VHF90's and me!

[Yorkie]

To get the set working if you wish to carry out tests pending getting a new pot Rob, you could always bridge the contacts either with croc clip leads or solder a link across each pole, making sure of course that you don't bridge the mains but wire each pole straight through. You can then switch the set on and off via the mains supply lead. Obviously only a temporary fix to enable you to do any diagnostics and voltage tests.

Towards the end of their active life, sets which had L, M & VHF were often only used on VHF, and when the VHF tuner valves (not noted for longevity) stopped working, the sets were pensioned off as a new transistorised set would - by the late 50s/early 60s - be cheaper than a repair/new valve. Hence, when such sets later resurface for a new lease of life by the likes of us, it's not uncommon for them to work OK on L & M bands, but be dead on VHF. If it's a lucky day, a new VHF tuner valve (ECC85 from memory?) will bring it back to life on VHF.

Who knows - in this case it may have been a faulty mains switch that caused the set to go dead and be put out to grass.

Glad to hear you are getting some respite and relaxation from long hours and the daily grind!

[pwdrive]

Hi Rob you were lucky, if you were measuring anything but voltage and current with the mains switched on to the set.

If that was a mistake and no harm was done that's fine but you need to get into a drill/routine when ascertaining the state of any mains powerd receiver etc, if not sure about anything always unplug from the mains and have a re think.
Reading ohms on the mains in etc with a meter that doesn't show any sign of stress etc could give you a false sense of security ie: my meter hasn't gone puff so the radio must not be plugged into the mains, you make a return path via your hands while poking around and ouch or worse.
If a radio of unknown quantity appears dead then the first check is to check continuity with a meter with the mains plug unplugged.

Good luck.

Lawrence.

[tiv48]

hi david did you mean ucc85?
rob t

[Yorkie]

Hi Rob,

Yes indeed. Both twin triodes, both B9A, but the ECC83 has 12.6/6.3V heaters, the UCC85 as used in the VHF90 is 26V.

[Bushbaby]

Hi All,
New switch has been wired in. I started using the lamp limiter, no glow at all. Gave it full juice and no dial lights BUT the radio has some life as I can see the valves alight (albeit a bit dim) and I can hear some crackling at the speaker at volume so its got some life there.

Any suggestions where to look next?

Many thanks

Rob

[tiv48]

hi rob can you confirm its vhf 90 and not a vhf 90A
cheers rob t

[Yorkie]

Are the bulbs ok - have you tested them Rob? (The set will work just as well without them as there's a shunt resistor in parallel with the bulbs).

If the valves are alight that means that the heater chain is intact.

If there is no glow from the lamp limiter that's a good sign.

Have you checked the HT or any other voltages per the circuit diagram? As I said earlier, the excellent 10-page Bush circuit has several key points at which the voltages are stated. If the voltages are low (as they will be to an extent with the lamp limiter in series), as the lamp isn't glowing, it should be safe to switch the lamp out of circuit so that the full mains voltage is applied to the set.

You can check if the audio stage is working by applying your signal probe to the centre pin of the volume control to see if you get a tone at the speaker. If not, there is a problem in the audio stage. Using your RF probe apply it to the centre pin of the volume control and see if you get any tunable stations on any bands. If yes on AM but not FM, problem in the FM tuner stage. If no signals, can you substitute the valves with a working VHF90 to see if the valves are all OK? EG, if the frequency changer valve (mixer/oscillator), even if the IF and audio stages are fine, you won't get any signals.

You can work your way to the front end of the set with the signal tracer RF probe to see if you have any sigs at the IF stages output and input.

You can check if the oscillator stage is working by putting another (working) set alongside the one you're working on, and if you 'rock' the tuning of the set under test either side of the same point on the dial, you should hear the oscillator beating with that in the good set.

Just a few pointers which might help Rob.

[tiv48]

if its a vhf 90 try a finger tip to the centre tag of the volume control and see if there is a loud hum.
the lack of dial lamps will be the bulbs them selfs or bad conections to the holders .the bulbs are in connected to the chassis at one end and hopefully neutral mains at the other shunted by a 38 ohm resistor C66 going short will burn out the bulbs and the resistor.

rob t

[Bushbaby]

Hi David & Rob,
Thanks for the advice which I shall try tomorrow if I am back home early enough.

Regards

Rob

[Bushbaby]

Hi All,
I'm back working on this set again after a long absence. At the moment, the heater chain is intact as it lights up and I can see that all of the valves are working. However, I have no signal (still) all I can hear is a slight hum (increases and decreases with volume) and a small click as I switch between mw and vhf (aerials connected of course).

Its time to use the signal injector but with my limited knowledge this one has me stumped.

Regards

Rob