TMK500 meter now working

[KeithsTV]

Well following on from this thread

https://www.vintage-radio.net/forum/...ghlight=TMK500

I decided to take a look at my TMK500 to see if I could resurrect it. It had suffered many years ago from a slight over voltage, the result of attempting to measure the voltage on a line output valve anode (approx 170v dc but with 6kv spikes that did the damage)

I first checked the ohms ranges although only the lower ranges functioned as the 15v battery had long since been consigned to the bin. Shorting the probes together the needle was all over the place and the zero control had little effect. Thinking it was the control I sprayed switch cleaner into it. The result? No functions at all. I checked across the probes with my DMM for any sign of voltage only to be greeted with virtually no volts. Opening it up showed the battery to have an expiry date of 1994 but it still showed about 1.3v. I also found a previous repair to the tracks damaged by the flashover from the line output valve

Checking the resistance between the probes showed virtually open circuit on all the dc ranges including the current. This looked suspiciously like a switch fault as checking all the resistors on the board showed they were approximately the correct values as shown in the circuit. I removed the PCB to reveal the switch. Now on these meters the switch contacts are part of the PCB and the switch rotor has two sets of sprung contacts that short out the appropriate pads on the PCB. The pads were all cleaned with switch cleaner which made absolutely no difference so the contacts on the rotor were checked. Both sets measured open circuit. Pushing them down did improve matters so the rotor was removed and the contacts checked. It’s a very crude system where the contact press down on a phosphor bronze spring. I guess over the years the crud has built up in the contact areas and turned a conducting contact into a non-conducting contact. Anyway a burst of switch cleaner combined with rubbing a piece of paper between the contact areas worked wonders as after re-assembly the ohms ranges were stable and the zero control actually worked.

I then tried the continuity buzzer, which failed to work but one replacement battery later and the buzzer works. The dc voltage ranges were comparable with my Fluke DMM, which incidently was bought to replace it. The current ranges were slightly out but there is an adjustment pot, which improved the accuracy on one range to the detriment of the other ranges. It’s currently set to a compromise setting until I can get round to checking the actual resistor values as one resistor looks a little burnt.

The AC voltages ranges were a bit out, again compared with the Fluke so I changed the dual diode pack with a couple of 1N007s, which has improved the readings.

So after 20+ years of non use and many years before that of thinking I’d irreparably damaged it after the line output incident I now have a functioning 43 year old meter that can take its place amongst my test gear.

I don’t think the continuity buzzer would be suitable for checking modern semiconductor circuits but it should be OK for general electrical continuity.

I also found the original receipt in the box. It was bought from Chas Young of Corporation Street in Birmingham. Anyone remember them?

Keith

[Peter.N.]

We used a couple of those as standard workshop meters - many years ago.

Peter

[Chris Parry]

Chas H Young: yep, just up the road from Aston University, where I was a student in the mid 70's. Eddystone dealer, located in rather sombre premises. Chris

[woodchips]

Ah, that Chas Young. They had a clearout auction when I was at Aston, must have been about 1978 because I had my Landrover at the digs. Bought loads of stuff, some of which I have just rediscoved after clearing my dad's stuff after he died. Bought a couple of Eddystone 770R (I think) receivers from there, not long been passed onto someone on this forum. Bob

[MrElectronicman]

I remember Chas Young, that's where I got my TMK from. The reason they closed down was because I went to Canada and they lost a lot of business.

Glad you got the meter working.
In my case the tracks on PCB actually wore out where the spring contacts ran. I am wondering if I could make a new board by photocopying the original?

[DragonForce]

Originally Posted by MrElectronicman

I remember Chas Young, that's where I got my TMK from. The reason they closed down was because I went to Canada and they lost a lot of business.

Glad you got the meter working.
In my case the tracks on PCB actually wore out where the spring contacts ran. I am wondering if I could make a new board by photocopying the original?

Yes you can. Scan it at around 360 DPI, then in painshop pro or photoshop (whatever you prefer), black in all the areas of copper, and whiten out the areas of no copper. Print onto transparent film and use it as a negative for photoetching. I've done this several times with stuff. It works well, and obviously, the more time and effort you put into the job, the better your results will be.

[MrElectronicman]

Yes you can. Scan it at around 360 DPI, then in Paintshop pro or Photoshop

Thanks, I will give it a try some time later in the year.