Pygmy Club four band

[Mike Watterson]

Oh, and the straight telescopic whip is 16 cm on the remaining base part.
I have the plastic shaft slightly reduced on the volume control/switch and the old knob fitted.
Strong station on OC1, maybe Radio China. The signal generator feeds at that point on scale at 13MHz approx and 12MHz approx. Or was it 12 MHz and 11 MHz?

Next a full alignment on all four bands, once I decide which is the correct tuning point on SW. I decided the pointer must line at edge of scale with tuner fully meshed.

[Mike Watterson]

No alignment yet. I got distracted by hum on the Advance Signal Generator and decided to fix two other faults on it while it was open. It's a pig to open.
One of the HT caps was replaced due to being leaking when I got it. Now the second has dried out. Audio socket was dead due to wire off. Added FET (J310) buffer on the "Full" RF out socket had died. I found that connecting anything to that, like a counter, caused distortion. So I'd added a voltage follower powered off the 6.3V AC via an onboard diode & two caps. Soldered between "full" RF out and the socket on the panel. Now ready to screw it together again.

[Mike Watterson]

Alignment of a sorts done. The highest band, OC1, is about 38m to 20m and is roughly correct now. Basically there is just the LO coil and the RF coil. 1st and last positions. The OC2 band depends on switching a fixed capacitor across those, hence the decision to solder low value ceramic disc capacitors across the backs of three and replace one entirely.
The only adjustments on MW & LW, apart from the two trimmers shared by all bands, is the ferrite rod coil positions and the LO coils for each.

I first rechecked the four IFT coils now that the "Full" RF out is working on the Advance signal generator. So the counter was connected. Either you short out AGC line or use the minimum signal possible. The rewound detector IF coil does clearly peak on a minimal signal. IF injected by 3 turns on ferrite and MW selected. In general the detector stage is normally less sharp due to the loading on it on any domestic set.

There are no individual LO and RF HF trimmer caps per band. So it's possible that the two trimmers on the tuner are just because it comes with them and that maybe they are supposed to be fully tight?

Next just the replacement whip and possibly polish out the abraded areas on the surface of the scale.

[Mike Watterson]

Conclusions:
The DK96 oscillation failing under certain conditions like touching  or extending an aerial seems to be related to condition. The original didn't work well on OC2 and not at all on OC1. A replacement DK96 didn't exhibit the aerial issue, in fact MW & LW receptiion was louder with a whip or touching the wire to the whip.

A beginner mistake not checking IFT coli continuity. That would have saved a lot of time.

Be suspicious of any component that's obviously resoldered, and  area where any leads  seem inexplicably snapped (or cut?). Be very suspicious of any part with far too long / original length leads and where one end goes to a place with lots of tags. One end of 33K HT feed resistor was to wrong tag, would only work well on MW.

Alignment
IF is first. Keep reducing the signal so AGC is at max gain. AGC action will make it hard to see correct peaks. IF was stamped on the IFT bases, but it can be anything from 128 kHz to 1700 kHz. Mixing up 455, 465, 470 etc will usually make tracking poorer. Below 455 KHz and above 480 kHz is very rare on domestic sets between 1937 and today. Always leave FM till after AM.
The first thing is to make sure tuning capacitor isn't faulty and you know where the mechanical alignment of the pointer is. Usually the lowest frequency, or a special mark at that end of the scale. Many tuning caps increase slightly in capacitance as fully opened due to rotor edges coming close to stator. One solution on LW/MW, if in doubt, is to put tuning to an empty part of the band on LW (or MW, only one) with rotor a bit past half closed and tune the LO coil for maximum noise with the two main trimmers about 1/4 turn open as the LW & MW RF tuning in the lower frequency range is little affected by the trimmers and RF tuning is usually set by coil position on the ferrite rod. Then move the scale pointer to that frequency!

