Nixies at 520MHz, The RCS 901 - UK Vintage Radio Repair and Restoration Discussion Forum

Pamphonica · 16th Jan 2014, 9:12 pm

I'm feeling fairly pleased with this repair. A rare beast – a 520MHz counter timer with 8 digit NIXIE display - an RCS Electronics Model 901M from about 1977 going by component date markings.
It was given to me FOC by a radio club member who couldn’t get it working.
No manual, circuit etc of course. So back to tracing the circuit and drawing bits of it out, armed with a copy of the 7400 series data manual.
It was clear that one of the main reset lines was stuck at “reset”. After a lot of circuit tracing I worked back to the monostables and found a suspect 74121. Pulses going in but no pulses coming out.
It was desoldered and a socket put in. Slightly difficult to put in as they used the IC pins on the topside to connect the power rails.
Anyway, after having to remove an apparently duff socket the second socket was fine and was equipped with a NOS 74121. To my delight the device sprang to life once the right buttons were pressed.
Even more to my delight was finding that it still has 25mV sensitivity right up to 80MHz, and that the prescaler still works, giving 25mV sensitivity right up to 520MHz.

What a really nice bit of kit - modern performance but with the comforting glow of the nixie tubes! It even has switchable 50-ohm termination on the input, just like Racal, HP and other top-end counters.
Useful and pleasing.....

Radio Wrangler · 16th Jan 2014, 9:42 pm

Spiffing!

That's a good useful counter. THe 7400 family monostables were always poor on reliability. The semiconductor line was optimised for fast switching, so essentially an analogue function was going against the grain.

David

Pamphonica · 16th Jan 2014, 9:55 pm

Yes, I have come across duff '121s before. I was hoping it was the '121 as it also has a '122 and I don't have any of those in "stock". I don't think I have fault-found good old original 7400 Series TTL for many years! Quite fun to get the logic probe out.

I just realised I should have added "RCS 901" into the thread title in case anyone is doing repairs in future and searches for it by model number.

(Mods can you oblige?) - sorted! Cheers, Sean

Jeremy

Andrew2 · 17th Jan 2014, 10:30 am

That's spiffing indeed, Jeremy. Most counters from that period are pretty poor things and they tend to look awful as well, but that's a nice-looking piece of clobber. Well done for getting it working.
I must admit that whenever I see a nixie readout I always think of petrol pumps! Six bob a gallon? Do you think I'm made of money?

Pamphonica · 17th Jan 2014, 7:54 pm

The display in the photo was the 901 measuring the output of a well-calibrated sig gen giving out 25mV at 520MHz. ~1 part in 10^6 error after 30+ years is not bad! I can't trim it out as I can't find any trimmer (the ovened oscillator is encased in styrofoam wich would have to be removed in case it's inside).
If anyone is doing fault-finding on these devices I have attached the very crude drawing I made of key bits of circuitry which might help someone.
- Jeremy

16th Jan 2014, 9:12 pm	#1
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Nixies at 520MHz, The RCS 901 I'm feeling fairly pleased with this repair. A rare beast – a 520MHz counter timer with 8 digit NIXIE display - an RCS Electronics Model 901M from about 1977 going by component date markings. It was given to me FOC by a radio club member who couldn’t get it working. No manual, circuit etc of course. So back to tracing the circuit and drawing bits of it out, armed with a copy of the 7400 series data manual. It was clear that one of the main reset lines was stuck at “reset”. After a lot of circuit tracing I worked back to the monostables and found a suspect 74121. Pulses going in but no pulses coming out. It was desoldered and a socket put in. Slightly difficult to put in as they used the IC pins on the topside to connect the power rails. Anyway, after having to remove an apparently duff socket the second socket was fine and was equipped with a NOS 74121. To my delight the device sprang to life once the right buttons were pressed. Even more to my delight was finding that it still has 25mV sensitivity right up to 80MHz, and that the prescaler still works, giving 25mV sensitivity right up to 520MHz. What a really nice bit of kit - modern performance but with the comforting glow of the nixie tubes! It even has switchable 50-ohm termination on the input, just like Racal, HP and other top-end counters. Useful and pleasing..... Attached Thumbnails __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk Last edited by Sean Williams; 16th Jan 2014 at 11:51 pm. Reason: to remove rouge formatting gibberish

16th Jan 2014, 9:42 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Nixies at 520MHz Spiffing! That's a good useful counter. THe 7400 family monostables were always poor on reliability. The semiconductor line was optimised for fast switching, so essentially an analogue function was going against the grain. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

16th Jan 2014, 9:55 pm	#3
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Nixies at 520MHz Yes, I have come across duff '121s before. I was hoping it was the '121 as it also has a '122 and I don't have any of those in "stock". I don't think I have fault-found good old original 7400 Series TTL for many years! Quite fun to get the logic probe out. I just realised I should have added "RCS 901" into the thread title in case anyone is doing repairs in future and searches for it by model number. (Mods can you oblige?) - sorted! Cheers, Sean Jeremy __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk

17th Jan 2014, 10:30 am	#4
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,038	Re: Nixies at 520MHz, The RCS 901 That's spiffing indeed, Jeremy. Most counters from that period are pretty poor things and they tend to look awful as well, but that's a nice-looking piece of clobber. Well done for getting it working. I must admit that whenever I see a nixie readout I always think of petrol pumps! Six bob a gallon? Do you think I'm made of money? __________________ Andy G1HBE.

17th Jan 2014, 7:54 pm	#5
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Nixies at 520MHz, The RCS 901 The display in the photo was the 901 measuring the output of a well-calibrated sig gen giving out 25mV at 520MHz. ~1 part in 10^6 error after 30+ years is not bad! I can't trim it out as I can't find any trimmer (the ovened oscillator is encased in styrofoam wich would have to be removed in case it's inside). If anyone is doing fault-finding on these devices I have attached the very crude drawing I made of key bits of circuitry which might help someone. - Jeremy Attached Thumbnails __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk