10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 - Radio Road

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10.7 mHz Discriminator Transformer Repair/Replacement

I recently bought new tubes for my H-K F50Xk FM tuner to do an alignment. During warm-up the unit caused a loud crash in the audio then smoke curled out of the tuner chassis. A B+ supply resistor to the plate circuit of the 6BN6 IF amp/limiter stage was fried. It turned out that the problem was caused by the gimmick capacitor built into the base of the P/N GL3654577 discriminator transformer. At first I thought the capacitor would be just a discrete part up inside the transformer assembly can, but as it turned out it is composed of just two metal tabs brought in close proximity between two layers of polycarbonate plastic in the bottom of the transformer coil form assembly.

I have written Harmon-Kardon to see if the original part is available, but I have serious doubts that it is. If a new part is not available, the only solution may be to find a way to get the bottom part of the assembly apart and cut away the gimmick capacitor tabs and then reassembly with a new gimmick capacitor comprised of some twisted insulated wires. The gimmick capacitor is shown on the schematics of many of these tube type tuners as a coupling capacitor between the primary winding and the secondary winding with no value specified. There is one physical capacitor of disc ceramic type inside the transformer assembly which is a 33 pfd N150 across the secondary winding. Two 1N541 diodes are also inside the assembly. Those diodes are a matched pair shown as Z3414016 on the schematic.

This tuner uses the same RF/IF cartridge as the Citation III-X FM tuner and also uses the same discriminator transformer and 6BN6 circuitry and possibly some other H-K AM/FM receivers.

Does anyone have one of these discriminator transformers? If Harmon-Kardon does not have one and no one here has one, I may have to try disassembling the bottom of the transformer assembly and make a new plastic base of the same size and thickness, drill holes for the leads, install some heavy wire, say 18 AWG as substitutes for the original flat metal tabs. The old plastic could be separated just above where the original base plate was and the new plastic base could be carefully joined and a new gimmick capacitor fabricated from some bi-filar insulated wire or something similar could be soldered into the space available inside the transformer case. My problem there is that I have no idea what the capacitance should be.

Does anyone have any idea what the value of the capacitance between the primary and secondary of such transformers should be? Perhaps some older tuners or receivers may have had a discrete disc ceramic or silver mica capacitor up inside the transformer assembly that someone has seen and recorded the value of, OR maybe some schematics may show a typical value?

Joe

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Topic - 10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 13:13:05 09/18/11 (13)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 17:20:47 05/15/13 (0)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - R. Mercure 14:41:42 09/18/11 (11)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 15:48:57 09/18/11 (10)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 04:01:06 09/19/11 (9)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - R. Mercure 09:30:51 09/19/11 (8)

RE: 10.7 mHz Discriminator Transformer Repair/Replacement - strijw426 11:08:35 09/19/11 (7)

RE: 10.7 mHz Discriminator Transformer Repair - strijw426 09:36:46 09/21/11 (6)

RE: 10.7 mHz Discriminator Transformer Repair - BFitz 12:02:54 09/21/11 (5)

RE: 10.7 mHz Discriminator Transformer Repair - strijw426 18:15:02 09/21/11 (4)

RE: 10.7 mHz Discriminator Transformer Repaired! - strijw426 17:12:12 09/23/11 (3)

RE: 10.7 mHz Discriminator Transformer Repaired! - BFitz 09:49:36 09/24/11 (2)

RE: 10.7 mHz Discriminator Transformer Repaired! - strijw426 14:54:07 09/24/11 (1)

RE: 10.7 mHz Discriminator Transformer Repaired! - strijw426 07:02:24 12/16/12 (0)

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