Superhet Kit Radio Pt 1

[Neil Purling]

I bought this radio at the BVWS Swapmeet at Lowton on the 23rd November.
It was actually on Forum member Igranic's table at a very reasonable £5.00. So I snapped his hand off.
A quick look on the day inside showed the usual suspects for an Octal superhet: 6K8, 6K7, 6Q7 and 6V6. Thankfully there was no odd o/p valve.
the dial glass had no name. The internals resembled a Premier, but what does that mean in these sets?
Getting the chassis out of the case was a minor challenge. One of the knob grub screw resisted my efforts to free it which included using a hair dryer. all screw slots had been filled with wax, a sensible precaution with a live chassis set.
In the end I had to destroy the knob.
With the knobs off and the chassis out it was immediately obvious that someone had been there before me. The volume pot had been changed, because the shaft had clearly been shortened with a saw. The new pot had a double-pole mains switch, only one pole being used.
A good few capacitors had been changed for grey plastic Plessey replacements, only two waxed paper capacitors were present. One was the input filter, which had started to sweat wax. The other was the 6K7 cathode bypass capacitor. There were no Mouldseals, the other capacitors being like much enlarged silver mica types. The forum opinion suggested that these TCC capacitors could well be mica type. The only one I did change was That Capacitor, it was before I asked the forum and I needn't have removed it. I broke one of it's tags so it couldn't go back.

I removed the 32+32uf reservoir & smoothing electrolytic & put my re-former on to it.
I gave the case a good clean out with hot, soapy water to remove the burnt dust and waxy traces.
At the end of the day the electrolytic was still showing enough leakage to keep the indicator neon on my re-former flickering. I was suspicious of that and put in a new German F&T 32+32uf can.
At the same time I tried some long disused CCL 3 section electrolytics that took only 4 hours for all 3 sections, which shows the original electrolytic had been blitzed.
The mains lead was a well perished pigs tail of rubber insulated & cotton covered wire. I came across some 2 core in my old mans shed and re-wired the mains switch to use both poles, maybe increasing safety.
I had no way of testing any of the valves. Two were loose on their bases so I used some pink nail varnish to re-fix them & left them above a radiator overnight to set the varnish.
It was odd that the radio should need so few components replacing. At this stage I was willing to trust those TCC & Plessey capacitors.

Tune in later for the 2nd thrilling installment.

[Neil Purling]

With the German F&T electrolytic installed (checked on the reformer too), the mains cable replaced, and the 6K7 cathode bypass waxie replaced I cut out the RF filter cap.
Now was the time to show the beast some volts.
A click of the switch, the dial light lit and all the heaters were lit. So far so good. After a few short moments a hum was audible in the speaker.
I sort of expected this. I tapped the volume pot a few times with a knuckle and the hum disappeared.
I could tune a station, but even at full volume the music was just about on par with a baby's whisper.
The replaced 6V6 grid coupler had a dry joint - pillock.
I re-soldered the dry joint and put my AVO on the terminals of the o/p transformer. Only 180V of HT there.
Now what is the voltage drop across a healthy Selenium rectifier supposed to be? If we assume the mains is 240V 70 volts drop sounds excessive.
I soldered a 1N4007 and a 100R 7W resistor on a tag strip and removed the metal rectifier from the circuit entirely.
An off-load test showed 338V, which is apparently right, being 240V X 1.414.
I screwed the tag strip to a lug on the heater transformer, as you can see.

A second try showed the knuckle tap on the volume control was still necessary. At 11:30pm my ears were assaulted by Tammy Wynette singing Stand by Your Man from magic 1161. Of course the volume was at max.
I put the chassis back in the case.
I could do no more that night, I needed my bed.


The first photograph shows the chassis after my work. You can see the nice fetching shade of pink I used to re-seat the 6Q7 and 6V6.
The second shows the amendment to the power supply. If I could I would have a new rectifier stack made, but what the hell. The originality has long since been violated.
The third image shows the compact nature of the set. At the time I was experimenting with other valves for the frequency changer and IF amplifier.
I have planted a Hytron VR53 (EF39) in the place of the 6K7. The Hytron seems to have more gain than a EF39, and is only equalled by a brand new metal 6K7 I have in my spares box.
The fourth shot shows the set after I replaced the dial glass.

I do need to change the mains cable. It's a foot or so too short.
Ideally I would like some fabric covered twisted pair cable like lamp flex was. This is the kind of cable remanant that was left on the radio.

[Sideband]

If it's anything like the one I have which uses the same valve line-up, it should work very well indeed. Somewhere in 'Success Stories' I think there is a write-up of the one I did.

I also had to replace the selenium rec and I used two BY126 in series with a 500 ohm 10 watt resistor. I chose the value of the resistor to give me about 220v on the anode of the output valve not having any idea of what the HT should be but this seemed to be reasonable for a 6V6.


Rich.

[Neil Purling]

Hello Richard.
How is Long Wave on yours? That is, if it is Long Wave. I would expect a coil pack with two RF and two oscillator coils but you only have one coil under the chassis with an associated trimmer. The oscillator coil?
How can you get decent Long Wave with the one coil? I would expect a RF & Oscillator coil for each band.

[Sideband]

Actually mine is a three waveband job so it must have been a de-lux kit!

All three wavebands work extremely well. The circuit of mine is based on the Repanco SH4, some information of which can be found here:
http://vintageradio.me.uk/radconnav/valvesuperhet/ (5th one down on list)

This only shows two wavebands so maybe Premier made SW an option for some kits only.



Rich.

[Neil Purling]

Footnote:
The valve holder under the 6V6 had intermittent contacts. Despite attention from my long-nose pliers on the pins and a blast or two of Servisol Super 10 there was no improvement.
I changed the holder for a 'proper' example by Mc Murdo. You can see the original in the photograph attached.
I also tested the 470K grid resistor. It was found to be 620K so I changed it.
I also changed the cathode resistor for a 470R example. The original was marked as 220R, but red 200R on test. No wonder the 6V6 was getting hot & bothered.
Out of curiosity I fitted a 22uf 63V electrolytic as cathode bypass. There wasn't one there originally. The difference means I get pounding full volume with a EBC33 in the place of the 6Q7. The EBC33 gives less amplification, but is screened with a metallised coat.
I can revert to a 6Q7 if I lift one leg of that electrolytic, but I don't think I will.
It's playing away in the kitchen now and the budgie is singing to it. Can't hear the radio, just the budgie at the moment.

[Nickthedentist]

I suppose a possible "bodge" with that pattern of valveholder would have been to establish which contact(s) were faulty by using a bit of wire to short each valve pin to its contact in the valveholder in turn, then actually solder the valve pin(s) to the dodgy contact(s)

If you used a tiny bit of solder, it would probably be retrievable if you needed to change the valve, though I suppose it could creep by capillary action and be pretty permanent. Maybe still valid though, as if/when the valve eventually failed, you could always do what you've already done, and replace both the valve and holder.

Nick.