Newb Stumped on a Derwent

[Station X]

Originally Posted by electrogram

Are you saying cathode current of 42 passes through both 470 and the 'measured' 5000

To be honest I don't really know. There's too much conflicting information. If both cathodes are connected, then there must be the same resistance between both cathodes and chassis. One can't be 500R and the other 5000R.

I'm not convinced that the 5000R exists as an actual component, as I can't see it.

[oldticktock]

Originally Posted by Station X

So with THAT capacitor connected you get 6V between pins 4 and 5 of the 42, and with that capacitor disconnected you get 23V between pins 4 and 5?

If that's the case then THAT capacitor is faulty.

no tiredness is getting the better of me I disconnected got better access and took the reading 23v. Re soldered back on with better access and reading is 23v. Sorry i'm a i think bed should be the order of the errr night.

[oldticktock]

If i touch one probe (the common) to pin 5 it springs to life!!!!!!!!!!!!! red (dc probe does nowt)

lots of volume quality not great though, th emore you turn the vol up it distorts, tone works well

This must mean something

[Station X]

Capacitor in circuit 6V measured.
Capacitor disconnected 23V measured.
Capacitor reconnected 23V measured.

That doesn't add up. Either 6V was wrong or you disturbed something when disconnecting and reconnecting the capacitor.

Given that you have 23V on the cathode I'd expect to see 23V between pins 4 and 5, so we're getting somewhere.

[oldticktock]

Originally Posted by Station X

Capacitor in circuit 6V measured.
Capacitor disconnected 23V measured.
Capacitor reconnected 23V measured.

That doesn't add up. Either 6V was wrong or you disturbed something when disconnecting and reconnecting the capacitor.

Given that you have 23V on the cathode I'd expect to see 23V between pins 4 and 5, so we're getting somewhere.

read it as 23v and i screwed! up maybe that's when the avo dying on dc happened. Now the mains powered Cossor seems to be very accurate. What about the common probe on pin 5 and life

[oldticktock]

OK bed beckons and what a long day...........

Thanks to all for your input and sticking with the newb it must be frustrating for you, but I sneakily think it brings out the remote sherlock in you. Got me hooked, I started this morning around 10:30 and now look at the time 01:00 Yeeeek. Lucky for me the wife is ok with it, means she can watch dross tv.

Can't wait to see how this fault hunt turns out.


Snooooooooze

[Station X]

I think the penny has dropped at last!

The "wire link" between the chassis/earth tag on the 75 and pin 4 (control grid) of the 42 wasn't a "wire link" at all. In fact it was a 510K resistor inside a length of sleeving. This was the grid leak resistor for the 42.

When the "wire link" was cut at the chassis/earth end it removed the grid leak resistor from the 42.

The grid leak resistor was replaced with another by shifting one end of another 510K resistor, spuriously connected across the 500R cathode bias resistor, from pin 5 to pin 4 of the 42.

This means that there were only two things wrong with the wiring as found:-

1. There was a spurious 510K resistor connected across the 500R cathode bias resistor. This would have had little or no effect on circuit operation as it would have been swamped by the 500R.

2. The 50uF cathode bias bypass capacitor was connected the wrong way round.

So we are back to square one.

THE VALVE VOLTMETER COMMON PROBE.

If i touch one probe (the common) to pin 5 it springs to life!!!!!!!!!!!!! red (dc probe does nowt)

lots of volume quality not great though, th emore you turn the vol up it distorts, tone works well

This must mean something

The valve voltmeter and the Derwent radio no doubt both have their chassis earthed via a three core mains lead? I suspect that the common probe is connected to the chassis of the valve voltmeter either directly or via a resistor. This means that if this probe is connected to anywhere other than the chassis it will disturb circuit operation.

When you connect the common probe to pin 5 you are providing a low resistance path from pin 5 to earth/chassis restoring circuit operation. You may be able to prove this by unplugging the valve voltmeter from the mains and touching the common probe on to pin 5.

If this theory is proved I would suggest that the way forward is to lower the value of the cathode bias resistor either by replacing it with say a 330R or by shunting it with say another 500R.

[PJL]

Well deduced Graham. I wonder if this might have more to do with earthing the 75 cathode. It would be interesting to see how the last IF is wired to the detector as it might give a clue as to whether it is meant to have the cathodes connected together.

