'62 Magnavox console sound problem

[schmiddj]

I'm a newbie trying to refurb an old Magnavox and it has a weird issue. To my non-audiophile ears this thing sounds absolutely gorgeous... for about 20 minutes. After 20 min or so, the sound starts to degrade noticeably in both channels, eventually cutting out completely in one channel and then sounding horrible in the other. Turn the thing off, wait 20 min, and it's fine again.

Seems like an obvious heat issue... but what to do? As far as I know, the thing has never been re-capped and it has its original tubes, so anything is on the table. But I thought tubes and caps were an overall sound quality issue, not this weird delayed timing issue.

Any ideas?

 

[jaymanaa]

No wonder, it's upside down, the heat can't dissipate. Are you getting any redplating on the tall output tubes. Maybe the rectifier as well. Hey, welcome to AK.:thmbsp: BTW, on stuff like this, once you know it plays, you could go ahead and do the refurbish. Nine out of ten times, that will cure your issue. You gotta do it anyhow.

 

[gadget73]

I'm going with leaky coupling caps. Usually when they get warm, they tend to leak more and what you're hearing is probably the tubes becoming saturated. The plates would be glowing red if this is the case. At this one's age, some new caps would be a good idea just for safety and longevity.

Also possible a resistor is going flukey from heat and changing value.

 

[buglegirl]

Hiya,

I would start by replacing the main filter cap (Tall Silver Can) as it sounds like B+ is going south after the cap warms up a bit and then shorts out.

Then move on to the coupling caps.

Frannie

 

[jaymanaa]

Hiya,

I would start by replacing the main filter cap (Tall Silver Can) as it sounds like B+ is going south after the cap warms up a bit and then shorts out.

Then move on to the coupling caps.

Frannie

Hmm, that's some good logic on the main filter cap, I'd have never thought of that. A meter will tell you, got one? Oh, and be careful, there's like over 400 volts DC running around down there the will kill you hammer dead. Always try to work with one hand, use clip leads to hold meter probes on and stuff like that. Just be careful, it never kilt me, but man it hurts like hell and will make you spit pennies.:yes:

 

[tubeampking]

Spit pennies. Nyuk nyuk...that's funny. All kidding aside if you're not sure of yourself leave it to a professional. ALWAYS discharge the cap before you go pokeing around in there. While building me second guitar amp I got impatient and didn't do this. I got a helluva jolt..felt like I got hit with a hammer...smelled hair on my finger burning....not fun






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[tubeampking]

Now that I think of it I head a very similar problem with my Maggie 8802. Turned out to be a coupling cap


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[Shadowdog]

Welcome to AK and the wonderful world of tubes & who would of thought they would be back!

For all old Tube Amps it's a really good idea to recap it fully (Power Supply, Coupling, & Bias Capacitors) as you risk any or all the Transformers and the Output Tubes.

Caps only have a life span of about 20 years.

Check the resistors without the tubes in also as they can go really high in value in some units.

Clean tube pins and sockets and switches.

If there is a Selenium Bias Supply etc. it also pays to replace it with a modern SS diode supply sometimes adding resistor(s) to get the voltage right.

Never run a tube amp without load (speakers or 25W - 16 ohm resistors) even when testing and the tubes don't like to be short cycled (let them cool well before restarting).

 

[schmiddj]

Thanks!

Spit pennies. Nyuk nyuk...that's funny. All kidding aside if you're not sure of yourself leave it to a professional. ALWAYS discharge the cap before you go pokeing around in there. While building me second guitar amp I got impatient and didn't do this. I got a helluva jolt..felt like I got hit with a hammer...smelled hair on my finger burning....not fun
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"leave it to a professional"?! What's the fun in that?

Thanks for the help everyone -- I knew joining up here would be a good use of my time.

Sounds like I'll start with that huge filter cap (after learning how to properly discharge the thing; i'm told that getting killed is frowned upon) and then move on to figuring out how to recap the whole shebang. Or at least figuring out which are the "coupling caps" -- already using google on that one.

Sounds like everyone is in agreement that the problem is likely NOT bad tubes.

 

[Shadowdog]

If you understand a schematic , the coupling caps connect to the output tubes from the driver tubes and on SE Amps there is one for each channel and on a PP Amp there is one to each output tube.

They are usually tube shaped with leads at each end for a total of two.
Sometimes the coupling caps are disk caps (not likely on yours) and they can last many years, but sonicaly they are lacking so many replace them.

Watch the polarity on the PS caps and on the Coupling Caps there is not an issue or any markings for such.

The voltage can be higher on the replacements and to save a fair amount of $ you can use individual caps for the PS placed under the chassis,
leaving the old Can Cap in place (unhooked) for looks. Some also re-stuff the can cap with pencil thin caps that can be found.

