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Old 23rd Feb 2021, 5:31 pm   #41
stevehertz
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Default Re: Pye M78F questions

Nearly finished the power supply, just one more component to fit that I'm waiting for, and finish the wiring. I mounted the regulator separately on a heatsink with insulated wires making the connections to the tag board.

I 'borrowed' the plastic housing from Thorn when I worked in the test department there over thirty years ago. They were used to house small electronic control circuits, PCBs, relays, connecting blocks etc. Most of the other components I had in stock, I just had to buy the transformer, regulator, and a C or two. A friendly forum member gave me the tag strip.

When that's done all I have to do is restore the radio..
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Old 23rd Feb 2021, 7:10 pm   #42
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Default Re: Pye M78F questions

Looking good so far Steve , keep us informed when you start to restore the radio , Mick.
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Old 23rd Feb 2021, 7:14 pm   #43
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Default Re: Pye M78F questions

Of course Mick, will do. I've got my tweezers ready.
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Old 25th Feb 2021, 4:48 pm   #44
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Default Re: Pye M78F questions

The power supply is now finished. I'm getting a perfect 1.4V LT, and 74V HT from it. I assume the Pye will be ok with the 74V? Not that much more than the 67.5V battery that it should run off?
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Old 25th Feb 2021, 6:39 pm   #45
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Default Re: Pye M78F questions

Hi Steve should be ok it will no doubt drop a little on load, Mick.
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Old 26th Feb 2021, 7:31 pm   #46
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I've started to restore the M78F. The valves are tested, although it doesn't look as if their emission is stellar on my Mullard High Speed valve tester, although the latter's CRT dot position is not a finite indication of gain, nonetheless they should be workers.

There's a bunch of four Hunts (2 + 2) 0.01 and 0.001uF caps at the bottom corner of the set, done those. There's a 0.01 waxy (aka 2M ohm resistor) on a piece of insulation board behind the aerial assembly, done that.

Now, there a couple or three waxy caps beneath the control panel that I'm struggling to see how I can get at them. The Trader sheet describes dismantling that part, but it's not making sense to me. The issue is, the control panel has the tuning cording assembly mounted on it, but the tuning cap itself is fixed to an adjacent panel below the control panel. The caps I need to change are sandwiched in-between! I haven't pulled all my hair out yet, but I'm getting near to it. Anyone recall gaining access to the components hidden below the control panel? How do you do it?!
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Old 27th Feb 2021, 4:25 pm   #47
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Default Re: Pye M78F questions

I figured out how to remove the control panel. Actually, the Trader does describe how it's done, I couldn't see for looking.. The trick is that you can release two screws on the tuning condenser's shaft to enable the control panel to be lifted and folded to one side - see the RHS of the photos. There's another two waxies amongst that rat's nest that I have to change. One I have done and the other I'm warming up to. The photo is before the first cap change.
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Old 27th Feb 2021, 6:11 pm   #48
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A rats nest is a good description . well done so far ,I bet you are itching to connect it up to your power supply , Mick.
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Old 27th Feb 2021, 6:22 pm   #49
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A rats nest is a good description . well done so far ,I bet you are itching to connect it up to your power supply , Mick.
Yes and no! Of course, I'd love to switch it on and be rewarded with a station or two. The reality is I bet it'll be 50/50 whether or not it works. We'll see. I've been doing this stuff for 45 years, I've gotta have one that works after restoration soon?
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Old 1st Mar 2021, 12:51 pm   #50
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Default Re: Pye M78F questions

Switch on time arrived. Somewhat of a disappointment.

1) I can tune in to just one MW station, the strongest, R5 Live.
2) Even with the volume up full it is very quiet, you have to place your ear against the speaker to tell what is being said.
3) There is a constant background 'hummy noise' sound that increases as you put your hand towards the set, but disappears when you actually touch the chassis.
4) No stations on LW even though R4 is a strong signal. There's the same hummy noise and a tendency to 'motorboat' although I am not saying it is actually motorboating in the true sense of the term.
5) The HT from my power supply loads down from 74V to 67V and the current draw is just 6mA.

