Admiral 14R12 Restoration

[old_coot88]

Originally Posted by Kevin Kuehn

So were the channel 3 coil modules already in the correct location? I'm not even sure if that A8 adjustment can cause the tuner IF to be out by 4MC. Something sure seems to be throwing your tuner IF output off.

Referring back to the screen shot in post# 197, if he's getting pic and sound correctly with the fine tuning around mid-range, then the osc. frequency is correct. The osc freq varies with the fine tuning, and has no bearing on the IF response curve. In fact the osc should not even be running during the alignment procedure.

(I don't have the schematic.) The tuner may or may not have a mixer plate coil adjustment. If there is, it may have some effect on the IF response curve.

Crist, was there a particular reason you opted to do an IF alignment?

[Crist Rigott]

Originally Posted by old_coot88

Referring back to the screen shot in post# 197, if he's getting pic and sound correctly with the fine tuning around mid-range, then the osc. frequency is correct. The osc freq varies with the fine tuning, and has no bearing on the IF response curve. In fact the osc should not even be running during the alignment procedure.

(I don't have the schematic.) The tuner may or may not have a mixer plate coil adjustment. If there is, it may have some effect on the IF response curve.

Crist, was there a particular reason you opted to do an IF alignment?

Yes, I wanted to learn how it was done. After replacing all the caps and resistors I thought it could use it.

[old_coot88]

Well first off, your set uses the old 24 mc IF (vs. the later standardized 45 mc IF). Superhet theory dictates that your set's local oscillator (LO) has to run at 24 mc above the received signal's frequency. Channel 3 video carrier is 61.25 mc. So, 61.25 + 24 = 85.25 mc for the LO. The two signals are mixed, yielding the 'beat' or Intermediate Frequency of 24 mc.

Since the video information on the carrier occupies a wide band width, the IF response must also be broad band, occupying a wide swatch on either side of 24 mc. That's why the IF strip is stagger tuned with an extra stage or two of amplification.

In contrast, a radio's IFs are tuned 'on peak' since audio bandwidth is all that's needed. Since the IFs are not staggered, less amplification is needed.

In a typical AM radio, the LO tracks 455 kc above the received signal, yielding the 455 kc IF. Most all FM radios use 10.7 mc for the IF, meaning the LO tracks 10.7 mc above the received signal.

Most all consumer gear uses 'high side' LO injection, meaning the LO runs above the received frequency. Some specialized communications gear may occasionally use low side injection.

Here's a listing of TV channel frequencies.. http://wiki.radioreference.com/index...on_Frequencies

[Kevin Kuehn]

Originally Posted by old_coot88

Referring back to the screen shot in post# 197, if he's getting pic and sound correctly with the fine tuning around mid-range, then the osc. frequency is correct. The osc freq varies with the fine tuning, and has no bearing on the IF response curve. In fact the osc should not even be running during the alignment procedure.

Agree with all that. I was simply suggesting that the beat frequency that the tuner is generating may not be what the IF is looking for(something other than 24MC). If I'm not mistaken Crist thought he had his IF aligned to a center frequency of 20.3MC, which he's based on what his marker generator is telling him. When he aligned it to where it's suppose to be he's not getting video. He also mentioned that he has changed caps and resistors in the tuner circuit.

[old_coot88]

Originally Posted by Kevin Kuehn

Agree with all that. I was simply suggesting that the beat frequency that the tuner is generating may not be what the IF is looking for(something other than 24MC). If I'm not mistaken Crist thought he had his IF aligned to a center frequency of 20.3MC, which he's based on what his marker generator is telling him. When he aligned it to where it's suppose to be he's not getting video. He also mentioned that he has changed caps and resistors in the tuner circuit.

OK I see what you're saying now. I shoulda re-read those prior posts closer.

Be good if he could post the tuner schematic showing the parts that were replaced.

[Kevin Kuehn]

I can post a scan of the tuner from Sams later tonight, gotta run now. It's really close if not the same as the 20Z1 chassis tuner which is available on the ETF.

[Crist Rigott]

Here is the schematic.

I replaced all of the resistors except R9. It was too buried to get to. I also replaced C15A and C15B. Then the only other cap that was replaced was C17, a mica cap. C17 was replaced with a 120pf 500v mica cap. C15A/B was replaced with 2 of these:

https://www.digikey.com/product-deta...5-1-ND/5812010

Just so you know, I used the same type of caps to replace those in the IF section but only with 1500pf for 1500pf and 3900pf for 4000pf caps.

C17 couples the tuner to the IF section.

[old_coot88]

Great. The two critical frequency-determining caps, C12 and C13, have been left alone. But I would double-check resistors R18 (10K) and R19 (4.7K) for correct value since they could affect LO frequency. (It's not necessary to disconnect those for checking.)

[Crist Rigott]

I checked those resistors and here are the pictures of each one installed. The first picture shows R18 a 10K and the second shows R19 a 4.7K.

I keep notes on each resistor removed and installed.

10K R18 was 9.81K and was replaced with a 9.8K
4.7K R19 was 4.92K and was replaced with a 4.64K

[Kevin Kuehn]

Can you verify that by adjusting the Ch 3 oscillator slug, that it will in fact alter your sound and picture tuning? There should be a definite point where both at their best. It's the same effect as adjusting the fine tuning control, only this should have a coarser range.

[Crist Rigott]

Originally Posted by Kevin Kuehn

Can you verify that by adjusting the Ch 3 oscillator slug, that it will in fact alter your sound and picture tuning? There should be a definite point where both at their best. It's the same effect as adjusting the fine tuning control, only this should have a coarser range.

Yes, it does exactly like you said.

[Crist Rigott]

I went over to channel 6 and my pattern generator and the video was unsettled. I tweaked the channel slug and got a great pattern. I only tweaked it just about a 1/4 turn or less.

[Kevin Kuehn]

Beings they're that close it's hard to imagine the IF is off as far as your marker generator is indicating. I suppose you could slowly tune your IF back in the direction of 24MC, intermittently checking if you can still get good sound and video by readjusting the Ch 3 oscillator coil. I don't know what else to suggest. Maybe Old Coot will have a suggestion.

[Crist Rigott]

OK. We'll see what he has to say.

[old_coot88]

I was gonna suggest essentially the same thing.. incrementally 'walking' the IF adjustments back, while looking for the best rendering of fine detail in the pic, with no ringing or smearing of fine detail. Probably have to re-tweak the Ch 3 osc slug along the way, to keep the fine tuning range centered.

There was a guy on here two or three years ago, a total newbie with a late 40s Dumont (I forget his name. Phil Nelson probably remembers). The guy did an IF alignment totally by eyeball, no instruments, no nothing. But damn if he didn't end up with a perfect pic, just as good as with a full instrument alignment. That might be the way to go with this one.

If you want to go the eyeball route and have a frequency counter that'll read 85.25 mc, set the Ch 3 osc slug to that frequency. (A turn or two of wire taped around the osc/mixer tube oughta provide a decent pick up.) With the fine tuning at mid range, the osc wants to be at 85.25 mc for a 24 mc IF. That way, you have a 'target' to walk the IF adjustments toward.

(Ch 3 carrier is 61.25 mc.)