Hello all,
I've hit a bit of a snag in the process of reviving an early 60s KB TV10, which is a late 405-only model. Thanks to Chris for scanning the manual to the library, which has been a great help.
It turns out the TV10 is a rental version of the TV15, which may account for some minor circuit discrepancies. Certainly the line output department appears different, or at least I can't reconcile what's on the set with what's shown on the circuit. I need to identify the black and red capacitor that goes from pin 8 on PL36 (V7) Like many of these caps, the label has parted company and is lost, so I have no idea of the value. The circuit appears to show pin 8 of V7 going direct to chassis...
It would be good to identify the rather tired-looking thin red resistor between pins 4 and 6 too!
Any thoughts appreciated!
Steve
That's a tricky one Steve, those electrolytics almost always lose the little band leaving you wondering.
If it were me the solution I would apply would be to put the cap on a bridge to determine its capacitance. Allowing for +/- 10% drift I would then select a cap within that range. To then cover myself I would put in a 400V, there's no way that cap was that highly rated but as It does not hurt replace it with a higher voltage component, you've go it covered.
As for the resistor I would take the same approach trouble is that resistor appears to have suffered a rather large dose of over cooking, it may well be way off its expected value.
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Pin 8 is the cathode so I would expect the capacitor to be something like 47uf and 50 volts.
There should also be a cathode resistor.
The resistor looks like a screen grid part as it goes from screen grid to an unused pin that is used as an anchor point.
Pin 8 is the cathode so I would expect the capacitor to be something like 47uf and 50 volts.
I agree, I have never come across one of those yellow & red Plessey caps rated higher than 100 volts.
Thanks for the replies chaps. Some good points there. Sadly I don't have a capacitance bridge (one to add to the shopping list).
I know these black & red caps are usually fairly low rated, so Ref's suggestion is perhaps a good place to start. Usually the printed 'wrapper' can be found floating around the bottom of the cabinet somewhere, but no such luck on this occasion! Also duly noted that the PL36 pin 6 is unused and therefore is just an anchorage point. I guess any set using a PL36 may be worth checking as to an idea of the value of the screen feed resistor.
Food for thought!
Steve
I guess any set using a PL36 may be worth checking as to an idea of the value of the screen feed resistor.
Well, the TV15 diagram you posted suggests 4700 ohms and whilst in no way implying that this is the case on your set, on my KV125 (VC51 chassis) it's 2200 ohms. I take it you've measured it?
The electrolytic also seems to have a paper cap across it, implying a feed that needs a goodly amount of decoupling.
Don't suppose you could put your AutoCAD Mark 0 (pencil and paper) skills to use and sketch out a little of the circuit as photographed?
Well now just as I had my pencil poised, a kind soul has intervened and pointed me in the direction of another KB set- the UV30. This appears to be the same chassis as that of the TV15 family, but it looks like the line output stage could just be the one used in my TV10.
I've not studied the diagram yet and probably won't get a chance to spend any time on it until the weekend, but it may well solve the mystery. I shall report back in due course...
Steve 
So other may study and help, here is the UV30 line stage
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The negative end of the electrolytic is connected to the PL36 cathode, so this looks like just a handy position to pick up HT negative for a decoupling capacitor.
The rest of the connections aren't completely clear in the photo but may be as follows:
The positive end appears to go to the centre spigot of the PY, so just another free anchoring point. It looks like the red wire feeds HT to this place and the choke feeds from there to the PY anode... making this an HT decoupler. C110 in Chris's diagram?
I wouldn't bother trying to measure its capacitance as it's probably kaput anyway.
The wax capacitor may be a heater decoupler: when suitably enlarged it looks as though it goes to pin 4 of the PY.
Pete
Looking through representative circuits of the period this morning, one thing I did note is that in all cases the cathode of the line output valve is returned directly to chassis - which considering that it is merely performing a switching action, would seem appropriate and fitted in with my general understanding of line stages.
KB actually made a convertible version of the TV10 series. Doubt if too many were ever converted to dual standard operation, at least any sets owned by ordinary people. Although I heard somewhere that a number were converted by a TV rental company.
Till Eulenspiegel.
