Author Topic: Magnavox console / amp repair  (Read 12852 times)

electra225

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Re: Magnavox console / amp repair
« Reply #45 on: August 06, 2016, 10:10:29 PM »
I hope some of this helps you.  Good luck.
I don't need Google.  My wife says she knows everything.

AlexanderMartin

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Re: Magnavox console / amp repair
« Reply #46 on: August 06, 2016, 11:05:55 PM »
Good luck, my offer still stands good sir if you think it gets too hard, troubleshooting weird problems is hard if you're not used to this kind of work.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #47 on: October 15, 2016, 07:08:58 PM »
I had to set this project aside for a bit because school was consuming all of my time but recently I have had a chance to pick up where I left off. I actually spent some time learning about how vacuum tubes work and studying the schematic to understand how this amp works. I decided to go ahead and take voltage measurements to see where I was.

I pulled the transformer out and tested all of its outputs (should be 600 Center tapped, 6.3, 6.3, and 5 volts.) and found them to be all more or less correct. (They were a bit high due to no load).

Next I installed the transformer back in the amp, and also installed a new Hum Potentiometer (the old one burned up due to the fault) I plugged it in with all the tubes, including the 5u4 rectifier, removed and the potentiometer didn't smoke or burn up. I tested and verified I had connected everything correctly and it was sending the 300 volts to the 5u4 socket. I then unplugged it, installed the 5u4 rectifier back in (none of the other tubes were installed) and plugged it back in. This resulted in the Hum Potentiometer burning up instantly and letting smoke out.

I unplugged it to do some resistance checking and made an interesting discovery. The 30 mfd capacitor was still holding a slight charge of about a volt, which wasn't surprising. The interesting part is that the 1 volt was making its way to the filament circuit of the tubes (the circuit that connects to the smoking hum potentiometer). This shouldn't be possible because they are physically separate with no tubes installed. I did some more testing and disconnecting parts of the circuit to figure out where the voltage was leaking through from and found that it was actually coming from the transformer its self.

After some more testing I found that the voltage from that capacitor was going to the 5 volt output of the transformer where they connect on the 5u4 socket (a normal thing) but the 5 volt and 6.3 volt number 1 outputs of the transformer were somewhat shorted together internally (tested at 3 kohm between output wires which is a lot but still too much) and allowing voltage to pass from the 300 volt DC side of things to the 6.3 volt filament circuit. This I believe is what was causing the tubes and hum pot on that 6.3 volt filament circuit to have issues.



I know this is pretty much a novel of a response but I would appreciate it if someone could verify that my logic is valid. I have learned a lot about these things but I am still new to this and I would like to make sure I am correct before I go trying to track down a new transformer.

AlexanderMartin

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Re: Magnavox console / amp repair
« Reply #48 on: October 15, 2016, 09:55:40 PM »
At this point I'd have to physically see what's going on before I'd give you any kind of advice, I'm not so good at translating online advice to real life advice, and vice versa.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #49 on: October 15, 2016, 10:18:06 PM »
I guess I am 99% sure the transformer in this thing is broken and that is what is causing my issues. It seems to have an internal short between the 5v and 6.3 volt rails which shouldn't happen and that is leading to voltages going to the wrong places.

The other tricky thing is that I have looked around and I have had no luck finding a transformer with these values.

voxACthirtee

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Re: Magnavox console / amp repair
« Reply #50 on: October 15, 2016, 10:57:56 PM »
It is tough to diagnose over the net but there's a couple things to try.

How close were the voltages in the 5v and 6.3v?

without a draw they won't "short" but should still read incorrectly when you test them. As in if they are internally shorted they should all read the same on the secondary. All might e the 6.3 voltage or something odd and lower
I've also never run across anything that was "sorta" shorted.
I've run across windings that are broken internally and don't work, or are ACTUALLY shorted to something else internally.

Did you check using "continuity"? (the beeper/buzzer setting on the ohm meter)- Checking each winding for continuity with itself(example-lead on each green wire for 6.3v) and then touch the other leads with one on a green 6.3v tap to see if there is continuity to any other secondary or primary winding.(there shouldn't be) best done with the amp unplugged, no tubes and perhaps the wires unsoldered(again)

assuming the transformer is ok.....

The balance pot balances a path of resistance to ground from the 6.3v heater taps. Since there is no 6.3v center tap, floating it above ground creates a false center tap and reduces hum. A lot of amps just start at 100ohms or so to ground(or to the business side of the power tube cathode bias resistor, as in this amp) on both 6.3v taps.
Using a pot lets you find the actual lowest hum point.
The pot actually should be something like a 1 or 2 watt pot though, not sure what they had, or you had in there.


So that being said, did you change the big power resistor and cap on the power tube cathode bias? If thats shot it might be passing the heater voltage directly to ground and that much current through that pot will make it go boom.

Also, try a new rectifier tube. Tubes can fail and short internally.


ALSO- does it look like you are the first person to work on this amp?
Because if you are tasked with solving someone else's hackery, you are in way over your head.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #51 on: October 15, 2016, 11:26:10 PM »
Starting from the bottom up, I am the first person to deal with this amp since it was built back in 1961 so I am lucky there. I replaced the big can cap with four individual caps and that did not resolve the problem. My readings on the secondary side of things showed the two 6.3 volt outputs putting out about 7 volts (which is fine since there was no load) and I didn't test the 5 volt output when I was doing the readings so just now I plugged the transformer back in and found the 5 volt output to also test at 7 volts.

