1959 Zenith Question- Capacitor?

[TedB]

Howdy Folks! I joined this forum because I stumbled upon a 1959 Zenith Rigoletto SFD 2575 L at a local shop, and now it lives with me. Hoping some gurus here might have a pro tip or two on an issue it has.

I love this thing, and want to keep it forever!!! Turntable works, stereo works. But there is a constant buzz in the background. The buzz/hum is a constant volume, it doesn't vary with the volume of the radio, nor when I change stations, nor when I use the turntable. Its always there in the background when the power is on.

Some online troubleshooting guides say it might be the capacitor, and I have to re-cap it. Does this sound right, or could it be something else? Is this a relatively straightforward procedure? I went to a TV and "vintage' radio service shop by me, but they only do solid state (false advertising if you ask me). So I'm on my own here.

 

[gadget73]

usually hum is a bad filter capacitor. Can you get the chassis numbers off the stuff inside? Its usually a number. a letter, and then some more characters. Often this is stamped in large ink letters somewhere, but it might also be listed on the model tag on the back.

https://www.radiomuseum.org/r/zenith_sfd2575l_ch_9d25_ch_7d31.html

maybe?

if so the tuner is a 9D25 and the amp is a 7D31.

 

[TedB]

usually hum is a bad filter capacitor. Can you get the chassis numbers off the stuff inside? Its usually a number. a letter, and then some more characters. Often this is stamped in large ink letters somewhere, but it might also be listed on the model tag on the back.

https://www.radiomuseum.org/r/zenith_sfd2575l_ch_9d25_ch_7d31.html

maybe?

if so the tuner is a 9D25 and the amp is a 7D31.

Thanks!!!!!! I'll post everything I find out.

 

[baconbadge]

Where are you located?

 

[TedB]

Where are you located?

Detroit area, East Side.

 

[RWood]

I've got one of those amps in a slightly different cabinet, the Grieg. You can remove the back panel and look at the power amp (bottom of the cabinet on the left) to verify it is a 7D31 amp. There should be a sticker on it with the chassis number.

I agree with your assessment of the problem - classic filter capacitor failure that is common to amps of this age. The filter cap is in a tall aluminum can, mounted on the amplifier chassis. It has at least 3 sections (three different caps with a common negative) and it is designated C1 on the attached schematic.

There are several ways you can fix the problem cap:
1) find a similar new cap online at AES. Values might be close but don't have to be exact
2) have a new cap made by Hayseed Hamfest
3) restuff the existing can with Nichicon Pencil Caps
4) use terminal strips under the chassis to mount new radial caps

There is actually a lot of room to work under the chassis, so I opted for 4) on mine. It is the least expensive way to do it.
While under it I also replaced all of the coupling caps and a few miscellaneous Electrolytic caps (bypass caps).

A couple of caveats on this amp in particular - - if yours has the "stereo expander" circuit it can be extremely confusing to work on or change out the output transformers. Also, this effect only works on the Phono and Tape inputs, as the tuner is strictly mono. Also, there may be speaker jacks on the back for "Radial Sound" speakers. The wiring to these also confused the hell out of me and I even had a pair of the extension speakers (I no longer do). I disabled these jacks and I just run it in stereo.

Let us know what questions you have.

 

[gadget73]

thats a strange output stage. Must be one of those deals with an out of phase signal used to sum the outputs into 2x the power level depending on the source material.