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Magnavox 142 - rebuild/modification thread

battradio said:
Did you ground the negitive lead of the output transformer to chassis. Try disconecting the feedback resistor to see if the whiste goes away , if it does swap the plate leads of the output transformer . The blue and brown .

http://www.edcorusa.com/products/458-cxpp100-ms-4_2k.aspx

Mark
Click to expand...

Nothing is grounded to the actual chassis except mother earth ground. I do have a comon circuit ground tht is copnnectd. ANd then the two leads of power, hot and neutral. The shcematic shows a 10000mmf cap from one leg of the power to mother earth ground/chassis ground - I do have that in there. I think that should have gone to common circuit ground though, now that I think about it.

Battradio, I thank you and hate you right now. I spent about 1.5 hours tracking the circuit and didn't find an issue. Switching the wires fixed the whislte. The unit sounds reasonable now. I am sure it needs tweaking. Time to get the volt meter out and seeing what I got where .

A couple of point to note:

1. The output tubes in the circuit are 6v6gt, not 6v6g as the circuit was designed for. How much difference will this make?

2. The ouput tubes are hot, but not as hot as I would normally think they should be.

3. The VR tubes glow a cool color. When I turn the volume up, the glow flickers a bit with the bass beats.

4. There is a slight PS hum. I cannot imagine with the amount of capacitance and chokes in here what is causing it.

All for now. I'll try and get some meausrments and post them in a bit.

Mark
 
Battradio, thanks for the time on the phone.
I corrected the few ground issues you pointed out. It is dead silent now. It is also quite a bit more dynamic too. I am going to call it a day and work on it after thanksgiving. I'll get all the measurements then and post my results.

Mark
 
Jcricket said:
Battradio, thanks for the time on the phone.
I corrected the few ground issues you pointed out. It is dead silent now. It is also quite a bit more dynamic too. I am going to call it a day and work on it after thanksgiving. I'll get all the measurements then and post my results.

Mark
Click to expand...

Hi Mark ,

Glad i could help .

Mark
 
What were the ground issues?
Im curious as I have a maggie amp that im going to rebuild and they have similar grounding structures and power supplies
 
mackinthebox said:
What were the ground issues?
Im curious as I have a maggie amp that im going to rebuild and they have similar grounding structures and power supplies
Click to expand...

When I built this I had an idea. I seperated the chassis from ground. I created a circuit wit ha common ground, but seperate from the chassis. I also did not "ground" the output transformer. I used the common as a common, but it needed grounding too. I grounded the cahssis to circuit ground, and the output common to chassis ground, and all of that to earth ground.

I hope that makes sense.

Mark
 
Hi Mark,
These chassis are nickel plated to prevent corrosion. That being the case, get a decent respirator, i roughen it up with 60 or 80 grit, use acetone to wipe it down. Your first coat of primer should be very light (like fog on glass).Wait till it's dry to touch(5-15 min)Add a couple more coats (allowing it to dry in between coats)Do the same with your top coat, start with a foglike first coat.
Use acetone outside and downwind (did I mention RESPIRATOR) or denatured alcohol.and GLOVES
All of the solvents are bad, (cumulatively) IME Sorry if I sound like a shop class teacher.:nono::D and/or preaching to the choir.
Back on topic. The nickel plating is a problem for paint. I'm liking this thread,tho! Dual 142 monoblocks, twice the power!
 
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bobabode said:
Hi Mark,
These chassis are nickel plated to prevent corrosion. That being the case, get a decent respirator, i roughen it up with 60 or 80 grit, use acetone to wipe it down. Your first coat of primer should be very light (like fog on glass).Wait till it's dry to touch(5-15 min)Add a couple more coats (allowing it to dry in between coats)Do the same with your top coat, start with a foglike first coat.
Use acetone outside and downwind (did I mention RESPIRATOR) or denatured alcohol.and GLOVES
All of the solvents are bad, (cumulatively) IME Sorry if I sound like a shop class teacher.:nono::D and/or preaching to the choir.
Back on topic. The nickel plating is a problem for paint. I'm liking this thread,tho! Dual 142 monoblocks, twice the power!
Click to expand...

