Decoupler test tool

[Terry]

A reference in a recent thread about replacing a screen decoupler in a TV IF stage reminded me of this, which I first wrote up many years ago. I don't know if this is the best place for it but I can't find a 'Handy Hints and Tips' forum. Mods, please move it if you think it would be better elsewhere.

Small decouplers in TV and FM IF strips of valved equipment are awkward to eliminate from suspicion although all that is usually needed is to bridge each one with a known good one. But trying to hold a small component in place when there are two wires about 10mm - and 200 volts! - apart can be tricky, particularly when you need to use the shortest possible wires. I made up a little tool for checking these little beasts many years ago and it paid dividends over the years.

All you need is a scrap of wood (mine was a glue block from an old cabinet), two panel pins and a 0.001µF ceramic capacitor - tubular for preference.

Knock the pins into the end of the wood block - the spacing should correspond to the PCB spacing* - you might have to need to place them on the diagonal, dependant on the size of your block. Cut off the heads with cutters to leave two points about  about 12mm long, then solder the cap between the two pins. Now you can shunt with immunity to shock!

* Of course, it works just as well in a wired chassis!

   

[pwdrive]

I used to use an old tooth brush handle with a 1,000pF back in the '60's, a must for some of the Murphy's.

Lawrence.

[raylec]

Good idea but you are only checking for lost capacity. When capacitors fail its the dielectric that breaks down, so you should check for leakage first, this means disconnecting one end from the circuit it is in. I have also found that a leaky cap will read low capacity, so your test may be trying to tell you that the cap is leaky

[pwdrive]

The one I used back in the day on TV's was called an "instability probe" a handy aid sometimes.

Lawrence.

[Terry]

Good idea but you are only checking for lost capacity. When capacitors fail its the dielectric that breaks down, so you should check for leakage first, this means disconnecting one end from the circuit it is in. I have also found that a leaky cap will read low capacity, so your test may be trying to tell you that the cap is leaky

My experience of the tubular ceramic decoupling capacitors used in most vintage TVs is that most go open circuit, causing the IF stages to go unstable. However, such instances were isolated and I never recall having to replace more than one in any set I repaired, thus it was not worth the time and money involved in carrying out a blanket replacement programme on any set, particularly if it was hand wired - much more efficient if you could accurately track down the culprit rapidly, which this tool does.

Usually a shorted ceramic would be self evident on sight if there was any blackening of the ceramic coating or, if the HT feed to it was fed through a series resistor (as was normal) the charred resistor would point the way. Voltage checks would pin point a leaky capacitor.

This tool is purely and simply for tracking down a specific known fault after other obvious alternatives have been checked.

[Murphyv310]

Hi.
I'd agree with terry. The small values go low or even O/C. They rarely go measurably leaky.
Bigger values tend to increase in value and I have seen motor run caps go wildly up in value, OK as cap droppers though as they are under less stress.

[Alistair D]

A simple way of measuring leakage in screen decouplers would be to run the set with the valve in question removed then measure the voltage drop across the screen feed resistor.

The main problem with the idea is that on sets with a series heater chain a suitable resistor would have to be connected across the heater pins to keep the rectifier generating the HT supply.

I like the philosophy of testing components without removing them first. Not only does it save time and cost it helps keep originality and neatness.

Al

[Refugee]

On a series heater circuit you can take the rectifier out and add a 1N4007 and a 100k 2W resistor in place of the rectifier.
Then just run through the entire set with a DVM.
The base of almost any duff valve can be made into a tool to do this.
The diode and resistor can be fed from an isolating transformer if it is an AC set. Without heaters even the valves can be left in place in both cases.

[Murphyv310]

Being fair there in no reason why the heater chain shouldn't be broken for this test. The proviso is to reduce the HT with a variac and in the case of a PY33 or other valve rectifier adding a temporary 1N4007 or external HT supply. Keep the HT at the sets nominal voltage to reduce any stress on capacitors.
It really is a bit of faffing around TBH as so many 60 year old caps are well past their use by date anyway, and should be replaced.

[ppppenguin]

Bigger values tend to increase in value and I have seen motor run caps go wildly up in value, OK as cap droppers though as they are under less stress.

Any idea for a mechanism for capacitance rising? The usual reason for apparent rise in C is a side effect of instruments that use charge/discharge to measure C. Such as the Peak ones and anything else that isn't a bridge. These will interpret leakage as increased C.