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Old 9th Apr 2024, 11:31 pm   #1
Techman
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Default Malden Electronics 7735 frequency counter

Earlier today I picked up some free bits and pieces from a local(ish) radio operator who I know and one of the items was this counter. He'd had a go at it and come to the conclusion that several of the Nixie type tubes had failed and that it was probably scrap. All the case screws were missing and the case halves were held together with tape, he'd also cut off the mains lead, presumably to use on something else. He didn't tell me a lot about it and when I opened it up later I found all the case screws in a plastic bag inside, so that was unexpected and good. I tacked a length of mains lead onto it and powered it up and it seems to count from its internal clock, but there's only two of the display tubes working, so I think he's probably right about it having several dud tubes. I've not looked any further into it at this stage and I see that Malden are/were associated with Venner. It's a nice vintage counter, but I think it'll probably end up as scrap for parts. However, I thought I'd post about it here first, just see what others think before I go any further into it, or pull it to bits for parts.
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Last edited by Techman; 9th Apr 2024 at 11:36 pm.
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Old 10th Apr 2024, 10:47 am   #2
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Default Re: Malden Electronics 7735 frequency counter

Have you tried swapping the Nixie tubes in the holders?
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Old 10th Apr 2024, 10:59 am   #3
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Default Re: Malden Electronics 7735 frequency counter

Those are not holders, they are spacers supplied with the tubes to align the wire leads in the correct pattern, to make it easier to insert into the PCB or a holder.

Before condemning the counter check the supply voltage for the display and the transistors that multiplex the display.

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Old 10th Apr 2024, 11:32 am   #4
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Default Re: Malden Electronics 7735 frequency counter

Apologies, but swapping round the tubes would show whether they themselves were faulty or whether the fault/s was elsewhere.
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Old 10th Apr 2024, 12:02 pm   #5
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Default Re: Malden Electronics 7735 frequency counter

From experience with repairing counters of another brand, with the same type of tube, which are also soldered in & very unreliable, the heat from desoldering & soldering the tubes can risk cracking known good tubes, in my case I had a spare board with working tubes, some of which failed shorty after being fitted to the repaired counter, despite using a desoldering iron and taking my time to avoid too much heat at once.
Checking voltages and the lower cost driver transistors, is where I would start. YMMV

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Old 11th Apr 2024, 6:46 pm   #6
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Default Re: Malden Electronics 7735 frequency counter

Venner / Malden 7735 and 7736. Also appears as Marconi TF2416 and BT1000F.
I have posted details and the manual etc in the past. Look under Venner or Marconi.
The indicator tubes are ITT / STC 5853S, widely about, and with japanese similar types. Used in early desk calculators. If you are stuck, I have some somewhere which could do I think.
It uses a single rather odd Fairchild device, CuL9960 bcd to decimal convertor in high speed DTL. Can be replaced with slight mods by more current TTL 7445/74145 or 7441 / 9315 and re-pinning.
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Old 11th Apr 2024, 8:08 pm   #7
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Default Re: Malden Electronics 7735 frequency counter

Quote:
Originally Posted by WME_bill View Post
It uses a single rather odd Fairchild device, CuL9960 bcd to decimal convertor in high speed DTL. Can be replaced with slight mods by more current TTL 7445/74145 or 7441 / 9315 and re-pinning.
wme_bill
The modification for 74141/K155ID1 needs an inverter for the BCD inputs, I've used a pair of 74HCT2G14 & K155ID1 to make adaptors for another obsolete device. The K155ID1 was still available new in bulk from the source country until very recently.

Note: Some TE using the Fairchild micrologic range, use slightly different logic supply voltages (i.e. not 5V), having said that they seem more reliable than the 7441/74141, from my experience.

Did anyone figure out what mystery "C16" or "C16B" IC Venner are using for the multiplexing?

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Old 11th Apr 2024, 8:23 pm   #8
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Default Re: Malden Electronics 7735 frequency counter

No help to you, but I saw those being chucked out by BT 30 years ago.
They seemed pretty reliable back then.
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Old 12th Apr 2024, 11:28 am   #9
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Default Re: Malden Electronics 7735 frequency counter

Many thanks for all the input folks.

