC's from left to right they are 7490, 7475 and 74141 driving the tubes.

[Vintage_RC]

Hi David, I should get a chance to take a more detailed look at what's going on with obvious supply rails tomorrow.

[factory]

The 9837 uses a similar looking smaller display board with two less tubes.
Hopefully there are other similarities that will help in the service manual;
https://archive.org/details/racal_Ra...Service_Manual

David

[Vintage_RC]

Thanks for the link David, have downloaded the manual. The display board certainly has strong similarities and uses the same 7490, 7475 and 74141 set of IC's so should help at least in that area. I hope to spend some time investigating tomorrow.

[buggies]

Noticed that there was an input cable attached in the photo - was there actually an input signal?
If there was (and the counter configured to measure it) then there could be a logic fault allowing the digital counter outputs to pass direct to the display drivers without being latched at the correct point.
Handy tool is a logic probe, preferable with monostable and beeper - otherwise it means hauling out the scope to look at the control lines.

[Vintage_RC]

Display Fault Investigation

Before opening the unit up for investigation, I took a look at the 74141 data sheet for clues about the conditions where all digits could be on at once. The answer is no such condition exists. All input conditions either light the appropriate digit or if invalid then the display is blank. This points to a possible power supply problem as suggested by David. Another clue is that the gate LED is off. This is always on barring the very brief period when the display updates, a possible indicator of no 5V rail. I then opened the unit and checked the two fuses, they were OK. With DVM at the ready I switched on and guess what? it appeared to be working fine as confirmed by applying a precise 10MHz input. Having inspected the unit I put it back together and left it soaking for several hours with no problems. The photo shows it monitoring the 10MHz output of my Quartzlock 2A locked to GPS time. This is a mod I did recently to the Quartzlock which I might write a few notes about on the forum. Coming back to the Racal 9839 fault, I have no idea what the problem was. Unless it reappears there is nothing more to investigate.

[Refugee]

I would whip the fuses out and clean them and the holders.

[Vintage_RC]

Originally Posted by Refugee

I would whip the fuses out and clean them and the holders.

Yes, I did do that while testing them