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38-8 code 121 run 2 alignment problem..... - Printable Version

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38-8 code 121 run 2 alignment problem..... - Guest - 11-07-2009

Hi Guys....
( Hi Ron, Chuck....long time, no hear...)

Here's the problem in a nutshell....
After recapping and re-res'ing, all tube voltages are now normal. IF alignment at 470 kc went without a hitch. My problem now lies with the ant/osc. circuits. I have no adjust for compensators 4A or 4B nor 7 or 7A. Checking the bias on the 6A8 showed a - 7 volts on the osc. grid, with g2 showing 170 volts and the plate and screen showing 180 and 90 volts respectively. So far, so good.
The RF alignments are another matter. Going by Philco's procedure, starting at 18 mc., no signal could be heard at this point. In fact, there was no signal heard throughout the entire range. A continuity check of the 4 coils were all on the advertised. Removed wax and did a visual of the coils and found no "greenitis" in the windings. Checked switch operation and continuity and that too passed muster. Began to suspect the mica caps and replaced (item 8) 3500 uuf. cap and jumped out the 250 uuf. cap (7B). Still no luck with SW signal alignment but on the BC band reception was "bunched" above 1100 kc. Adjustments of 7 and 7A still have no effect. A electrical and visual check, out of circuit, showed no shorts or physical damage. A cap check of the internal 250 uuf cap showed it to be within tolerance. The same procedure was done with 4A and 4B with the same results. Tuning cap was looked into...burnished rotor contacts and washed out stator. No apparent shorts throughout rotation. Checked all riveted ground lugs, all proving 0 ohms.
Replaced the 6A8 with a known good one, no effect. In fact all tubes, when checked, had good "English" and no shorts.
Going back to the books, I have info up to Run 5. But since this is a pristine Run 2 version, I wasn't going to do any mods as of yet.
Intuitively, I still suspect the problem lies in either the 7 and 7A compensator or the coils.

Ruminations welcome....
Tnx,
...Jim

Re: 38-8 code 121 run 2 alignment problem..... - mandh - 11-07-2009

Did you try putting your scope on the oscillator side of the tuning cap to see if the oscillator is working over the entire range and at the right frequency?

Re: 38-8 code 121 run 2 alignment problem..... - Guest - 11-07-2009

mandh writes:

"Did you try putting your scope on the oscillator side of the tuning cap to see if the oscillator is working over the entire range and at the right frequency?"

Is that what that thing is for?? Icon_eek Icon_biggrin

That's the next step. Usually, I have little need for the scope and don't think of using it for most operations. But come Monday morning, I'll be warming that up as well.

...Jim

Re: 38-8 code 121 run 2 alignment problem..... - Guest - 11-10-2009

Well the scope did the trick!....

I was about to connect it to the osc. side of the tuning condenser when I found the broken wire leading to the osc. post!

It was hiding in plain sight.

Tnx...
...Jim

Re: 38-8 code 121 run 2 alignment problem..... - mandh - 11-11-2009

Glad you found the problem Jim. Sometime the obvious is overlooked. I'll contact you before the next meet in Seaford for the tube shields.