Before I Drive Myself Nuts!!

[Steve_P]

Have you got the Bush or Trader sheet?

Set it for the highest voltage. Our electricity is usualiy around 240v, so use the biggest tap.

With a meter across the plug, the reading should be 400 Ohms. (200 on the middle tap.) Give or take.

If you're a coward, remove the cap across the mains.

When you remove a valve (any one, doesn't matter) then the reading should go very much higher, like to Nothing. Make sure the valves are in the right places (new to you and you don't know who's been there before) and then switch on the set and plug and switch on the plug, Keep your hands on the plug switch so that you can cut the power fast if you need to.

The valves should all light and if you're lucky you should hear sound. As in speech or music, not fizzes and bangs.

Cheers,

Steve P,

[Bushbaby]

Hi Steve
Thanks for guidance and will do as you indicate. I have both the trader and service sheet. My only concern is why the lamp limiter light came on indicating a short. I'll have another look at the switch again - I replaced this and it does suggest that I haven't wired it correctly.

Regards

Rob

[Skywave]

Rob - when you say that he lamp lit up brightly, was that an initial brightness that gradually faded to a dull glow or did it stay bright? The former condition would suggest (possibly) normal surge current, but the latter definitely suggests a dead (or near dead) short, in which case I'd snip out the a.c. mains by-pass capacitor in the set and try again.

P.S. I love the thread title - I think we've all been there at some time or another!

Al.

[Andrewausfa]

Hi Rob,

I am right in thinking you've completely re-capped the whole set already, per your recent thread?

Andrew

[Bushbaby]

Hi Andrew
Yes, it's been completely recapped and retired.

Regards

Rob

[Bushbaby]

Ok,
Last post before bed!
I have checked the voltage setting - its set at 240/260, the highest setting.
I have connected the Avo across the plug and removed a valve (well several but one at a time) each time - about 200R reading, the meter needle does not move. I have checked the lamp limiter again - the light is on and doesn't dim.

Now what i have checked is another pot of the same type (not connected to the radio of course). I connected the same two terminals as I wired for the mains connections in the radio - 200R reading again. I therefore assume I've selected the wrong two terminals! 64 thousand dollar question and I think I know the correct answer - the terminal for +ive to the mains should have the highest resistance when measured to the chassis. Is this correct? Does it matter which one of the pairs is taken for mains connection??

If I've time I shall have another look tomorrow.

Regards

Rob

[Skywave]

I have checked the lamp limiter again - the light is on and doesn't dim.
Rob

Whoops! That signifies a dead - or near dead - short. I think you are getting very close to the cause with the ON / OFF switch.

Thoughts arising.
This exercise amply demonstrates the value and usefulness of a lamp limiter. A Variac wouldn't have given you such an instantaneous clue, although you could have obtained an equivalent indication using a Variac + a.c. voltmeter. But not only is the lamp limiter much less bother to use for this test, it also completely eliminates the risk of damaging an expensive Variac - and replacement 60-watt light bulbs are a lot cheaper !

Al.

[pwdrive]

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

Hi Rob, when you say "zero R" with switch off we are assumming a dead short (meter needle at FSD and not at infinity) if this is the case then it sounds like the mains switch is shorting out L & N in the off position, if that is the case then clearly something wrong there so double check the switch and it's wiring configuration, also be aware from a safety point of view of making sure it is finally wired so the chassis is not live (sometimes easy to forget when in a fluster with another fault)
If the switch and wiring and mains filter cap is then all ok then do that R check again across the mains plug using Steves figures for approximate R reading.
If all that goes ok then before connecting to the mains you need to check the HT side of the rectifier for any low readings, if anything suspicious then you will need to sort that out before applying power. When checking the HT side of the rectifier for resistance reading the meter needle should kick over then return as the Resevior/filter caps charge up. I have not got the circuit in front of me but the meter should retorn and settle and read several K ohm from HT to chassis, this value will depend if there are any bleeders (voltage dividers) in the design.
Keep at it and work methodically.

Lawrence.

[Yorkie]

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

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.

Just to be clear about how the two pole mains switch (S2 on the circuit), should be wired, see the attached diagram.

If you've wired it correctly, any problems aren't with the switch, but downstream of it. Looking at the circuit, if all was well, you have two 200 Ohm resistors (R35 & R36) in series with the heaters (assuming you have the mains tap set to 240V), so there is at least 400 Ohms, plus the total resistance of seven valve heaters in series. If - as you say - you've changed all the caps that would normally be changed, you shouldn't have any leakage to the chassis on the HT side, so if all is well, the resistance across the mains input with the switch closed, should certainly be in excess of 400 Ohms. C60 (across the mains input) should be a Class X cap, and really, you'd be better off without it as Steve has said - you won't notice any difference.

You mentioned the AVO7 Rob, and that 'you must use it more'. If you do, it will lead you a merry dance, and often give misleading results. Whilst the Avo 7 is fine for checking resistances or current, and voltage in certain circumstances, it's a most unsuitable instrument for radio and TV servicing work, because it only has an input impedance of 1,000 Ohms per Volt. (AVO 8 is 20,000 OPV). I don't want to muddy the waters of this thread, so I'll post something about that in another thread.

A nice collectors' item on the shelf, but best not used in radio work.

Hope that helps a bit Rob - best of luck with the VHF90.

Keith Wevill (AKA 'Keith's TVs') gave an excellent talk at the Golborne Swapmeet last Sunday in which he explained the effects of various multimeters in diagnostic work, and in the use of 'Scopes. He has since sent the notes of his talk to those who attended, and I've been in touch with Keith to request his permission to place the notes on the forum, which he has granted, so long as we credit him as being the author, as of course we will. Keith's talk was excellent, and the notes are very well written and illustrated, so I'll upload them shortly now that we have Keith's kind permission.

[pwdrive]

I agree David regarding the AVO 8, the industry standard and still available at reasonable cost.
Once Robs set is sorted it would pay him to do several resistance checks on the different types of receivers he may have that are working ie:

AC/DC type with thermistor fitted: resistance across mains input (typical value to be expected)
Ditto but no thermistor fitted.
AC only sets, ie mains transformer primary (typical value to be expected)
Same then for typical resistance readings from HT to Chassis:
Sets with screen grid bleeder network(s)
Ditto without.
Write typical values down for reference, after you've serviced a few you will know what to expect automatically.
Rob is clearly on the early stages of the learning curve so any practical hints will help him and others in a similar situation.
To me efficient fault diagnosis/finding is an art, but an art that is easily achhieved by first understanding the basic building blocks of the receiver, what each blocks function is and what you can expect to see test wise from each block.
It is sometimes easy to be tempted to just jump in and hope for the best, if you are lucky you might just crack the fault and be ok, a methodical approach is always best though, once you know what to expect out of each stage of the receiver then it's relatively plain sailing so far as fault finding goes, I am generalising here so no offence to the seasoned repairer/engineer.
I personally still like to diagnose and fault find down to component level as the faults arise, it's the only real way of learning, then once all up and running you can use the scatter gun method if required to increase the reliability factor of the receiver.

Lawrence.