The Fisher 70-RT Repair

[PioneerPro]

I bought a Fisher 70-RT tuner a few years ago without the intent to use it for anything but a show piece. The thing is just gorgeous if you as me and I have polished it up like new. I want to show it off to friends especially because of the cool Green Eye tuning tube at the top. It was working just fine when I cleaned it up and set if on the shelf but here it is a year later and I can not get the Green Eye to work anymore.

I plugged it into my Dim Bulb Tester and it lit up pretty bright but honestly I don't know anything about Tube Equipment other that it draws a lot of power. Anyway I removed each bulb one by one to see which one my be causing a short and Bulb V-14 appeared to be the culprit. I have included a bunch of pics below, the Brown, Green, Orange, Gold transistor if I am seeing the colors correct sometimes reads a value and sometime reads OL, I am thinking maybe that's the problem??? I could be wrong since I have never seen a Service Manual like this but it appears to be the R67 resistor and should have a resistance of 15,000 ohm, so it is way way off.

Once again I know nothing about tube equipment and don't know where to start. Any advise would be appreciated. This things is just too damn cool not to be fully working.

Thank You, Ray

 

[PioneerPro]

Here it is all polished up.

 

[gadget73]

V14 is the rectifier, if removing it cut the bulb down, it sounds like a short on the output side. Most of the time this means one or more filter caps are shot, which is a very typical problem with tube gear of this age. Most of the time thats going to be the can cap, usually they have a couple caps in there.

 

[PioneerPro]

V14 is the rectifier, if removing it cut the bulb down, it sounds like a short on the output side. Most of the time this means one or more filter caps are shot, which is a very typical problem with tube gear of this age. Most of the time thats going to be the can cap, usually they have a couple caps in there.

Is a MICA Capacitor and a Can Cap the same thing, because the Service Manual says there are 3 Mica Caps, 13 Molded Tubular caps and a ton of Ceramic Caps.

Can you point the Can Caps out in the pictures or are they from the top of the radio? Are you referring to C69 and C70?

 

[gadget73]

The can capacitor is a multi-section electrolytic. C69 and C70.

This is the Sams, different component numbering, but the ones in red are high voltage, the ones in blue are low voltage. Both are probably due for replacement but the red ones will stop being a problem if the rectifier tube is removed.

easiest thing to do here (IMO) is get a pair of new caps from Hayseed Hamfest made to match the original config, that makes it a like for like swap.

The molded paper caps can be swapped for any sort of film cap. Illinois MWP, MWR, Cornell 940, 942, or whatever you like.

 

[PioneerPro]

OK, I think I need to do some studying on how to test the Can Caps then proceed.

If there is a way to safely bypass these two caps and get the Magic Green Eye to work, I would be ok with that because like I said it is just a showpiece for me but I want the Green Eye to work and thats all I care about.

And I am not saying I did this correct but these are the readings I got from the Can Caps
C69 - 33mfd 33mfd 50mfd and 33mfd
C70 - 253mfd and 257mfd on one side and 219mfd and 219mfd

The Brown Green Orange Gold resistor that is supposed to be 15 kOhms is registering 12.6 kOhms
The Brown Green Black Silver resistor that is supposed to be 15 Ohms is registering 16.5 Ohms
The Yellow Purple Red Silver resistor that is supposed to be 4.7 kOhms is registering 4.9 kOhms
The Yellow Purple Black Silver resistor that is supposed to be 47 Ohms is registering 50.7 Ohms.

Does this give you a picture of what may be going on?

 

[gadget73]

you can't bypass them, they are the power supply filters. They need to be there, and if they are bad they need to be replaced. The values generally aren't the problem, they fail leaky, meaning high voltage shorting to ground. Only way to test for that is to supply them with high voltage and measure leakage current.

If you don't actually want the tuner to work as a tuner, get a VU meter that uses an eye tube. It will actually do something and be less involved than fixing this just to make a single tube glow.

 

[PioneerPro]

I went back and checked the dates and everything was working fine 6 months ago. There is nothing leaking from the bottom of these Caps and so I don't want to order new ones if this is not the problem. Can you tell me how to test the Caps will connected and powered in the radio? How do we confirm it is indeed the C69 and C70 Capacitors?

 

[gadget73]

I don't meant to sound rude here but unless you're comfortable working with high voltage DC, I'm going to suggest putting the cover back on and not messing with this.


if I were trying to do this without replacing any parts, first thing I'd do is remove all of the tubes, plug in a solid state rectifier, and use my variac. Wire one voltmeter across R81, and another across C2A. Start bringing up the voltage, stopping before it hits 250 volts, or when the ammeter starts to climb.

