38-10T chassis Restoration begins - Printable Version
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38-10T chassis Restoration begins - radiorich - 09-23-2023
Hi Everyone,
I just started my lasted restoration Model 38-10T "Bullet".
The first issue was getting the center tuning knob off it was frozen on the shaft after using some cardboard and mini plastic prybar it came off ,
Here are a few photos of the Chassis as you can see there a missing Electrolytic can !
So far as everything looks in good shape no burned or mouse eaten part .
I will be checking all the resistors plus that bias voltage dividing network so should just shotgun that Dividing network?
Sincerely Richard
P.S. this looks like it will be a fun project !
RE: 38-10T chassis Restoration begins - radiorich - 09-23-2023
Hello Guys,
My first item of business was to do the one Bakelite block that will house the two safety capacitors !
I just got cleaned out and ready to install the new X & Y safety capacitors .
Sincerely Richard
P.S. I will take a photo and show some more progress !
RE: 38-10T chassis Restoration begins - radiorich - 09-23-2023
Here you go the finished bakelite block ready to install !
Sincerely Richard
RE: 38-10T chassis Restoration begins - RodB - 09-24-2023
Richard, when you reinstall the block do you rewire the power switch to apply power to the caps or leave it as is to apply power when the cord is plugged in?
RE: 38-10T chassis Restoration begins - morzh - 09-24-2023
Rod
I would recommend simply installing 1M resistor across the caps (the right two holes on the picture) and not to think of it anymore.
RE: 38-10T chassis Restoration begins - radiorich - 09-24-2023
Hello Rob,
Well to answer your question on this set I will leave it as is but I will be adding a fuse to the radio between the line cord and the bakelite block .
Sincerely Richard
RE: 38-10T chassis Restoration begins - radiorich - 09-24-2023
Hello morzh,
Well well put and Yes the one meg resistor good idea !
Sincerely Richard
RE: 38-10T chassis Restoration begins - RodB - 09-24-2023
Mike, wouldn't that be true if the chassis were tied to earth ground through a three wire power cord? Otherwise there is no complete circuit with a two wire cord. Or is it just smoke and mirrors? My concern is if you leave the unit plugged in all the time, there's power on the caps all the time. What can go wrong??
RE: 38-10T chassis Restoration begins - radiorich - 09-24-2023
Hello Rob,
Yes there is a chance the capacitors could short !
But with any of my sets they are never left plugged in when not on.
Far as this set it will be plugged into self contained bucking transformer in a enclosure which holds a bucking transformer plus fuse and outlet and powerswitch .
Sincerely Richard
P.S. good source for bucking transformer i found in 12 yard lights !
RE: 38-10T chassis Restoration begins - radiorich - 09-24-2023
Hello guys,
Todays task is to stuff the one of the two electrolytic cans which houses a 4ufd and 8ufd .
Sincerely Richard
RE: 38-10T chassis Restoration begins - morzh - 09-24-2023
Rod
Here's what I think is there:
1. Two-prong plug:
Normally you would have your switch on the "Live" wire and the "Neutral" is the one permanently connected.
- With the caps in front of the switch, upon plug-in the caps connected in series will charge to the whatever voltage there is at the moment of unplugging, as the chassis is insulated. So you have 7.5nF charged to up to potentially 170V.
- With caps after the switch they will fully discharge via the load, uf the radio is hot or vial sheer DC resistance of the primary if cold (plug-unplug). So no danger here, but there is another problem - every time you make the switch you create a spark and this will eat the metal out eventually; I think this was one of the thoughts behind having the switch after the caps, as the transformer when the radio is cold is simply an inductance and should not create sparks, and when it is the break, then the transformer looks like DC resistance and also should not spark too badly.
2. 3-prong plug:
Same thing pretty much, only the lower cap never charges much due to the Neutral and Chassis being under the same potential, and the charge kept by the cap is twice as large (15nF vs 7.5nF) and the energy is 4 times, so the pinch will be more painful.
So to have the benefit of not sparking and not having charged caps the switch is better off being where it is now, and a 1mOhm resistor will discharge the caps in 7.5ms.
There is nothing wrong with the caps plugged all the time. These caps in the pic are Y2-rated (or they should be) and so fail open. If X-cap is used (why?) then there should be a fuse in series, as X-cap is not guaranteed too fail open.
And you are right, there is no comp[lete circuit, and this is why without bleed resistor you could get a little shock.
RE: 38-10T chassis Restoration begins - David - 09-24-2023
Time for what might be a stupid question. Looking at Nostalgia air calls for a 6A8G your set has a metal tube would that make a difference?
RE: 38-10T chassis Restoration begins - radiorich - 09-24-2023
hello morzh,
yes, those are X&Y rated safety capacitors !
RE: 38-10T chassis Restoration begins - morzh - 09-24-2023
Rich
Yes, I know these.
Y-rated caps are specifically made to be connected from AC wires to safety GND and are used in, for instance, off-line power supplies as bridge caps or common mode filter caps, all of which connect various parts of the AC line rectifier or the lines themselves to GND. Therefore they are supposed to fail open as in the situation with accidental GND disconnect (touch current).
In the days of yore there were no X/Y rating. They just counted on high enough working voltage to guarantee from the electrical breakdown. Which, to my knowledge, still resulted in some untimely demises of a few human beings, including some more or less famous ones.
RE: 38-10T chassis Restoration begins - radiorich - 09-25-2023
Hello morzh ,
Yes , I bet there was and a bunch of house fires too paper caps where just pure junk .
Sincerely Richard