Nickle plated.............
Hmm, that might explain some of this. On the next one, I will sand this, and yes I do wear a resperator.

I changed the 2.7k feedback resistor to a 3.3k. It made a huge difference in the sound. Much more open and airy.

I did this for two reasons. The original output tranny was tapped for 4 ohm speaekrs. The new tranny is 4/8/16. Also, the old tranny was 3.8k impedance, the new is 4.2k. I am not sure if that last point is relevant or not.

More to follow..........
 
This new paint won't stick to factory paint , the Magnavox chassis are zinc plated , wish it was nicklew the plolihing them would be alot more rewarding .
 
questions, confusion, and measurements

Hey Folks,
I was able to get the time to do some meausrements.

The measurements were consistent from tube to tube, so I am only going to list the values of one of each.

TUBE pin schematic Measurement
6v6gt 2/7 heaters , 17.5 volts??? What
6v6gt 3 plate 295v , 286
6v6gt 4 screen 305 , 288
6v6gt 5 screen 0, .2/.3
6v6gt 8 cathode 18.5 , 17.5

5y3 4/6 290 , 315

6sl7 1 screen -.1 , 1
6sl7 2 plate 95v , 78.6
6sl7 3 cathode 1.5v , 1.17
6sl7 4 screen 0 , 3.2
6sl7 5 plate(b) 140 , 123
6sl7 6 cathode(b) 1.7 , 1.92
6sl7 7/8 heaters , 17.5


0D3A's pin 2 287.6 (note voltage before 2k 10w resistor is 375.6)

So, a few questions. HOW in the heck am I getting 17.5 volts output on each leg of the heater supply??? This is going to eat my tubes fast, right?

It appears I need to drop the value of the 2k 10w att resistor a little - maybe 1.8k, maybe 1.5k? This should raise the striking voltage on the 0D3A's a little. Is this correct?

All readings were taken with the tubes in.

What about the 6+sl7 pin 4 variance. Is this in the resistors - the two 220k and 20k located between pin 5 of v2 and v4?

Any help would be appreciated.
Mark
 
Jcricket said:
It appears I need to drop the value of the 2k 10w att resistor a little - maybe 1.8k, maybe 1.5k? This should raise the striking voltage on the 0D3A's a little. Is this correct?
Click to expand...

Nope. The resistor changes the current flow through the VR tubes but the tubes themselves set the absolute voltage you get. Did you measure the voltage between the two OD3s? Perhaps one isn't pulling its weight. It could be related to a memory effect that the tube manuals talk about, so if they were run at one current level and then switched to another they might not regulate to the same voltage. They also talk about age effects and I'm sure they never extrapolated that out to 50 or 60 years. I'm not sure it will make much difference either way since 288V really doesn't seem to be too far off of 300v.

Put a couple of different ones in there and see if it changes. Also, measure the voltage drop across the 100 ohm resistor to see how much current the screens are drawing. I'd be curious to figure out what the split between VR current and screen current is.

Did you measure heater volts on AC?
 
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You probably have the heaters referenced to DC voltage from the outputs cathode.
(normal)

You need to measure AC volts there for the heaters.. :) (1 heater pin to the other)

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Scuzzer is more then likely right on with those VR tubes, I've had to try a few to get
"correct" voltage from them, some worse then others, but try a few there to "set it"
 
What is your configuration for the B+ that goes to the output tranny's pwr?

Rectifiers, cap, choke, cap? (nothing else is in there) ? What is the resistance
of that choke? You really want the plate voltage higher then the screens, that
is if you really want to do this right the plate volt should be higher then screen.
(many inexpensive vintage designs just tapped off the screen voltage and let it
be higher by just running both screen an output to the same place, not the best)
 
I saw that, an I'm trying to figure why there is such a voltage drop to those outputs?
No resistance of that choke, an I want to make SURE the configuration he settled on.

If still using the 5Y3's? You can use 5V4's or even 5AR4's to get a few more volts then.
(personally I'd use the 5V4's in place of the 5Y3's anyway, that's what I do here)
 
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