Racal Frequency Counter - Page 1
Forum section: electronics / repair

author: veedub565
author_rank: Frequent Contributor
timestamp: July 15, 2015, 07:37:50 pm
content: Hi I'm trying to fix an old Racal 1992 Frequency counter (option 04C) and was hoping someone might have some more experience on this than me. Here's how far I have got so far.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. All power supply rails and clocks are ok, 10mhz reference is coming out of the back ok.It is supposed to display model number and software version when it boots up, but since it wasn't even getting this far I suspected the EPROM. I was able to swap the EPROM from another identical known good unit and my faulty one now boots as far as displaying the model number (1992) but then it freezes there and will not go any further.It's obviously a issue with the EPROM causing the original fault, but I cannot now understand the new fault. Both EPROMs have exactly the same software version sticker on them so they ought to be identical? If it works in one it ought to work in the other.Any idea's ?

author: Andy Watson
author_rank: Super Contributor
timestamp: July 15, 2015, 08:02:50 pm
content: Ha ha! So you're the one that out bid me?Clearly the Eprom is ok. Remember that Eprom will be sharing the bus with the display/keyboard chip and custom timer chip - if the selection logic for either of these is broken they could be fighting for control of the data bus. Given the half-filled digits shown in the photo I would be concentrating on the display/front panel - at least if that works it would give it half a chance of displaying a fault message.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 15, 2015, 08:10:57 pm
content: lol I wondered if anybody would recognise it, for that price it was worth a punt. Looks like it may be a paperweight though.The (original) EPROM must be duff as it won't even post the model number at boot And when I put the EPROM in from another unit then it gets a bit further along the boot process and does display the model number. The display works fine.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 15, 2015, 08:22:57 pm
content: The difference in Eproms might just be down to their ability to drive the bus and/or slightly quicker reaction time to chip select etc.. The seller said something about the eprom chip select being dodgy - I think I would look further afield at everything that shares the data bus. I assume you've got the service manual:http://www.ko4bb.com/manuals/index.php?dir=Racal

author: veedub565
author_rank: Frequent Contributor
timestamp: July 15, 2015, 08:39:42 pm
content: Yeah thats a possibility I hadn't considered, and as you say if multiple chips are fighting for control of the bus that would cause it to freeze up. I checked out the EPROM chip output enable (active low) as this is what the seller was on about. It originates from the MPU and with the original EPROM it's high, with the new EPROM it's low.I also swapped over the MPU and custom ASICS from the working unit just to be sure the originals are ok. I'm going to check the original EPROM in the reader/writer just to see if it can be read ok (or not)I have a do have a paper copy of the service manual.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 15, 2015, 09:37:55 pm
content: Clutching at straws here, but, one small difference between the faulty unit and the known good unit is the faulty on has the 04C option (basic crumby 10mhz) and the known good one has 04T option (better tcxo with oven?) Maybe, although the eproms look identical and have the same software ver. Maybe they are still programmed slightly differently, maybe when it boots up it checks the accuracy and/or stability of the internal 10mhz ref. The eprom from the working unit will be expecting to find the oven tcxo quality ref, and when it doesnt it sits and waits for it to settle down before allowing measurements to take place. Hence apparently freezing/not booting.Will try swapping over the oven tcxo tmoz and let you know. Long shot but you never know.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 15, 2015, 09:43:09 pm
content: Quote04T is just a TCXO, no oven04E is the high stability oven - you want this if you can get it04A/B are the standard OCXO options, the A being fast warm up and B being slightly more stable.04C is a basic crystal, intended for use with an external standardQuoteUnlikely - the oscillator in these just sits on a separate board at the back with a connector that carries power and the 10MHz back - there's no way for the CPU to know what type it is.In fact I'm pretty certain the firmware is the same for all the 199x counters - it works out what model from which front panel is presentOh, it's worth replacing the PSU caps in these models, they are a known weakness especially the big 680uF ones.I know you said power rails were OK, but double check them for ripple.

author: edavid
author_rank: Super Contributor
timestamp: July 15, 2015, 09:50:10 pm
content: I know these units are prone to keypad problems... does a stuck switch cause them to hang?

