Superstar 121 lost channels

[Radio Tech]

Had one of these come in. No TX. Found final bad. No big deal. Repalced and all was fine.

Put it back on the bench one more time before calling the guy. Noticed that the low channels was dropping off about 8 channels down below 1. Checked high and no out put at all.

On channel 20 radio dead key is 2 watts. As you go hiher in frequency (channel 35) over half the power is gone. Same below channel 20.
Radio is no longer broadbanded enough for the normal 40. I have seen this problem years ago. But can not remember what it was. I tuned the VCO and made no difference. I ws thinking there was a zener diode that supplied voltage to the coil. Can not find it on the schematic. Checked for cold joints also.

Any ideas to jarr my memory?

Thanks.

 

[Moleculo]

I'm not familiar with that radio and don't have the schematic. So take this for what it's worth. Does that radio have Cans to adjust the coils on the band pass filter? If so, did you check to see if anyone monkey'd with them? That would definately mess up the power output across the band.

 

[Radio Tech]

Yes but I do not have a service manual on this model. I have to check off the schematic.
A momnet ago I was making some checks and watching the TX on the spectrum. I notice the frequency was drifting off center by 15 kc. Then by just touching L17 the frequency would center. On the next key it would change again.

So sems that there is something heating in the vco section.
I am done for tonight though.
Going to kick back and lok over the schematic.

Thanks

 

[Radio Tech]

Still looking info on this problem. I may get a chance to get back to it this week end.

 

[freecell]

"Then by just touching L17 the frequency would center. On the next key it would change again".

were you still in contact with L17 when you re-keyed?

 

[Radio Tech]

At the time no. I have found it would change with out touching it. It seems that when you key up the radio, and looking at the spectrum analyzer the "spike" is anywhere with in 30 kc's on either side of the displayed frequency.

Really sounds like a bad ground somewhere in the vco circuit. I am going to go back and solder some joints in the area and see if it will stabilize.

 

[freecell]

it takes a fairly large amount of stray capacitance to shift L17 15 - 30 khz. and that would be readily supplied by your contact with the inductor. i've seen this happen when the enamel covering the wire with which the inductor is wound is excessively worn because some screwdriver jockey has been in there twisting the slug relentlessly. if you run out of options try swapping L17 out with a known good replacement. FYI.

 

[Radio Tech]

i've seen this happen when the enamel covering the wire with which the inductor is wound is excessively worn because some screwdriver jockey has been in there twisting the slug relentlessly. if you run out of options try replacing L17 with a known good replacement. FYI.

You may be on to something there. About 4 tuning cores were broke in the radio. There was no heat compound behind the driver ,final, and the 2sa1012. The wax cover the vco coill had been removed and you can tell some one has been having a good ole time with it.

Going to take your advice and go ahead and replace it.

 

[freecell]

"You may be on to something there. About 4 tuning cores were broke in the radio."

that's usually a reliable indication that some idiot hobbyist has been in there with the wrong tools twisting his ass off. if you back the slugs out of the bottom of the cans and invert them you can usually re-install them and they will work. if over half of the slug has been turned into crumbs then replace the entire toko can assembly.

let me know the results.