Feedback FM610 Digital frequency meter
Thought it was about time I wrote about something again! So, I've had need for a decent working frequency counter for a while, my old RS one had become a random number generator, somehow managing to count things that were not there (not even connected to anything!) and when it had been working, I didn't trust it's accuracy.
I spotted this Feedback FM610 frequency counter on good old eBay last week, nearing the end of the listing and very cheap too, so I slapped in a bid! Won it a few hours later for £15 plus 8 for shipping. It arrived nicely packed and working, the display was sitting a bit wonky, so I decided to have a look inside, even if the display wasn't wonky, I was going to open it up just to see what's in it!
Once opened, the reason for the wonky display was obvious, it's on a separate plug in module that had come a bit loose, probably because there is nothing to hold it in except the springy terminals in the connector, so I measured up and glued a block of hard rubber on the inside of the top case so the display can't get pushed back, that should keep it in.
The display itself is quite interesting, being made up of 3 dual 7 segment neon Panaplex displays, labelled 'Beckman SP352', looks to be a stop-gap between Nixie tubes and VFD's. It's nice, bright and easy to read from a distance. Looking at these displays on their own on eBay shows them to be quite expensive, 1 easily being the same cost as the whole counter cost me!
The counter is in excellent cosmetic condition, so no complete disassembly for a scrub, I just gave it a wipe with a micro-fibre cloth and some foam cleaner. It would also seem that the accuracy is pretty much bang on, I have a signal generator app on an iPhone, which can output anything from 0 to 20kHz, and trying various frequencies from the iphone resulted in the counter agreeing with what was on the phone display!
I took pity on the old RS counter at that point, thought I'd give it one last shot at repair before trying to give it away or consigning it to the bin. I fired it up and sure enough, the display was going nuts, so I pulled the boards out of it and had a good look at them, found some damaged solder joints and ripped up print on the connections to the display PCB, surely it can't be that easy?! I remade the joints, and repaired the damaged traces, connected up 9V and to my amazement up came a stable display! I quickly connected a signal to one of the dangling BNC's and the 'gate' started flashing on the display, and it actually gave a sensible reading, it wasn't quite right but it did it! There are instructions for calibrating the internal 1mHz clock on the bottom of the case, so I used the new counter connected to the internal clock out on the back of the RS unit, and sure enough, the frequency was off, so I tweaked it to read 1mHz on the counter. It then read the same as the new counter, so I boxed it back up, gave it a scrub, and now I have 2 working frequency counters!
The Feedback counter can only read up to 9.99999mHz, then the over-range LED lights, but it carries on reading frequencies past that. Another interesting feature is that it has up to 1KV input range!! Might be useful for telly work, although I don't really fancy trying it...
It also shows that my signal generator is severely out of calibration!! So no wonder the Ekco AC86 I tried to align with it doesn't work very well!
That's a very nice counter you've got yourself there Lloyd, looks hardly used and at a very good price too, I can see why you would be pleased on all fronts.
Isn't it funny how the things turn out, you intend to replace one dodgy unit, end up looking a little deeper at the first then end up with two. I think we've all been in that position with kit. I've often fancied getting a nixie counter but of course so do many others and prices match accordingly.
Nice write up, thanks for sharing ?
I'm curious to know what the Motorola TV is, lurking behind and above the counter in your first picture Lloyd ?
To understand the black art of electronics is to understand witchcraft.
I've often fancied getting a nixie counter but of course so do many others and prices match accordingly.
I've got two - an Advance and I think a Marconi-badged version of one. The Marconi's yours for the cost of the postage if you want it, Chris.
I've often fancied getting a nixie counter
Yes, I looked at some of those too, and was put off by the prices! I’ve still got that Fluke multimeter with the Nixies in it, doesn’t see much use as it’s so big, but they do look nice when lit 🙂
I'm curious to know what the Motorola TV is, lurking behind and above the counter in your first picture Lloyd ?
The little Motorola is a 7VT2 from across the pond! I got it at the NVCF in 2014, there was a guy there with a number of American sets, so I bagged this wee beastie while I had chance! Never seen anymore there since, even though the same guy is there every year. I’ll start a thread on it soon, I have started work on it, but put it to one side whilst I wait for some parts 🙂
A good find for £15 with plenty of resolution too! Those gas plasma displays were used in a lot of high end equipment in the early 80s, in particular Datron multimeters and calibrators. It;s nice to see that its in good working order. The Datron calibrators were left on 24/7 where I worked and some of the display segments and annunciators started to get a bit dim. But equipment of that era was so reliable! Frequency meters often get better with age as the reference oscillator gets more stable as it ages. Working for 3 decades in a calibration facility, I remember one particular unit that read true to the last digit without adjustment (a Racal 9901) for over 20 years. Without an ovened standard, I remember.
