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Old 3rd Nov 2021, 7:25 pm   #1
Radio Scotland
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Default Pilot Little Maestro pre war AC/DC 1939 restoration project.

Continuing on from this thread

https://www.vintage-radio.net/forum/...d.php?t=185208

First up iv'e never worked on a valve set before, Although i understand the basics of how they work and i'm well aware of the dangers especially with a live chassis set. I don't have a variac or isolation transformer, But plan to knock up a dim bulb tester, The radio won't be powered up till its been checked and had all faulty parts replaced. I'm kitted up with the following gear,

Signal generator RF/AF
Scope
Meters for alignment
Various Multi meters, Digital & analogue
Trimming tools etc etc,

As stated no variac or isolation transformer or valve tester,

This might be a long running thread as i have more important stuff to get on with first,

But my first question is does this diagram for a capacitor dropper look correct ? Component placement values etc ? I'd rather use a safety cap rather than a motor run cap.

Lots more questions to follow at a later date.

Cheers all
Jay.
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Old 3rd Nov 2021, 8:08 pm   #2
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Default Re: Pilot Little Maestro restoration project.

Looks roughly right to me. Would have to double check the values, but from memory they look about the same as what I used on one of these sets. I agree with using x2 type caps for the dropper.

One point though; your diagram appears to show a link from the live via its resistor to both the rectifier anodes and ground?
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Old 3rd Nov 2021, 8:09 pm   #3
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Default Re: Pilot Little Maestro restoration project.

Scratch that; I see its C35 rather than a link. Just the image resolution and my tired eyes!
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Old 3rd Nov 2021, 8:09 pm   #4
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Default Re: Pilot Little Maestro restoration project.

That diagram looks similar to way I fitted a capacitor dropper. I did mine in 1983 though, using the capacitor dropper as fitted in the Thorn 1580 hybrid portable TV which was still available from Thorn as a spare part. I fitted it above chassis behind the tuning capacitor, very ugly and the back bulges slightly.

I don't know what else you could use now except a motor run cap. There is a thread describing fitting it in the space occupied by the reservoir/smoothing cap with a modern reservoir/smoothing cap below chassis.

See if the valve heaters have continuity, once you have them lit it should be easy!
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Old 3rd Nov 2021, 8:21 pm   #5
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by agardiner View Post
Looks roughly right to me. Would have to double check the values, but from memory they look about the same as what I used on one of these sets. I agree with using x2 type caps for the dropper.

One point though; your diagram appears to show a link from the live via its resistor to both the rectifier anodes and ground?
Yeah that caught me out too till i took a closer look,

Quote:
That diagram looks similar to way I fitted a capacitor dropper. I did mine in 1983 though, using the capacitor dropper as fitted in the Thorn 1580 hybrid portable TV which was still available from Thorn as a spare part. I fitted it above chassis behind the tuning capacitor, very ugly and the back bulges slightly.

I don't know what else you could use now except a motor run cap. There is a thread describing fitting it in the space occupied by the reservoir/smoothing cap with a modern reservoir/smoothing cap below chassis.

See if the valve heaters have continuity, once you have them lit it should be easy!

Cheers, Yeah i'll check the valve heaters, Audio transformer, Field coil etc.

Was thinking of using a couple of safety rated caps in parallel, That way i don't need to worry about them shorting or leaking at a future date,


Cheers
Jay
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Old 3rd Nov 2021, 8:35 pm   #6
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Default Re: Pilot Little Maestro restoration project.

Heres how i'm thinking of approaching the resto.

Good visual inspection,
Change coupling/Decoupling caps, & Filter caps
Check valve heaters for continuity.
Check audio transformer/Field and voice coils
Obviously do the dropper mod,
Change on/off switch to switch the live side
Replace wiring where and if needed
Lube pots etc etc

One done power set via dim bulb,
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Old 3rd Nov 2021, 8:45 pm   #7
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Default Re: Pilot Little Maestro restoration project.

Hi Jay, despite the theory cap droppers will work when feeding both HT and heater circuits with one class X2 cap as I and others have done it several times. This further reduces the heat in the cabinet

Ed
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Old 3rd Nov 2021, 8:51 pm   #8
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Jay, despite the theory cap droppers will work when feeding both HT and heater circuits with one class X2 cap as I and others have done it several times. This further reduces the heat in the cabinet

Ed
Good info, Never knew that, i'm not familiar with cap droppers, So working from a schematic of the above mod would be good if anybody has one they can upload.


Cheers
Jay
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Old 3rd Nov 2021, 9:53 pm   #9
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by Radio Scotland View Post
Was thinking of using a couple of safety rated caps in parallel, That way i don't need to worry about them shorting or leaking at a future date,


Cheers
Jay
Yes, that's how I got to the value required in one of these sets.

Quote:
Heres how i'm thinking of approaching the resto.