author: grumpydoc
author_rank: Super Contributor
timestamp: July 15, 2015, 09:57:03 pm
content: Quote from: edavid on July 15, 2015, 09:50:10 pmI just tried powering my bench 1992 up with a key pressed - it will stop any other keys working but the unit still boots

author: Andy Watson
author_rank: Super Contributor
timestamp: July 15, 2015, 10:07:08 pm
content: Quote from: grumpydoc on July 15, 2015, 09:43:09 pmYes, I think that's right. The 1991/1992 software is identical - it detects the difference by link on the HF module. The clocks are interchangeable on plug-in modules - as is the IEEE/GPIB and battery option - it's all the same firmware.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 15, 2015, 10:18:52 pm
content: One thing I've found with my 1992 is that theremustbe an internal clock - even if you've got it connected to an external standard. On bootup, the internal clock is used until it recognizes that an external reference is connected. Then it switches over to the external standard. So, if the internal clock is flakey, the unit might not boot properly.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 16, 2015, 03:32:32 am
content: Quote from: edpalmer42 on July 15, 2015, 10:18:52 pmI have a couple 1991s and one which does not get used all that often does hang on boot up initially atfer sitting for some time unpowered. This behaviour is generally repeatable too after some months of no power !!.Suspected a dodgy connection on the timer IC socket since this beasti gets quite hot in normal operation so thermal cycling issues and given its age !. But now you mention the clock perhaps thats also suspect at the connector !.Usually the unit starts working once the cover comes off so tracing the culprit has not been definative. (and yes I have changed the socket, not that!)And definately change the 680uF caps.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 16, 2015, 05:07:49 am
content: Quote from: lowimpedance on July 16, 2015, 03:32:32 amIf by 'timer IC' you mean IC39, a ceramic IC that normally runs smoking hot, that's the chip that detects the external reference and switches over. They're custom chips that are used in all the Racal 199x counters. I've got two units that are dead because that chip died. I've thought about replacing it with a CPLD or FPGA, but since I can barely spell those names, it's probably not going to happen.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 16, 2015, 06:54:48 am
content: Heh yeah thats it, I was too lazy to check the manual for the ref./ desrcription.On the unit that I do use more regular I made a sort of heat pipe type of contraption the bolts to the side of the chasis and is adjustable to apply light pressure contact to the top of the IC and hopefully conduct some of that HEAT away, (does seem to work!)Perhaps I could post a picture if any one is interested .

author: veedub565
author_rank: Frequent Contributor
timestamp: July 16, 2015, 07:36:59 am
content: Thanks for all the info guys. I have now confirmed for sure that the original EPROM is dead, I plugged it into an external reader and it could not read anything from it. So it looks like that's one fault dealt with.I'm making a copy of the working EPROM to plug into this unit now.So now the issue is why does it freeze during boot, and could this be what blew the EPROM in the first place (hence making a copy)Plan now is to:- double check the power lines for ripple- check the internal 10mhz reference- and also have a look at the chip select/address lines.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 16, 2015, 09:11:01 am
content: Ok an update:- Power rails look ok and reasonably ripple free.- 10Mhz internal ref looks okLooking at the address lines on a scope shows they are clocking high/low ok.I also had a look at the data lines, and it looks like there is two different signals one on top of the other. I've ruled out (by way of replacement) IC22, IC18, IC19, and IC39 so that only really leaves IC20I'm guessing it's possible IC20 is constantly sqwaking on the data line, talking over the top of other chips and not shutting up when it's supposed to.I've removed IC20 and ordered a new one which should arrive in a day or two. Will let you know how it goes.

author: grumpydoc
author_rank: Super Contributor
timestamp: July 16, 2015, 12:40:57 pm
content: What 'scope do you haveDon't forget that if you look at a typical MPU data bus with a 'scope you tend to see what looks like a jumble of signals because the timing can be quite irregular.the best way to look at a data bus is, if you have a DSO with decent memory depth, is to capture activity as a single shot then expand the time base and scroll back and forth.If you have an analogue 'scope and the CPU is executing just a small bit of code in a loop (eg waiting for a key press) then you can sometimes get a stable trace by adjusting the hold-off control.If multiple devices are trying to drive the bus at the same time and one is at a "1" with the other at "0" the bus will take up an intermediate value (and things will get warm).However also remember that the bus only needs to be at a "clean" data level at a specific point in a read or write cycle and you get periods when nothing is actively driving the bus so it can take up odd values at those times as well.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 16, 2015, 01:01:44 pm
content: I'm using a 500MHz HP InfiniumI did check the working unit and the data lines looked pretty similar to my faulty unit so it may not be IC20, at least it will rule it out though once it's replaced. I've also ordered IC21 which is the chip select latch. That then eliminates a lot of the microprocessor system. Won't have the new parts till next week now.It's a shame there isn't a documented boot sequence so at least we'd know what its doing when it boots.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 16, 2015, 01:30:23 pm
content: I don't think IC20 dirves the data bus (unless it's properly broke). It only has inputs on the data bus. Have you checked that the chips have power and ground on the actual pins of the chip?

