Racal 1992 - Page 1
Forum section: electronics / testgear

author: Yoanndlc
author_rank: Contributor
timestamp: October 12, 2024, 08:43:52 pm
content: Hi,I had a hold racal 1992 and I had some trouble.When I connect the ext 10Mhz ref on channel A, I have a good reading during 20 second and it start to give bad frequency.I checked the ext 10Mhz and it is stable.I checked all the power supply and they are ok.I see that the temperature of IC39 is around 65°C.If you have a 1992, did you observe the same temperature on this device?I did the Fault Finding Flowchart, and I begin to have an issue at fig 7.5.If you had the same issue, I am very interested because I spend 6h and I do not see where it is comingThanks in advanceYoann

author: Andy Watson
author_rank: Super Contributor
timestamp: October 12, 2024, 10:30:58 pm
content: U39 is an ECL ASIC - it runs very hot.I have a vague (>25 years ago!) recollection of a similar problem with a 1991. It would operate correctly for 5-10 minutes after power-up. After a while the channel wouldd become increasingly insensitive. The problem was traced to the potentiometer that sets the hysterisis on the channel comparator (R149 - channel A, R150 channel B). There was an internal short within the potentiometer. After a while it would heat-up and move the hysteresis voltage.

author: Yoanndlc
author_rank: Contributor
timestamp: October 12, 2024, 10:59:31 pm
content: Thank you Andy,I will check the potentiometer.I also read on the forum that the IC socket could also be a problem. During my tests, I used contact cleaner, and it worked better for a few minutes.I will try to change the socket

author: coromonadalix
author_rank: Super Contributor
timestamp: October 13, 2024, 12:53:39 am
content: be very careful with the chip, it is unobtanium if i recallyou had the regulators on the frame who sometimes become intermittent, they are under mechanical stress and they sometime break

author: shabaz
author_rank: Frequent Contributor
timestamp: October 13, 2024, 02:03:08 am
content: I have an even older one of their counters. The IC sockets become unreliable for the unobtainable chips as others have found too.The legs on some of those ICs are so weak, they can get damaged even when taking care lifting out of the socket. I didn't trust myself to insert it into a brand new socket, so in the end I permanently soldered it in.

author: Yoanndlc
author_rank: Contributor
timestamp: October 13, 2024, 01:20:46 pm
content: Thanks, guys.The first thing—and not the least—is to desolder the old socket! I had to remove the SMD parts underneath before removing the solder joint on the 40 pins.I agree with you; the pins seem to be very weak. I think I will follow your advice and permanently solder it in.

author: Yoanndlc
author_rank: Contributor
timestamp: October 13, 2024, 07:53:35 pm
content: Hi,I finally changed the socket, but I'm still experiencing the same issue.When I select the check function, I read 10.0000000 MHz. With the 10 MHz connected to input A, when I select channel A, I initially get 10.00000000 MHz for a few seconds, but then it becomes random and the number of digits decreases.When it goes random, if I decrease the number of digits, I can read 10.000 MHz.Do you have any idea where this issue might be coming from or where I can check the signal?Thanks in advance,Yoann

author: Yoanndlc
author_rank: Contributor
timestamp: October 13, 2024, 07:56:34 pm
content: One display when I ask all the digit but it become randomAnd one display when I decrease the number of digit.

author: Yoanndlc
author_rank: Contributor
timestamp: October 16, 2024, 12:34:53 pm
content: Hi,I started to check what is happening on the digital side.On IC39 Q3 input do not look good, it is not going to 0v.Is somebody has a scope screen shop of the Q pins on IC39?Thanks in advanceYoann

author: TheDefpom
author_rank: Frequent Contributor
timestamp: October 16, 2024, 09:19:47 pm
content: I think I remember something similar on a unit I repair in a video quite a while ago, I had an issue with I think a flip flop, or maybe an op amp (I don't remember) in the front end.It MIGHT have been this one (not sure), it is one of my very early videos, so quality isn't the best.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 18, 2024, 11:14:14 pm
content: I only have the probes to monitor two lines simultaneously, however, the signals look clean, 0-5V.

