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 Post subject: Picture coming in between channels
PostPosted: Sep Mon 13, 2021 6:56 am 
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Joined: Apr Wed 01, 2020 2:51 am
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I have an old black and white Magnavox set from 1986.
I used a 75ohm coax adapter and hooked it up to the vhf screws and connected it to a vcr.

Issue is, the channels are incorrect and I'm having a hard time getting a good clear picture. It has the vhf UHF double click knob setup. With the VCR set to channel 3 I can get a picture with the knob set to channel 2.... But I have to fiddle with it for a while before it comes in at all. I tried another VCR that could be set to channel 4… and I only get a clear picture if I hold the knob between channels 4 and 5. The fine tune wheel does nothing on channel 4.

Has the tuner drifted or something similar? I opened it up and tried to find a point of adjustment on it, but I don't see any kind of adjust set screw anywhere. This tv has been in the family for a long time, when I last used it worked fine, but got put up in a closet after the digital transition. This is my first time trying to connect it to a VCR since the rabbit ears won't do me any good with DTV.


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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Mon 13, 2021 1:23 pm 
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ethefire wrote:
Issue is, the channels are incorrect and I'm having a hard time getting a good clear picture. It has the vhf UHF double click knob setup. With the VCR set to channel 3 I can get a picture with the knob set to channel 2.... But I have to fiddle with it for a while before it comes in at all. I tried another VCR that could be set to channel 4… and I only get a clear picture if I hold the knob between channels 4 and 5. The fine tune wheel does nothing on channel 4.
I think by "double click knob setup" you mean that is has the separate UHF and VHF tuners. So these are mechanical tuners.

The fact that it worked, was unused for a long time and now the channel selector has to be fiddled with or shows the channel on the wrong number indicates the mechanical contacts are dirty and need cleaning.

Normally as switch contacts are used (like in a mechanical tuner), the oxidation is rubbed off of them frequently just by changing channels several times a day. However, with years of disuse oxidation builds up and can prevent metal to metal contact. At that point the oxidation must be cleaned off to regain proper operation.

Assuming this might be a little daunting for you, you might try something simple with the VHF tuner. Just rotate it around a bunch of times to attempt to clean oxidation off of the mechanical contacts. By a lot I mean like full rotations 20 or more times and then try it again. If that is any better at all but still "fiddly" try another 20 and see if that gets any better. You are literally trying to "wear" the oxidation off of the switch contacts. Alternatively you could just keep flipping back and forth between 2, 3, 4, and 5 twenty or more times and let the rest of the channels remain unusable due to oxidation.

If this doesn't work it is time to open up the TV and resort to cleaning the tuner with some type of contact cleaner. I will not attempt to describe that for your set because I am not familiar with that and will leave that to someone who has dealt with a 1986 Magnavox B&W TV.

Best of luck.

Curtis Eickerman

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 1:08 am 
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Location: Chapel Hill NC
I think by 1986 Magnavox (a brand of North American Philips by then) had fully switched over to varactor tuning, with mechanical detents. The "fine tuning" wheels were pots to trim the channel tuning voltages. When I left there in 1985 that was the case, at least on the color sets.

My 1983 Magnavox C2331S service data warns, "Tuner Control Unit Assemblies are non-repairable," though provides schematics and parts list. The tuning voltage divider resistors could drift in value, of course, and bypass caps could fail.

Whether this kind of tuner is easier or harder to repair than the mechanical kind with lots of contact, I don't know.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 1:23 am 
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AJ2X wrote:
I think by 1986 Magnavox (a brand of North American Philips by then) had fully switched over to varactor tuning, with mechanical detents. The "fine tuning" wheels were pots to trim the channel tuning voltages. When I left there in 1985 that was the case, at least on the color sets.


That is what I was thinking also.
Could still be problems with the selector contacts, but they would be handling DC voltages instead of RF.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 2:24 am 
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AJ2X wrote:
I think by 1986 Magnavox (a brand of North American Philips by then) had fully switched over to varactor tuning, with mechanical detents. The "fine tuning" wheels were pots to trim the channel tuning voltages. When I left there in 1985 that was the case, at least on the color sets.

Would that be a pot with mechanical detents with a fine tuning pot to trim the voltage?
What I am familiar with is a rotary switch to select either a particular tuning pot or a tap on a big voltage divider.
In either case there is something to clean with contact cleaner.
A mechanical tuner would also be fixed with contact cleaner.
I usually use DeoxIT D by Caig labs. Some people claim that you should not use DexoIT on pots. But Caig's web site says that it is good for pots and I have had good luck with it.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 5:21 am 
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So the question is, is how to get deoxit inside the tuner to clean it? The tuner on the inside looks like a silver box about the same dimensions as a pack of playing cards. Would I just spray it into any creavice on the tuner? I'm assuming there isn't anything that can get messed up blindly spraying? Or is there?

I also noticed that the tuning shaft is greased. Would deoxit mess with that?

For reference, the model number is BG3741AK01.


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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 11:48 am 
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Location: Belrose, NSW, Australia
Try not to get it on the coils, it will de-tune them. If this happens, an alcohol wash (be generous!) and a thorough dry should restore it.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 2:46 pm 
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Location: Dallas, TX
ethefire wrote:
So the question is, is how to get deoxit inside the tuner to clean it? The tuner on the inside looks like a silver box about the same dimensions as a pack of playing cards. Would I just spray it into any creavice on the tuner? I'm assuming there isn't anything that can get messed up blindly spraying? Or is there?

I also noticed that the tuning shaft is greased. Would deoxit mess with that?

For reference, the model number is BG3741AK01.

Does the tuner shaft go into the silver box? I've seen separate sections of tuners where the box in on the board holding the rest of the TV circuits and with wiring going to a separate unit on the front of the TV that had the tuning shaft.
Pictures would help.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 9:47 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
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Location: New Hampshire
This will ALMOST always apply.
The TV is B&W so should have 2 tuners. UHF & VHF. the bigger one is VHF.
The fine tune is multi turn & if you put the TV on channel 3 it can tune up to ch4
or down to ch2 while on ch3.
Try that first.
sometimes you also have to push in & turn the FT knob. Watch the gears & make sure they are moving.
For cleaning best to send a pix first & can advise safest way.

good luck
Zeno
LFOD !


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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Tue 14, 2021 10:23 pm 
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Joined: May Mon 24, 2021 3:33 pm
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Location: Chapel Hill NC
From your description, the actual RF tuner is that box the size of a pack of cards, but the control board (with the tuning pots) is likely on or just behind the front panel of the TV set. If no mechanical shaft goes into that tuning box, spraying it will have no good effect since there are no moving parts inside.

It's possible that this is a Japanese set with a Magnavox name, in which case I bow out of the discussion.

Photos of the set and of the tuner area will be most helpful.

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 Post subject: Re: Picture coming in between channels
PostPosted: Sep Thu 16, 2021 5:15 pm 
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Joined: Nov Sun 11, 2018 12:32 am
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The NAP B&W sets didn't use a preset varactor tuner. It was either a classic clunker or on the really cheap stuff a continuous slide-rule affair like a radio.


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