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 Post subject: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Mon 05, 2018 6:06 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 684
Location: South Jersey East of Philly
Not a peep on FM...AM is fine. Checked/changed the 6BA6 and 6BE6, good. Antenna hooked up. Plate/screen Voltages on 6BA6 and 6BE6 good...but no signal on pin #1 of either tube (no neg. voltage). Just looking for ideas...Unfortunately I don't have a scope or signal generator as it is a hobby and usually I can sort out these problems eventually. I did put a transistor radio, on a FM station and swept the dial on the Magnavox tuner...and at one point I could hear the transistor radio FM station cut out...so I assume the oscillator is working(?). Just looking for ideas.
https://www.tubesandmore.com/schematics ... -co/cr-234


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 7:06 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2415
Location: Lexington, KY USA
Thanks for the schematic link! This is a serious radio, with a PP 6L6 audio output.

Basic things to check include coil and winding continuity, the bandswitch, and voltages on each stage.

It is often useful to check bandswitch contacts and the connected coil with one measurement. Say, from the tube grid, to wherever the other end of the coil is supposed to connect.

Did the second radio detect the local oscillator signal at the correct frequency? Delta F between the dials of the two sets should be 10.7MHz.

Have any parts been replaced?

Is there noise on FM, or silence? If the front end is dead, the set should have enough gain in the IF to make the discriminator output fairly noisy.

Ted


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 7:27 am 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 684
Location: South Jersey East of Philly
Thanks for the suggestions Ted...working thru them now. The radio on FM is silent, except for the normal amp hiss when you turn up the volume. No indication of anything going on AFA static, background RF noise. I can touch the 3 FM terminals on the gang tuner and nothing. Even touching my finger to the grid of the 6BA6 and 6BE6 produces little or no sound on the FM mode. I going thru all the resistors and caps..changing the out of spec ones. All tubes have been checked. All IF cans seem to have continuity. I am double checking everything again now including wafer switch contacts.
Could you write the procedure for using the second radio to check the 'bad' radio's oscillator..to make sure I am doing it right? Thx...Brian


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 8:28 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2415
Location: Lexington, KY USA
For checking the oscillator frequency, just repeat what you have done already and jot down the dial settings for both radios.

The two frequencies should differ by the IF frequency of the radio you are testing.

Note which one is higher and you can tell whether the LO is above or below the station frequency. Different radios used different schemes, but chances are that a 6BE6 FM tuner will have the LO below the station. The extra 21.4MHz would be pushing the tube too far. The 6BE6 barely works for FM.

It looks as if the schematic indicates permeability tuned IF transformers. The problem could possibly be the fixed capacitors inside the cans.

Does the tuning indicator respond as you tune across strong local station frequencies? That would be a clue.

Ted


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 5:15 pm 
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Location: USA
You said you were changing resistors and capacitors...does that mean old paper capacitors are still in there? I have a similar set (CR-321). It was quite a bit of work before FM was working again; there were a number of paper capacitors and an open candohm resistor that had to go. My set doesn't receive well with the existing FM dipole antenna, it works much better with just a single 3-ft wire.

Quote:
Could you write the procedure for using the second radio to check the 'bad' radio's oscillator..to make sure I am doing it right?


Ted already mentioned this and you performed this test already. Note the dial setting of both radios when you turn the knobs. Each radio should "hear" (you used the words "cut out") the other's oscillator 10.7mhz away from the other's dial. For example: if radio A is on 100.0mhz, you should get a response when radio B is tuned to 110.7mhz.

Also check 6H6 (discriminator).

Unless you have a signal generator, don't touch any of the FM alignment adjustments.

_________________
"I got a bad feeling about this." (Han Solo)


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 7:27 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 684
Location: South Jersey East of Philly
OK..I re-did the radio test and tuned the good radio to 94Mhz. I swept the dial on the dead Maggie FM to around 106Mhz when I heard the good radio 'react'. So I think this shows the Maggie oscillator is working...maybe a little off. But close enough for now.
For the 6H6 discriminator I tried other 6H6's I had, and put in germanium switching diodes temporairily, but no change.
All FM tuning transformers have continiuty on each coil side.
I checked the wafer contacts on the function switch...all seem OK.
When I opened this chassis up originally, there was only evidence of one cap being changed...all else untouched. That leads me to think it died one day and was just put into storage.
There are some old caps still in...although I individually temporairly subbed new ones in to see if anything helped...no dice.
The tuning eye isn't working on either band...so I'll have to address that also.
I still think the problem is in the front end, since no audible signal or sound change other than amp hiss of any sort is getting thru to the speaker.
Keep the ideas coming...Thanks!


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 7:43 pm 
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Posts: 2345
Location: USA
Quote:
...there was only evidence of one cap being changed...all else untouched...There are some old caps still in...although I individually temporairly subbed new ones in to see if anything helped.....Keep the ideas coming....


This is not a new idea: replace the electrolytic and paper capacitors. I think this set has either "Micamolds" or "bumblebees."

_________________
"I got a bad feeling about this." (Han Solo)


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Tue 06, 2018 10:23 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2415
Location: Lexington, KY USA
Sounds as if the local oscillator is indeed working. You might check that it still works at the high end of the dial. Just follow it up by tuning both radios at once, or alternately in small increments.

Are the expected circuit voltages listed in the service docs?

I don't think I saw any on the schematic pages, so there may be a chart, or voltages are sometimes listed on a tube location layout drawing.

If nothing is listed, you might post a chart of what you measure here. I suggest calling out both the tube element and pin number for each measurement.

If you lack a signal generator, you can try tapping the grid of each stage with a screwdriver to see if you get a click.

Ted


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Wed 07, 2018 4:38 am 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 684
Location: South Jersey East of Philly
Starting at the beginning stage----RF---When on the working AM band, I get a -2v grid voltage on the 6BA6 pin #1, and -14v on the 6BE6 mixer grid pin #1. When on quiet FM band, I get no voltage on either input grid. Shouldn't there there be some sort of bias voltage generated by the FM antenna circuit on those grids?


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Wed 07, 2018 9:32 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2415
Location: Lexington, KY USA
How are you measuring the grid 1 voltage on the 6BE6? A meter lead can kill the oscillator even on the AM band.

Old time VTVM DC probes had a resistor at the tip to limit the capacitive loading effect.

Grid 1 of the 6BE6 is the oscillator grid. The mixer grid is grid 3 on pin 7. The signal goes in pin 7.

If the oscillator is running, there will be some negative voltage on the oscillator grid.

The RF amplifier may have no negative AVC voltage on the grid for the FM band.

I'm more interested in the voltages for the IF and limiter stages not used for the AM band.

Ted


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 Post subject: Re: Dead FM on 1951 Magnavox CR-234
PostPosted: Mar Wed 07, 2018 2:47 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 684
Location: South Jersey East of Philly
Turned out that the alignment was way out of whack by a previous diddler. Almost gave up on it, but finally found the right sweet spots, and she came around.


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