02 December 2017

New instrument in the lab: Schlumberger 1240 multimeter

During one of my time travels in the 1970's I brought home a Nixie-based multimeter: Schlumberger 1240. Three and half digits in a compact desktop case. It is the same instrument of Weston 1240, and the Heathkit IM-102 shares a lot with them.

Schlumberger (Weston) 1240 multimeter from circa 1972.
It was given away as working except for the 200mA scale. Not a big deal if you want ot make a clock out of it, no? The first power up confirmed both its working state and my suspicion that the half digit neon bulb was broken. Just feed a variable voltage in the 20 V scale and let it go beyond 9.99 V. Time to open it up, without a manual/diagram/parts list to be found online.

Well, Weston made also model 1242, a 4.5 digits multimeter that is aestetically similar, and the manual is available online (not complete and some pages were poorly scanned). At least it shows how to extract the circuit board.

Top of the board with discrete logic.
Two notches later, I had the two-layer through-hole board on my desk. Meet another 1970's hand-drawn PCB, with charming curvy traces and no ground plane! There are three Burroughs B5855S Nixies.

The "half digit" was a 25 mm tall neon lamp with an illuminated bar of about 15 mm and long leads. Initially I suspected the driver transistor was gone but I begun removing the lamp first: only two solder points to redo in case it works rather than three short leads of the transistor. Well, one lamp leg broke in the process and I couldn't lit it with my high voltage DC source.

Bottom of the board with curvy traces!
Looks like it is not easy to source a neon bulb with this size in 2017, and temporarily a shorter one will do the job.

But something else happened ...

17 November 2017

Linear PSU failing with overvoltage

According to timestamps on components, I have owned this linear power supply for 30 years (rms K135). It powered my first CB station (Midland Alan 48) and who remembers how many other devices. Lately it has been powering a 12V LED strip in the shack-lab.

One of these days I wanted to take a picture of a multimeter (will be feature on the blog, don't worry) and I needed both something to measure and less light in the shack. So I unplugged the LED strip and wired the PSU to the multimeter.

To my great surprise I read 24V. What?! That would have burned the LED strip and everything else I had connected in the meantime. But, with the LED strip powered, it behaved as expected and outputted 12V or so. Of course I cross-checked the reading with several DVMs and they all agreeed.

A bad capacitor? I opened up the PSU and I was greeted by a large amount of dust. And two 78S12 in parallel, as I remembered. Hard to remove the dust, but once components were in the clear, I still had 24V out without a proper load.

Capacitors looked OK. No leaks, no deformation. Unwillingly I reached for the solder side of the PCB and disconnected one 78S12 at a time (just the output pin, lazy me). One regulated at 12.5V, the second at 24V. Well, it didn't regulate at all then!

I temporarily replaced the bad one with a 7812 and the output is still 12.8V.

So, watch out for this simple technology too! Until today I would test the Amps rating of a PSU. Now I will check the open circuit voltage!

07 November 2017

My first HF RTX, in 2017

Got home a bit early tonight from work and didn't want to stare at a screen, so I finally dug out my first HF transceiver for a quick check-up.

It is an Icom IC-728, 100W on HF bands, AM/CW/SSB, triple conversion receiver. I think I bought it in 1993 after the high-school graduation. The price was a bit less than 1'000'000 lire (that's about 820€ in 2017's value).

First I did a visual inspection of the circuits for leaking capacitors: no signs. Then I grabbed the DC power cord from the IC706 and powered it up. It was exactly as I remembered it when it was last used .... 20 years ago or so. Whew!

I reached for the coax that enters my little shack and inserted the hot pole into the antenna socket. At last HF noise filled my room!! Why just the hot end? Because there's no antenna on the other end! It's a coax left over from my balcony experiments 9-10 years ago (seek blog archives if you want). But it does provide few metres of pseudo-antenna. Just don't press the PTT.

I quickly tuned around the bands. I found CW stations on 40m (and I could even make sense out of it!), some AM broadcastings too.

I caught myself few times looking for a "MENU" button, but this radio has no menu: each key has one function (some have two). That's it. All functions are one press away.

Then I wanted to reduce the incoming noise from the open line antenna and ... no DSP, of course! All you get is a Noise Blanker and Pass-Band Tuning: neither of them is effective against modern QRM. Well, I "retuned" my ears and it was fine.

I love the smooth effect of the large tuning knob. I could keep tuning for hours.

I will do my best to put it on the air even if propagation doesn't seem to be helping.

(I didn't take a picture. I will next time I take it out of the box ... hopefully soon!)

29 October 2017

Failing old LCD displays

Following my recent interest in old numeric display technologies I came across old calculators, and started collecting them. In early 1970's there was a lot going on in the research of display technologies, so while Nixies were fading out, VFDs taking their place where there could be a lot of environmental light, LEDs and LCDs were entering the market.

Overheating causes early LED displays to fail faster, especially if run at full brightness.

Sharp EL-5103S
Early LCDs on the other hand lost the "vacuum" inside and became unusable. You can see the failure as a darkened area in the corners/border and there's nothing you can do.

But I wouldn't expect this kind of failure from my Sharp calculator made in late 1980's or the VHF transceiver of early 1990's! Unfortunately replacing an LCD is not easy and you can hardly fit a replacement in the same space. So these devices just turned into (my personal) museum pieces.

The upper area of the display shows signs of air leak.

Too bad for the scientific calculator that served me in high school and at the university. But it looks like a modern replacement costs less than 10€! And I will look for some old battery powered pocket calculator as well, possibly with LED display.

24 October 2017

Old instruments with a warm glow

With the purchase of the Philips PM6645 frequency counter I double the number of instruments sporting Nixies as display device in my lab.

Here they are, the PM2422 Multimeter and the PM6645 Counter both from Philips:

Philips PM2422 (left), Philips PM6645 (right).
I think the multimeter is slightly older than the counter, because the darker front panel looks more 1960's to me. It has a red-coated glass, while the 6645 has clear glass and clear tubes. Both devices are huge compared to today's standards, even if inside there is a lot of room. Probably back then workbench space was not an issue, maybe not an important cost in companies budgets?

Both Philips instruments sit on my oscilloscope, a Tektronix 7000 series, also from 1970's.

Why not add two more nixies in the picture with my biNixie clock?

19 October 2017

New toy: 500 MHz Philips frequency counter

At the Mercatino by ARI Biella in October 2017 my Nixie-radar noticed a frequency counter with as many as 9 tubes. It is a Philips PM6645 500 MHz device, with excellent sensitivity and 10 MHz OCXO reference. Considering that I had a 100 MHz kit-built counter, given the price and the specs I couldn't let it on the table.

At 80€ the beauty came home with me, an early Xmas present:

It mounts common ZM1005 Nixies. It is built with still commonly available parts in case something breaks

17 October 2017

Keeping jump wires in order during maintenance

I think this tip fits the "tribal knowledge" section launched at GQRP reflector and printed in SPRAT magazine.

There are some times when a lazy builder has used jump wires and pin headers to join two boards, rather than fitting proper block connectors. The situation is depicted in the picture below.

Then maintenance time comes, you don't remember neither the wiring scheme nor where the documentation is ...

So, you need to remove a bunch of jump wires but keep knowledge of their order. You have an easy option: take a picture and let them loose. Unfortunately they don't come in many colors, and some colors can be easily mixed up. And the simple idea kicks in: transfer one side of the jump wires on a free piece of pin headers so that they stay in the same color order but can be moved out of the way. The picture helps nevertheless, but order and wire bends will be preserved.

Now ... time to do some shopping for block connectors in various sizes!