Thanks for the possible offer of indicator tubes, Bill.

A member has just offered me a scrap board from a Marconi counter that had working tubes before it was scrapped. My exact tubes are Rodan GR-116D, 6FY. The tubes on the board I've been offered seem to be the same number wise other than having 4FY instead of 6FY as the large printed number. What do you folk think might be the difference, if any?
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Old 12th Apr 2024, 1:41 pm   #10
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Default Re: Malden Electronics 7735 frequency counter

Probably a batch or date code, hopefully we'll see a successful repair.

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Old 28th Apr 2024, 7:18 am   #11
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Default Re: Malden Electronics 7735 frequency counter

Just checked my TF2417 and the tubes are also ITT / STC 5853S. I have a manual for this as well for interest sake.
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Old 30th Apr 2024, 6:49 am   #12
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Default Re: Malden Electronics 7735 frequency counter

Probably just manufacturing date.

Quote:
Originally Posted by Techman View Post
My exact tubes are Rodan GR-116D, 6FY. The tubes on the board I've been offered seem to be the same number wise other than having 4FY instead of 6FY as the large printed number. What do you folk think might be the difference, if any?
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Old 23rd May 2024, 4:59 pm   #13
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Default Re: Malden Electronics 7735 frequency counter

Quote:
Originally Posted by factory View Post
Probably a batch or date code,
Ha ha, it seems there's a bit more to it than that after all.

Been very busy and up to my eyes in other stuff at the moment, but just got round to looking at this counter again. A very kind forum member sent me a board from a scrap counter of similar design with good tubes fitted. It turns out that the pinouts are different between the 6FY and the 4FY, even though the main type number is the same. The second photo below shows the difference between the replacement tube on the left and the original on the right, the first photo shows the board with the replacement tubes as originally fitted - it's from a seven tube counter. It may be possible to cut the print on the board and jumper that errant pin to the other side, but there may be other differences that I haven't appreciated yet. I'll probably put it to one side for the time being and come back to it when I get a bit more time to play around with it. All suggestions welcome.
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Old 23rd May 2024, 7:13 pm   #14
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Default Re: Malden Electronics 7735 frequency counter

Those tubes should have both the left & right pins, they are for the left/right decimal points, why they have been cut off I have no idea.
Guess you should work out how many of the decimal points are actually used and rewire if necessary to use the other side.

Edit: The cut pins seem to be due to the poor track layout, Racal managed to fit all 14 pins without cutting some off.

David

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Old 24th May 2024, 12:07 pm   #15
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Default Re: Malden Electronics 7735 frequency counter

Quote:
Originally Posted by factory View Post
The cut pins seem to be due to the poor track layout, Racal managed to fit all 14 pins without cutting some off.
You're absolutely right!

This is certainly something I hadn't appreciated on first sight and assumed it was how the tubes were originally manufactured, but can now see that the pins have just been cut off. There's actually two missing, one of the side pins and another in the main row - always expect the unexpected!

Anyway, with a bit of drilling and a couple of wire jumpers and side cutters to another of the pins, the first left hand Nixie tube has been successfully replaced. It's a bit of a bodge but it's working and arguably the bad track layout on the board and cut pins on the tubes is an original bodge by Venner/Malden anyway.

Two more Nixies to replace - lucky I've still got six more to play with, just in case there's any accidents as mentioned in post #5 regarding desoldering and soldering, but so far so good!

I noticed that the last original working tube on the right seemed to take a moment to come on compared to the other two and also looks slightly pink in the picture, although that could be a trick of the photo, so possibly it's also on its way out.

So it looks like the job's going to be doable with the parts that I've got.
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Old 24th May 2024, 2:23 pm   #16
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Default Re: Malden Electronics 7735 frequency counter

That pinkish glow looks like the lilac-pink glow of a gassy valve and that tube is going to air perhaps?
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