 

[tubeampking]

Discharging the can is very simple. Take a test lead...insulated alligator clips on both ends...Attach one end to the chassis first
The other gets attached to the plate of one of the output tubes. For your amp that would be pin 7. Be sure to attach to the chassis FIRST. Let the there for a minute or so before you start work. I leave the lead attached the wholen tjme I'm working on the amp. And definitely change out the coupling caps...there's like 4 of them...about $10 investment. If they're not leaking they will soon enough.





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[century tek]

If you are going to replace the main filter anyway, don't waste your time learning how to discharge it. Just short it with a screwdriver. 99 percent of the time, they start to drain after turn off so you might get a weak spark if that.

Plus, try changing out the 5U4 rectifier tube. I had the same problems once in an amplifier such as yours.

Yes, change out all capacitors anyway. It's just what you do with this 50+ year old equipment.

 

[schmiddj]

redplating

Some excellent advice above. I checked and it does indeed seem like 2 of my 6BQ5s are redplating. I switched them to different sockets and the redplating stayed at the original socket (didn't follow the tube). So I guess that means the coupling caps are probably the problem.

And I guess there is no harm in changing the rectifier tube too, or is that just a waste of money if it isn't redplating?

 

[cademan]

Sometimes it can be the rectifier tube but since you say you have a couple of redplating tubes, it is usually the coupling capacitors that causes this but replace all capacitors anyway. If you do this and it cures the problem, leave the rectifier tube as is.

 

[gadget73]

I'd change the coupling and filter caps and see what that gets you. Rectifiers are pretty tough.

 

[kirkendoll]

Does all 4 of your output tubes have that smokey look to them on the inside of the glass, if they do then you got lucky and have Mullard output tubes which are highly sought after.

And welcome to AK :thmbsp:

 

[tubeampking]

If you are going to replace the main filter anyway, don't waste your time learning how to discharge it. Just short it with a screwdriver. 99 percent of the time, they start to drain after turn off so you might get a weak spark if that.

Plus, try changing out the 5U4 rectifier tube. I had the same problems once in an amplifier such as yours.

Yes, change out all capacitors anyway. It's just what you do with this 50+ year old equipment.

Its not a waste of time to learn proper safe procedure. DONT use the screwdriver technique...ever. If you're just starting out why not learn the proper methods ?

It could absolutely be bad tubes. One is worse and as it gets very hot it starts to hog the load or spread the load to the others and as they get loaded another weak one starts to go and it all sounds like crap and you lose a channel. However leaking coupling caps could make this happen, too. Observe the amp playing in a dark room to watch for the plates to start glowing pink to light red. If you see that swap the tubes to different positions to see if the redplating follows the tube. If so get a replacement. If it stays with the socket, it's probably leaky coupling cap or caps.

Odds are it is not the main filter. If you don't have a significant hum in the audio a "shorting" main filter would not drag things down without a serious hum developing or popping a fuse.

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[No Money]

Its not a waste of time to learn proper safe procedure. DONT use the screwdriver technique...ever. If you're just starting out why not learn the proper methods ?






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What he said. The screwdriver technique can be very dangerous. Do it the right way. Far to many "I've always done it this way" people either end up getting hurt or causing others to get hurt.

 

[century tek]

Its not a waste of time to learn proper safe procedure. DONT use the screwdriver technique...ever. If you're just starting out why not learn the proper methods

Well, being a tech repair for over 40 years, I have never had any safety issues by doing this. Just make sure it is turned off and unplugged when doing this. The only thing you are doing with a resistor to ground is bleeding it off slowly and not creating a spark. Doesn't really matter if the can is going in the garbage anyway, It's just not a good idea with new capacitors as it tends to strain the capacitor and break down the electrolyte. I have replaced more capacitors from people doing this with new one's in their equipment.

Just don't do anything with it powered up. No matter how careful you might think you are. The screwdriver method won't hurt anything and just don't be dumb enough to be holding on to the metal shaft of the screwdriver when doing this. Always keep one hand in your pocket.

Not saying not to learn the right way, just saying with the old can, it really isn't a safety issue. Like Jaymanaa, I have gotten bitten more than I can count fingers and toes but that was when I had things powered up.

 

[cademan]

Now, there is nothing wrong with using the screwdriver method. If turned off and unplugged, there is no way that there is ever going to be 400+ volts on the capacitor. Besides, this will just discharge it in an instant if there is anything left inside.

I just don't do it with new capacitors when testing equipment. It won't hurt the old capacitor because you are going to be replacing it anyway so who cares if it hurts it or not. I have been doing this for 30+ years and yes, I know the right way of doing this.

I have also gotten jolted by capacitors that have had a residual charge but as mentioned, it will not even have near the operating voltages inside as if it were plugged in and turned on. If you get a tingle from one (and you will if you stay in this hobby), it isn't going to kill you but it can sure piss you off!