Voltages on Cossor 484 mains radio

[Dick Glennon]

Hello again, Can anyone please tell me the voltages I should have within reason on the anode and screen on V 2 MSPen B, 7 pin.

Thanks again
Dick.

[Herald1360]

The Cossor service data gives the following voltages when read using a 1000ohms per volt meter (though it doesn't say whether it's used on its 30V, 300V or 600V range to get them):

Anode 80V

Screen 20V

Since the anode has some 200K and the screen 1Meg to the HT supply, I would think these readings are more meter dependent than actual electrode voltages.

Measuring the voltages between anode and screen and HT+ with a decent DVM and comparing the results when divided by the relevant resistances with the expected anode and screen currents of 0.9 and 0.2 mA may be more informative.

[FERNSEH]

Hi Dick, The Cossor MSPenB is a straight RF pentode. Infomation can be found here:

www.r-type.org/exhib/aad0250.htm

It would appear that the screen grid voltage is much lower than the anode.
Typ: Vsg = 100 Va = 250.

DFWB.

[vinrads]

Hi Dick my ref book states Anode 200v and Screen 100v , Mick.

[Station X]

Duplicate threads merged and some posts, which are no longer needed, deleted.

[Dick Glennon]

Hello again, Does anyone know where I might get the page of the 484 service sheet with capacitor values on it. It is not on the service data on the top of this page. Regards Dick.

[Top Cap]

Hope this helps, note that some values are not available and, for example, 0.0003uF would be replaced with a 0.00033uF or 330pF.

[Dick Glennon]

Hi,

Thanks Top Cap and thanks to everybody that has helped with my query.

Dick.

[Dick Glennon]

Hello again, I am still trying to get my 484 working, and I wonder if someone could please verify the following,V 1 41STH (7 pin) anode pin no., osc.anode pin no. and screen pin no. V 2 MSPenB, (7 pin) anode pin no. and screen pin no. V 3 420T, (7 pin) anode pin no, and screen pin no. Regards, Dick.

[Herald1360]

Try these:

41STH http://www.radiomuseum.org/tubes/tube_41sth.html

MS PEN B http://www.radiomuseum.org/tubes/tube_mspenb.html

420T http://www.radiomuseum.org/tubes/tube_42ot.html

[Dick Glennon]

Hello again,

Still not sorted with my 484. I got to the stage that I was getting very clear but very low sound on Radio Eireann, longwave. I started checking resistors. R7 seemed burnt so I took it out and checked it. It read 245K but on Wireless Trader it reads 25K (Osc. anode load). I replaced it with a 25K which gave no sound. I put back in the original resistor but alas no sound. The osc. anode voltage on the 41STH is 96 volts yet on the Trader sheet it reads 220 volts.

Any advice?

Dick.

[PJL]

From Trader 252, the anode load is R8 (25K). If the anode is 96V, the voltage across R8 will be about 250-96V or approx 150V. I=V/R so the current is about 6mA.

The Trader 252 quotes 145V (4.5mA) for the anode. The oscillator amplitude is automatically controlled by rectification of the signal on the grid which creates a -ve bias for the valve. 6mA would be quite reasonable if the oscillator was not working and the grid was biased at 0V wrt cathode.

I suggest the following:
- If you have not already done it, replace C3, C4 & C6
- Clean the waveband switch and pins and socket of the 41STH
- Measure the HT voltage.
- Measure the voltages on all pins of the 41STH

Cossor made cheap sets and economised on parts. Most sets have decoupling for the oscillator anode supply. I would try adding a 0.1uF 630V from HT to earth.

[Dick Glennon]

Hi,

I have checked C3, C4, C6 and cleaned contacts. I will check voltages on 41STH, and also m/a and report back. Does it matter where I connect 0.1 capacitor to earth?

Thanks again.
Dick.

[PJL]

I suggest you keep it near to V1 on the HT end of R8. Have you replaced the electrolytics?

[Dick Glennon]

Hi again, Still no sound on Cossor 484, well this is really not true, I have plenty of sound but not stations on any band. I now know that the frequency changer is working as I tried it in another set. I substituted valve 2, MSPenB with a similar valve but this made the buzzing worse.

When I put my finger near V2 the buzzing gets louder, and, when I put my finger on the top cap the noise gets much louder still. I have replaced all resistors, 3 electrolytics, and all ordinary capacitors. More help would be greatly appreciated.

Regards,
Dick.

[Cobaltblue]

Hi Dick

Just a quick suggestion have you checked R6 ( The gain control? )

It's in the Cathode circuit of the FC.

These are a bit prone to going open circuit.

If it is open circuit you will find that the Cathode of the FC is at a high voltage.

As a quick and dirty test you could put a 2.2K 1W resistor in parallel with it.

It could be that before you replaced the Caps C3 and or C6 were leaking enough to keep it sort of working.

Cheers

Mike T

[PJL]

Originally Posted by Dick Glennon

Hello again, Still not sorted with my 484.I got to the stage that I was getting very clear but very low sound on radio eireann, long wave. I started checking resistors. R 7 seemed burnt so I took it out and checked it. It read 245K but on Wireless Trader it reads 25K (Osc. anode load). Ireplaced it with a 25K which gave no sound. I put back in the original resistor but alas no sound. The osc. anode voltage on the 41Sth is 96 volts yet on the trader sheet it reads 220 volts. Any advice, Dick.

If R7 is the oscillator anode load it should be 25K - were the voltage measurements with the 245K or 25K? It's unlikely to work with 245K.

[Dick Glennon]

Hi, I am going to take all voltages again. It says on the service sheet that they should be taken with the volume at minimum and the lower end of the medium wave with no signal. I have also noticed that the hum noise is greater when the volume is turned down and the long wire ariel is plugged out. Could the volume control be wired wrong way to earth. All resistors have been replaced as circuit. Dick.

[Dick Glennon]

Dick here again, Valve voltages are as follows:-

V1
Pin 1=194
pin 2 98
pin 3 223
pin 6 103
pin 7 305.

V2
Pin 1= 0
pin 2 113
pin 3 0
pin 6 0
pin 7 30.

V3
Pin 1= 0
pin2 0
pin 3 305
pin 6 5.5
pin 7 287.

Rectifier
Pin 1= 350
pin 2 351
pin 3 411
pin 4 409.

The heater pins measure 2.16 to chassis.

When I put my finger on to chassis and move it I get a scraping in the speaker. Hope someone can enlighten me further.

Dick.

[Dick Glennon]

Hi Cobault, Would I be right in thinking that you mean R 5 instead of R 6.Regards,, Dick.