Voltages on Cossor 484 mains radio

[Cobaltblue]

Hi Dick on the Trader sheet 252 yes its R5 which is the gain control.

Cheers

Mike T

[Dick Glennon]

Hi Mike T, Noise has got worse by doing that. Dick.

[Cobaltblue]

Hi Dick

If your gain control was at maximum that should have made no difference.

I think its possible you gain control is faulty.

ISTR that on this set it is ganged with the reaction capacitor

It looks like you have another problem though.

Is the metalisation on the 41STH ok and is it connected or is the valve base loose?.

You should check the dc resistance of the RF coils as well.

Cheers

Mike T

[Dick Glennon]

Hi Mike T, I actually have 2 41Sth valves and they work very well in another set. Could you elaborate some more about the method of checking the dc resistance of the RF coils as I am only learning about the workings of Valve Radio. Regards, Dick.

[PJL]

Hi Dick, Mike has already given you the answer to your problem. The cathode voltage of V1 is much too high. You should measure the resistance between the cathode and the chassis to confirm there is a fault in the cathode circuit. You will then need to trace the circuit back to see where the problem is. It is most likely a faulty volume control.

[Dick Glennon]

Hello again, I have taken the volume control apart, there is nothing broken inside, but as I put the ohmmeter on it I am getting a general reading from 9 ohms to14K but with erratic readings up to 26K. Would it be possible for me to use a 10K potentiometer seperate from c 33 300pf variable to see if that cures the problem. C 33 is reading from 0 to 460pf. Sorry for being such a nuisance. Regards Dick.

[Cobaltblue]

Hi Dick

Apologies for my late reply I was away for the weekend my wifes birthday then Harpy.

No one thinks you are a nuisance, it can be tough fixing old Radios until you have experience.

The Pot changes the gain of the front end whilst at the same time the capacitor varies the reaction (positive feedback).

The Pot is almost certainly wirewound so it might be a bit tough on a 10K linear carbon pot.

I haven't got a 484 to hand but you could fix the gain at the maximum by shorting the pot out and relying on the reaction for the gain control should be OK as long as the aerial isn't to long.

You can then re-check the voltages.

The coils will all have relatively low resistances from less than an ohm for SW coils to maybe tens of ohms for the long wave coils.

If the set has been damp stored open circuit coils in Cossor sets is common due to Green spot. The copper reacting with the moisture.

Cossor did not tend to varnish coils.

We are all rooting for you to fix this set.

Cheers

Mike T

[Dick Glennon]

Hello again, I have made a little bit of progress with the radio. I shorted out R5 on full volume and with the reaction (c33) turned down low I am getting very low sound on radio eireann 252 long wave, but as I turn it up the sound goes. I have got hum but not very loud but when I hold the outer braid on the lead going to the top cap of V2 the hum decreases and I can hear the sound more clearly. The interference is still there in the speaker when I touch the chassis with myfinger. Any more ideas on how I can increase the volume.. Regards, Dick

[Cobaltblue]

Hi Dick

What sort of Aerial are you running to this set?

Did you check R9 the grid leak to V2?

High value resistors like this one are very prone to going very high in value.

R9 and C12 will be inside the coil can itself so you should check both of them.
It is essential that these components are in good condition as V2 operates as a leaky grid detector.

I am also wondering if there is a short circuit between the outer and the inner of the connection to V2 top cap, although I think that's less likely.

Turning up the reaction usually results in a bit of squealing followed by a plop followed by just noise as the reaction control is turned up.

Let us know how you are getting on.

Cheers

Mike T

[Dick Glennon]

Hi again, I have checked R9 but Ido not have a new c12 but it reads ok. My ariel is a longwire. I have put a good earth into the earth socket and now I have grand radio eireann with very little hum but very low volume. Could the output valve be gone low?. Regards, Dick.

[PJL]

Hi Dick, I suggest a full set of valve voltage readings so we can see what's going on.

[Dick Glennon]

Hello again, Here is a list of the valve readings on the cossor 484. I had it tuned to medium wave on the low end, with the volume control at minimum. V1 pin 1= 182, pin2= 103, pin 3=225, pin 6=103 and pin 7=279. V2 pin 2=107, pin 7=29. V3 pin 3=308, pin 6=5, pin 7=305. V4(rectifier) pin 1=352ac, pin 2=353ac, pin 3=412, pin 4=410. When I take the reading on pin 7 on V3 a very loud whistle starts after about half a minute. It goes when I turn the volume almost fully on. Regards, Dick.

[PJL]

V1
Pin 1 (ta) =182 (145)
pin 2 (tg) =103 (a few volts negative)
pin 3 (g2) =225 (180)
pin 6 (k) =103 (1.2V)
pin 7 (ha) =279 (255)

V2
Pin 1 =0
pin 2 (a) =107 (50)
pin 3 =0
pin 6 =0
pin 7 (sg) =29 (20)

V3
Pin 1 =0
pin 2 =0
pin 3 (sg) =308 (260)
pin 6 (k) =5 (6.2V)
pin 7 (a) =305 (230)

Rectifier
Pin 1 (a) =352
pin 2 (a) =353
pin 3 (f) =412
pin 4 (f) =410

You still have a problem with the cathode of V1. It should read much lower. Measure the resistance from V1 pin 6 to chassis. There is a break somewhere in the circuit, V1pin6 -> R4 -> R5 -> chassis.

[Dick Glennon]

llo again, First of all I want to say a very sincere THANK YOU to all who are helping me to solve my Cossor problem. The resistance from pin 6 V1 to chassis with volume on min is 14.45K, and with volume on max is 320 ohm. The D.C volts on R5 vol min=120 volts and with volume at max=0 volts. Thanks again, Dick.

[PJL]

OK, let's have another go. I had not realised R5 was 10K and the cathode could be so high. Looking at your voltages again there is nothing I can see that should stop it working.

When you receive a station, is the volume at maximum? What happens as you adjust the volume?

[Dick Glennon]

Hi, As I turn up the volume I get Radio Eireann for about three quarters of the way but then I loose it again. I will put in a different V1 (41STH) and V2(MSPenB) and take the voltages again. Thanks, Dick.

[PJL]

You are probably losing the signal when the IF starts oscillating so let's get rid of the regeneration and see if you can get it louder. Disconnect C33 from V2 anode and try again. This time you may need more volume to receive anything.

Did you check R9 (3M)?
Have you got a signal generator?

[Dick Glennon]

Hi PJL, That made no difference to the volume, Yes, I have a signal generator, but I am not too familiar with its workings. I could give it a go as there may be someone that might be able to help me out. Regards Dick.