Any ideas?
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Old 1st Mar 2021, 3:09 pm   #51
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Default Re: Pye M78F questions

I s your power supply earthed? switch to MW and tune a station short the middle connection of the v/c to the hot end sig in ,I had a similar fault it turned out to be the v/c o/c worth a try . Mick.
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Old 1st Mar 2021, 3:15 pm   #52
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I s your power supply earthed? switch to MW and tune a station short the middle connection of the v/c to the hot end sig in ,I had a similar fault it turned out to be the v/c o/c worth a try . Mick.
The power supply is in a plastic box and the mains transformer has no earth connection either, so just L&N going to the power supply. I'll try the volume control. Cheers.
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Old 2nd Mar 2021, 11:18 am   #53
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Default Re: Pye M78F questions

The volume control is indeed o/c. What to do about that?! It's about as 'special' as you can get! Totally non-standard mounting, non-standard shaft, difficult (2M) value. At first I thought it was classic case of the connection tags not making contact at each end of the track. I tried pushing them down to make better contact and I also tried soldering them with the addition of flux but they wouldn't solder. But here's the rub, applying test prods to the track at different places is very hit and miss in terms of getting a reading, the track just seems to be devoid of conductivity.

Vinrads, what did you do?

Anyway, I rigged up a 100k pot with series resistors each side to approximate to 2M ohms and it made no difference to the performance of the radio.
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Old 2nd Mar 2021, 11:26 am   #54
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Default Re: Pye M78F questions

Try a 2M resistor with the wiper wire connected to the max volume end and and see what it sounds like then?
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Old 2nd Mar 2021, 11:51 am   #55
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Try a 2M resistor with the wiper wire connected to the max volume end and and see what it sounds like then?
Why would that be any different to the '2M' load that I've already tried albeit in the form of a 100k pot with an 820K resistor at each end? The previously mentioned faults are all there, it's not just a volume thing. My test set up should at least have given half volume, not whisper quiet.
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Old 2nd Mar 2021, 12:00 pm   #56
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Default Re: Pye M78F questions

Fair enough. If the original pot was log law and your end resistors were equal in value you should have got considerably more than half volume.
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Old 2nd Mar 2021, 3:58 pm   #57
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Hi Steve.

It looks like you have other faults as well as the volume control duff. All I can suggest is that you solder a 2meg resistor across the volume control then connect the wiper to the input end. Then start fault finding ie check all valve voltages. I repaired mine by grafting a 2 meg track onto the original, connection was made using conductive paint, it works.

Mick.
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Old 2nd Mar 2021, 10:00 pm   #58
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Default Re: Pye M78F questions

Thanks Mick, makes good sense. Did you use a track from a vintage pot or a new one that I could buy the same one? As it's a volume pot it should be log, but the Trader sheet doesn't state log. Did you use log?
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Old 3rd Mar 2021, 10:23 am   #59
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Default Re: Pye M78F questions

I'll place a detailed wanted ad for a 2M pot in the wanted section. The main issue being the diameter of the track to accommodate the existing wiper yet also allow the volume knob to fit over it.
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Old 5th Mar 2021, 7:18 pm   #60
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Default Re: Pye M78F questions

I got a 2M pot from Cricklewood. Having dismantled it and removed the track, for fit reasons I had to remove excess material from its outer edge using a belt sander, and material from the inside using a Dremel. See photo. It's quite a complex set of 3D measurements that I have to meet in order to be able to use the 'frame' of the original pot and the slider and knob that all work and fit together uniquely. I'm getting there, but presently the innards stack height is too high for the slider/knob assembly to work effectively if I simply lay the new track on top of the other. So I have to consider removing material from the original track bed. I'll get there!
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