Author Topic: Need some FM help on a tube console  (Read 182 times)

thermofin

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Need some FM help on a tube console
« on: September 10, 2018, 06:01:13 AM »
I bought this console a few weeks ago (Model 1ST657S) and have finished re-capping it and replacing a couple weak tubes. I have reconditioned the changer and it is working well. When I brought the unit home it fired up and I was able to get both AM and FM reception.
After the re-cap I am no longer getting anything from the FM. AM works, and the phono sounds great. I have also connected a Bluetooth to the aux tape and it sounds very good.   
Any help would be appreciated, I'm hoping it may be something simple that I have overlooked.

19and41

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Re: Need some FM help on a tube console
« Reply #1 on: September 10, 2018, 06:50:47 AM »
Mine completely failed after recapping as well.  I found that the socket for the dual AF amp tube had a pin socket loosen and not make good contact.  I took the chassis, removed all the tubes and used a fine pair of Hemostats to compress each of the contacts in the tube sockets that could be done.  It worked out that the filament wasn't getting current.  Re stressing the contacts fixed that problem.
"Any sufficiently advanced technology is indistinguishable from magic."
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Bill

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Re: Need some FM help on a tube console
« Reply #2 on: September 10, 2018, 06:53:07 AM »
I have the same unit, but mine is unrestored yet.  It plays but the RH channel is weak. 

The first thing I would do is double check everything that you touched in your rebuild process.  Check all solder joints, and if there were multiple items soldered on the same connector that they all got re-soldered, tube pins, connectors that you unplugged and plugged back in, broken wires...just anything that got touched. 

Bill

electra225

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Re: Need some FM help on a tube console
« Reply #3 on: September 10, 2018, 08:42:05 AM »
Do you have a good antenna?  Did you clean the function switch?  The FM oscillator tube, either a 6EA8 or 6DT8 is uber-critical in those tuners.  Sub another tube to see if that helps.  Wiggle the tubes.  Good luck.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Motorola Minion

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Re: Need some FM help on a tube console
« Reply #4 on: September 10, 2018, 01:55:16 PM »
Your tuner is assumed to be a 79 series, based on your model which is close to my 1962 1ST659. See tuner portion of Sams 655 attached
 
In mine, I did replace the 6EU7 (AF amp) socket mentioned above but I assume your FM issue is a stopped oscillator or non-functioning mixer.
#1 try the 6DT8 tube, but also check your mixer diode M3 on the sams. My note at the bottom does not apply as the 79 series is a variable inductor similar to what Zenith uses, simpler and better selectivity IMHO. .

Because your AM band works, the signal circuit path beyond the tuner is dual-band and probably not be the issue. Only if there was a silver mica issue in one of the two 10.7 Mc IF transformers, is it possible to lose FM but not AM.  My note below does not apply as the 79 series is a variable inductor similar to what Zenith uses, way better IMHO.

To compare: I recapped a 57 series tuner which has a variable condenser, then I put it together without cleaning the rotor grounding contact springs. ::) As its tuned, it pops in and out due to the intermittent connection.It has a separate RF amp tube and 6EA8 for oscillator and mixer as do most Magnavox produced before 1961.
 It stayed on-station when I played it yesterday, but after I turn it off - I need to run it back and forth so it tunes.
Tubes - Magical - Tubes

Dave

thermofin

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Re: Need some FM help on a tube console
« Reply #5 on: September 10, 2018, 06:02:53 PM »
Thanks for all the help and suggestions. As it turns out Electra225 and Motorola Minion are right on with the 6EA8 tube which turned out to be defective. I didn't bother checking the tube since I bought it as a replacement from "Tubedepot" which will be my last purchase from them. The tube was dead.... I swapped the old tube that is a little weak and the FM fired up perfectly.
I guess you live and learn. Any suggestions for a good tube source would be appreciated.
This made my day!