If I test resistance between the two leads of the 6.3 volt out I get 0 ohms and if I test resistance between the two 5 volt leads I also get 0 ohms. If I test from either of the 6.3 leads to the 600vct leads I get no connection (good), but if I test between either of the 6.3 volt leads to either of the 5 volt leads I get about 5 kohms. I have taken these resistance measurements of the transformer a few times now and I have noticed that each time I run it with a load on it (and burn up that hum pot) the resistance between the 6.3 volt output and the 5 volt output gets lower (closer to a dead short)

I tried a new rectifier tube and that didn't solve it. I also verified that it is putting out the correct 300 volts DC. The issue is that the 300 volts DC is somehow making its way over to the 6.3 volt heater circuit.

The original hum pot that burned up was a .5 watt unit and I replaced it with another .5 watt part.

along with replacing the big can capacitor I also replaced the coupling capacitors along with the 100 ohm cathode bypass resistor. This also did not correct the issue.

TC Chris

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Re: Magnavox console / amp repair
« Reply #52 on: October 15, 2016, 11:43:07 PM »
I wonder if you could get away with using a separate 5 V filament transformer for the rectifier--or by replacing the 5U4 with some silicon diodes.

Chris Campbell

voxACthirtee

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Re: Magnavox console / amp repair
« Reply #53 on: October 16, 2016, 12:50:51 AM »
Its grasping at straws at this point but did you change the power tube cathode bias cap as well?

Obviously you are getting too much something through that pot.
Since the 5u4 has the DC voltage on the 5v AC heater line and if the 5v and 6.3v are shorted then there you go. The other issue would be if the pot is allowing too much heater current to ground. that would be through a bad power tube bias cap/resistor.



Also, better to test the transformer for continuity. A transformer is full of windings. You test ohms by sending a small current. That signal inside a transformers windings can give you readings you might not be interpreting correctly. Use continuity. If it beeps, there's continuity.
If it don't, there isn't. keep it simple.
I've had one, if memory serves, that shorted once it got hot. But that was just once.

The ohm readings of a transformer are pretty meaningless. A transformer is a lump of iron or steel with wire windings of different "ratios". You can't test the ratios with an ohm meter(there is a process involving using a 1v supply and doing the math on the other side)
Sometimes you will see ohms labelled on a schematic, but its just for that transformer and its unique windings. Typically it will just show what either side of a secondary should be from a center tap. Most schematics don't have this though.
Sometimes you will see an output transformer labelled with a "5k" or "6.6k" or "8k" impedance. But that again ultimately relates to the tube its technically made for and the ratio its wound to, to get a 4/8/16 ohm secondary. No matter where you put a meter on a "6.6k" OP transformer you will never get a reading close to 6.6k.

Ohm meters can help you identify the secondaries and center taps of a winding if you find an old unidentifiable transformer. But thats just about measuring the taps, finding the largest reading between 2 points

and also, as TC said.
Try a plug in solid state rectifier. Disconnect the 5v taps.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #54 on: October 16, 2016, 12:49:46 PM »
I am going to give that solid state rectifier a shot. I don't have any diodes of that rating laying around so I went ahead and just ordered one for a few bucks.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #55 on: October 22, 2016, 02:56:22 PM »
Well the solid state rectifier showed up yesterday so today I went ahead and put that in, cutting the 5 volt filament wires from the power supply and taping them up so they no longer connect to anything. I installed yet another new hum pot and when I powered it on I got no smoke. I put it back into the console and connected everything and it works perfectly. The receiver works and picks up stations and the amp makes actually very nice quality sound. I would call it a total success.

My question though is should I be concerned about the capacitors in the receiver. The amp has been completely re capped but the receiver is still stock. I notice it has a large can cap on the outside the same as the amp did and I would assume it has more individual caps on the inside as well. The sound also gets very quiet on one side when you turn it to fm stereo instead of fm mono so I would guess the stereo receiver could use some attention as well.

AlexanderMartin

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Re: Magnavox console / amp repair
« Reply #56 on: October 23, 2016, 01:50:30 PM »
Wow, great job on finding the problem! I am impressed.
As for the tuner, you will want to recap that was well as check the resistors and make sure it's all square. The dead channel could be a number of things, open resistor/cap, bad tube, or even something as simple as dirty connectors/tube sockets.

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #57 on: October 23, 2016, 10:23:08 PM »
I think you may be correct that cleaning the pots and selectors will help since wiggling the various tone adjuster pots and the speaker output selector makes the sound better or worse. I found some old radio shack pot / selector cleaner at my dads house that I planned to use but its all dried up so I have been looking around and people recommend the DeoxIT brand stuff although it seems they have various types. What type should I consider using for this application?



I am also honestly very impressed that I was able to find the fault and repair it.

impaladb

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Re: Magnavox console / amp repair
« Reply #58 on: October 24, 2016, 08:32:42 AM »
Use MG-Chemicals  Nu-Trol. It cleans and lubricates pots and selectors. I get it at amazon in the 12oz can.

Motorola Minion

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Re: Magnavox console / amp repair
« Reply #59 on: October 24, 2016, 12:07:52 PM »
Great to hear you got the amp PS fixed ;D. I just replaced a 5Y3 tube with two silicon rectifiers on a 1963 Zenith where the original transformer's primary melted down. I did this for a different reason, I wanted to save the 10 watts of drain on the power transformer I had just replaced. I left the tube in the socket but no wires are connected to it now. I used a terminal strip from Radiodaze that allowed the 500 volt secondary winding to terminate safely to the new rectifier diodes.

After replacement using a transformer from an older Zenith amp, voltage into the first electrolytic PS cap was supposed to be 368 but it measured 415 volts DC under load. It is also a good idea to put some resistance ahead of that first cap, because that voltage hits the cap with 475 until the output tubes warm up and it starts drawing current. Based on the 100 ma.current draw, a 500 ohm resistor at 7 watts to brought voltage down where it was supposed to be.
Tubes - Magical - Tubes

Dave