Before I Drive Myself Nuts!!

[Bushbaby]

Hi All,
Back on the VHF90 (yawn!!).

All wired up, valves in, connected to variac. Start with 25v across the Variac terminals to the switch (off) 25v measured across the mains leads from the switch to the variac. I've turned the radio switch on and then measures 6v at variac terminals and 6v across the switch. Have I got a problem (short) if so, have I wired the switch correctly??

What next?

Regards

Rob

[Mark R]

Bin the variac, check the wiring for the mains and HT, then do the Sir Sydney method, it can only go bang!

cheers Mark

[pwdrive]

Depends what the variac is rated at, unplug radio switch on and measure the resistance across mains lead with an analogue meter, if it's an AC/DC set it should measure approx the sum of the heater resistances + the dropper, if that reads ok then make sure a correctly rated fuse is fitted and monitor the expected current consumption with an analogue meter while slowly advancing the variac.
My own personal way is to publish and be damned ie: plug it in and switch it on at full bore, if anything smokes it will take you straight to the area of the problem.

Lawrence. (Edit) Whoops Mark beat me to it..

[Joe]

Same here.

- Joe

[Bushbaby]

Thanks for the advice, Stupid question, what will be the sum of heater resistances + dropper. Theres a learning exercise for me here, how can I calculate it for myself?

Thanks

Rob

[Yorkie]

Hi All,
Back on the VHF90 (yawn!!).

All wired up, valves in, connected to variac. Start with 25v across the Variac terminals to the switch (off) 25v measured across the mains leads from the switch to the variac. I've turned the radio switch on and then measures 6v at variac terminals and 6v across the switch. Have I got a problem (short) if so, have I wired the switch correctly??

What next?

Regards

Rob

Don't need a variac Rob - I know they have their advocates but I'm not sure when or why this variac malarkey took a hold. About the only reason I can think of for which I'd consider using a variac would be to reform smoothing caps by bringing the voltage up slowly over time, but I've got a cap reformer anyway, and personally, I'd only do that with the valves removed and with the variac connected across the caps - not via the mains lead (which would be useless in a set with valves removed as there'd be no power from the rectifier to the smoothing caps).

I'm not even sure that valves take kindly to being run at below their correct voltage via a variac.

Better I think to run the radio through a lamp limiter - if there are any problems, the bulb lights up - not the set. If the bulb doesn't light or glows only dimly, and the set seems to work OK, switch the mains directly to the set, by-passing the lamp limiter, at which point the volume might increase slightly.

If the set works OK, and you want some reassurance that all is as it should be, you can put a meter on the DC voltage range across the resistor between the smoothing/reservoir caps to see what voltage is being dropped, then if you divide the resistance of that resistor into that voltage, you'll reveal what HT current is being drawn by the set, (Ohm's Law: I = V/R), and whether it is what you'd expect it to be. Then you can carry out voltage checks compared to the circuit to see if they look about right.

Good luck with it.

[Bushbaby]

Hi All,
I've taken David advice and put the radio on my lamp limiter. The lamp immediately lit up brightly. Does this suggest that I've wired the switch up incorrectly or is the problem likely to be further on?

Thanks

Rob

[pwdrive]

Thanks for the advice, Stupid question, what will be the sum of heater resistances + dropper. Theres a learning exercise for me here, how can I calculate it for myself?

Thanks

Rob

Hi Rob, no worries, with set unpluged switch on and give the forum the resistance reading across the mains plug pins L & N and also from the resevior cap (rect. cathode) to chassis, use an analogue meter. When doing the latter measurement you need the meter lead with the -Ve battery voltage on it connected to chassis and the +ve voltage to the resevior. Note pos and neg meter test leads don't always mean pos and neg voltage on them when switched to ohms on some meters.

Lawrence.

[Bushbaby]

Ok,
Using an Avo 7 (I must use this more so please excuse me if I am reading this incorrectly - lesson for me here) with the AC set to resistance and DC set to ohms, across the mains leads is zero R (switch off) and a shade under 200R (needle to the rhs of meter) with the switch on. From the body of the dual cap (I assume you mean the reservoir cap) the resistance is also just under 200R. Dose this suggest that I have a short to earth somewhere on the ht side?

Thanks

Rob

[Joe]

If it's reading zero ohms when switched off the only things that can be shorting are the plug, flex or switch. I can't see it being the first two...

- Joe