Marconi TF2430 frequency counter

[Techman]

Returning to one of the frequency counters discussed in this thread:-
https://www.vintage-radio.net/forum/...d.php?t=136103
It's the first frequency counter of this model that I obtained that was originally an easy fix and mentioned in this thread, and not the second counter that needed a replacement chip and repair to the display board. This new thread could possibly be combined with that previous thread and this paragraph deleted if thought fit by the moderators.


I switched on the older of my two Marconi TF2430 frequency counters the other day and noticed that it was just displaying zeros and not counting the frequency being applied. It's actually had quite a lot of use monitoring frequency as a 'sniffer' counter with a single turn of wire around a coax feeder. The other counter which was repaired earlier with a replacement chip etc. has had less hourly use as a bench counter and is still working perfectly.

I checked the internal clock and all seems to be working as it should, with waveforms where they should be, unlike when the other counter was faulty. Applying a 100mV signal to the counter input on the front panel produced an approximate 200mV waveform at C9, a 'composite' type 1.5 volt waveform at the output of the Schmitt trigger IC3 at the R35/36 junction and a 1 volt pulsing waveform after IC2a2 at the top end of R41, so guessing all is in order so far. It was then that I noticed that the 'hold' indicator light was illuminated on the front panel, regardless of its switch position and I think that this may be the next big clue as to the fault.

I haven't done anymore with it, or done comparisons with the other counter, as that one is now in use monitoring frequencies. I also checked the various power supply outputs, and they're all working normally. I popped the case back on the counter for the time being so that it didn't get messed up, but intend to return to it soon, as although I've got a second one to use, I'd like to get this one working again if possible. The circuits are available in previous posts from when I repaired the other counter a couple of years ago.

[G0HZU_JMR]

It might just be a switch or connection issue with the HOLD button.

I've still got my old TF2430 but it rarely gets used. However, I dug it out today and powered it up and it is still working well apart from one dodgy LED segment. It's definitely the segment because I recall swapping the display digits around and the dim segment followed across to the new display location. I did manage to find a supplier of the original black LED segments used in this counter but I've not fitted the replacement yet.

In terms of accuracy, mine rarely needs adjusting. It usually starts up about 7 Hz high (10 Hz high on a cold day) and over the course of about 30 minutes it slowly drifts to be within a couple of Hertz of 10MHz. It's doing the same today. I've used a lot of frequency counters over the years but the little TF2430 takes some beating for general CB/ham servicing in the HF bands. The sensitivity and display stability are excellent and this is almost the perfect counter for this type of work.

One thing to be way of is that they don't like being operated in a cramped location as the regulator at the back panel can get very hot. Also, always allow air access to the vents on the underside at the rear. This keeps the regulator at a cooler temperature. Mine doesn't have the tilt stand so I have to prop it up to avoid this issue. Laying it flat on a hard surface is a bad idea in terms of thermal management. The 10MHz reference will probably not be as stable when used like this either.

[Techman]

Thanks for the reply, Jeremy. Yes, I agree that these are a very nice counter for the sort of work you mention. I didn't buy them because I was aware of how nice they were to use, I had no idea, but as it has turned out, they are actually the perfect counter to use and I wouldn't want to use any other. Both mine start slightly high and drift down to frequency the same as yours does. The one that's used on the bench is operated with its front stand opened out and holding the front end elevated, but the other counter that's developed the fault is permanently sat flat on the top of an old Ham Jumbo MK1, although the feet allow for reasonable air flow I would have thought and the part of the circuit at the rear hasn't failed (yet) and it is sometimes left running for hours within a partially closed cupboard and forgotten about. This particular counter actually has a lot more ventilation slots in the sides of the casing than the later one does. I wonder why they modified the casing on the later model to have less ventilation? Perhaps a more constant temperature inside the case gave better long term frequency stability, or perhaps they found that there was a problem with stray RF affecting the circuitry in some situations with the older and more open case design, however, it's the one with better ventilation that's failed this time.

It's a good point you make regarding the actual 'hold' switch itself as sometimes it's the most obvious things that are overlooked. However, having taken another look at it last night, checking the voltages at TR20 gave the following - Collector/R91 = 0.07v with 'hold' off, and 0.03v with 'hold' on. Base/R90 = 0.7v with 'hold' off, and 0.73v with 'hold' on, all WRT chassis, so this basically confirms that the switch at least is working. The transistor function on its junctions is also fine. I next lifted one end of R90 which, as expected, put the 'hold' light out, but still no counting, so my suspicion is now directed to IC8.

Below is the part of the circuit under inspection with the parts highlighted and the circuit path marked out with green marker - any other thoughts before I go any further? It looks like this chip may have been removed at sometime in the past, and certainly this counter has had a lot of previous attention with several of the other chips having been removed sometime in the past and replaced with fitted sockets on the board, although IC8 is unfortunately not in a socket.

[Dickie]

I'm not sure your measurements confirm the switch is working. I would have expected a larger voltage variation. Can you measure the actual switch voltage (point"AE", or pin 1 IC8)? It should go from 0 with hold out to +5V with hold "in"