RCA 9x571: Static in reception

Static during reception with this model is often caused by internal leakage in the IF transformers caused by silver creap in the internal capacitors that causes leakage current between the primary and secondary.

Note: use care when desoldering and soldering to these IF transformer pins because excessive heat will cause the plastic base surrounding the pin to melt and the pin to loosen which can cause internal connections to break.

This leakage can be checked by disconnecting the secondary (both terminals) of the IF transformer. Test one transformer at a time.  Temporarily connect the two wires together that were disconnected from the transformer secondary so the remaining radio circuitry will not be an open circuit. Connect a high impedance DC voltmeter, such as a vacuum tube voltmeter or a digital voltmeter, to either of the disconnected secondary terminals. Turn on the radio and check for the presence of any DC voltage reading on the meter. This might be a jumpy reading. A transformer with good capacitors will not have any voltage during this test.

If leakage is found it can usually be eliminated by disassembling the IF transformer and making repairs. Sometimes careful visual inspection will show black wiskers on the mica capacitor sheet between the primary and secondary sides which can be scraped away. Sometimes the mica sheets can be carefully cut or broken to separate the primary side from the secondary side. Sometimes the mica sheet capacitors must be disconnected or removed and replaced by temperature-stable ceramic or mica capacitors.

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Dear Paul,

please look over this interesting text:

http://www.radiomuseum.org/forum/pictorial_example_how_to_repair_if_transformers.html


Kind regards
Rolf

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Hello Paul,

Myself and a friend have this past week finished restoring a 1951 Zenith H-615 Long Distance MW radio which is around the same period as this RCA Victor. Like the RCA it has two IFTs,    failure of the two capacitors in the first transformer was causing static and failure of the filter capacitor in the second transformer was causing distortion. I gather that this is a very common problem with US built radios of this period.  Repair entails removal of the IFTs from the chassis, removal of the outer casings, careful detachment of the fine wires from the existing capacitors which are built into the base of the transformers, removal of the old capacitors and finally installation of new capacitors.

I hope to do an illustrated article for RMOrg shortly covering the restoration of this Zenith radio which will cover fault diagnosis, repairs to it's IFTs, replacement of other capacitors, faulty resistors and tubes, broken dial cord and the restoration of it's bakelite case and it's handle back into near mint condition.

Regards.......Howard

 

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Here is a link to repair of a similar IF transformer that may be helpful:

http://www.radiomuseum.org/forum/pictorial_example_how_to_repair_if_transformers.html

For the USA radios (Zenith, GE, and RCA) where I have found silver migration in the IF transformers, only in one case did I have to use a procedure this extreme.  In all other cases I was able to either remove the offending migration by scraping the mica sheet using a toothpick or a dental pick, or I was able to cut or break the mica sheets to cause a gap between the primary and secondary sides. To accomplish these repairs I remove the IF transformer from the chassis then I remove the outer aluminum can. No additional disassembly of the IF transformer is usually necessary. I work under a bright light and carefully inspect for the best (least intrusive) way to solve the migration problem for the individual IF transformer.

I strongly recommend making a drawing of the connections to the IF transformer before disconnecting any wires. Include any terminal numbers or color dots that can be seen surrounding the IF transformer pins.  Before the can is removed the can location with respect to the base should be marked. This can be done by making a small stripe using a permanent magic marker on the side of the can and on the base or by making careful notes on your drawing. Once the IF transformer can is removed, I again strongly recommend making a drawing of the coil wire connections.

If it is necessary to install new capacitors, it may be possible to measure the capacitance of the original damaged capacitors.  This should be possible if the silver migration has only connected one side of the original capacitors together, otherwise the primary and secondary capacitors will be connected in parallel by the silver migration. It is necessary to unsolder the coil wires before making this measurement.

With the correct capacitor value connected, the IF transformer adjustment will "peak" near the center of the adjustment range during alignment. It is definitely important that the adjustment has sufficient range to go beyond the peak resonance in both directions. Be sure you are using the correct IF frequency. 

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