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 Post subject: Help ID The Number Of This Problem Motorola Tuner ca. 1960
PostPosted: Mar Sun 15, 2020 4:40 am 
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Got a nice 1960 Motorola SK-28B 1960 radio/phono console in the shop. The FM only gets stations from 94 to 107 MHz, but they are on the low scale of the dial (88 to about 98 MHz). I can't figure out this yet...changed all tubes, and verified resistors and disc caps...no change. IF's are peaked. The main thing I notice is the two flat head osc. alignment adjustment screws on the FM module (see pic) don't have any effect. I managed to get into the FM module a bit and inspect inside, but didn't do any work in there..hoping the problem was elsewhere easier to get to.
I can't find a schematic for the Motorola AM/FM tuner it came with..Looks like the chassis number is HS-788B (see pic) but I may be reading this wrong. This Motorola tuner was a drop in dealer option. The power amp is HS-768 and the pre-amp is HS-774..I have the schematics for those.
If anyone can locate the tuner schematic and/or suggest some reason for the shift of the frequency range, let me know...Thanks!
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Last edited by Tbirdkid on Mar Sun 15, 2020 2:57 pm, edited 1 time in total.

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Sun 15, 2020 7:03 am 
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Edit: I just read after being a little non-observant you already had these schematics in Beitmans Vol. R20 1960 page 101 where I found them
(Note: The chassis are not the same numbers but Pre-amp is HS-769-and HS-774 and Power-Amp as HS-768 and HS-793) :oops:
so maybe try this (I don't know if it has the tuner)
Sam's set 491 folder 11 (Note: it says Quasar in the universal schematic locator program maybe a TV?)
Hope this Helps.

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Sun 15, 2020 3:04 pm 
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I have that Sams folder; doesn't include tuner. And neither Beitmans nor Sams lists the HS-788.

You may have to just list the tubes, and we can see if there's a similar Motorola model.

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Sun 15, 2020 8:14 pm 
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I found an AM/FM tuner that matches the component layout of your HS-788B chassis in my Motorola service manual. Described as “optional drop-in tuner” for several Motorola stereo cabinets, it is HS-783.786 (also found in Sams 483-10). I did not find HS-788B in the manual, possibly a later version than the coverage of my manual.

Tube complement: 6BQ7A, 6BA6 (2), 6BE6, 6AU6, 6AL5, 6X4, EM81/6DA5 (tuning eye).

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Mon 16, 2020 6:28 am 
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Meade...The tube lineup is correct on the drop in tuner you found...sounds like a match, as mine was described as a drop in tuner too. Any way I can get a copy of that schematic/info from your Motorola service book?


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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Mon 16, 2020 6:45 am 
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Sure! I'll scan it tomorrow.

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Mon 16, 2020 2:33 pm 
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Cool...Thanks!!!!


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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Mon 16, 2020 5:51 pm 
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Manual sent to the email address I have on hand from January 30.

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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Fri 20, 2020 6:21 am 
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Thanks for MAG for the service info!
Follow-Up---After digging and exhausting all the possibilities within the FM tuner module, I did a final desperation move, and put 2 pf cap across the RF trim cap and got almost all the stations back, except for the ones around 88 MHz. I was about to settle for that, but I tried a ECC85 in place of the 6BQ7 and got the complete dial now. I have to assume the FM Osc and RF trim pots are bad. I can't fine tune the band for peak performance, but what I got is pretty good. I didn't have any trim pots in the 2 pf range, and to replace the existing ones would require a pretty much impossible task due to lack of parts and the construction of the FM module.


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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Fri 20, 2020 7:54 pm 
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No trim pots here, those are adjustable mica capacitors and they rarely if ever go bad.

-David


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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Sat 21, 2020 3:07 pm 
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Quote:
No trim pots here, those are adjustable mica capacitors and they rarely if ever go bad.

OK, they are adjustable caps, sorry for the mis-speak. Then why did my solution work, after replacing all the caps in the module didn't work? Those two mica caps had no effect on the tuning/tracking at all.


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 Post subject: Re: Help ID The Number Of This Problem Motorola Tuner ca. 19
PostPosted: Mar Sat 21, 2020 4:44 pm 
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Location: Boston, MA USA
The trimming capacitors have only a very small range. If the tuning capacitor and oscillator coil are not positioned correctly when you do your alignment, the trimmers will not be able to be adjusted to resonance, which is what you are seeing. Very common.

-David


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