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Fisher 700-T questions - Printable Version

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Fisher 700-T questions - SolderSplash - 12-21-2015

I'm working on a Fisher 700-T, and I found this site via Ron's excellent restoration thread here.  I have two questions:

1) This is my first restoration, and I've read plenty about replacing the electrolytics.  I've never seen a bad cap before, but I'm guessing this one is no good:

[Image: https://lh3.googleusercontent.com/-wDL03GdHLO8/VndDYqMljAI/AAAAAAAAAIE/B4lzzBNtiM8/s400-Ic42/Fisher%252520700-T%252520Bad%252520Cap.JPG]

Yes?  It's C20 on the schematic (Serial 10001):

[Image: https://lh3.googleusercontent.com/-ayvuo7sLD0c/VndDcP8e6cI/AAAAAAAAAIE/xz1fntDigtY/s800-Ic42/Fisher%252520700-T%252520Schematic%252520PS.JPG]

It appears to be smoothing after the diode bridge.    I had done a visual inspection before powering on, but missed this one.  There are smoothing caps before it (for the +63V supply) and a fuse and other smoothing caps after it (for the +15V, +24V and +30V supplies), so I think I just lucked out.  

2) Everything seems to work great, except the "Selector" switch.  It appears to be stuck on "FM AUTO".  It seems the aluminum selector shaft is frozen in a collar that is secured to the mounting bracket.

I thought, "Great! I can handle mechanical issues."  Then I started looking closer... it looks like the stack of sections have dozens of soldered connections to various parts of its surroundings.  And I don't think I can slide the shaft in or out of the mounting.    One end of the shaft has a retaining ring (arrows):

[Image: https://lh3.googleusercontent.com/-Adp6j77I6O8/VndDUf_aUYI/AAAAAAAAAIE/23t7N-kO1hE/s800-Ic42/Fisher%252520700-T%252520Retaining%252520Ring.JPG]

The other end, which runs through the selector disks, has a crimped-on spring (1), which presses a ball bearing (2) in a captive ring (3), and rolls in crimped detents (red highlights) of a plate (4).  A large nut grabs the mounting bracket; a small nut on the other side backs it up (previous pic):

[Image: https://lh3.googleusercontent.com/-Ylf275xbl14/VndDWn4SLKI/AAAAAAAAAIE/ovaYsxsjASw/s800-Ic42/Fisher%252520700-T%252520Detents.JPG]

So.

Does anybody have some magic incantation that's useful here?  I used a toothpick to wick a little WD-40 in, at the retaining ring and the plate, but it's not any better.  I can grab the D-shaped shaft end and turn it gradually, while bracing the bracket.  The ball bearing and spring appear to move properly, riding across the detents, but golly it's difficult!  The collar and shaft look clean, and the grease (lithium?) at the detents is still goopy.

I have verified that the phenolic disks and rings, and the wipers and contacts are all free.  They need cleaning, but they're free to move.

Suggestions?

Thanks,
Devin

RE: Fisher 700-T questions - Ron Ramirez - 12-21-2015

Devin

Welcome! Allow me to answer your second question first.

Regarding the function switch, I had the same problem with my 700-T as the switch in mine was also frozen. My solution: I applied liberal amounts of PB Blaster to the function switch shaft at the retaining ring - let me borrow your photo:

[Image: https://lh3.googleusercontent.com/-Adp6j77I6O8/VndDUf_aUYI/AAAAAAAAAIE/23t7N-kO1hE/s800-Ic42/Fisher%252520700-T%252520Retaining%252520Ring.JPG]

I did this several times over several days. Each time, I clamped a pair of vise grips onto the end of the shaft and carefully turned the shaft each time I applied PB Blaster.

After several applications, it began to work properly.

Once it loosened up, I worked in some 3 in 1 Oil and let it go.

It is still a bit stiff but it now works properly with the knob, with no need to resort to vise grips to turn the switch. I use my 700-T as a daily driver in our living room, mainly as an amplifier for our HDTV and occasionally use its FM function as well. It's been a year or so and the switch has not frozen up again.

Now to your first question - yes, C20 is bad. You should replace all of the electrolytics in the power supply and audio amplifier sections, especially the two electrolytics that couple the output transistors to the speaker outputs. That's what I did with mine, and it continues to play well (it didn't work when I bought it).

RE: Fisher 700-T questions - Ron Ramirez - 12-21-2015

Here's a photo of my working 700-T from February 2015.

[Image: http://philcoradio.com/images/phorum/Fisher_700-T/700-T_15.jpg]

It is still in the early 1960s Radio Shack credenza, and still works well.

RE: Fisher 700-T questions - John Bartley - 12-21-2015

Just as a supplement to Ron's answer :

The capacitor can that you illustrate in your picture is probably both C19 and C20 in one can, so replacing both of them is a good idea.

Ron is correct about the lubricant. To get your lubricating/penetrating fluid into the control more easily, heat the threaded body (the collar) of the switch with a big soldering gun or use a heat gun with a nose cone to focus the heat. The lubricant will penetrate more easily and completely and the switch should free up properly.

RE: Fisher 700-T questions - SolderSplash - 12-22-2015

Great! Working on that switch, and caps are on order...

Say, I can't tell from the schematic. Do you know what voltage the lamps get from the transformer? The dial pointer lamp needs to be replaced.

Thanks,
Devin

RE: Fisher 700-T questions - SolderSplash - 12-28-2015

Hmmm... There is a difference at C17:

[Image: https://lh3.googleusercontent.com/-5no5FN3nF-o/VoHG8lD8WcI/AAAAAAAAAIg/R2Fgeiduai8/s800-Ic42/Fisher%252520700-T%252520C17.JPG]

I have two 200uF in parallel instead of one.  It looks original, so I assume it was intentional.  It seems like this would just slow down the beacon response time... maybe?  My serial is 15732B, so I was using the first manual.  The later service manual still shows 200uF for C17, so it doesn't seem like a change on the way to a later version.

Maybe this unit had a unique "Service Procedure"?  Oh well.

Any suggestions on the dial lamp voltage?  Although the beacon is 12V, I can't find anything about the transformer, nor a Service Procedure that says what it should be.  If nobody knows, should I just fire it up after recapping and measure, then guess what should go in series with a 56ohm resistor (R72)?

Thanks,
Devin

RE: Fisher 700-T questions - Ron Ramirez - 12-28-2015

As I recall, the winding for the dial lamps is 6.3V.

I used some LED replacement fuse lamps in a few of my Fisher receivers before the supplier doubled the price on them and I quit buying them as a result. Anyway, they were designed for use at 6.3V and they work just fine.

I have no idea why you would have two 200 uF electrolytics in parallel at the C17 position. The early manual and a later manual (above serial number 30001) both call for a single 200 uF cap for C17.

If you drop in a tiny LED to replace the dial pointer lamp (which is what I would do if I had to replace one), you might have to adjust the value of R72 plus add a diode in series so that the LED will be happier than by using raw AC.