Zenith console rebuild: Tuner ID

Hi all:
I have a console pull from a friends that I want to refresh, but need a little ID help to get started. It's similar to the unit in this thread: zenith pp 6bq5 console amp. I would like to recap both the amp and preamp, no sense in turning down a good preamp/tuner, and some of the power amp's circuitry routes back through the pre. Unfortunately the chassis code is rubbed off the pre. Here's some pics:

Power amp first, then shots of the pre/tuner. Although it doesn't match the rubbed lettering, I think the pre may be an 8B24 or 9B25, based on this or this.

Any help? This will be my virgin build, the amp shouldn't be too hard, the pre's pretty tight, though.

Thanks.

Underbelly: amp and pre

Nudie underbelly shots, power amp first, pre second.

It is probably a 9B25 or an 8B24. Which confirms what you saw on Radio Museum

What I did was take your amp chassis, 7B31, and plug it into the Sams Online Index to see what folder it is in. Then click on the contents of that folder to see what goes with it.

My advice since this is your first attempt is to do the power amp first - lots of room to move and it is easy to see the capacitors that will need to be swapped out. Then run it with the tuner/preamp and evaluate. You may be able to use it as is or next do the preamp section. Doing tuners is tricky, as you saw, plus it might need alignment when done after the new caps. That's a job for a pro or a long-time radio guy.

RWood

RWood said:

It is probably a 9B25 or an 8B24. Which confirms what you saw on Radio Museum

What I did was take your amp chassis, 7B31, and plug it into the Sams Online Index to see what folder it is in. Then click on the contents of that folder to see what goes with it.

Click to expand...

I did just that after my initial post, and ordered 453-15.

RWood said:

My advice since this is your first attempt is to do the power amp first - lots of room to move and it is easy to see the capacitors that will need to be swapped out. Then run it with the tuner/preamp and evaluate. You may be able to use it as is or next do the preamp section. Doing tuners is tricky, as you saw, plus it might need alignment when done after the new caps. That's a job for a pro or a long-time radio guy.

Click to expand...

That's my plan, power amp first, then pre, and only as much of that as necessary. I'd like to stay away from the tuner section completely if possible.

Thanks,
TB

8 tubes, so it must be 8B24 (Zenith chassis numbers always start with tube count, letter is year). If there's a tube I missed, then 9B25.

I believe I have folder, if I can find it. I DO have the pages for the 7B31 scanned. tbavis(at)rochester(dot)rr(dot)com

Looks like 9 to me. I've seen your posts on prior Zenith rebuilds, thanks for the schem offer. I've already ordered the folder from Sam's.

More to come, I'm sure.

TB

All right, I'm back at this with a schematic. Keep in mind a few things: this is my first rebuild; the unit was free, so I'm not heavily invested in it; and my diagnostic skills end after checking voltages and continuity. I'm thinking I'll rebuild both the amp and preamp/tuner now, since their circuitry seems to be pretty well linked via the two umbilicals between them. If the pre/tuner ends up a bust (as in, if I screw it up), I know I can separate the power amp and modify it for stand alone use.

So, I'm thinking of replacing all caps in both units. Any that should stay? Any other components that could use a swap before power up? And finally, any recommendations on cap brands? There sure are a lot out there.

Schematic can be found here:Zenith 2560 Console Pull

TerryB123 said:

So, I'm thinking of replacing all caps in both units. Any that should stay? Any other components that could use a swap before power up? And finally, any recommendations on cap brands?

Click to expand...

Caps that I would leave alone are the ceramic and mica caps in the tuner/preamp. So change out the cylindrical paper (wax) caps and the electrolytics. In your tuner/pre there are a couple bypass caps that are electrolytic (I see one in a cardboard tube all the way at the top of the chassis) - those should go.

Other components. I redid a Zenith 7D31, very similar to your power amp, and found that the "Current Balance" pots were of very poor quality. I replaced mine with 2W units but also considered using a single resistor in place of it.

The large wirewound and ceramic resistors are probably fine, but I would check the smaller carbon comps. Remember that if they are connected to common point (such as gound) with other resistors in the circuit, you'd need to remove one lead to get an accurate reading. I often replace resistors in the power supply just because they see such heavy use and high voltage.

Film caps by Mallory 150 series are what I typically use, but similar caps like those from Just Radios or Weber are very affordable. Many opinions on caps; I try not to worry to much about it.

Hope this helps