08-15-2008, 03:52 AM
originally written by Norm Leal:
A radio with defective audio interstage or intermediate freq transformer can often be repaired without replacing or rewinding the transformer. For many of us, it is difficult to find a replacement that fits. In case of the IF trans, the early radios did not standardize on a freq, and a exact replacement may be nearly impossible to find. The trans coupling can usually be replaced by capacitor/resistor coupling often without any loss of performance. In the case of Audio Interstage trans, connect a capacitor from the plate of the driving tube to the driven tube grid, the value is not critical. I suggest 0.1 mfd @ 400 v. Then, bypass the open winding with a resistor. Again, the value value is not critical. If the plate to B+ side is open, try using a 22k @ 1 watt. If the grid winding is open, a 100k 1/2 watt will work. I think you will be surprised @ the results. The freq resp will probably be better than orig circuit design.
You may do the same with IF trans also. With IF trans, use small caps between the plate of the driving tube to the grid of the driven tube. This will allow peaking of the 1-side of the trans which is still working. Caps around 200 pf will usually work. Again, the resistor value used to bypass the open winding is not critical. The same values used for AF circuits will work. I have suggested rather low resistance values so almost any circuit will work. In the case of tetrode types 24A, it is important to keep the plate voltage above the screen, and tubes like the triode 27 should also be used with fairly low plate resistors.
Thar ya go techs!! Bypass those bad trans, and no need to rewind, unless "originality" is paramount in your restorations,!!??
A radio with defective audio interstage or intermediate freq transformer can often be repaired without replacing or rewinding the transformer. For many of us, it is difficult to find a replacement that fits. In case of the IF trans, the early radios did not standardize on a freq, and a exact replacement may be nearly impossible to find. The trans coupling can usually be replaced by capacitor/resistor coupling often without any loss of performance. In the case of Audio Interstage trans, connect a capacitor from the plate of the driving tube to the driven tube grid, the value is not critical. I suggest 0.1 mfd @ 400 v. Then, bypass the open winding with a resistor. Again, the value value is not critical. If the plate to B+ side is open, try using a 22k @ 1 watt. If the grid winding is open, a 100k 1/2 watt will work. I think you will be surprised @ the results. The freq resp will probably be better than orig circuit design.
You may do the same with IF trans also. With IF trans, use small caps between the plate of the driving tube to the grid of the driven tube. This will allow peaking of the 1-side of the trans which is still working. Caps around 200 pf will usually work. Again, the resistor value used to bypass the open winding is not critical. The same values used for AF circuits will work. I have suggested rather low resistance values so almost any circuit will work. In the case of tetrode types 24A, it is important to keep the plate voltage above the screen, and tubes like the triode 27 should also be used with fairly low plate resistors.
Thar ya go techs!! Bypass those bad trans, and no need to rewind, unless "originality" is paramount in your restorations,!!??
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