Before I Drive Myself Nuts!!

[Joe]

I've just checked mine. With an ohmmeter across the plug it reads infinity when switched off and 4.7kΩ switched on.

In THIS case an AVO 7 is the best thing to use as that's what the measurements quoted on both the Trader and Bush sheets were taken with.

- Joe

[pwdrive]

Sounds about right.

Lawrence.

[Yorkie]

In THIS case an AVO 7 is the best thing to use as that's what the measurements quoted on both the Trader and Bush sheets were taken with.

- Joe

So it was Joe, so Rob’s choice of the AVO 7 was the right thing to do in this instance! A bit puzzling that they used an AVO 7 in 1956 when the set came onto the market.

I guess that on the 10V range, when checking the cathode voltages on say V2/V3/V4 (1.3V - 1.4V) putting the AVO7 on the 10V range across the 150R resistor wouldn't have much effect, in that it would parallel 10K with 150R and would only reduce the 150R to 147.78R – a reduction of only 1.5%, and in any event, the resistor is unlikely to be spot on 150R anyway. But were the AVO7 on the 10V range to be placed across a 1 Meg resistor in a potential divider, it would reduce the overall resistance at that point to just 9,900 Ohms with drastic effect on the reading. The important thing is to know what the effects are in any given situation and whether they’re likely to be negligible or significant. In the case of Rob’s VHF90, then clearly, when the set is working, the AVO7 should give results for current and voltage similar to those on the data sheet.

Interestingly, the Trader Sheet (1293) says that for alignment purpose, either an AVO8 or a Valve Voltmeter should be used.

So does the maker’s sheet for VHF alignment, so it pensions off the AVO7! Seems a bit inconsistent.

None of which is of the slightest help to Rob in his trials and tribulations.

[pwdrive]

I might have posted this before, appologies if I have, having a bad day so far.
The link is for the CZ1, CZ2 etc range of commonly encountered thermistors and their characteristics as used in many radios/tvs etc as heater and HT surge limiters. I believe the CZ1 is used in the Bush VHF90?

http://www.domino405.co.uk/documents/Brimistors.pdf

Lawrence.

[Bushbaby]

Firstly Rob, when you say 'zero Ohms' do you actually mean just that? IE, do you get the same reading as you would if you shorted the test prods together, indicating a dead short (needle of the meter full scale deflection to the RH side? If so, that indicates a dead short, and yes, you've wired the switch wrong. Or do you mean you get 'no reading' - IE, the needle doersn't move at all, in which case that isn't 'zero Ohms' but infitinty - an open circuit in other words.

I must have been tired last night when I was doing this! The Avo was a model 8 and it was reading at the LHS - which I assume to be infinity and not zero! Therefore, David you would be correct and this would be open circuitwith the switch in the off position and 200R when switched on. If so, have I still wired the switch incorrectly?

Regards

Rob

[pwdrive]

Hi Rob, following your latest post it sounds as though the switch is ok but double check to be sure including L & N polarity as regards live chassis avoidance.
The 200 R reading sounds low if the CZ1 thermistor is fitted in theseries heater chain, the specs for this thermistor can be found in the link I posted earlier.
You will need to re confirm that 200R reading and investigate before proceeding any further, the mains filter capacitor would be the first check followed by a good visual inspection of the wiring asscotiated with that part of the circuit including unintended blobs of solder rattling around, once sorted then you can turn your attention to resistance readings across the HT supply. Checking the mains side of things and the HT side of things by resistance checks is always the way to go before applying any power whatsoever. Note: I have also posted a link to an article on variacs, if you have not already done so it would pay you fuse the output with a suitably rated fuse, the input should already be fuse protected, read the article and it will become apparent. Your doin' ok Rob.

Lawrence.

[Steve_P]

Chop out the mains filter cap, C60 on the Bush Sheet, C66 on the trader sheet. Remove V7 (UY85), Don't plug the set in but turn it on at the switch. It should read nothing across the plug pins.

If it still reads 200 Ohms, then it's either the switch or the mains setting that's wired wrong or faulty.

Check for blobs of solder or short circuits too.

There should be no reading at all with V7 disconnected. Now put V7 back and remove the valves and check the resistance across the mains plug and then put the valve back before moving on to the next one. Do them in this order....

UL84
There are 2 UF80's - remove the one nearest the UABC80.
The other UF80.
UCH81.
UCC85.
UABC80.

If you find that one valve, when removed does not cause the reading to drop to 0, then check the capacitor before it on the chain and the valve before it.

If you get through this without any trouble, try giving it some mains.

I tend to just give it mains and see what goes bang, but that's just me.

Cheers,

Steve P.

[Mark R]

I tend to just give it mains and see what goes bang, but that's just me.

Cheers,

Steve P.

Ah yes the Sir Sydney way, works for me every time. It gives a starting point, and wakes you up!

cheers Mark

[Bushbaby]

Guys,
I think I've finally lost it!!
Just to recap (excuse the poor pun) I have replaced C51 on the trader sheet with a suppressor cap - correct or not. C60 on the trader sheet is the 0.5uF silver picopack type cap - I did think that this was an electrolytic and to be honest I haven't taken any notice of it whatsoever!

Help, I feel I'm slowly drowning in treacle!!

Rob

[Tedzed1]

Try this Rob.