I have acquired an American Bosch 1205A radio. Something drastic happened to some of the electronics.
Some of the wiring has obviously gotten hotter than normal and the dog bone resistors seemed to change value somewhat. They read about 30% greater than the color code. Is this normal to increase value as they age? Is it possible to tell the wattage of the old dog bone resistors by their physical size?

Other than these things the power transformer is good and I'm in the process of recapping. The tubes have no direct shorts, all the coils are fine and the filaments are good.

Thanks

:I have acquired an American Bosch 1205A radio. Something drastic happened to some of the electronics.
:Some of the wiring has obviously gotten hotter than normal and the dog bone resistors seemed to change value somewhat. They read about 30% greater than the color code. Is this normal to increase value as they age? Is it possible to tell the wattage of the old dog bone resistors by their physical size?
:
:Other than these things the power transformer is good and I'm in the process of recapping. The tubes have no direct shorts, all the coils are fine and the filaments are good.
:
:Thanks

For wattage to size relationships see this page on my website:

http://www.philcorepairbench.com/resistcompare.htm

Shows relationship between old and contemporary sizes.

Chuck, The Philco Repair Bench

::I have acquired an American Bosch 1205A radio. Something drastic happened to some of the electronics.
::Some of the wiring has obviously gotten hotter than normal and the dog bone resistors seemed to change value somewhat. They read about 30% greater than the color code. Is this normal to increase value as they age? Is it possible to tell the wattage of the old dog bone resistors by their physical size?
::
::Other than these things the power transformer is good and I'm in the process of recapping. The tubes have no direct shorts, all the coils are fine and the filaments are good.
::
::Thanks
:
:For wattage to size relationships see this page on my website:
:
:http://www.philcorepairbench.com/resistcompare.htm
:
:Shows relationship between old and contemporary sizes.
:
:Chuck, The Philco Repair Bench

Thanks Chuck.

It is normal for resistors to drift higher in value as they age, especially from heat, even if the heat is only mild. If you wish to save the old resistors, as they are no longer available, you can parallel new resistors with them, and perhaps hide the new resistors under the old resistors. There is a formula for paralleling resistors to get a specific value, but I forgot it. I will have to get back to you on this. At any rate, you can experimentally come up with the original value by trying various resistors in parallel with the original resistor in question. The resistors must be somewhat higher than the original value. Control grid bias resistors can actually be off by up to 30% sometimes, and the radio will work well as long as no condensers have ANY leakage. Bass notes will be accentuated with higher values on the control grids. If you wish, try the radio with both correct values and with the drifted values, and you can see which you like better. Resistors handling B current should be very close to original value, however. These would be resistors supplying plate and screen grid and cathode voltages. The plate of the first AF tube can have a slightly drifted resistor, though. This, like the control grid resistors, must be experimentally tried. Try the radio both with and without a drifted value at this place, and see which one you like better.

T.

:It is normal for resistors to drift higher in value as they age, especially from heat, even if the heat is only mild. If you wish to save the old resistors, as they are no longer available, you can parallel new resistors with them, and perhaps hide the new resistors under the old resistors. There is a formula for paralleling resistors to get a specific value, but I forgot it. I will have to get back to you on this. At any rate, you can experimentally come up with the original value by trying various resistors in parallel with the original resistor in question. The resistors must be somewhat higher than the original value. Control grid bias resistors can actually be off by up to 30% sometimes, and the radio will work well as long as no condensers have ANY leakage. Bass notes will be accentuated with higher values on the control grids. If you wish, try the radio with both correct values and with the drifted values, and you can see which you like better. Resistors handling B current should be very close to original value, however. These would be resistors supplying plate and screen grid and cathode voltages. The plate of the first AF tube can have a slightly drifted resistor, though. This, like the control grid resistors, must be experimentally tried. Try the radio both with and without a drifted value at this place, and see which one you like better.
:
:T.

Thanks Tom. I think I've found a place that encapsulates modern resistors in a mold that looks like the old dog bones. Not real cheap at $1.50 each but I only need a few. Soon this radio will live again.

Mike M.

Mike, Do you have a link to the place that does this work?
I have found that many of these 'dogbone' resistors are way off, and now make it a habit of checking each one. I didn't know they would drift off their original spec.
What I have done in some cases is to 'repaint' one or two as an experiment,to reflect their 'new value' and they looked good. Don't know of the reliability of them keeping stable though.
Take care!
Gary Rabbitt
:Thanks Tom. I think I've found a place that encapsulates modern resistors in a mold that looks like the old dog bones. Not real cheap at $1.50 each but I only need a few. Soon this radio will live again.
:
:Mike M.

:Mike, Do you have a link to the place that does this work?
:I have found that many of these 'dogbone' resistors are way off, and now make it a habit of checking each one. I didn't know they would drift off their original spec.
: What I have done in some cases is to 'repaint' one or two as an experiment,to reflect their 'new value' and they looked good. Don't know of the reliability of them keeping stable though.
:Take care!
: Gary Rabbitt
::Thanks Tom. I think I've found a place that encapsulates modern resistors in a mold that looks like the old dog bones. Not real cheap at $1.50 each but I only need a few. Soon this radio will live again.
::
::Mike M.

Gary,

The website is http://www.oldradioz/id154.htm.

I e-mailed the person and got a response. He said it would be about a two week delay in shipping but that's ok if the resistors look authentic.

Hope this helps.

Mike M.