RGD R135 3 band radio

[Crackle]

I have added the voltage readings from the RGD R135, in blue, to the schematic alongside the green voltage readings from the working ITT Colt.
   
I still have not found the problem or problems causing the radio to be effectively dead on all bands.
I have changed T303 and T305.
I noticed that C369 on the collector circuit of T304 was not fitted, I do not have a 75pf cap so temporarily removed C369 from the ITT circuit board, I can confirm the ITT still worked without the cap and fitting it to the RGD made no difference.
I have also changed a few resistors; R319 was 540R, R307 was 110R, R334 was 260R, R303 was 954R. These all made slight differences to the voltages but none resulted in a working radio.
I am beginning to wonder if there are shorts in the IF cans. I will have to check to see which of my meters has the best low resistance resolution and check them against the ITT.

All suggestions would be most welcome.

Thanks
Mike

[Refugee]

T305 readings are showing short circuit B-E.
You should be able to measure it on a low ohms range with the set switched off.

[Terry]

T305 readings are showing short circuit B-E.
You should be able to measure it on a low ohms range with the set switched off.

No, Mike has already replaced T 305 and there is no external component to cause such a fault.

Mike, your short circuit IFT theory is a red herring for two reasons:

1. If the IFT was s/c, there would still be a voltage drop across R322, so the collector voltage would still be approximately normal;

2. A silicon transistor needs ~600 - 700mV between base and emitter to turn it on but you only have 100mA, so the transistor is turned off. Thus there is no voltage drop across R322!

Try connecting your meter directly across the base-emitter junction to see that the 100mV figure still exists and is not a myth created from the rounding of the individual readings.

The chances of a PCB powered by a 6V battery breaking down is, I think, highly improbable but a resistance check across the junction will rule this out (assuming there is nothing physical bridging the tracks as you would have surely found that when changing the resistor.

Final thought for now: is C329 leaky and, thus, pulling the bias voltage on T305 base high?

No, thinking on my feet, not that! Is it C331 that is leaky, thus dragging the emitter voltage low - because there is obviously current flowing through R319 - and it's got to be going somewhere!

Actually, having worked all the way through that lot, I think a check of C331 should be your first move!

[Crackle]

A couple of things I have confirmed are; I do appear to have an AM oscillator, or at least I get interference on MW when I sweep across the dial on the RGD.
I checked the IFT's against the ITT and there are some differences of up to an ohm on some of the AM ones, but they do all more or less match. Maybe they used thinner wire on the RGD, they certainly used inferior resistors.
I have also changed C352 the OP dc blocking cap as although this came up very well on my Peak ESR tester it looked like it showed signs of leakage.
I swapped IFT L313 with the one from the ITT and it made no difference.
So next I will try your suggestions Terry.

Thanks
Mike

[Crackle]

Hi Terry
You very clever person, your last suggestion to try first was spot on.

Here is the naughty little C331 capacitor, doesn't it look innocent, no marks or damage.
   
It was fluctuating between 5 and 8 mA when connected to a 6 v supply.

I have found this little radio one of the most difficult to repair. Its lucky it is a fairly rare one, (I have never seen this model before, in the 7 years of scanning Ebay for RGD transistor radios)  it nearly got slung out the window on at least 2 occasions.

You have saved the day again, its working on FM & MW, I have yet to check LW but that shouldn't be an issue.
Thanks very much Terry and all who contributed.

Mike

[Terry]

Hi Mike, I'm glad we got there in the end!

Some of the voltages in Post #10 made no sense at all and having any available dat,a including on the ITT circuit, reference to the HT line was really confusing!

The readings you took using the ground rail for reference made a lot more sense. Without them, I doubt you would have had it working by now!

[pwdrive]

Those types of capacitors used to go duff back in the day too.

Lawrence.

[Refugee]

Those ceramic capacitors do indeed go short circuit all be it not as often as many other types.
Tubular ones are a bit less reliable.

[Crackle]

Both the ITT Colt and the RGD R135 are back together now.
I had to find some screws to help hold the RGD together.

Here is the RGD R135
           

I have 4 examples of the ITT Colt, here are 2 early models in both silver and wood effect
       
and 2 late models again in silver and wood effect.
       
It would seem that ITT had their radios made in both Malaysia and Hong Kong.

I was surprised to find another radio which uses the same chassis and case as the early ITT Colt
   

And here is yet another radio with what appears to be the same chassis.
https://www.radiomuseum.org/r/itt_junior...c_103.html

I have modified the RGD service manual http://www.kbmuseum.org.uk/other_service...135_me.pdf

Thanks again to all.

Mike

[Crackle]

I forgot to mention that when I bought this radio it came with its original box, instructions and guarantee card. The radios original box was a bit damaged and squashed in the post, but I have managed to more or less get it back into shape.
So I am really pleased to get the radio working again to add to my collection of the transistor RGD radios.
   
Mike