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3rd Nov 2021, 7:25 pm | #1 |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
Posts: 671
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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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3rd Nov 2021, 8:08 pm | #2 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,731
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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? |
3rd Nov 2021, 8:09 pm | #3 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,731
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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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3rd Nov 2021, 8:09 pm | #4 |
Heptode
Join Date: Jan 2006
Location: Cornwall, UK.
Posts: 501
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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! |
3rd Nov 2021, 8:21 pm | #5 | ||
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
Quote:
Quote:
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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3rd Nov 2021, 8:35 pm | #6 |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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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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3rd Nov 2021, 8:45 pm | #7 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
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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 |
3rd Nov 2021, 8:51 pm | #8 | |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
Quote:
Cheers Jay
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3rd Nov 2021, 9:53 pm | #9 | ||
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,731
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Re: Pilot Little Maestro restoration project.
Quote:
Quote:
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3rd Nov 2021, 10:05 pm | #10 | |||
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
Posts: 671
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Re: Pilot Little Maestro restoration project.
Quote:
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4th Nov 2021, 1:35 am | #11 |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
What should the voltage rating of the dropper cap be ? 250v or above ?
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4th Nov 2021, 6:17 am | #12 |
Pentode
Join Date: May 2017
Location: Ware, Hertfordshire, UK.
Posts: 215
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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 |
4th Nov 2021, 1:16 pm | #13 | |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
Posts: 671
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Re: Pilot Little Maestro restoration project.
Quote:
Jay
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4th Nov 2021, 2:22 pm | #14 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,731
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Re: Pilot Little Maestro restoration project.
The X2 caps I use are rated at 305VAC.
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4th Nov 2021, 2:41 pm | #15 |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
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4th Nov 2021, 4:31 pm | #16 |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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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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4th Nov 2021, 4:41 pm | #17 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
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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. |
4th Nov 2021, 4:51 pm | #18 | |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
Quote:
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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4th Nov 2021, 5:06 pm | #19 |
Heptode
Join Date: Jan 2006
Location: Cornwall, UK.
Posts: 501
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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. |
4th Nov 2021, 5:13 pm | #20 | |
Heptode
Join Date: Jul 2020
Location: Larkhall, South Lanarkshire, Scotland.
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Re: Pilot Little Maestro restoration project.
Quote:
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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