Can you help me fix this Zenith?

[Carmine]

For a while now, I've had this partially operational Zenith Emperor in my living room. I would really like to get it up and running for the holidays, as I think it would be fun to load some Christmas DVD/CD/Albums and play them through both the TV and the stereo for holiday entertaining.



If you recall, the CRT's blue gun dies occasionally (as it is here). It seems to come back if I zap it, but is dead again within a few days. For this reason, I've never bothered to really dial in the convergence or any of the other TV issues. (Sound doesn't tune well with pic, etc.)






When I go it, one of the stereo channels was DOA... In fact, some of the amp output transistors were just plain missing. I brought the amp chassis to Capt' Moody, along with a Magnavox "parts" stereo chassis. I re-installed it and it worked for a few hours over the course of some days. However, one day I turned it on and it immediately "popped" and shut down entirely.





I just pulled out the chassis again, and there is a 3 amp fuse that blew straight to hell. I am willing to document this thing step-by-step with pictures if others on here are willing to guide me on diagnosis/repair.



Here are the pros/cons...

Pro:

• Better than average electronic knowledge
• Fairly good at soldering
• Access to some parts
• money to buy parts

Con:

• No schematic
• Have only replaced one capacitor in my life
• a zillion other things compete for my time

Can I test capacitors with a VOM for shorts? If your instructions are just to shotgun the whole thing, it will never get done. But I can give meter readings, etc. Can this be done, and where would you start?

[ctc17]

What are the chassis numbers of the tuner, amp and tv?

I guess the crt is a given and fairly easy to replace. The hardest part of that is the convergence.

As far as capacitors go I see very few bad ones on zenith sets. I have a feeling the fuse blowing issue is a shorted output transistor or rectifier diode. I have also see situations where something like a diver transistor will randomly short causing one of the output transistors to short. These issues can be tough to troubleshoot.

[Carmine]

Originally Posted by ctc17

What are the chassis numbers of the tuner, amp and tv?

I guess the crt is a given and fairly easy to replace. The hardest part of that is the convergence.

As far as capacitors go I see very few bad ones on zenith sets. I have a feeling the fuse blowing issue is a shorted output transistor or rectifier diode. I have also see situations where something like a diver transistor will randomly short causing one of the output transistors to short. These issues can be tough to troubleshoot.

I agree with you about the caps... I have several 40+ year old Zenith sets that fill the screen and are rock-solid with OEM caps. I think it's some kind of dead short too.

As for the stereo chassis numbers, you'll love this... The paper glued inside the cabinet appears to have been damaged for decades, right where the model number should be...



I can only read a partial number that says: **?**5T24-8N24

It's so brittle that if I tried to move the paper, it would simply disintegrate.

[sampson159]

nice looking set!i had a zenith roundie that had the same blue gun issue.a gentle tap on the lowest setting would bring it back for a few days at a time.finally got tired and replaced it with a hi-lite crt.not a good swap.looked good in a rca,but wasnt good in the zenith.convergence might hace a coil unwinding on the panel.found this on many zeniths.popping noises sounds like caps.should be an easy fix.great find!

[freakaftr8]

Agreed. Shorted recitifier diode due to excessive ESR from a filter cap probably, or just a filter cap that went south for the winter entirely.
Check those cans!

[Carmine]

Originally Posted by freakaftr8

Check those cans!

How does one do this? Do I measure ohms across some connection looking for 0 resistance (short)?

[zenith2134]

An ESR meter would help with checking those can cap's...you could of course use an ohmmeter to check for dead shorts like you described...Measure across the can itself or the neg/common lead which are all at the same potential, and each section of the cap *with the set turned off*

on a side note, I always thought it was odd that the tube compliment inside most hybrid SS sets emphasized 'turning power off before removing transistors' ...i mean it isn't such a great idea to pull tubes while it's running either,,,not to mention that they'd be hot as heck.

[ctc17]

Get yourself a 100 watt minimum light bulb, if you can get a 150 watt outdoor flood light even better. If you dont have a socket to screw it into solder wires to it. You can cut the plug off a lamp and strip the wires too.
Tie the bulb across the back of the fuse with the fuse out. If you have a short the bulb will light up fairly bright. If working properly the stereo should only light the bulb a tiny bit if at all.
First thing I would do is remove the output transistors and see if the short goes away. If so you can easily test the transistors using the diode check on a multimeter.