[oldticktock]

Ok head still fuzzy read your post looks like you have it even if i only partially understand. Trouble is so much has gone on its "wood for the trees now"

I have attached a diagram of how it looks now with a couple of queries and if someone can suggest what I do next.

[Station X]

I'm asuming that your drawing is a true representation of the wiring as it exists at present.

The 50uF capacitor is the wrong way round. It should be connected negative end to chassis and positive end to pin 5 of the 75.

Do not replace the dotted 510K. It isn't needed.

Green, Yellow, Brown indicates a 540 ohm resistor.
---------------------------------------

Try the common probe to pin 5 test with the valve voltmeter disconnected from the mains supply ie mains plug out. Is performance still improved? If not this indicates that 500/540 ohms is too high a value. It's possible that the value is changing when current flows through the resistor. Not unknown with carbon resistors.

I'll call the 500/540 the cathode resistor. Try reducing it in value to 330R and take new resistance and voltage readings. You should be aiming for about 16V DC from cathode to earth ie "across" the cathode resistor.

[oldticktock]

Originally Posted by Station X

I'm asuming that your drawing is a true representation of the wiring as it exists at present.

The 50uF capacitor is the wrong way round. It should be connected negative end to chassis and positive end to pin 5.

Do not replace the dotted 510K.

Green, Yellow, Brown indicates a 540 ohm resistor.
---------------------------------------

Try the common probe to pin 5 test with the valve voltmeter disconnected from the mains supply ie mains plug out. Is performance still improved? If not this indicates that 500/540 ohms is to high a value. It's possible that the value is changing when current flows through the resistor. Not unknown with carbon resistors.

I'll call the 500/540 the cathode resistor. Try reducing it in value to 330R and take new resistance and voltage readings.

If the

Yes drawing is accurate as of now.

I will change the 50uf around again I thought when someone suggested turning it, it seemed weird to have + to earth

I will leave out the 510k where one used to be and will replace the 540 with a 330

I will try the probe test

Cheers Station X

[Station X]

I will leave out the 510k where one used to be and will replace the other with a 330

I hope that by other you mean the 500/540 and not the other 510K!

I expect this to be working by tea time today!

[oldticktock]

ok before doing anything else I tried the probe test

Valve Volt Meter disconnected from mains, probe on pin 5 of 42.... nothing

Valve Volt Meter connected to and switched on, probe on pin 5 of 42.... station heard albeit distorted lots of vol etc

Now I will rotate the 50uf and install the 330

[oldticktock]

Originally Posted by Station X

I hope that by other you mean the 500/540 and not the other 510K!

I expect this to be working by tea time today!

Yes when I typed that I thought you would think that and edited the post. I'm getting there slowly but surely. Have to say this is fun.

By tea time would be great, I would so love to have this working for xmas. I've got myself a little am transmitter AM250 and want to send this radio some good OTR broadcasts sit down in front of the fire with a whisky and a cigar and soak up the atmosphere.

[oldticktock]

Ok changes made, radio fires up and can now be heard.

Volume was loud but distorted

now volume has dropped off (vol at max) but still can be heard but very very distorted.

[Station X]

Voltages? Resistances ?

Try disconnecting one end of the 50uF.

[PJL]

There are a number of Philco sets with the same lineup. In all cases the 42 cathode is earthed and grid bias is taken from the energising coil which is in the -ve HT path.

Has this been hacked about due to a speaker change?

[oldticktock]

Ok Will do.

I have to nip out while I can, will be back later to post update.

Back in about 1 hour (she who must be obeyed).

Steps in the right direction now..... I see the summit..... head down and push on.

[Station X]

PJL.

Quite possible I'd think. That O/P TX held in with one screw doesn't look quite right.

We have to work with what we have though.

[oldticktock]

Originally Posted by PJL

There are a number of Philco sets with the same lineup. In all cases the 42 cathode is earthed and grid bias is taken from the energising coil which is in the -ve HT path.

Has this been hacked about due to a speaker change?

pin 3 - Grid 2 goes to HT rail
pin 5 does goto earth see my lastest diagram

speaker and all associated looks original