11-24-2005, 11:59 PM
It's happened again...different 89 lowboy this time...same trouble in the oscillator coil. And this one came with a Type 77 det-osc tube as standard equipment (somewhat more reliable than the older models using a Type 36 in this position).
The hair dryer was not effective this time; I had rewound the cathode (tickler) winding after hitting the coil with the hair dryer, but all the set would do was squeal. Amazingly, when I put the test leads of my DMM in parallel with the tickler winding, the set would work properly.
So this time, I pulled the coil, along with coils from my 19TX and the leftover coil from the previous 89L, completely unwound all three windings on each, and baked the forms in the oven for an hour at 225 degrees.
Sealed the forms with lacquer, then proceeded to completely rewind one of them and reinstall it in this 89L. I'll rewind the other two coils later.
It seems to have worked; at least now the locals are coming in loud and clear, without the squealing it had previously. I'll know more tomorrow about how well it works, and if it is working on the lower end of the band as well - all our locals are on the high end, and it's hard to pick up anything in the workshop area of my basement.
Edit: I aligned it this morning and all went well for the most part, although the IF broke into oscillation at one point; that turned out to be a very simple problem which is now corrected. Also, I had to recone the speaker, so now it sounds really good in its cabinet. But it is now apparent to me that unwinding all of the windings from 89 oscillator coils, baking the forms, sealing them and then rewinding (with new wire) is going to have to be standard operating procedure from now on in order to assure a good, stable repair on these sets.
The hair dryer was not effective this time; I had rewound the cathode (tickler) winding after hitting the coil with the hair dryer, but all the set would do was squeal. Amazingly, when I put the test leads of my DMM in parallel with the tickler winding, the set would work properly.
So this time, I pulled the coil, along with coils from my 19TX and the leftover coil from the previous 89L, completely unwound all three windings on each, and baked the forms in the oven for an hour at 225 degrees.
Sealed the forms with lacquer, then proceeded to completely rewind one of them and reinstall it in this 89L. I'll rewind the other two coils later.
It seems to have worked; at least now the locals are coming in loud and clear, without the squealing it had previously. I'll know more tomorrow about how well it works, and if it is working on the lower end of the band as well - all our locals are on the high end, and it's hard to pick up anything in the workshop area of my basement.
Edit: I aligned it this morning and all went well for the most part, although the IF broke into oscillation at one point; that turned out to be a very simple problem which is now corrected. Also, I had to recone the speaker, so now it sounds really good in its cabinet. But it is now apparent to me that unwinding all of the windings from 89 oscillator coils, baking the forms, sealing them and then rewinding (with new wire) is going to have to be standard operating procedure from now on in order to assure a good, stable repair on these sets.
--
Ron Ramirez
Ferdinand IN