I see what look like mismatched output transistors there. I works on high power amps used in nightclubs and its absolutely critical the output transistors are matched, not only the number but the tested gain figure as well. I know its not that critical on low power stuff like this but when I see two totally different looking cans its very suspect. Im really looking at those output transistors.....
Generally its cool to 'stick anything' in there on low power consumer stuff but not always.

Is there a model number on the back of the entire unit?

[radiotvnut]

I don't know about this stereo; but, I've had several '60's era solid state console stereo's that needed a full electrolytic recap in order for them to work properly. Most of the offending caps were the solid black electrolytic caps that were marked "callins", "whale", etc. The short in your stereo could be due to a shorted rectifier diode or a shorted output transistor. If everything is not just right in the output circuit, the output transistor will short again. It may not short instantly; but, it will happen.

[Carmine]

Here is a picture of the underside of the amp chassis. It looks a lot simpler than the typical TV chassis, so maybe this will be good for me to learn on... Perhaps somebody could us a MS Paint or Photoshop program to show me where to test for a shorted diode, or shorted cap?

[Carmine]

Picture of the top-side.

[Carmine]

Originally Posted by ctc17

Tie the bulb across the back of the fuse with the fuse out. If you have a short the bulb will light up fairly bright. If working properly the stereo should only light the bulb a tiny bit if at all.

Not quite sure what you mean by this. I should solder the connection across the fuse, effectively using the 100 watt bulb parallel to the fuse? This will also require a new (unblown) fuse, right?

First thing I would do is remove the output transistors and see if the short goes away. If so you can easily test the transistors using the diode check on a multimeter.

I get this part. I will do this first because the connections are very easy to get to.. I don't even need to remove them from the sockets.

Is there a model number on the back of the entire unit?

According to the brochures that Doug posted a while back, this is model X8580H in 1967. The TV is Chassis number 25MC46Z, if that helps date this as a 1967?

Page 19 on this PDF link:

http://www.mediafire.com/?nwqgjoeymtz

[ctc17]

Leave the fuse removed using the light bulb AS the fuse. That way the bulb tells you the level of short and you dont blow fuses over and over testing.

The output transistors that drive the speakers are on the heatsinks. The big ones below look like the drives and the small little plug in ones are predrivers.

This uses driver transformers between the driver and output, so it makes it fairly tough for a problem in the driver or before circuit to fry the outputs. If it turns out the drivers are shorted, it could very well be a leaky cap if caps are used to isolate the output transistors from the speakers.

Start by pulling the big transistors out one at a time, measure between the two pins then between the case and each pin using the lowest ohm scale setting. If one is shorted it will read close to 0 ohms.

The other way is the check it with diode test on the meter. A reading of .500 volts is common in some directions but none should read 0 or shorted.

[ctc17]

Do I see 8N124.. ? that would probably be the chassis number.

Check the two rect to see if their shorted, then check all the ebc on all the sockets to see if there is an shorts, OR remove the transistors and check them above. The main rectifiers are the ones studded into the chassis with the blue wires. I also think i see a small one. You may need to unbolt the stud and lift the rectifier out of the chassis. I bet that chassis is positive, thats why all the caps are isolated.

Looks like one of the driver transformers has been replaced and in that case it was most likely smoked if a driver shorted in the paste.

[Carmine]

Originally Posted by ctc17

Do I see 8N124.. ? that would probably be the chassis number.

Check the two rect to see if their shorted, then check all the ebc on all the sockets to see if there is an shorts, OR remove the transistors and check them above. The main rectifiers are the ones studded into the chassis with the blue wires. I also think i see a small one. You may need to unbolt the stud and lift the rectifier out of the chassis. I bet that chassis is positive, thats why all the caps are isolated.

Looks like one of the driver transformers has been replaced and in that case it was most likely smoked if a driver shorted in the paste.

It's 8NT24, so jives with what's legible on the pasted layout.

Thank you for pointing out the rectifiers. I thought they would be those square heat-sink looking things I've seen in TV chassis. When you want me to check for a short, in what two places should my multimeter probes be? One on the soldered connection, one on the chassis? Or should I unbolt them?

On the driver transformers, all four of the rivets appear to be original, no signs of them having been removed, no scratches on the chassis, etc.

I did check the E & B terminals with my meter set on diode. All four outputs had similar readings, .005 in both directions (polarity). The two big drives had different readings based on polarity of E & B, .115 and .490, but were similar to each other... Maybe .117 and .450

I'll try the light bulb thing in a while.