Author Topic: Racal Frequency Counter  (Read 8991 times)

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Offline Andy Watson

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Re: Racal Frequency Counter
« Reply #25 on: July 22, 2015, 01:03:03 pm »
On my unit the ripple is 50hz and on the working unit it's 100hz.
i haven't got the circuit to hand but I am reasonably sure that the supplies are full-wave rectified - so it should be 100Hz. This needs to be sorted before anything else.
 

Offline Stray Electron

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Re: Racal Frequency Counter
« Reply #26 on: July 23, 2015, 02:42:48 am »
 I was going to suggest that you could have a stuck address or data line but I see that you've already checked for that. But I can tell you from bitter experience that trying to troubleshoot a computerized device with just scope is absolutely the pits!  You REALLY need to get a logic analyzer or something like a Fluke 9010 Troubleshooter that's made for troubleshooting computers.  I have both. Where are you located? Have you considered that the meter may be working but the display subsystem may not be? (Edit:  OK I see that you replaced the front panel.  Are address decoders also on it or just the displays themselves?)  It certainly sounds as if the CPU is running and communicating. I don't know if your meter has an HP-IB interface but it it does you should see if it will accept a command and return a reasonable response.
« Last Edit: July 23, 2015, 02:57:55 am by Stray Electron »
 

Offline veedub565

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Re: Racal Frequency Counter
« Reply #27 on: July 23, 2015, 06:59:16 am »
it's not booting

When you switch it on, during the boot process  it's supposed to display the model number and then the software version. It's getting as far as displaying the model number then freezing, hence not booting. If it was a display issue then it wouldn't display the model number properly (if anything at all)

As Andy said I need to sort out why I have 50hz ripple instead of 100hz before going any further. That may also be the problem if it's upsetting the microprocessor system.
« Last Edit: July 23, 2015, 07:04:55 am by veedub565 »
 

Offline Stray Electron

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Re: Racal Frequency Counter
« Reply #28 on: July 24, 2015, 01:06:43 am »
it's not booting

When you switch it on, during the boot process  it's supposed to display the model number and then the software version. It's getting as far as displaying the model number then freezing, hence not booting. If it was a display issue then it wouldn't display the model number properly (if anything at all)



  Oh? In the OP you said that it didn't display anything but random lights, etc. Let's be clear, is it displaying the model number or not or only sometimes?

  Quote: "Racal-Dana 1992 (option 04C) - Does not boot up, when you switch it on it displays random lights, characters, sometime everything lights up sometimes nothing lights up."

 

Offline veedub565

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Re: Racal Frequency Counter
« Reply #29 on: July 25, 2015, 01:10:35 pm »
Read through the thread and you'll see that this problem of displaying random digits etc was resolved (Faulty EPROM) The EPROM was replaced and software copied over from the EPROM in an identical working unit. However it still does not boot, the new problem is it gets as far as displaying the model number and then stops. You could argue I should start a new thread for the new problem? but I decided to keep the repair of this unit within one thread to provide a continues fault finding process.


The issue with 50hz ripple instead of 100hz was a red herring, it was just the test leads picking up interference.

I went back to looking around the microprocessor circuit and found that IC19 pin2 IRQ line is stuck low (Active low) I know for a fact IC19 is ok as I have tested it in a working unit. I traced back and found the output from IC28c is stuck. All inputs to IC28c are correct (compared to a workijng unit) so it looks like it may be IC28c is faulty. A stuck IRQ line would tie in with the symptoms, it obviously starts to boot but then the CPU gets stuck in a state of constant interrupt.

Waiting on new parts now and then I'll let you know.

 

Offline veedub565

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Re: Racal Frequency Counter
« Reply #30 on: July 27, 2015, 07:22:20 am »
Ok I can confirm that this is now fixed and working.

Fault 1: Not powering up, displaying random digits etc
Fix: EPROM was faulty, replaced and copied software over from working unit.

Fault 2: Not booting fully, gets as far as displaying 1992 and then freezes
Fix: IC28 was faulty, IRQ line stuck low causing microprocessor system to be in a constant state of interrupt.

Hopefully this helps someone else fix one in future.

 

Offline sbeube02

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Re: Racal Frequency Counter
« Reply #31 on: July 27, 2015, 07:55:26 pm »
Nice fault finding. What was the IC28 chip?
 

Offline veedub565

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Re: Racal Frequency Counter
« Reply #32 on: July 28, 2015, 09:37:57 am »
Just a standard TTL chip 74LS10 or 20 I think.
 

Offline lowimpedance

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Re: Racal Frequency Counter
« Reply #33 on: July 29, 2015, 01:00:19 am »
Just a standard TTL chip 74LS10 or 20 I think.
The manual says its a 74LS10 but the instrument has a 74LS20 installed !! quite different.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline veedub565

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Re: Racal Frequency Counter
« Reply #34 on: August 04, 2015, 07:43:01 am »
yes I noticed that, i think mine was fitted with a 74LS20 too though.
 


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