Hello Phorum
Getting started on my 38-12, I see some previous replacement caps under chassis. Not sure if that work addressed the filtering caps, also I have posted photos of the antenna transformer wiring. Is it correct? Thank you for any guidance you might be able to provide.
I have restored at least three 38-12, one 38-14 and one 38-15, which are more or less the same radios.
You could search for 38-12 and my name (and there are others' names with the same radio).
I see that the AC filter cap (large backelite in the center) was re-stuffed.
I am not shure from the pictures of what's going on with the Plus of the electrolytic in the corner. Is it being used as a post? If yes, is the case of the original cap isolated from the chassis?
Now, the main question is: why are you planning to address the radio? It's been recapped before: does it not work, or you have other considerations?
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
This is my first attempt at recommissioning a tube radio.
It was donated to me.
I read up on the process.
I understand that a Variac is recommended, but without being aware of the previous cap replacement, I elected to make a brief functionality check. A long wire antenna was connected, and the chassis grounded to a radiator. The radio powered up without issue, all five tubes' filaments lit up.
The audio was a buzzing in the background with a whine superimposed on it.
The audio was loudest at the 1700 kc end, decreasing at lower frequency. One station was clearly received at about 1000 kc, broadcasting the BBC World Service. I did not wait fot station identification, and unplugged the set to begin my physical inspection.
The original item 28 12 mfd. Condenser is still present and wired into the 8-position terminal strip at the center of the chassis.
The original Item 26 4 mfd. Condenser has been removed. In that corner of the chassis, the positive lead of the blue 22 mfd.,450 volt XICON cap is joined to three wires, feeding item 22, the output transformer on the speaker, the speaker voice coil and the base for the #41 output tube.
The negative lead of that cap is connected to the item 30 .01/.01 mfd. bakelite Condenser.
The negative lead of the new, blue 10 mfd. cap is connected between one end of the 8-position terminal strip and the socket for tube 84.
Make sure the old capacitors are not wired into the circuitry.
If a cap is insulated from the chassis, it's plus then could be used as a soldering post (make sure it's minus then is not). Or vice versa, if you want to use its Minus, do not connect the Plus.
The smaller cap, electrically connected to the chassis, cannot be used. You said it has been removed.
Your noise could be due to the modern electronics/light (LED, CCFL) interference etc.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Thank you for sharing the thread of 08-17-24.
I have two questions, having read thru and examining the great photos. First, what has been done to the 0.01, 0.03 and 0.05 mfd. tubular wax caps, they still look original, and please explain more about the bakelite cap 3903-DG, item 30 on the schematic. Has it been modified?
This is my first radio, and I'm pretty unfamiliar with standard practice.
Since the caps are tubular, they're built by coiling the paper and foil with leads attached, inserted into a cardboard tube, sealed at both ends, then dipped in wax. Mike did what a lot of restorers do, he restuffed them by pushing out the old guts and replacing with a modern cap, sealed the ends with hot glue, then hired 50 honey bees to wax them.
Excellent info, thanks so very much.
Can you suggest a source for the stuffing caps?
Digikey, Moser, etc ? I dealt with them before I retired, but mostly surface mount chips.
There are people who sell these non-name caps, and they are good. As long as it is 600V cap, you could stick it just about anywhere. There are some names here in teh Resources; then there is Justcapacitors, Tubesandmore (Amplified parts it is called today), and yes, Digikey and Mouser (Moser is a watchmaker).
I buy mostly from Mouser when I need to by a lot of everything; if I need a very few parts, I might go elsewhere.
For the purpose of radiorepair, any paper cap could be replaced by a film cap.
PS. DO not replace mica capacitors unless during troubleshooting one is proven bad; they do not go bad on their own due to age.
PPS> And, yes, like Rod said, I restuff the caps. I replace the resistors only if needed (badly out of tolerance/open/burnt).
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
I have finished identifying and testing the caps and resistors on this 38-12, except for the final resistor on my list. On the schematic it is identified as Item 25, a 250 ohm, 1/2 watt. However the color code appears to indicate 270 ohm, 20%.The colors are red, pink (violet), brown and silver. Photo attached.
I lifted one leg and it measures 306 ohm.
What should I use to replace it?
Only two other resistors were out of tolerance:
Item 9, a 2 Megohm (no wattage speced) tests 2.36 Meg, and Item 11, a 40 Kohm measured 49.5.
Thanks for your patience and assistance.
So glad to get started on this job, and I have several more radios I plan to work on.
The resistor seems original. I probably would leave it be, considering that it is within tolerance for 270 Ohm and not that far out of tolerance for 250.
2M resistor is fine.
40K is also fine.
If you want to replace them, do it, but it will work fine as is.
PS. Did not see the original photos of teh whole chassis; the middle tube, the 75, the 2nd Det/1st Audio, is supposed to have a shield around it. If you decide to align without it, you might get squealing.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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