Zenith Amp

[William]

Sometime ago an acquaintance gave me a Zenith console. He is always finding things, nice things, bringing them home, and then not taking care of them. He said if I wanted it come get it or it was going to head to the curb. When I arrived, we headed to his shed, which is not a sealed building, and when he opened the door mice took off. Oh great, it's another mouse hotel. Sure enough, I found where mice had made a nest. We got it outside; I cleaned out the mouse nest and we loaded into my truck and headed home.

Before it went to my barn, which is mouse proof, it received a three-part scrubbing to get rid of any mouse poop/pee. After cleaning, it went to the barn. About a year ago I pulled the amp and brought it home thinking if it was here, I might do something with it. Well, last week I plunked it down on the bench to see just what it had to offer. Zenith chassis number is: 5F29 is a single ended 6BQ5, 5Y3, 12AX7, 12AU7 amp with lots of wax paper caps.

The first thing I did was test all the tubes and found them all testing good with no shorts. That is good news, now for the next step. I hooked up my bench speakers, plugged in the amp to my Variac contraption and slowly started to increase the voltage. When I got to about 60 volts, I started to smell old hot electronics, so I felt the PT and it was still cold and there was no smoke. I also noticed the tubes had a faint glow. I continued to increase voltage until I reached 80 volts and 22 watts, and everything seemed the same, so I powered it down and hooked up my little KLH tuner. I wanted to see if it was making any sound. Repeating the process, I slowly started to increase the voltage and this time I noticed the smell was a little stronger and I also noticed the watts had doubled. I forgot to mention that this amp has a factory fuse which has not blown so I continued to increase back to 80 volts which did produce a kind of distorted noise/music. At this point the draw was 55 watts. Starting to think about increasing the voltage a little more when I notice a slight crackle sound, I reach over and touch the PT and its temperature is climbing rapidly. It's not hot/hot yet but it will be soon. I immediately pulled the plug and walked away so things could cool off.

Since the weather here in wonderful west Michigan is cold and damp today, I decided to see if the PT was bad on this Zenith amp. The first thing I did was see if the fuse had blown and sure enough it did but not during my power up as I was still reading volts and watts on my contraption when I pulled the plug. So why would it blow after I shut it down? Next, I pulled the rectifier tube, brought the amp up to 117 volts and still reading 55 watts, on the Variac contraption and was about to test for voltages on the PT when I noticed a strong PT kind of smell. I touched the PT, and it was getting hot in a hurry so again I pulled the plug and as I did that I looked, and I still had a voltage and watt reading on the contraption. I think I may have just encountered my first bad PT. So here I am sharing my adventures in Zenith console stereos and trying to decide what to do next. Here's a couple of photos so you know what I have been working on.

[electra225]

Zenith is not known for robust power transformers. Sorry for your trouble. And I know the sinking feeling when you find a dead PT. I wonder if your Zenith was bitten by the same PT eating bug that bit my little Olsen amp? Maybe we need to find a couple cheap PT's and see if we can get them going.......

Have you made any investigation as to what caused the high current that killed the PT? What size fuse was there? Could someone have put in a fuse of too high amp rating?

[William]

The fuse was correct per the schematic, 3AG 2amp. That was what was in it and that is what I replaced it with. I have tinkered a little this afternoon trying to figure out what happened but I'm not sure if I'm doing the correct testing.

Bill

[hermitcrab]

I had the same problem... but I think my amp is a 4F29...mine does not have all those paper caps , mine has ceramics. was playing the zenith one day and noticed all at once the sound dropped down and became distorted and I could see smoke .. the fuse was good ... found out it was the PT as well , ... but not the secondary , it was the primary that shorted ... I assumed due to the dampness in the basement the PT wrapping just broke down ... I had a NIB PT on the shelf I had bought for a RCA tombstone radio with a shorted PT some years before. the specs were close and had the windings the zenith needed so it got used for that.. been playing fine ever since ...unfortunately replacements are not cheap... I think the one I used from antique radio supply was like 60 bucks back then , can't imagine what they want for them now...what year is your zenith from?

[William]

Elton, the Sam's folder is dated 12-61 but somewhere I read it was a 1959, but I could be dreaming that. The rectifier tube, which is Zenith branded, has a date code of 60-48 so that might be the correct answer. EIA code 188-5, General Electric.

