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Bright orange screen grid - Printable Version

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Re: Bright orange screen grid - Music in a bottle - 07-27-2009

kruc Wrote:Codefox, I'm keeping the same capacitance the sets were designed for, I just wondered if my 400v caps that were at their upper voltage limit. I don't have an issue with the slightly higher price of the 630volt caps, I just happened to get some 400v units at the time. I'm trying to stay away from electrolytics whenever possible when I rebuild a unit, that's why I'm using plastic caps whenever possible. FWIW, I found 600v plastic caps at AES. B+ shouldn't be an issue after replacing the filter with the same capacitance.
Music, my 20 lists a primary of 105-125volts. However at 125 the secondaries will put out a higher voltage than the tube filaments and plates were designed for. Recent threads on ARF discussed using a bucking transformer or a resistor in series with the primary. The obvious issues are finding space for a transformer or adequate area for heatsinking a wirewound resistor. I have a Japanese superhet with a 100v primary I run off a variac as I haven't gotten around to reducing the input voltage permanently.
Dave
I don't know, but the Variac just struck me as a sure bet for protection, and though I know very little about how to use test gear, it does look impressive and makes me at least "feel" like a certified electronic tech! 8)

Re: Bright orange screen grid - kruc - 07-28-2009

Wait until the first time you accidentally bump the variac knob and send 150 volts into your precious 70. You'll be looking for a better way to reduce voltage after the magic smoke clears.

Re: Bright orange screen grid - Alan Douglas - 07-28-2009

Quote:In the 1939 season they began putting the output transformer under the chassis in most models. These tend not to survive while the speaker-mounted outputs are usually still good.

Could also be an outside supplier, where the earlier transformers were wound in-house.

115 volts was very common in the 1920s, but 120 and even 125 volts were standard in some areas. I have the exact percentages at home (I'm at work now).

Re: Bright orange screen grid - Arran - 07-28-2009

kruc Wrote:Codefox, I'm keeping the same capacitance the sets were designed for, I just wondered if my 400v caps that were at their upper voltage limit. I don't have an issue with the slightly higher price of the 630volt caps, I just happened to get some 400v units at the time. I'm trying to stay away from electrolytics whenever possible when I rebuild a unit, that's why I'm using plastic caps whenever possible. FWIW, I found 600v plastic caps at AES. B+ shouldn't be an issue after replacing the filter with the same capacitance.
Music, my 20 lists a primary of 105-125volts. However at 125 the secondaries will put out a higher voltage than the tube filaments and plates were designed for. Recent threads on ARF discussed using a bucking transformer or a resistor in series with the primary. The obvious issues are finding space for a transformer or adequate area for heatsinking a wirewound resistor. I have a Japanese superhet with a 100v primary I run off a variac as I haven't gotten around to reducing the input voltage permanently.
Dave

In my experience 400 volt condensers are usually fine in most old radios, I have a number of .047 mf 400 volt ones that I have used and none have failed. The typical plate voltage in an AC set isn't any more then 275 volts running, and maybe 350 volts on start up, most circuits will not see anywhere near that. One area where I would use a 630 volt unit is a situation where if the condenser fails that the B+ would short out of find a low resistance path to ground, some tone control circuits could have this happen. Electrolytics will work fine in place of a large paper condenser, just make sure that you get one of the right voltage and observe the polarity correctly.
For the Japanese radio I would take a look at junked printers, scanners, and other assorted consumer electronics for a bucking transformer, some were 25 or more volts at a high amperage, I would then build it into a box with a standard electrical outlet with the transformer in series with one side of the line.
Best Regards
Arran

Re: Bright orange screen grid - kruc - 07-29-2009

Arran, I'm using plastic caps to get away from electrolytics that dry out eventually. Same for paper caps as I've had them disintegrate before I could get both leads unsoldered. So much for testing a component that's in 2 pieces. I use better capacitors and higher tolerance resistors than Philco used so hopefully my radios will be playing long after I'm gone. My concern was only for the working voltage on my Philco 20's filter caps, but I've just gotten some Solen 630v units to replace the black mess I melted out of the can. For the Japanese radio, a wire wound resistor screwed to the chassis will be the simplest solution to get 100v to the primary.
Dave

Re: Bright orange screen grid - Arran - 07-29-2009

kruc Wrote:For the Japanese radio, a wire wound resistor screwed to the chassis will be the simplest solution to get 100v to the primary.
Dave

Dave;
A resistor may be a simple solution but I wouldn't call it the best, heat is the enemy of electronics and if you can avoid adding more of it it's a good idea. Suitable bucking transformers are just about everywhere for the taking, and all of the other parts needed are available from the local hardware store. I'm the sort who prefers to find a way to operate a radio as it was originally designed, even if using modern parts.
Best Regards
Arran

Re: Bright orange screen grid - kruc - 07-30-2009

The Japanese radio was a kit I bought in Akihabara in 1991. The directions are in Kanji, and it was a fun mental exercise to put together. Akihabara is a neighborhood in Tokyo with small shops that sell caps, resistors, transformers, tools, tubes, kits, and computers. I wish I had a place like it that I could visit for parts, though the closest example in this country was probably the old Radio Row in NYC. As the radio is only a black steel chassis with an aluminum faceplate, a gold ww resistor wouldn't hurt it even if it here mounted on the back.
Dave