Ferguson 208UL Thermistor

[scubado]

Hi I have a Ferguson 208UL & need to find out the resistance in ohms of a possibly 3 Watt Carbon based resistor in order to buy a replacement. It was soldered to the outside case of the on/off control knob & the 3 'o'clock terminal on the back of the knob.

Any help or clues will be much appreciated.

Cheers, Tony.

[ukcol]

Can we have a picture Tony?

Colin M

[Nickthedentist]

Hi Tony and welcome,

As Colin implies, this sounds a bit fishy. 3W is quite a beefy resistor and it seems odd that it's connected between ground (the "outside case") and one of the terminals.

How do you know it's a 3W one?

What are the colours/numbers on it?

Does it look original, or botched-on by a dabbler?

Are you sure it's not the mains supression capacitor, which could indeed be connected to one of the terminals on the switch portion of the on/off/volume control, and ground? These tend to blow themselves apart, but can safely be removed and not replaced.

Nick.

[Herald1360]

At a guess. I'd say that resistor was a capacitor. There are no power resistors anywhere near the volume control but there is a mains filter capacitor 0.01uF from the switched live terminal of the mains switch (part of the volume control) to chassis (presumably the case of the control).

This is an instant replace part- it's directly across the mains and should either be removed altogether or replaced by a proper "X" rated part of 10nF. "X" capacitors meet current safety standards for across the mains connection, that old 1000V part doesn't. If it hasn't failed open circuit it will probably go bang at some point if left there!

There's another, even more safety related part- the 0.005uF capacitor between the aerial input and the radio circuit itself. This must be replaced by a "Y" rated part of 4.7nF. If this part is leaky and the set ends up with line connected to the chassis (perfectly possible- the set will still work just fine), then mains voltage will be directly accessible on the aerial input connector "Y" capacitors meet requirements for connection between the line and earth or parts which are accessible to touch.

It's a "short" superhet so there's no audio amplifier anode to output valve grid capacitor to worry about and the tone correction is straight across the output TX primary, so failure there won't be catastrophic, just a "won't work very well" sort of problem.

Any more questions, just ask away!

Edit: You diagnosed the culprit while I was typing, Nick!

[Vicboduk]

Here's a picture from my 208UL. I think Tony is referring to what looks like a thermistor. What is odd is that I can find no reference to it in either of the two datasheets that I have!
I think it is in parallel with the lamp (red wire). As all the neutral return current flows through the lamp this will enable the set to run should the lamp blow. Probably a later modification, I can check for you this weekend.

As stated by Nick and Herald there is indeed a mains suppression capacitor (C29 on trader sheet) mounted across the switch. Its value should be 0.01uF NOT the 0.1uF as shown in my picture (the only X rated cap I had to hand at the time)

Vic

[Nickthedentist]

Ah, I think you've cracked it, Vic!

Presumably if Tony fits the right pilot lamp (6.2V, 0.3A or thereabouts), he could omit the thermistor with no ill effects, other than the set stopping if the bulb blows.

Note that the Trader Sheet says that the lamp was omitted altogether in early production.

N.

[scubado]

Hi thanks for all your replies, I'm new to this site & can't tell a resistor from a capacitor but willing to learn. I've uploaded a pic where you can see the solder joint on the on/off knob and the remains of the solder joint to the left, and the remains of the resistor is in 3 pieces in front of the valve holder. Hope this gives more clue to what I'm after.

Tony

[ukcol]

That is (was) a thermistor. I don't have a circuit of the 208UL and the 208U circuit that I do have does not include a thermistor. Power thermistors are very difficult to source now but if someone has a circuit of the 208UL we may be able to work out a "work round" or find someone who has the right thermistor.

Colin M

[Radio Wrangler]

Picture in post number 5....

I've never seen someone voluntarily add a Rifa capacitor to anything since the 1980s!

There are some power thermistors used nowadays but they all seem to be designed for inrush current protection in SMPS and are intended to go to a very low resistance when running and aren't suitable for this application.

David

[Nickthedentist]

As above, your radio should work without the thermistor so long as the bulb that lights the dial is present and correct.

Nick.

[kalee20]

It's a thermistor. The radio will work without it - it is only there to bypass the pilot lamp and allow the radio to still work if the bulb fails.

If you don't have a bulb, you can sort-circuit the bulb or short-circuit the thermistor (same thing!) without any problem whatever, and enjoy using the radio.

This is a cracking little set, despite having no AF amplifier valve.

[Vicboduk]

Tony, your picture shows that the thermistor fitted in your set is considerably larger than fitted in mine. Mine measures as 9mm long x 3mm in diameter- which seems too small for something that may have to dissipate >2 watts. I thought I would see what happens if I disconnected the lamp. There was a drop in volume for about 1 second, it then returned to normal.
However there was a lot of smoke coming off the thermistor, it was obvious it was under stress!
I removed power quickly.

Resistance checks showed that it was around 2.6kΩ when cold and reduced to < 50Ω when hot.
I didn’t want to leave it too long to find the true value. Bear in mind that a 6.2v 0.3A lamp will have a hot resistance around 20Ω.

Vic

p.s. Re-iterating what others have said, when aligned correctly this set performs remarkably well. Definitely worth the effort to get it going again.

[scubado]

Hi all just purchased a new bulb to see what that does. Problem is I removed the valves for safe keeping & didn't make a note of their order, can you please tell me the correct order left to right looking from the rear.
Thanks for all your advice.
Tony

[Herald1360]

Not sure about looking from the rear, but V1 UCH42 next to tuning gang. Next across is V4 UY41, then V2 UAF42 then V3 UL41.

[kalee20]

Yes - as Herald1360 says. This happens to be the order you wanted, left-right, looking from the rear.

Be very careful plugging and unplugging these valves. There is a glass pip on the side, to ensure correct orientation. Many valveholders have a spring which is designed to hold the valve in place by acting on the pip, but it can also break off the pip. And UL41's in particular are in short supply.

[turretslug]

Originally Posted by Vicboduk

Tony, your picture shows that the thermistor fitted in your set is considerably larger than fitted in mine.

Size and resistance wise, that sounds somewhat like a Brimar CZ3 or similar, if so, they were rated for 0.2A, so would be a good device for this sort of application, if placed/wired appropriately. They may be titchy, but that keeps them hot, ergo low resistance, and ensures low thermal inertia.

[scubado]

Hi fitted a new bulb & everything works, although the sound is is not loud I'm more than happy that it is working.
Thanks for all your info, I even learnt what a thermistor is
Cheers
Tony

[Vicboduk]

Congratulations Tony, your first vintage repair. You are now officially off the first run of the ladder, let’s not stop there!

These sets are easy to work on and perform very well when set up and have plenty of volume with a decent aerial, 6ft or so, try this first. Your low volume could also be due components going out of tolerance or leaky, particularly capacitors. For low volume I would check the following; (reference numbers taken from manufacturers data sheet) C25, 25µF 25v cathode bypass capacitor and C20 0.02µF 160v agc decoupling. Ideally it is wise to change all wax type capacitors. (agc type).
On my set I found it was also necessary to change R1 22kΩ screen biasing and R5 1MΩ agc feedback. In the case of C25 I re-stuffed the original component with the nearest preferred value of 22µF 25v. Here’s a picture showing locations.

Vic

[robin coleman]

I too have embarked on one of these sets. Unfortunately I keep getting bulbs blowing, which did not happen when the thermistor was intact.
Is there anything I can use in its place?
Regards
Robin