Repairing K-Tran Type 3/4" IF Transformers

100pf is a good starter choice for 455-6..

IF's can be tested for peak using a generator, w/or/wo modulation (sweep). It does not apply any high voltage to the plate windings so a mica defect may not be revealed.

Isolate the generator with a 10k carbon resistor in the hot lead and use an RF voltmeter or a VTVM with a de-modulator probe. Modulate the RF signal. You can hand plot the curve... If sweeping, demodulate for the scope. Be sure the probe has high impedance or make a suitable network.. I suppose one could isolate the plate side with caps then apply 200 VDC via a 50K resistor. That should reveal any bad mica. Sweep at 60hz so service scope can be used.

Or use a junker AA5 and take out its 1st IF and install clip leads.

Chas

I need to work on this K-Tran IF repair some more. I am hoping that the (2) coils are all right. The wire leads to the lugs are very fine and subject to breakage and this may have caused it to become intermittent in the first place. It may not have been the mica caps at fault. I am not certain that I got the replacement ceramic caps right (100pF). It is an Output IF can and sometimes these use different values on the primary vs. the secondary windings. I only measured one side of the mica wafer that came out of the can. I had some trouble doing that as I said before. They looked the same however. Since then I have heard about a better way (see Radiotvphononut video) to measure the caps before removing them from the IF can (assuming they are not shorted). I might give that a try next time.

Since last time, I have got in stock (4) more Merit IF cans. I am looking at a way to check out IF cans ahead of time with a signal generator. I might need to build up a simple cathode follower circuit to isolate the IF can under test from the low output impedance of the signal generator. The model currently in use is a Jackson 640. It is working well and needed but little restoration after receiving it. Just a bit of realignment and a tube substitution (3 X 6J5).

73, Rod  WB6FBF

I managed to measure the capacitance of the dual silver mica capacitor removed from the defective Hallicrafters/Automatic K-Tran style Output stage IF transformer. They are of necessity rather small and thin in size. At first, I tried to solder two fine wires to one half of it. Since the solder wouldn't take to the silver contact, I used an insulated alligator clip to clamp the two wires to the other half of the cap and measured it on my Eico 950A and got about 120 pF for it. Then I measured it on the lowest capacitance range of one of my digital multimeters and got 103 pF. I saw some small NOS 100 pF ceramic caps for sale from an eBay seller recently that I am thinking of buying some items from and hope to give them a try soon.

I could use some help to pick suitable replacement mica or ceramic capacitor values for an Automatic Mfg. Corp. 3/4 Inch 455 kHz Output IF can out of a Hallicrafters SX-99/110 receiver. This is for the 6SK7 2nd IF Amplifier stage to 6H6 Second Detector and Noise Limiter stage. The Hallicrafters part number (a fairly common one for them) is either 50-300242 or 50C242. 242 seems to be the operative part number. This IF can uses a cup core design and the coils read about 16 Ohms. I think the coils are OK and that the silver mica cap connections were at fault for making it intermittent-no doubt throwing off the resonant frequency. I got the flat one piece mica cap out. It was held in tightly by a central rivet. The coil connections themselves look OK. 

I just got in an SX-110 receiver and after initial checking, recapping and removing some audio modifications I found it to have intermittent operation especially on AVC. Signals would cutout, the S-Meter would drop to S0 (indicating high plate current in the 6SG7 1st IF stage) and the AVC voltage would go positive from several volts negative to zero. After some checking of the various AVC and cathode bias circuit resistors of the RF, and IF stages, I found that the RF gain control wiper arm was not making contact with the rest of the pot so after some cleaning of the control I managed to get the RF gain circuit working properly again.

Performance was improved but the main intermittent problem was not solved. After some thinking I decided to go ahead with an IF alignment even with the intermittent operation. My idea was that maybe something will show up to point to the problem during the alignment.

I set up my equipment and started with the alignment finding the crystal filter series resonant frequency first and then resetting the BFO for a zero beat. Next, I started on aligning the IF transformers. The 1st and 2nd IF cans went fine but the 3rd IF can gave trouble. First of all, the wiring and components going to the IF can blocked access to the bottom slug under the chassis. I had to bend these leads aside. I noticed right away that moving these leads caused a change in the AVC output voltage. I was monitoring it at the output of R14 (47K Ohms) with the TS-375A/U VTVM. This was helpful to find as it turned out. Further, trying to adjust the slug to a peak on the meter was nearly impossible as the reading would jump around. I concluded there was something bad about this 3rd IF can and didn't waste much more time on it beyond re-soldering all the leads going to it. That not helping, I disconnected all the leads and pulled it out of there.

I had a NOS spare in the form of a Merit BC-353 Miniature 3/4 Inch chassis mount 455 kHz Output IF can. This is a nice style IF can in that the resonating caps are small discrete tubular ceramic caps and the coils are wound on a single threaded form with (2) small adjustable cores. After installation, I checked and adjusted the alignment for the stage and it went fine. I think I got it solved.

Afterwards, I turned my attention to the old Hallicrafters/Automatic IF can. I would like to get some new caps for it pending further investigation and some recommendations. I hope to get small enough replacements that will fit inside the IF can. There is a bit of room in there for (2) such capacitors.