Newb Stumped on a Derwent

[Station X]

SPLITTING THE BIAS.

The 75 needs a bias of -2V and has a specified anode current of 0.9mA.
2/0.0009 is 2222R, so I'll use a 2.2K cathode resistor.

I'll bypass the 2.2K with a 25uF capacitor, because that's what's used in the GEC 4750, which also has a 2.2K cathode resistor for the AF amp.

I'll leave the 0.0033uF between anode and cathode, as that's how the 4750 does it.

Not very scientific I know, but it should provide a starting point.

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1. Disconnect the 0.0033 capacitor from the junction of the 330R and the 50uF capacitor. Reconnect the disconnected lead directly to pin 5 of the 75.

2. Remove the wire between pin 5 of the 75 and the junction of the 330R and 50uF.

3. Connect a 2.2K resistor between pin 5 of the 75 and chassis.

4. Connect a 25uF capacitor between pin 5 of the 75 and chassis. Negative end to chassis and positive end to pin 5 of the 75.

5. Power up and measure voltages:-

Chassis to pin 5 of 42. Should be about 15V
Chassis to pin 5 of 75. Should be about 2V.

Report results.

6. See what hapens if you inject a signal at the PU socket.

7. See if the radio works.

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I'd wait a while for other member's comments on this post before starting work.

[Station X]

Sketch showing suggested modifications attached.

[oldticktock]

Hi Station X

which electrolytic would best suit?

The last one just has u not uF is that pico

[oldticktock]

sorry picture this time

[PJL]

Bottom one is fine. There is a wire that runs from 75 pin 5 into the IF transformer. Make sure you leave that connected to pin 5.

[oldticktock]

Originally Posted by PJL

Bottom one is fine

Thanks PJL why did it only have u and not uF newbs find this odd

[Station X]

Seconded. There'll only be a couple of volts across it.

[oldticktock]

Originally Posted by PJL

There is a wire that runs from 75 pin 5 into the IF transformer. Make sure you leave that connected to pin 5.

will do.

Not starting until you are all happy for me to do so

[oldticktock]

2.2k red,red,blue.green?

[Station X]

Red, red, red plus a tolerance band.

First figure 2.
Second figure 2.
Number of zeroes 2.

Check with a meter

[oldticktock]

Only have 2.7 or 1.8

[Station X]

Go for 1.8 then.

Brown, Grey, Red?

[oldticktock]

yes

[Station X]

Plenty of knowledgable members have viewed the suggested modifications without making any adverse comments, so I'd go for it.

I'll refrain from making any predictions about having it working by tea time. That backfired yesterday.

[oldticktock]

Ok off we go then, iron warming up

[oldticktock]

Ok it's much better now, fade off cured
lots of gain
stations tune in

however

lots of background noise and voices are raspy

Station X you the man

[oldticktock]

just going to check voltages now

[Station X]

Voltages?

Clean the wavechange switch, speaker plug, valve pins, volume and tone controls. Is the distortion still there when using the PU socket? If so that would indicate that the problem is in the AF stages. If not it's further back in the IF or RF stages.

[oldticktock]

...
voltages

Valve 42
pin1.....11vac
pin2.....260vdc
pin3.....270vdc
pin4.....2vdc
pin5.....16vdc
pin6.....0

Valve 75
pin1.....0
pin2.....150vdc
pin3.....VVM set to 1.5v read 4 on the scale
pin4.....2vdc
pin5.....3vdc
pin6.....11vac

I noticed that heater pin6 on the 42 and heater pin1 on the 75 read 0. Looking at the physical wiring. there is as follows

Physical wire from 42 pin1(H)........... to pin6(H) on the 75
Physical wire from 42 pin6(H)........... to pin1(H) on the 75.

interesting that the link between 42 pin6 and 75 pin1 both read 0 (wiring fault?)

signal added to P.U. and Vol pot, distortion is still present and all sorts of sounds. reminds me of a kid listening to radio luxembourg remember the inteference.

[Station X]

You have to measure heater voltages by connecting your meter on the AC range between pins 1 and 6. Don't do this with your VVM though.

42 readings are fine. I'm still thinking about the 75.