author: Yoanndlc
author_rank: Contributor
timestamp: October 23, 2024, 11:21:34 am
content: Thanks you very much.Sorry for the delay, I was waiting for the capacitors.I changed all the chemical capacitors and I still have the trouble.I am very far Andy to have a clean signal like you.I took two screenshots, one with triangular shape is Q3 yellow and Q2 pink when I am on FREQ A functionThe second is the same but I am in CHECK Function.

author: Yoanndlc
author_rank: Contributor
timestamp: October 23, 2024, 11:25:37 am
content: Pictures

author: Yoanndlc
author_rank: Contributor
timestamp: October 23, 2024, 01:00:18 pm
content: The signal have got on the bus between ic19 and ic22 is not looking good too

author: Andy Watson
author_rank: Super Contributor
timestamp: October 23, 2024, 01:27:12 pm
content: As a sanity check - I would check the power supply voltageon the actual pinsof IC39 and IC19.

author: Yoanndlc
author_rank: Contributor
timestamp: October 23, 2024, 07:44:02 pm
content: It is what I was thinking but the power supply are correct.They use a RC filter to filter the vcc, there is some variation on the vcc pin but I decrease the serie resistor and there is no improvement.Thanks to you

author: Yoanndlc
author_rank: Contributor
timestamp: October 31, 2024, 04:18:53 pm
content: Hi Andy,Could i ask you a last measurement ? Could you make a scope screenshots of the pin 3 of ic20?please.Thank you very much in advanceYoann

author: Andy Watson
author_rank: Super Contributor
timestamp: November 03, 2024, 12:59:57 am
content: What are you expecting? I think pin 3 is part of the data bus - therefore it is an apparently random signal which will include periods of no-drive - i.e. floating. The image below is a typical capture of IC20 pin 3. This is exactly the sort of signal that I would expect.Note: My scope is slightly over-reporting the voltage. The peaks should be about 4.9V

author: Yoanndlc
author_rank: Contributor
timestamp: November 03, 2024, 09:25:57 pm
content: Thank you very much Andy. Your signal is very close to mine.I was expected a clean signal with 0 or 5v but sometimes I have triangular signal.But You seems to have the same.I think it is coming from IC39 Q3 output, I will try to add a comparator on this pinI let you know.Best regardsYoann

author: Yoanndlc
author_rank: Contributor
timestamp: November 19, 2024, 05:08:42 pm
content: Hi,Just to wrap up this post:I tested the pin with a comparator, and it was working. However, I found the IC39 on eBay.I replaced it, and now everything is working perfectly.The issue was indeed with IC39.I’m very happy! Thanks to everyone for your help.Yoann

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2024, 06:10:08 pm
content: Those sockets I got 2 turned pin sockets, soldered the one to the board, and soldered the other to the IC, solving both the intermittent connection problem, and the fragile leg issue as well. Was not in a Racal, though there was one in the rack, but in the custom boards there that were doing digital logic and such, where the one leg on a fusible link prom was intermittent, and there was no way i was going to easily change that 16 pin wire wrrap socket, as all the wires had been wrapped and soldered during manufacture, so just a new socket with turned pins, so you could reach to the contact point with solder, was the solution, and the other socket with the IC soldered to it fixed the broken leg tip. One hour of work, after identifying the issue, as opposed to the guess of at least a week to get to that particular socket, buried in the middle of a stack of boards.

author: edpalmer42
author_rank: Super Contributor
timestamp: November 19, 2024, 06:33:10 pm
content: Since IC39 is only available as a salvaged part, you should add a heatsink to lower the temperature and hopefully extend the chip's life. Heatsinks are available in stick-on or clip-on versions but they're getting hard to find. One such clip-on model is:https://www.ebay.com/itm/266786380596My junk box coughed up a similar heat sink that I put on my 1992.Ed

author: Yoanndlc
author_rank: Contributor
timestamp: November 24, 2024, 05:58:59 pm
content: Thanks Ed,I will add a heat sink, i will follow your adviceBrYoann