Marconi TF2600B video voltmeter

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Joined: Sat Oct 22, 2022 9:06 pm

Marconi TF2600B video voltmeter

Post by Zenith »

This came from a rally in the summer. It was marked "OK, £5". The seller took the view that we were made for each other. I didn't have the heart to offer £2.50, so I handed over the fiver. They then threw in a couple of ham projects, which have a few useful bits on. It was a "get rid of it" stall.

I know someone who has one and he's always thought it was a useful piece of kit. I thought that being Marconi it probably didn't have ASICs and hybrids and the manual would be on the WWW. I was walking past the stall later and they called me over and handed me a copy of the manual. Just as well, as it doesn't seem to be available on the WWW.

It powered up and it gave a more or less reasonable indication at 1MHz and over, but dropped to nothing at AF. One to sort out in the dark days of winter. The dark days of winter are now here.

The power supply lines are +30V and -9V and the manual gives the acceptable voltages and ripple. The 30V line was OK but the -9V line was lower than the specification and it had a lot of ripple. I replaced C73 with a 470µF and D13 with a selected 9V Zener measured to give 9.2V.

The relevant parts of the circuit diagram are attached. It's a photo rather than a scan, because the scanned version caused problems.
Next I looked at it and there's an AC output brought out on a BNC socket. The block diagram shows an amplifier/attenuator stage which feeds an follower stage which is brought out on the BNC socket and also feeds the meter circuit.

Block diagram
I tried feeding a signal to the input and checked the output from the BNC socket with a scope. The output from the BNC socket was more or less right at all frequencies, but the meter response tailed off to nothing at low frequency. It seemed most likely a problem with a capacitor in the signal path, maybe dried out or possibly a bad joint. There were several candidates, but the prime suspect was C46, a 10µF 63V electrolytic. This is a rather strange part made by Siemens. The bottom is epoxy filled. It has to be low leakage and handle a wide range of frequencies. I decided to replace it with a 10µF 50V tant. The old capacitor has a capacitance of 800pF at 1KHz. It has sort of crack on the top surface.

With that done, the meter more or less behaved even at low AF. I couldn't see much point to replacing other capacitors which appeared to be giving no problems.

I worked through the calibration procedure using a new Siglent function generator, which Santa brought, and a DSO. Results were hit and miss until it dawned that the wafer switch was dodgy. I cleaned it with IPA and cotton buds, then applied contact cleaner with a small brush. I don't think it's a great idea to spray contact cleaner onto twitchy RF circuits. The cotton buds and IPA removed a lot of grime.

The calibration worked out fairly well, although adjusting a preset cap for one range seems to affect others, so it's an iterative process. If I try it a few days later it can be a little out. It may be temperature sensitive. I don't think it had ever been adjusted since it left the factory and maybe 50 year old preset caps can be temperamental. The 1mV range is slightly out, but correcting that would involve replacing a select on test resistor which is very hard to access under the wafer switch. Some of the other ranges are adjusted by select on test components, but none were sufficiently out to worry about.

It's about as right as I can easily get it. It was interesting to play with, but I wonder just how useful it will be. The DSO gives good enough voltage readings and is very convenient. I also wonder how good these were when they were new.

It might be departing via ebay when the clutter next becomes too much.
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