Pilot Little Maestro pre war AC/DC 1939 restoration project.

[Radio Scotland]

Originally Posted by Simon Gittins

Yes, C10 is the most important one. C13 and C14 might be reformed but changing them is an option. C15 is worth changing (it needs to be a Class X type as it's filtering the mains) also C12 as if it fails the output transformer is at risk.
We normally suggest not changing too many capacitors at once as it's easy to make a mistake so you could try the set after changing those five and then change others as required.
At some point C1 should be changed in case someone touches the end of the throw-out aerial (needs to be a Class Y type).
C6 will probably reduce the screen voltages and C4 will probably mean the AGC won't work properly. C11 will probably be causing low volume. C5, C7, C8 and C9 could cause trouble as well.

Just the kind of info i was looking for Yes i was thinking the same thing about C1. I'll take your advice and change the five you recommended and go from there,

The problem i'm having with replacement caps is the price, Can buy a pack of ten polyprop all the same value for about £4, But all the values i need vary,

Next job is testing the resistors, Getting there bit by bit.


Thanks simon


Jay

[Radio Scotland]

Originally Posted by Simon Gittins

C11 will probably be causing low volume. .

The only other electrolytic besides the smoothing caps, I might have one i can use but wont have a high voltage rating, So i'll need to see what voltage is present at that point,



Cheers
Jay.

[Radio Scotland]

I Have 4 X 100 OHM 100 WATT Aluminium power resistors that i'm thinking of combining to obtain the correct HT Voltage. The shocking plan is this.......

Get a piece of mains wire and connect 2 X of the above resistors in series (200ohms) and put them in series with the mains cable live. And measure the voltage..




What voltage should the rectifier see at its plate(s) ? 200 volts ?

[Radio Scotland]

Resistor values, Tested in situ, Tested values on the left, What they should be on the right.

Not looking good

[Simon Gittins]

I forgot to say that C12 should be rated at 1000V as the output transformer has voltage pulses higher than the HT line.
C11 doesn't have to be a high voltage type, the original working voltage should be marked on it and was probably 25V.
Post 63 - that test won't work! your meter will likely have a resistance of 10M and will just show the mains voltage if no current is being drawn. 470 ohms is a good starting point and you will be looking for about 200V dc between cathode and chassis. I suggest you start by not connecting anything to the anode and get the valve heaters and scale lamp working properly first.
Post 64 - the resistors were mostly 20% tolerance and don't have to be exact. If they measure high in the circuit then they have drifted high but if they measure too low then this may be due to another path. R3 is significantly high. R5 reads very low, this is probably due to leakage in C8. R9 reads low, this is probably due to leakage in C11.

[Radio Scotland]

Originally Posted by Simon Gittins

I forgot to say that C12 should be rated at 1000V as the output transformer has voltage pulses higher than the HT line.
C11 doesn't have to be a high voltage type, the original working voltage should be marked on it and was probably 25V.
Post 63 - that test won't work! your meter will likely have a resistance of 10M and will just show the mains voltage if no current is being drawn. 470 ohms is a good starting point and you will be looking for about 200V dc between cathode and chassis. I suggest you start by not connecting anything to the anode and get the valve heaters and scale lamp working properly first.
Post 64 - the resistors were mostly 20% tolerance and don't have to be exact. If they measure high in the circuit then they have drifted high but if they measure too low then this may be due to another path. R3 is significantly high. R5 reads very low, this is probably due to leakage in C8. R9 reads low, this is probably due to leakage in C11.

Some good info there simon I should have a replacement in the parts bin for C11. No voltage marking on the original.

Yeah i didn't think that test would work without a load, Plan was just to do a mock up circuit to check voltages, I don't wanna get the heater voltages wrong and pop them, Yes that was my plan, Get the heater circuit working with no anode voltage applied.


So what resistors would you change ?


Thanks Again.
Jay.

