Premier Midget FM Oddity

[cathoderay57]

The metal rectifier is toast. Applying mains to one end (off-load) produced 227v AC at the other and 9v dc. I have removed it and will fit a 1N4007. J

[cathoderay57]

I've fitted a 1N4007 silicon diode in place of the metal rectifier, with a 100R surge limiter in series together with a fuse. I've replaced the ECL82 triode input (0.1 uF) and output (0.01uF) coupling capacitors since both were leaky Hunts components. The electrolytics were all OK and the 16+16 uF HT smoothers reformed OK with leakage less than 1 mA. At first I tried powering up with a 100mA HT fuse but it ruptured on the power-up surge. I replaced it with a 300mA fuse, which is the next higher value that I had to hand. After the set had warmed up I got background hiss. A mild 100Hz hum was also present even with the volume control at zero. I think 16+16uF smoothers are unlikely to be sufficient for a half-wave supply so I will probably replace them with 47+47uF caps to reduce the background hum. On-load, HT at the rectifier cathode is 284v and smoothed HT 172v. I stuck a length of wire into the aerial socket and was rewarded by BBC R1, Radio Wales, R3 and R4. These appear 180 degrees out of phase with the dial, since R1 appears at the LF end. Whether the constructor marked the dial the wrong way around, or something else is amiss remains to be investigated. Performance (while it lasted) was strong and loud considering it is only a makeshift aerial. I tuned up and down the dial again and while the background hiss remained, the signals disappeared but at one point I could just get R1. I'll have another read of the FMT1 manual and have a think about what to do next. J

[cathoderay57]

A few more pics, including the underside. I prised open the tabs retaining the mains switch onto the volume control and cleaned the contacts with switch cleaner, which fixed it. I've reconnected this double-pole switch and removed the toggle switch. The tuning caps are fully meshed when the dial is at the HF end of the VHF band, which is clearly wrong and explains the reversal of stations that I mentioned earlier (I wasn't going nuts after all). The most obvious explanation is that the dial cord has been threaded the wrong way around the tuning capacitor drum. This should be an easy fix. Shutting up shop for today. J

[Neil Purling]

That is a massive heater transformer. 6.3V @ 2.5 amps. Do you know the actual current drawn by the heater chain yet?
I never thought I would see one of these cases housing a FM radio.

[Sideband]

That's a well-made chassis. Someone went to a lot of trouble to build that. It looks like they used the Jason layout for the FM part and then added the power supply and output stage.

With my own Jason, it has a separate power supply to which I want to add an output stage. I have several ideas but an ECL82 seems the most practical. I toyed with the idea of push-pull ECL80 just for fun but I think I'll have instability problems.....

[cathoderay57]

Neil, heater current measures a shade under 2A AC which is about right for 4 x EF91 plus one ECL82 in parallel. Looks like there's just enough headroom for a scale lamp, although there isn't one fitted. The ECL82 produces plenty of volume so seems like a good choice. I sorted the tuning scale inversion by simply slackening the locking screws then rotating the tuning drum on its spindle (the TC has no stop so it rotates continuously until the pointer reaches the end of the scale). The only problem I had with it was crackling and loss of signal when rotating the tuning, which was solved by cleaning the TC shaft earthing strap and the screwthread on the oscillator beehive trimmer. I've printed a better tuning scale (the original markings were scratched into the paxolin panel) and I'll post some photos of it inside a wooden Premier Midget cabinet when I've cleaned it up. I've kept the white bakelite cabinet for an original AM Midget superhet. J

[cathoderay57]

Here are some pics. I decided to transplant the innards as-found and put the homebrew FM set into the wooden cabinet. Reason was the cream bakelite cabinet was just about perfect so I wanted to put the AM superhet chassis into that to make a reasonably original set. The superhet isn't finished yet because I'm waiting for knobs and an OPT rewind or replacement. The eagle-eyed will spot that the output valve in the superhet has a top cap. While it is supposed to have a 6V6 it arrived with an EL32 installed. Not sure if this was its original valve since the Premier construction notes say that certain component changes were made to their kits depending on availability. Cheers, J

[G6Tanuki]

The EL32 and 6V6 were often interchanged, the WWII PCR receivers were originally specified with the EL32 but most ones had a 6V6. The bushed hole in the chassis for the EL32 grid wire to the top cap remained even though it was unused.

BTW the IF cans in your AM radio are the same type as used in Military radios like the PCR, WS18 and WS19, I guess Premier got another bunch of surplus bits.

[Sideband]

Originally Posted by cathoderay57

On-load, HT at the rectifier cathode is 284v and smoothed HT 172v.

I think Jason quote 180 - 200V so you are in the right area. That's for the tuner at any rate. You could probably play about with the HT arrangements for the ECL82 and run that at a slightly higher HT if the output is a bit low.

[cathoderay57]

Thanks for the info. I guess in the days when these kits were on the market, more than a few people had the skills to put them together with a reasonable expectation of success. The Jason kits were sold with "pre-aligned" coils. Probably it was the same era when the keener constructors were building TV sets using government surplus green CRTs etc. Since I used to be in the RAF, the late Gerry Wells used to call me "government surplus". I miss his sense of humour.... J