The Fine Things are Always Hand Made
I have been struggling with the oscillator in my Allwave 15 project for over 2 weeks now. I thought I had started a thread here about it, but now I can't find it.
To begin with, I have aligned the IFs according to the procedure, to 265 khz. They were off pretty far, making me wonder if I had a custom IF, but there are no markings in the chassis to indicate that.
Briefly, my oscillator was dying at the low end of the red and green bands, at about 6mhz on the red band and at about 13 mhz on the green band. (I don't care about the blue band.) After implementing all of the suggestions here, to no avail, I acted on a suggestion and ordered 3 NOS tube of 3 different brands. Among the three, the RCA worked the best, just barely covering the red band, but still leaving me with nothing below 12 mhz on the green band.
On a suggestion from Norman on ARF, I removed the tuning cap, and cleaned and tightened the screw connections to the tuning gangs. Here is where I made an extremely bad decision. I have encountered a lot of sets where previous servicers had bent the segmented fins on the tuning gang, attempting to correct dial linearity problems, making proper alignment impossible between BC and SW bands. The fins on mine were bent a LOT, some of them sprung out 15 degrees or so, and even twisted quite a bit in some cases. I made the horrible decision, while I had the cap out, to just straighten them all. Oh boy. Now my sensitivity is poor on all bands. My oscillator dies at about 6 mhz and below on the red band and 14mhz and below on the green band and dial linearity is out the window. BC still works across the whole band, but sensitivity is much reduced.
My set is very late, and has a lot of changes from the technical instructions. I have no trimmers on the tuning gang. I have only 4 trimmers on part 55, the SW antenna coil trimmers. What should be #1, I believe is on top of the BC antenna coil.
I set out to try to recover from this mistake working on the red band first, as it has no padder, so dial linearity has to be done by adjusting the tabs on the tuning cap. I can adjust the 10 mhz end just fine, but there are 2 peaks on that trimmer. If I use one of them, I have to keep pushing the tabs inward as I advance down the scale. By pushing them nearly to shorting together, I can get the dial linear down to about 8 mhz, but it is hopeless below that. If I use the other peak, bending them outward is necessary to get the frequency correct, but is is off so far that I would have to bend them a LOT, and I don't feel comfortable doing that.
I have read the section in the technical instructions that talks about using the "lower setting" on the blue and red bands and the "upper" setting on the green band. I am not sure which of these is which when adjusting the high end trimmer on the red band.
I guess, what I am hoping, is that someone here has been through this and can help me with some step by step instructions. I do not believe that the trimming tabs on the tuning condenser should have to be bent as wildly as mine originally were to get this thing to work.
HELP!! I know I screwed up. I was so close. Dang.
Remove the tuners from the chassis. Connect modern handheld digital capacitance meters to each section. Starting with the rotor set one sector from fully un-meshed, adjust the first sector such that all sections have equal capacitance. Rotate to the next sector and repeat. When finished, check and readjust as necessary. Adjustments should be made by rotating sectors leaving the high frequency end (lower capacity end) at the position it was when starting.
Well, yes, I can do this. It is pretty much what I did when I straightened all the tabs. I'm pretty sure, right now, that my 3 gangs will all be very close. Unfortunately, that doesn't help me with dial linearity and oscillator drop out at the low ends. These things were crazy bent when I got the set. With them all lined up, the set doesn't work. I would like to find a solution that doesn't involve bending the snot out of all those tabs. I would also like to avoid that nasty job of removing that tuning cap again, as I don't think it will accomplish anything. Sure hope someone has such a solution.
Thanks for the suggestion, and I truly appreciate the effort to help, but I am certain it will not fix my problem, as this is what I have essentially done already, and caused most of my problems.
Somewhere in the Scott service literature they warn the tech NOT to mess with the bent tabs on the tuning - they are set at the factory via some mysterious testing process and must not be adjusted in the field. Given that you've probably made that tuner unusable, I have some parts chassis here, I could pull off a tuner from another AW15. It may not match the coils you have exactly, but it will be better than the straightened tuner you currently have. I suppose we could even swap the coil assembly from the same chassis....just thinking out loud now. I do know there are at least two different coil assemblies for the AW15 however.