[davebms]

Hello all. as others have said probe leave resistors alone (poss r5 may be a problem) i have tacked in a modern in parallel to get one down to near)
grid coupling a recommended change. for testing i have used just 650 volt
caps, caps power resistors do not need to be in the set. i use fly leads poss a bit cardboard for safety i think its best to try to get radio type noises from set.THAT always makes me happier, if you change many components and set not work. Thats not good

dave

[Radio Scotland]

Originally Posted by davebms

Hello all. as others have said probe leave resistors alone (poss r5 may be a problem) i have tacked in a modern in parallel to get one down to near)
grid coupling a recommended change. for testing i have used just 650 volt
caps, caps power resistors do not need to be in the set. i use fly leads poss a bit cardboard for safety i think its best to try to get radio type noises from set.THAT always makes me happier, if you change many components and set not work. Thats not good

dave

Your right dave i'm getting carried away, I'll change the bare minimum amount of parts, i.e the ones that could cause damage on power up and take it from there, More parts that are changed more room for error,


Cheers.
Jay

[Radio Scotland]

Turns out R5 is 13.7M, wasn't getting a good connection first time i measured it, R9 Is 350 ohms even with C11 Disconnected which is dead.

Schematic shows C11 As a 10uf electrolytic, But its actually a big wax bomb

[Radio Scotland]

Must be a dry electrolytic, I'll re-stuff it tomorrow,

[Radio Scotland]

Got most of the parts on order, But need to know what type of capacitor i need for C12 (Tone correction) ? I know the value and voltage rating, But unsure of the actual type needed ?




Cheers

[Simon Gittins]

Any .02uF 1kV non-electrolytic type would be ok but there don't seem to be many that are sold singly these days and those that are seem to be ceramic types, here's one example:
https://www.mouser.co.uk/ProductDeta...xfWiv%2Fk0w%3D

Another thing to be aware of when buying resistors is that even if the wattage seems suitable they are not all rated for higher voltages. Here is one example that is rated at 350V:
https://cpc.farnell.com/tt-electroni...0k%20resistors

[Radio Scotland]

Originally Posted by Simon Gittins

Any .02uF 1kV non-electrolytic type would be ok but there don't seem to be many that are sold singly these days and those that are seem to be ceramic types, here's one example:
https://www.mouser.co.uk/ProductDeta...xfWiv%2Fk0w%3D

Another thing to be aware of when buying resistors is that even if the wattage seems suitable they are not all rated for higher voltages. Here is one example that is rated at 350V:
https://cpc.farnell.com/tt-electroni...0k%20resistors

I Can get a ceramic type on ebay, But didn't think they were suitable for audio circuit application.

Didn't know resistors had a voltage rating, Learn Something new everyday as they say.

Iv'e restuffed C11 With a 10uf ..50 volt capacitor audio output cathode bypass, According to your earlier post anything above 25 Volts should be ok ??


Cheers simon,


Jay

[Simon Gittins]

Yes, that will be fine and looks much better than a modern component.
Next step is to get the valves glowing!

[Radio Scotland]

Originally Posted by Simon Gittins

Yes, that will be fine and looks much better than a modern component.
Next step is to get the valves glowing!

Will do when the parts arrive.


Cheers for all your help


Jay

[Radio Scotland]

Meant to say - Evox Metallized Film Capacitor 22nF 1000V 5% ordered for tone correction.

[Sideband]

Originally Posted by Radio Scotland

I Can get a ceramic type on ebay, But didn't think they were suitable for audio circuit application.

Yes they can be used for audio and they are. In this case it doesn't really matter since its only a tone corrector. It's more important to get a suitable voltage rating.

I've come across 0.01uF ceramics as audio coupling caps on several occasions.

[Radio Scotland]

Originally Posted by Sideband

Yes they can be used for audio and they are. In this case it doesn't really matter since its only a tone corrector. It's more important to get a suitable voltage rating.

I've come across 0.01uF ceramics as audio coupling caps on several occasions.

Thanks sideband. Was just going by what i read online, I ended up buying the caps mentioned in my last post,


Jay

[Radio Scotland]

Tested all the wax bombs (in situ) and all give a reading albeit sky high. Only open one was C11 Since restuffed.

Problem is with restoring one radio and buying parts one at a time is costs quickly rocket.

I have a spare 22nf ..1000 volt capacitor, Could it be used to replace any of the remaining waxies ? I know value isn't super critical. As mostly coupling/decoupling.


Remaining wax bombs

C4 - .05UF
C6- .05UF
C7- 0004UF
C8- 005UF

Cheers
Jay

[Radio Scotland]

HT Resistor- 10 watt rating Really ?