Solartron 7060 Repair

[Sam__]

I just got a Solartron 7060 from an auction in an unknown state. First success is it was a full blown G model with V/I/R features. Then I turned it on and turns out only the resistance wasn't working properly. Winner!

After a solid 4 hours of troubleshooting I managed to narrow the resistance reading issue it down to a single reed relay that wasn't switching the current source to the I- bus. Swapped that out and it was happy. Until I went to plug the board in and intermittently it would show the resistance and current modes as "no function", aka it hadn't recognised the board that handles those functions was plugged in. Narrowed this down to the old school pin connectors Solartron seemed to love in that era and made a shim to go under the PCB to hold the pins in place. The connections were now totally fine. Unfortunately during the process or putting it all back together something went awry and I'm now looking at a screen like the following:

Normal boot:




Then it sits on the two dashes. Not taking any measurements.



The sample LED on the analogue PCB doesn't tick at all making me think something isn't triggering any samples to be made.

At this point, a full day of debugging and getting so close to a working device, I'm taking the evening off and having a beer. I'm posting this on the off chance of some sympathy or that someone has seen this failure mode before. Cheers all.

[Sam__]

After further investigation this evening (I couldn't keep away!) signs are pointing toward the digital side of things stuck in some kind of state, unable to sample from the ADC. Still not really sure what's going on here...

[Kleinstein]

I recall that failure of old days opto-couplers is quite common. Like old electrolytic caps these belong to the usual suspects. As already noted, connectors and other contacts also are not that reliable.

[Sam__]

Thanks for the info!

I checked the opto-couplers and they seem okay on initial investigation. Although they aren't being actuated as it seems to be in a paused state.

This morning I think I have found something that's a bit off. This part of the circuitry, that should generate clocks for the UP and DOWN count on the counters, does not seem to be operating correctly. The d-types are not flip-flopping as expected.


After finding an excellent online simulator (https://logic.ly/demo/) I build up a small part of that circuit and the sim gave different results. From the datasheet of the counters it seems they expect a clock alternating betweeen the UP and DOWN inputs. At the moment the device is seeing clocks on both pins at the same time. I may have to replace the 74LS74 d-type chip by the looks of it. Sadly I don't have any to hand so will have to order some and wait.

The other question is, is anything else faulty?

[Kleinstein]

The signal from the flip-flops to the counter also depends on the inputs to the D pins. These two would be similar and not having +VE high and /-VE low at the same time.

The 74S74 is faster than the 74LS version.  74ALS  may be OK as a lower power replacement.

[Sam__]

The signal from the flip-flops to the counter also depends on the inputs to the D pins. These two would be similar and not having +VE high and /-VE low at the same time.

The 74S74 is faster than the 74LS version.  74ALS  may be OK as a lower power replacement.

You are correct so I attached a scope to the inputs of one of the flip flops. The CLR and PRE pins are tied high, and checked with a DMM. So the only inputs that should influence the output are D and C. IC24.2 being the D input and IC24.3 being the clock input. With the stimulus shown on the scope I would expect the output to be toggling between states.


But here's the Q output of both of the flip flops. Constantly high.


This makes me think the two nets on the outputs of the flip flops are being held high somehow, and the flip flop seems the most likely culprit.

Edit:
I just snipped the Q and _Q_ pins on the top flip flop and the outputs are still static. Meaning they aren't being pulled by another device. So it's time to replace and go from there. Sadly it doesn't look like I can get any delivered quickly. So until next weekend, we wait. I'll also get some sockets just in case something is going to damage the replacement IC.