Calculate current across R81. With no tubes, it should be extremely low, milliamps at best. If its significant, measure across the choke, 80, and R79 to narrow down where the current draw is coming from. Isolate as appropriate to cut down on the leakage.


if all of that sounds like a foreign language, put the cover back on.

 

[Mr Boochie]

I went back and checked the dates and everything was working fine 6 months ago. There is nothing leaking from the bottom of these Caps and so I don't want to order new ones if this is not the problem. Can you tell me how to test the Caps will connected and powered in the radio? How do we confirm it is indeed the C69 and C70 Capacitors?

If those are the original cap cans, they prolly need to be replaced. That is a 1950s-era tuner. Those cans are over 65 years old.

You can get a good capacitor tester, but it's gonna cost around the price of those cans. And it'll prolly test them as "need replacing."

Just a thought.

 

[DOBIEGUY]

Ray
You did not mention if the unit receives FM (or AM) radio or not.
I’m assuming you have an antenna connected.
I would start by giving the input selector a thorough DeOxing.
Work it back forth multiple times and spray it again.
Are you trying to operate and trouble shoot this while plugged into the dim bulb tester?
At better than 65 years old I’d say if it worked 6 months ago it should work today.
I had one as good looking as yours. Worked correctly.
Good luck.

BTW
As posted above…….. Watch your fingers in there!

 

[Mr Boochie]

At better than 65 years old I’d say if it worked 6 months ago it should work today.

Thing is, "works okay" could mean it turns on and maybe gets an FM signal — for a minute or two. Once those old electrolytic caps start heating up, things could definitely change. And not for the better.

If you remove a 65-year-old can from the chassis, it usually has next to zero electrolytic fluid left in it (it weighs 1/4 or less of what it did new). Those cans are not safe for continuous use.

Just my 2¢ worth of 2¢.

 

[DOBIEGUY]

@Mr Boochie
I understand this all to well. Seems the OP is just interested in getting the tuning eye to work.
The 6U5 (V12) gets its grid voltage right off the selector switch. (and it is only a few volts)
I’m waiting for an answer on the dim bulb question.
A 6 volt filament on a dim bulb tester ………

 

[PioneerPro]

I don't meant to sound rude here but unless you're comfortable working with high voltage DC, I'm going to suggest putting the cover back on and not messing with this.

Yes I am comfortable working on this high power old stuff and I know how to discharge caps.

I understand this all to well. Seems the OP is just interested in getting the tuning eye to work.

Correct I just want to get the Green Eye to work just to show friends, I am not concerned that the radio works.

These Cans are pretty old but my wifes Cans are old too but they still look pretty.

I'll order new Can Caps and hopefully things will be operational.

 

[Mr Boochie]

Yes I am comfortable working on this high power old stuff and I know how to discharge caps.

Correct I just want to get the Green Eye to work just to show friends, I am not concerned that the radio works.

These Cans are pretty old but my wifes Cans are old too but they still look pretty.

I'll order new Can Caps and hopefully things will be operational.

Good luck!

What I would do is pull tubes one by one until the pull affects the tuner's "eye." Keep only those in the unit to run that eye.

It might make the tuner more stable in the long run.

 

[PioneerPro]

I went to Hayseed Hamfest website to order the two Can Caps.

Once again this tub stuff is 100% new to me and I don't know what I am doing.

I need one of each below. C1A is a 40mf 300v 4 pin cap and C2A is a 1000 and 500mf 25v cap.

I see nothing on there site with that description and am not really sure what to order. Some part numbers would be helpful. They do not have a phone line just an email.

HAYSEED HAMFEST LLC

Hayseed Hamfest manufactures new single and multi-section can-capacitors for vintage radio, audio, and electronic gear.

hayseedhamfest.com

Thank You

 

[DOBIEGUY]

@Mr. Boochie.
If you like Google the tuner a full schematic is available.
Not difficult to trouble shoot the eye circuit.
Looks like the OP pulled the cans already. Certainly not the way to go in my book.
The price of H/H cans is a high price to pay to look at a green eye. (And more than the tuner is worth)
To each their own. I’m out.
James

 

[notdigital]

Yours is a custom job. Best to call saying you need a 4 section cap of xxx values and xxx voltage. You’d want to make sure he builds them with the same symbol designation so you’ll be appropriately guided by the schematic. That said, hope you took pics of the factory connections.

 

[Mr Boochie]

40/40/40/40 @ 350VDC can.

For the 1000 and 500mfd @ 25 VDC cap can. I'd prolly cut the original line and insert equivalent solo caps.

 

[notdigital]

Yup! 4 caps all the same! Easy Peasy! The other one just 2 caps. Boochie hinted at what is easier/cheaper: just undermount 2 axial and be done!