On another note about Frequency: As well as the 198kHz Radio 4 carrier being a recognised standard maintained by NPL; I seem to remember that you could use a broadcast video signal as a low frequency standard, 15.625 and 50 Hz from the composite sync pulses. I think that the whole Television network had to be synchronised so that there was no jitter when changing video sources during a broadcast. All cameras, VTRs, OB equipment had to be synchronised for smooth transition. That meant that the sync pulses were a very accurate time/frequency source. I'm not sure whether that's the case with digital video signals we use today though!
Colour subcarrier in the PAL days from the BBC was 4.43361875MHz +/- 1Hz. The pulses were derived from a master oscillator which was a rubidium gas oscillator.
Digital video pulse generators are generally locked to GPS these days. The acceptable levels of jitter on an HD digital video signal are measured in picoseconds.
I smiled when I saw this meter. I bought a working one from the same source about 17 years ago. It arrived and worked so I left positive feedback, (no pun intended). This was a mistake because the next time I switched it on I had an unresponsive and garbled readout. Not surprisingly the seller was unimpressed and, as I was reluctant to dispose of it, I put it in the loft. During a recent clearout I discovered it and took another look. This time, being a little more determined, I found that the high voltage driver ic had lost interest in the situation. It's a Beckman DD700 and they are available on ebay. My meter has worked perfectly and, given its limited frequency range, has become a favourite. Somewhere I have a vesigal circuit sent by the company which took over Feedback and if I can find it I'll post it here in case you find it useful.
That’s great that there is another one of these out in the wild! Good to hear you got it working after it packed up too. I actually haven’t used mine recently, it’s been buried in a cupboard in the workshop for a while, and I’ve been trying to tidy up that cupboard, now I’m finally ‘furloughed’ (funny word that, I’d never heard of it before!), and I came across the meter today and was wondering about where to put it so it could be easily accessed when needed, I still haven’t worked that out yet! But I may put it on the shelf above the bench, and bury another piece of test gear in the cupboard instead, the Minolta TV colour analyser that hasn’t been used ever might as well go in there!
I got my hands on the same model FM610 Frequency meter, which is also malfunctioning. Though I'm not so lucky as you were, because I have a more difficult problem, than you did.
Originally any signal, with the frequency displayed higher than 120, on any range, will first show as "all zeros" and after increasing the signal, displaying some digits unrelated to the actual frequency. So it was only correctly displaying up to 120Hz on Hz/kHz range, and up to 1.2kHz on MHz range, while the gate trigger is working fine all the way up to 10MHz.
With no damages or bad soldering visible, and not having any circuit diagram or service manual, and after checking basic voltages and elcos, I almost gave up the fixing. Then I went curious about the DIL switches (S13, S14, S15) on the main board, what they do, and tried to flip them. With the correct combination of them I managed to get correct display on Hz range up to 999Hz (with overload indication after that), but only up to 12kHz on kHz range, and 120kHz on MHz range. Although this is an improvement to the original situation (usable for audio signals), the kHz and MHz ranges are still limited to the "120" digits, with the same symptom as before.
As I did not expect that the previous owner has played with those switches, now I'm not sure what should be the correct setting of them for the normal use, or are they used for servicing/repair.
Therefore I would kindly ask you to check the positions of those switches 1/2/3, on S13, 14, 15, on your working meter, and let me know them.
I'll be most obliged if you can do that for me, the electronics enthusiast, and may be suggest where to find the schematic diagram for that meter!
Stay safe and BR,
PS, anybody else, please let me know of the possible source of Service Manual or schematic diagram for FM610 made by Feedback UK.
I’ve popped the lid of and had a look at the switches, in mine they are all set with 1 on, and 2 and 3 off, I’ve no idea what they do.
best person to ask about a circuit diagram is Jonathan, aka 6.3V (one post above my last one), as he sent it to me, but I don’t want to go passing it on without permission!
thanks a lot being so kind to look at your meter, and so prompt with the response.
The sw settings you describe were the same on my meter when I got it. That would mean they are normal settings, and it appears that switches might be used for troubleshooting purposes. I got better result (higher possible frequency displayed with No1 of S13 in the ON position, which I think has to do something with the time base of the gate. While S15 is dealing with position of decimal point on the display.
Also thank you very much for the hint on the source of ckt dia !
Stay safe and kind regards,
Hi there - the filament V !
In case you have read my and Lloyd's correspondence about FM610, you might be already familiar with my problem.
Anyway, I'm stuck because without ckt diagram it is almost impossible to fix that meter.
Before I haven't read your post carefully enough, so I didn't notice that you are mentioning a vesigal circuit of the FM610, which I'm desperately looking for. I was already in touch with the company which claims that they have bought a part of Feedback (LD Didactic GmbH), but they couldn't help me with ckt diagram nor Service manual.
So I'll be very grateful to you if you are willing to provide me the documentation you have about FM610.
Of course I'll share with you any information that you need, which I might have.
My e-mail is email@example.com in case you want to use that way.
Hi, I've found the circuit and will email it to you. You are free to share it. It's a poor scan and this is all I could get from the organisation that holds it. Hope you have success with this instrument. Keep us updated