Iteratively repeat adjusting RF & LO trimers at HF end (about 260 KHz or 1400 kHz) and recheck lower end peak RF, no lower than 600kHz, or the LW scale equivalent, but only on MW. Or if there are now LW trimmers and LW is more important, then LW only. Doing this resulted in loud R4 LW and louder RTE LW. Then the pointer did seem to line up to a logical point on the scale fully closed. It's MUCH easier with alignment instructions. Never align RF & LO coils and trimmers at the ends of the scale. See actual MW alignment instructions and around 1400 kHz to 1500 kHz and about 600 kHz are typical.

If there are bands created by switching fixed parts with no trimmers or separate coils then don't align them. Check switched in capacitors if madly out.

Other thoughts
Use a shunt regulator (two suitable rectifiers for 1.3V to 1.5V) on the LT PSU so any inadvertent short doesn't blow the filaments. On this set the B7G power connector had corroded pins, sockets and wires. I had to cut back all four wires to reliably resolder them. Connect LT first and disconnect last. Check voltage on LT- to HT- bias resistor from time to time (it's 180 Ohms to 1000 Ohms depending on O/P valve type and HT. Series filament sets don't need it as the O/P valve f+ is usually at LT+ and the DAF96/DAF91/1U5 etc is usually f- to LT- = 0V. On series sets the DAF91/1U5/DAF96 etc should be 1.3V to 1.35V, also that's a reasonable range for external LT on a parallel set. Similarly, the 6.3V valves are absolutely fine at 6.0V AC, in fact an accurate 6.0V is better than a poor 6.3V that's really 7V. The actual range is about 5.75 to 7V. Life about halves with every 10% increase from 1.35V or 6V.

Testing Frequency Changers
Even an AVO VCM163 is poor for Heptodes, Octodes and Nonodes as well as Triode-Pentodes and Triode-Hexodes. You can only do a basic emission test, the gain tests are only of significance for the Triodes or treating g1 to g2 as as triode on the single valve types. You need either a known good valve or a known good radio.

[Murphyv310]

Hi.
It's well known that valve testers don't simulate real life situations. Manufacturers would often say in manuals to substitute a new valve if there was doubt and of course for good reasons.
Even after selling on many valves recently I've kept all my battery valves, new and used. Over the years I've found huge variations between new ones especially the DK versions. Performance varies greatly especially those sets with short waves. A Zenith transoceananic I had used a very rare US F/C that would work after a fashion with a DK91 or 92, I can't remember what one but even the one that did work you had to try many before it worked best. In fact I later discovered a DK32 was the best valve, fortunately the chassis was already drilled for an octal, my set came with an adaptor plate and B7G base so was easily reversed.

[Mike Watterson]

The 1L6 is similar performance to the DL92, and a DL92 will work, except the capacitances are different, so not a plug in replacement for the shorter bands. Fine on MW. Many USA Broadcast only sets also used the 1L6, it's not rare. The problem is Transoceanic collectors hoarding them, thus the price is high.
The DK40 is very good and can be used in earlier TO before they changed to the 1L6 B7G. It was used instead of the 1R5 / DK91 for SW till the DK92 came out, so you'll see it in European / UK sets that are otherwise B7G. The DK96 may not even be as good as the DK91/1R5 for SW, hence SW sets using the DK92 up till maybe 1958 in the UK & Europe.

I've figured that sawing off the bottom of a military B7G pin protector is good to make a B7G plug. I have a load of them. My wire is a little thin for the pins, so I'll tin it with the horrible solder I got by accident.
The 1j24b seems to have very low capacitances, so since two work as a DK91 / DK96, I will try them on the Pygmy on SW and then add parallel resistor for 50mA and small capacitors to create a plug replacement 1L6. I have two models of Transoceanic, one RCA clone and one Hallicrafters clone and only one good 1L6

I have a McMichael 1949 2V set. Neither KK32 nor an adapted DK92 works on the shortest SW. The other 3 bands are fine. I'll try the dual 1j24b in that too, even though technically it's a separate oscillator and Pentode mixer and not a pentagrid. Most circuits work with either as the pentagrid/heptode/octode/nonode g2 is a sort of anode.