Admiral 14R12 Restoration

[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.

[Crist Rigott]

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.

I'm not really sure if they were in the correct position on the turret. I lost the metal insert that goes into the plastic channel dial so I can't use the correct dial. I'm using another dial just to be able to turn the channel selector. I looked into the tuner and when the channel 3 inserts were making contact with the wipers, then I used some masking tape to mark the dial. I know it's a bit hokie, but my friend has a metal piece I just haven't gotten to go over his house.

[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.)