19and41

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Re: Need some FM help on a tube console
« Reply #6 on: September 10, 2018, 06:13:30 PM »
I have bought a couple from Viva tubes.com. 
"Any sufficiently advanced technology is indistinguishable from magic."
Arthur C. Clarke

TC Chris

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Re: Need some FM help on a tube console
« Reply #7 on: September 10, 2018, 08:39:28 PM »
This is the valuable lesson--check the tubes and their sockets.  My mono 1956 model has one tube that has a touchy socket, and sometimes doesn't work until I wiggle it.  Yeah, I should fix the problem, but meanwhile  rely on wiggling.  It' always an easy strategy.

Chris Campbell

electra225

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Re: Need some FM help on a tube console
« Reply #8 on: September 10, 2018, 08:41:09 PM »
About the only tubes in those tuners that will make it totally dead on FM, but okay on AM is the 6EA8/6DT8 oscillator tubes.  If you have a 6U8 or a 6GH8 (A) on hand, those will sub for the 6EA8 and may provide better resistance to FM drift.  If AM works there is no SMD in the FM transformers, since the signal for both bands passes thru all the IF transformers.  SMD on FM would also be SMD on AM.  The fact that the radio worked on AM places suspicion squarely on the 6EA8.

I don't have 6EA8's to spare.  Glad you got it sorted.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Motorola Minion

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Re: Need some FM help on a tube console
« Reply #9 on: September 11, 2018, 11:08:11 AM »
I have collected tubes since I could pull them out of sets found in rural, farm dumps and at curbsides. Some tubes, like the 6EA8 show up a lot, as they were used extensively in 1950s TV tuners and FM sets. What I see on epay is much ignorance about tubes, listing them by generic (tv or radio, ham, etc) and hyped terms (gray plate, gold-toned getter, etc.) 

I encourage anyone who sees a tube caddy or box of random tubes for sale to buy it cheap. It could have been cherry-picked of its 12AX7s and 6BQ5's but ANY hoard of tubes contains at least a few good ones, of the right type. 6AU6's are everywhere, yet some listings make them sound rare and unique somehow. 

A repairman's caddy is worth $30-50 with half never-used boxed tubes and used tubes stuck in boxes ??? (when the new one didn't deliver a cure). I used to have time to weed out all the ones showing shorts, gas, open heaters

Tubes are not handled and shipped in a way that does not shake at least a few to failure.
Tubes - Magical - Tubes

Dave

TC Chris

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Re: Need some FM help on a tube console
« Reply #10 on: September 11, 2018, 08:54:56 PM »
there is no SMD in the FM transformers, since the signal for both bands passes thru all the IF transformers. 

I don't the schematic at hand to peek.  I know older AM-FM radios often used two sets of IF transformers with the same amp tubes, but I always figured that when the set was operating on AM at 455 kHz IF (or whatever was chosen), the FM IF transformers weren't passing signal because they were tuned for 10.7 mHz.  Now maybe if the SMD gives a dead short on the input winding, maybe it would be a problem.  Explain, please. 

Chris Campbell

thermofin

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Re: Need some FM help on a tube console
« Reply #11 on: September 13, 2018, 01:51:53 PM »
I have noticed when I initially turn on the changer to play a record, I get some static out of the left channel until the system warms up and then all is clear. Would this be a weak or defective tube? Possibly one of the EL84 tubes in that channel?
Any help would be appreciated.

electra225

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Re: Need some FM help on a tube console
« Reply #12 on: September 13, 2018, 04:15:06 PM »
I would suggest that you clean tube sockets, wiggle tubes, wiggle 6EU7's, and clean the function switch, and external speaker switch thoroughly, before I would suspect an output tube.  6EU7's are notorious for developing inter-electrode shorts. 

Chris, if you refer to a schematic, you can see how the IF's are connected on AM and FM to use common tubes.  They typically have separate converter tubes for AM and FM, but use common IF amplifiers.  I'm probably not explaining it right.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

TC Chris

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Re: Need some FM help on a tube console
« Reply #13 on: September 13, 2018, 08:17:46 PM »
No, you're right on the concept.  Same tubes, different IF transformers (455 kHz vs 10.7 kHz). But unless the SMD causes a short on the unused transformer primary, it shouldn't affect the other band's performance.  Is SMD a problem on rimary, secondary, or both?

Chris Campbell