19-08-2018, 03:21 PM (This post was last modified: 19-08-2018, 03:28 PM by Crackle.)
I have been using this layout drawing which identifies EBC on the transistors. It doesn't seem to be wrong, comparing it to the schematic.
The layout drawing is as seen from the copper side of the board.
I would check those low value electrolytics. The Capcon, Kapco, Taicon and Aloha low value ones are nearly all bad by now. The Nippon ones are always nearly ok.
Danndad
Ooh! What a nasty combination of NPN and PNP transistors!
It's doing my head in going back and forth from the circuit to the text!
The VDR is part of the bias potential divider for both types, which is distinctly odd.
Your voltages bear no relationship to the few shown on the circuit. Have you checked the voltage on the -7V line at point A WRT to ground?
There is definitely something very wrong there as the emitter of T304 is connected directly to ground (positive) yet you measure the voltage WRT the negative line as 0V!
Obviously, if there was a short across the supply like that, all the voltages would be zero!
Have you correctly identified Point A, Mike?
Perhaps, as a sanity check it would be useful to have a fresh set of readings WRT to ground.
If the VDR is o/c it will definitely screw up the bias supplies for all of these stages - some data on the VDR would be useful.
19-08-2018, 07:23 PM (This post was last modified: 19-08-2018, 07:28 PM by Crackle.)
Hi Terry
That is exactly what I have done. The way it has been done on the circuit is very confusing, and I believe may even be wrong.
Can you reconcile the official voltages with what I have marked up on this working ITT Colt chassis, which is an identical chassis to the R135 (apart from the dial mounting method)
Schematic showing voltages from a working circuit are marked in green. All measured with +ve probe connected to the +ve ground.
I will start to compare them with the non working circuit when my back is feeling less tired.
thanks
Mike
p.s. I started to check some of the resistors in the R135 and most of the ones I checked are high. e.g. R303 was 950 ohms
(19-08-2018, 05:45 AM)Crackle Wrote: If it is a VDR they are supposed to be used for high transient voltages like lightning.
Not necessarily, Mike. Foe example, they are often found on the earthy side of the brightness control on TVs. They allow the brightness to rise at switch-off to discharge the EHT quickly and prevent screen burn.
I can't find any data for this particular VDR but I have found one with a clamp voltage as low as 14V.
I've no idea just how low the clamp voltage can be, though.
If it's a voltage-dependant resistor, it's possible that it's being used to hold the 1.5V bias point at a more consistent voltage as the battery wears down.
I also wonder if it's a thermistor? Perhaps to hold the bias constant in the face of ambient temperature changes? Probably less likely...
It reminds me of the funny little voltage regulator in some Tandberg sets that is usually playing up by this time. Those are 2 terminal shunt regulators, and can be replaced by Zeners or a string of normal diodes. Not the same as this, but reminiscent nonetheless...
Anyway, here's what I'd be tempted to do:
Temporarily connect up a variable resistance - an old pot will do if you don't have a decade box. Adjust it until there is 1.5V at the point indicated. Measure the value of resistance needed, and substitute the nearest preferred value resistor. OK, it's not ideal, but until more data about this mystery component comes to light, it'll get the set working, hopefully.
PS: I wouldn't worry about carbon-comp resistors being high. Certainly, I wouldn't replace them unless I was sure that they were definitely causing a problem. I note that they've fitted a few carbon-film resistors here and there - presumably these were more critical to the operation of the circuit, and these are likely to still be comfortably in spec after all this time.
Hi Lawrence
I dont think the problem is with the VDR, my question regarding it was more of a learning thing.
I had my meter probe soldered to the wrong spot, there is actually 1.4v across the VDR.
Sorry if that was a red herring.
But I am now comparing with the voltages measured with regard to +ve ground taken from a working ITT Colt, and getting more frustrated every second.
I am going to replace T305, its a silicon NPN, as I am getting a 0 volt reading on its collector, and there should be about .7v there.
I have checked and replaced R319, it was about 90 ohms high. There is approx 560 ohms between its collector and ground, so the IF primaries seem OK.