author: veedub565
author_rank: Frequent Contributor
timestamp: July 16, 2015, 03:35:45 pm
content: Checked the chips around the MPU circuit (IC39,18,19,20,21,22,23,24,25) and they all have 5v and gnd on the chip itself.Won't be able to do much more now until the new parts are fitted next week as I've already removed IC20.

author: gfiber
author_rank: Contributor
timestamp: July 16, 2015, 09:25:02 pm
content: You might take a look at Gerry Sweeney's blogwww.gerrysweeney.comif you have not. He had a Racal 1999 counter that would lock up. It appeared to be a faulty break or connection on the main board but turned out to be a faulty 10 Mhz Osc connector. In another video he built his own design High Stability Osc for that unit and also replaced all the switches. Ian Johnston atwww.ianjohnston.comreplaced all the switches in his Racal 1992, but I do not remember him having any other issues than bad switches.Some of the above may help

author: rastro
author_rank: Frequent Contributor
timestamp: July 17, 2015, 02:50:35 pm
content: I purchase the same model with the same problem and it turned out to be a stuck key(s). The key caps can be carefully pulled/removed from the front panel. Then remove the rubber switch contact and ensure there is no debris between the gold contacts on the underling circuit board. This should take less than 15 minutes. I would do this before starting to trace signals.-rastro

author: veedub565
author_rank: Frequent Contributor
timestamp: July 21, 2015, 10:34:20 am
content: Well it isn't IC20I tried swapping the entire front panel assy from another working unit and it didn't make a difference so that rules out a stuck key or display fault.back to the drawing board.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2015, 09:42:39 pm
content: That doesn't leave much! Do you have a scope with enough channels to monitor the address decoding gates - they're only two-input, output ors?

author: veedub565
author_rank: Frequent Contributor
timestamp: July 22, 2015, 09:47:44 am
content: Not to hand no, but I could borrow a 4 channel one.I had another look at the supply lines again, initially dismissed the ripple as it was quite small (200mv p-p) but I had another look today and compared with the working unit. On my unit the ripple is 50hz and on the working unit it's 100hz. My theory's a bit rusty but 100hz is full wave rectification and 50hz is half wave, so maybe the bridge rec is faulty. I had a look at it and D12 is 30k ohms both ways across pins 2-3 compared to 2Mohms on the working one. So may be worth having a closer look or replacing D12.*Edit* Replaced D12 and it's not that. more investigation required.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 22, 2015, 01:03:03 pm
content: Quote from: veedub565 on July 22, 2015, 09:47:44 ami 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.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 23, 2015, 02:42:48 am
content: 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.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 23, 2015, 06:59:16 am
content: it's not bootingWhen 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.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 24, 2015, 01:06:43 am
content: Quote from: veedub565 on July 23, 2015, 06:59:16 amOh? 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."

author: veedub565
author_rank: Frequent Contributor
timestamp: July 25, 2015, 01:10:35 pm
content: 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, thenewproblem 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.

author: veedub565
author_rank: Frequent Contributor
timestamp: July 27, 2015, 07:22:20 am
content: Ok I can confirm that this is now fixed and working.Fault 1: Not powering up, displaying random digits etcFix: EPROM was faulty, replaced and copied software over from working unit.Fault 2: Not booting fully, gets as far as displaying 1992 and then freezesFix: 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.

author: sbeube02
author_rank: Contributor
timestamp: July 27, 2015, 07:55:26 pm
content: Nice fault finding. What was the IC28 chip?

author: veedub565
author_rank: Frequent Contributor
timestamp: July 28, 2015, 09:37:57 am
content: Just a standard TTL chip 74LS10 or 20 I think.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 29, 2015, 01:00:19 am
content: Quote from: veedub565 on July 28, 2015, 09:37:57 amThe manual says its a 74LS10 but the instrument has a 74LS20 installed !! quite different.

author: veedub565
author_rank: Frequent Contributor
timestamp: August 04, 2015, 07:43:01 am
content: yes I noticed that, i think mine was fitted with a 74LS20 too though.