Well, this is kind of a "cost only" job for a family member from Arizona. I'm in it pretty deep already. If I changed the tuner, I would want to change the coil wheel, hopefully with a matched pair. There has to be something wrong with the pairing of this coil wheel and tuner. As I received it, before I messed with with the factory tuning of the trim tabs, this radio would not work at the lower 30% of the red and green bands. After trying everything else, I was fooling with that hoping that the problem was a previous servicer messing up the trim tabs. I have it working again, better than it was when I got it. My only problem is from about 13 mhz down on the green band. There is nothing else to change. It just doesn't work, and I don't think anyone could make it work. I would like to talk to my cousin about it. If you have a coil wheel and tuner from the same radio, I would like to know what the cost would be when I contact him. Can you give me some idea?
Thanks for the offer, and trying to help.
As I mentioned, there are two different coil wheels for AW15 sets, and they are not interchangeable (learned this the hard way). Can you send me a picture of the one in this set. The view from the bottom (coils, not contacts) is fine.
I have ordered a new coil wheel and tuning cap from Kent King. There is just nothing else to try on this problem. I got a little grief on the forums for fooling with the trim tabs on the tuning cap, but, darn it, neither of these upper short wave bands worked when I got the set. They both work much better now, and having eliminated everything else, I was left with trying to tinker with those little trim tabs.
While I am waiting for those parts, I have decided to tackle another possibly imaginary problem:
The performance on the broadcast band is very passable on this set, but I don't think it is as good as it should be with a dedicated RF amp. There are a couple of symptoms that have me a little baffled. There is the little cam actuated antenna switch in the Broadcast antenna coil can as well as the 1500 mhz antenna trimmer. I have 2 symptoms in that can.
1: The antenna trimmer for high end RF tuning is on top of the coil form in this can. It will not peak. The more I back it out, the better it gets, until I run out of travel. I am working on a 70 foot long wire if that matters.
2: If I tune a shortwave station and open up the contacts on that antenna switch for shortwave, the station goes away. When I tune even a relatively weak broadcast station at 1400 and open the broadcast side of the antenna switch it makes almost no difference.
That coil has a winding on the outside of the form and another coil down inside of it, clear down at the bottom. I have continuity from the antenna post to the center wiper of the switch, but I'm having trouble following it after that. The newest schematic I can find has quite a few differences, especially in this area of the set. It is almost impossible to find the connections to the 2 coils on this form.
Have any of you fine folks encountered anything that might save me some time trying to ferret this out?
Curious, what is the black wire laced to the set cable??
My customer wanted FM, so I put in one of the David Winter FM modules. I installed a micro switch, and engineered a little cam on the coil wheel so when you select the blue band it disables the AM and activates the FM module and you tune it with the tuning dial. The FM works great. You can see the little actuator I put on the wheel and the red microswitch, and the FM module is visible at the lower left of the coil wheel in the diagram. The wire is from a 5V linear power supply I installed in the Amp chassis to power the FM and switch the relays. Normally you can power the FM with the filament supply, but the 2.5v filaments of the AW15 were not useable for this, so I had to run the wire and lace it to the receiver cable.
I've been waiting a couple of weeks for a new, matched set coil wheel and tuning condenser from Kent. Got them installed today. No real difference. Broadcast sensitivity is a trifle better. Red band works down to about 5mhz and green band to about 13mhz, so pretty much the same as before. Now that I have 2 coil wheels, I am going to start fooling with the windings on the coil forms for the upper bands and see what I can accomplish with that. There is nothing else left in the circuit to work on.
On the other issue of mediocre broadcast sensitivity, I still find it odd that I can open up the antenna switch contacts, even on a weak BC station, and it makes almost no difference. I spent 3 hours this morning tracing out every wire in that antenna switch circuit and Broadcast antenna coil. All is apparently as it should be. I say apparently, as there is no diagram I can find that matches my set, which I think is a very late model. I had Kent throw in a broadcast antenna coil as well. It looks and reads just the same as my original, so I don't expect it to make any difference in broadcast sensitivity, but I will put it in there so I can say I have eliminated ever part in the circuit.
That's about it. I'll post something here if I make any progress on either problem.