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Old 13th Jan 2021, 5:08 am   #1
gcwills
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Default Advance TC11 repair

Hi all,
Recently an Advance TC11 frequency counter came into my possession. It is identical to one that I used many years ago in my first job working for a university here in Australia so it had quite a bit of sentimental value.
I think part of the attraction of the TC11 was the nixie tube display, so I was keen to see if it could be brought back into working order. Under test, it had a few issues - sometimes it failed to count and when it did count, the frequency range was inoperative.

Here is an image of the counter:
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Old 13th Jan 2021, 5:32 am   #2
gcwills
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Default Re: Advance TC11 repair

So I set to work to restore the counter.
Firstly it needed quite a bit of cleaning and straightening of the aluminium top, bottom and side panels. The counter had attracted a fair amount of dust so all of the boards and insides were cleaned.
Thanks to several members here especially Dekatron (Martin) who provided a great OCR of the manual I was able to test the power supply and confirmed that all voltages were what they should be. After some time tracing the circuit I realised that the 100Khz timebase clock was intermittently disappearing from the input of the timebase board. A bit more investigation revealed that the 100kHz timebase oscillator was routed back into the timebase/power board through a small 3.5mm socket at the rear of the counter. This socket allowed for an external timebase to be used with the counter. However, the switching contact in the socket was making intermittent connection causing intermittent timebase drive. Cleaning of the contacts in the socket was all that was required to get the timebase operating reliably.
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Old 13th Jan 2021, 5:39 am   #3
Radio Wrangler
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Default Re: Advance TC11 repair

That's rather good-looking. It's a full universal counter, not just a frequency measurer, it can be used to count events and time things.

Too many of these things are raided for the nixies by people wanting to make clocks, so it's great to see it saved.

You probanly have some dirty switch contacts to work clean, then board connectors and maybe IC sockets.

David

Edit: crossposted congratulations on the success
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Old 13th Jan 2021, 4:00 pm   #4
factory
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Default Re: Advance TC11 repair

Good to hear you got it working again, I reckon you have found a common fault with the TC11/12 counters in that switched jack socket for the external reference.
Just had a look through the pictures of mine and noticed a previous owner had replaced the switched jack socket with a separate switch & BNC input.
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David

P.S. I tend to avoid mentioning the brand name of the displays when posting about anything that uses them, I don't want to help people who are only interested in robbing vintage equipment of them.

Last edited by factory; 13th Jan 2021 at 4:09 pm.
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Old 14th Jan 2021, 4:21 am   #5
gcwills
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Default Re: Advance TC11 repair

Thanks guys for the comments - I have appreciated the resources available here. They were of great help in repairing the counter.

Having sorted the first issue of an intermittent timebase, I began tackling the non functional frequency counting range. Quite a lot of time was spent in trying to understand how the frequency range functioned. I learnt that in common with other frequency counters, the input signal after conditioning is applied to a gate. The gate is switched on and off by a selected division of the timebase. So the output of the gate is the number of cycles of the input signal allowed through the gate as determined by the timebase which is then counted by the following counter circuitry.

So all that to say that my counter had a faulty gate. The input signal was not passing through the gate in frequency mode. Two ICs - IC1 and IC14 act together with transistors VT1-4 as the counter gate. All of the transistors were checked and tested fine which left IC1 and IC14. These are ancient uL926 RTL JK flip flops from the mid 1960s. Replacing them presented a problem - I located some from the USA - however at US$50 plus post each this was not an option. I reasoned that it might be possible to replace them with a CMOS CD4027 dual flipflop so I jury rigged one in place of the two ICs. These ICs are specified to operate at the 3.3V used by the RTL family. However the 4027 was not a suitable substitute and didn't work in this circuit. I then tried a 74HC73 and 74HC76 as the HC family will operate at 3.3V but they also failed to work. So I needed to locate some old stock RTL ICs.

I sent out an SOS here and on several electronics forums for uL926 ICs. Eventually I received an email from a guy in the US who kindly offered me some uL923 ICs. The 923 is an epoxy 8 pin JK flip flop vs the 926 metal 10 pin device. The main difference is that the uL923 lacks the "preclear" pin on the 926. However the preclear pins are not used in the TC11 so it looked like a pair of 923s would substitute.
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Old 18th Jan 2021, 5:50 am   #6
gcwills
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Default Re: Advance TC11 repair

So eventually the ul923s arrived from the US (thanks Jan!) and I installed a pair to replace IC1 and IC14. I was delighted to confirm that they did the trick and the Advance TC11 was finally fully functional.

Here they are installed on the main board:
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