Good visual inspection,
Change coupling/Decoupling caps, & Filter caps
Check valve heaters for continuity.
Check audio transformer/Field and voice coils
Obviously do the dropper mod,
Change on/off switch to switch the live side
Replace wiring where and if needed
Lube pots etc etc

One done power set via dim bulb,
Sound like a good plan. You might find changing the on/off switch to the live side causes too much hum, but you will have to wait and see. Certainly better to be on the live side.
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Old 3rd Nov 2021, 10:05 pm   #10
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by agardiner View Post
Quote:
Originally Posted by Radio Scotland View Post
Was thinking of using a couple of safety rated caps in parallel, That way i don't need to worry about them shorting or leaking at a future date,


Cheers
Jay
Yes, that's how I got to the value required in one of these sets.

Quote:
Heres how i'm thinking of approaching the resto.

Good visual inspection,
Change coupling/Decoupling caps, & Filter caps
Check valve heaters for continuity.
Check audio transformer/Field and voice coils
Obviously do the dropper mod,
Change on/off switch to switch the live side
Replace wiring where and if needed
Lube pots etc etc

One done power set via dim bulb,
Sound like a good plan. You might find changing the on/off switch to the live side causes too much hum, but you will have to wait and see. Certainly better to be on the live side.
Sure i read another thread on this radio where the above mod was done without issue
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Old 4th Nov 2021, 1:35 am   #11
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Default Re: Pilot Little Maestro restoration project.

What should the voltage rating of the dropper cap be ? 250v or above ?
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Old 4th Nov 2021, 6:17 am   #12
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Default Re: Pilot Little Maestro restoration project.

hello all, my set was done with a cap dropper,the motor run cap is very
good ,there is no flair of pilot lamp ,i think the charge time makes smooth
start up ? a metal clad resistor on ally bracket for the ht

dave
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Old 4th Nov 2021, 1:16 pm   #13
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by davebms View Post
hello all, my set was done with a cap dropper,the motor run cap is very
good ,there is no flair of pilot lamp ,i think the charge time makes smooth
start up ? a metal clad resistor on ally bracket for the ht

dave
Cheers dave, The more info i can get on this mod the better, Much appreciated.


Jay
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Old 4th Nov 2021, 2:22 pm   #14
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Default Re: Pilot Little Maestro restoration project.

The X2 caps I use are rated at 305VAC.
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Old 4th Nov 2021, 2:41 pm   #15
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by agardiner View Post
The X2 caps I use are rated at 305VAC.
Cheers
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Old 4th Nov 2021, 4:31 pm   #16
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Default Re: Pilot Little Maestro restoration project.

Chassis has been removed from the cabinet, Few things iv'e spotted so far are the tuning shaft is bent. The speaker voice coil seems to be rubbing, Hopefully its just dirt/dust, Small tear on the speaker cone. Crumbling wire insulation The smoothing caps have been replaced and the original canned one has been removed Some modern looking wire had been taped to the original line cord. Replacement aerial wire connected to the band switch ??

Other than that it looks all original, But i'm no expert, What do you think ? Looks like the above repairs were done a long time ago, Chassis is thick with dust and the odd petrified spider,

Tested the valve heaters and heres the results.

6A8GT Seems to be open circuit.
6K7GT -5 OHMS
25Z4G (Should be a Z6 ?) 15.5 OHMS
25A6G 11 OHMS.
6Q7G (If thats what it is) No markings. 3 OHMS.

Dial lamp ok, 6.5v mazda

Some look kind of gassy, Not much getter left, But as stated i'm a valve novice..

See pics.

Cheers
Jay.
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Old 4th Nov 2021, 4:41 pm   #17
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Default Re: Pilot Little Maestro restoration project.

If one of the valves has an open heater, then there is a good chance that someone powered up the set without any kind of series resistance, ie replaced mains cord without realising that it was a dropper. Seen this a few times!

I have seen these sets with both Z4 and Z6 valves. I would start with cleaning, re-capping and installing your capacitor dropper.
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Old 4th Nov 2021, 4:51 pm   #18
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by agardiner View Post
If one of the valves has an open heater, then there is a good chance that someone powered up the set without any kind of series resistance, ie replaced mains cord without realising that it was a dropper. Seen this a few times!

I have seen these sets with both Z4 and Z6 valves. I would start with cleaning, re-capping and installing your capacitor dropper.

What was left of the cord looked original, Actually looked like a mouse had eaten through it, Maybe it had been stored in an attic.

Thanks for the conformation on the Z4/Z6 Valves.

The above order of work was roughly what i was thinking too.


Cheers
Jay
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Old 4th Nov 2021, 5:06 pm   #19
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Default Re: Pilot Little Maestro restoration project.

Worth checking the speaker field coil at this stage, should be 650 ohms.

I expect you've seen this page already:
https://www.vintage-radio.com/repair...per-calcs.html
It's a good idea to include the discharge resistor across the dropper capacitor as in Post 1.
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Old 4th Nov 2021, 5:13 pm   #20
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Default Re: Pilot Little Maestro restoration project.

Quote:
Originally Posted by Simon Gittins View Post
Worth checking the speaker field coil at this stage, should be 650 ohms.

I expect you've seen this page already:
https://www.vintage-radio.com/repair...per-calcs.html
It's a good idea to include the discharge resistor across the dropper capacitor as in Post 1.
Yeah the field coil is next on my to do list. Cheers for including the value of it.

I have seen that page, Its answered a few of my questions but the maths just go over my head.


Cheers
Jay
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