Bill

[hermitcrab]

I was told that consoles are like cars and some are mfg before the actual year... although someone could have put that tube in the set afterward... but the tube with 48th week of 60 seems it would make it a 61 which would explain the date on the sams ... mine I believe is a 63 or 64 ...

[William]

I would agree, it could be either a 60 or 61. I will dig around in the console cabinet and see if I can find a number or date. Do Zenith model numbers indicate years?

Bill

[hermitcrab]

I don't think so .. their radios from the 30's and 40's... the first number was usually the tube count ... then the letter designated whether it was a floor or table model then the model number followed..... which could still be true since your chassis starts with a 5 and you have 5 tubes on the amp...

[William]

Good to know, thanks Elton.

Bill

[Motorola minion]

As far as Zenith console model numbers go, its not much easier. Ill share what I have since I memorize numbers like this.

Zeniths I have had: 2 - 1959 consoles, 1 each - 1960 and 1963 and 1964. Model numbers are SFD2505, SF177, SF183, MK2635, MT1971.

The amp and tuner chassis numbers are ultimately the most useful overall, but model numbers are only good for finding the Sams folder or Riders you need. Fortunately I have factory service books for all 1960, 61, 63, 64 Zenith Consoles, which are very detailed in a model index.

In general, Zenith years are coded by first letter in chassis number but rarely tie to a model unless its a TV
A=1958, B=1959, H=1960,

Example 1: 1959 amp chassis 5B23 used in the SFD2505 phono-only and has single ended 6BQ5 driven moderately.
Where 5=tube count, B=1959, 23=sequential number given to each subsequent version
Example 2: 1959 amp chassis 7B30 used in the SFxxxx? phono-only and has P-P 6BQ5 driven harder.
Where 7=tube count, B=1959, 30=sequential number given to each subsequent version
Example 3: 1963 amp chassis 3K31 used in the MK2635 with TOTL tuner/preamp and has single ended 6BQ5 driven
harder. Where 3=tube count, K=1963, 31=sequential number given to each subsequent version

[Motorola minion]

Here are the manuals, https://www.ebay.com/itm/403174580636?h ... R5TkuJn-YA
this one is a bit pricey and I already have it!

[William]

Thanks for the info, Dave. I always enjoy leaving something new.

Bill

[TC Chris]

What were they drinking at Zenith HQ back then? "A=1958, B=1959, H=1960" and then "K=1963." ???? Maybe they went on holiday in 1961 and 1962? And forgot the next letter after B??

Chris Campbell

[Motorola minion]

What were they drinking at Zenith HQ back then? "A=1958, B=1959, H=1960" and then "K=1963." ???? Maybe they went on holiday in 1961 and 1962? And forgot the next letter after B??

Chris Campbell

.
Same thing at RCA in those days, especially TVs! Model numbers were all about sales logic I guess.

I was just looking at my VHF-only Z tv L=1964, last year for no UHF. J=1962 (Z's first color tv) but 1960 may have been C.

H is actually 1961 - MH2635, with 3H32 SE6BQ5 amp driven hard , 12H26 non-stereo tuner

[Motorola minion]

What were they drinking at Zenith HQ back then? "A=1958, B=1959, H=1960" and then "K=1963." ???? Maybe they went on holiday in 1961 and 1962? And forgot the next letter after B??

Chris Campbell

.
Same thing at RCA in those days, especially TVs! Model numbers were all about sales logic I guess.

I was just looking at my VHF-only Z tv L=1964, last year for no UHF. J=1962 (Z's first color tv with 29 tubes) no I don't have one.
1960 may have been C or G, Ill check but Z console model numbers from 59-60 usually begin with "SF" or "SFD".

H is actually 1961 - MH2635, with 3H32 SE6BQ5 amp driven hard , 12H26 non-stereo tuner is a near TOTL model, only the 20+20 watt pp amp would make it top dog.

I had one what I think was a 1965 (M) model but all tube, MT1971 with a 4L21 preamp/se6BQ5 and 9 tube tuner with MPX. Same year saw solid state introduction, a year or two later than Magnavox and RCA.

[electra225]

How hard did Zenith drive their 6BQ5's? Magnavox usually takes the rap for that at 355 volts on the plate. Did Zenith go more than that? They also run hot, maybe warmer than the 50C5. That doesn't seem to hurt them.

[Motorola minion]

I have several schematics of Zenith stereo push-pull EL84 amps, spanning 1959-64. These were very similar but one of them has the 5U4's highest supply voltage at 420 volts DC

[electra225]