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 Post subject: Motorola 58 FRC power transformer
PostPosted: Aug Fri 13, 2021 3:15 am 
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Joined: Sep Wed 27, 2017 4:07 pm
Posts: 10
Hey all
I got a request from my wifes uncle to look at a couple of radios he got from his Mom when she passed away. He said he's had them for 20 years and they never worked. I'm hoping to get them working again for him.

It didn't take long doing the normal initial checks to find that the HV winding on the power transformer was open, bummer :(

I've been looking for some time and haven't came across the same model. I'm hoping someone could possibly help me cross reference to another model.

There is a Motorola 65T21 for sale for a reasonable price but not certain the Transformer would match up. Any suggestions would be much appreciated.

Both schematics are on Nostalgia air. They use a similar tube line up except for the Rectifier and output tubes.

The 58 FRC uses a 5W4 & 6V6 respectively
The 65T21 uses a 5Y3 & 6K6 respectively

I'd be open too a new one as well, if someone could help match one up as well.
Thanks a bunch
Bob


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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Aug Fri 13, 2021 4:41 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 4047
Location: Austin, Texas
Bob,

This is difficult to read but it looks like the B+ is 165V. You will need to look for a transformer with something in the range of 350VAC to 400VAC center tapped HV winding.

A HV rating of 70mA should be OK.
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Mot 58FRC.jpg
Mot 58FRC.jpg [ 52.85 KiB | Viewed 693 times ]

Here's the link for the 58 FRC schematic: https://www.radiomuseum.org/r/motorola_58frc.html

Jay


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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Aug Fri 13, 2021 11:17 pm 
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Joined: Sep Wed 27, 2017 4:07 pm
Posts: 10
Thanks Jay
I do have those figures for my 58 FRC. Your voltage/ ma ratings suggested, will be very helpful if I have to go the NEW route.

Was wondering if you could tell by the schematic for the 65T21 if that transformer might fit the need. That schematic doesn't have voltage ratings unfortunately, so it would be somewhat of an educated guess. There is a 65T21 for sale for a reasonable price, that's why I was wondering if it might work :?:


Bob


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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Sep Sat 04, 2021 11:35 pm 
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Joined: Sep Wed 27, 2017 4:07 pm
Posts: 10
Gentlemen
after alot of searching, the only affordable {$35.00},I found was a used transformer the seller listed it as:

HV 330v - 0 - 330v 70ma
5v 2a
6.3v 3a

I bought and installed it. The HV output only measure 316v, the 5v & 6.3v windings were right on. I powered it up on my variac but before getting to full primary voltage {117v} the resistor highlighted, started smoking. I quickly backed it off.

I replaced all the paper caps, E-caps and resistors. I've double checked all my work and didn't find any wrong connections. When I got this unit the power Trans - HV winding was burn't. wondering if someone could help with why that resistor would be getting so hot?
Thanks Bob
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photo.JPG
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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Sep Sat 04, 2021 11:51 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 11127
Location: Sunbury, Ohio 43074
Any excessive load on the power supply could cause that resistor to overheat. I'd start by triple checking against the schematic, not against what you found. It's possible the unit had a <ahem> previously existing condition from some other misrepair.

Next would be making triple sure you got all the old electrolytics and paper caps replaced, and with the correct value, voltage, and in the case of the Electrolytics, polarity.

This is all preliminary. It's possible there's something dragging the supply down, which led to the transformer burning open. Once the above is done, try pulling all the tubes. If there is still smoke being sacrificed to the gods, you've got a wiring error, or a shorted or leaky part (probably a cap)....should be easy to find with an ohmmeter if it's that bad.

When you replaced that multiple section can where the resistor is, what did you use? A new can? or new individual caps ?

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Sep Sun 05, 2021 6:12 am 
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Joined: Sep Wed 27, 2017 4:07 pm
Posts: 10
Barry
Thank you for the tips. I was getting ready to reply to your individual questions/points when I decided to take your advice and recheck through with my ohm meter, when, and I'm embarrassed to say, I discovered that I accidentally used the lug that also serves as the mounting tab, on the 5 lug T-strip I used to mount the 3 E-caps, essentially grounding that resistor, I un-mounted the T-strip from the chassis and wala, she's playing again. I always check and recheck my work as I go, this one slipped thru.

A follow up question, I don't know what the HV output of the original transformer was, but I'm getting to high of voltage on the Plate of the Output tube, according to the schematic it should be 215v but is coming in at 275v. I'm wondering if this transformer has to much output? If so would I need to add resistance to the B+ and B- feeds ?

Bob


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 Post subject: Re: Motorola 58 FRC power transformer
PostPosted: Sep Sun 05, 2021 6:42 am 
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Joined: Sep Wed 27, 2017 4:07 pm
Posts: 10
I just noticed in the schematic, in the voltages table, that it list the Rectifier cathode voltage as 225v. Is that what the transformer HV output should be ??

Bob


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