1973 Magnavox. One dead channel.

[JunkForLess]

I am working on repairing a 1970s Magnavox stereo console. The Left channel works perfect, but the right channel is dead at the speaker. This happens in all modes (Tape, Phono, FM Stereo). Now, I do have audio from the right channel via the "Tape Record" output, so this confirms the problem is in the right channel AF and Output amp? I also tested the speaker itself. I attached a schematic of the Right channel: AF and Output.


In addition to a recap, I found that R101 and R107 were both open (see arrows). I replaced R101 and R107, but they both become very hot. MUCH MUCH hotter than the left channel. Also, transistors Q21 and Q22 became very warm so I removed power. Q21 and Q22 both test fine (even after the heat). The voltages at Q21 and Q22 were both pulled down low. So a ton of load is being put on the output transistors and R101, R107. The left channel also had low voltages, but no heat. I believe the problem in the right channel was pulling the whole power supply down, too.


I am just now learning about amps, so forgive me for using terms incorrectly. I believe R101 and R107 have to do with the push pull system of Q22 and Q21? I read somewhere that you can run into oscillation problems and thermal runaway when this section acts up? I can't find any other bad parts. Tested Q20, Q18 and Q29, all the resistors.. Really looking for some advice.

Thanks.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Don't worry about oscillation yet.

Oscilloscope would be helpful, or a battery operated handheld amplifier/speaker to trace.

Voltages at all the test points in range?

Is D5 working or is it open?

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Thanks for lending a hand.

D5 tests good.

I do own a scope.

[petehall347]

Re: 1973 Magnavox. One dead channel.

R112 ?

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Yes, and I did test them removed.

[lotas]

Re: 1973 Magnavox. One dead channel.

You have tested capacitors C79, C82 and ... others for capacitance and ESR, for 50 years they have already dried up.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Wait, was it heating up quiescent or with a signal connected to the input?
Open D5 was the only thing that I could think of that would cause a world of pain for the resistors and output transistors... other than that I'm at a loss, short (pun intended) of a short somewhere...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

I have the input set to AUX(tape) with nothing connected. The volume is all the way down. I have 8 ohm 100 watt dummy loads for each channel.

I replaced all the electrolytic caps in the AF and output, including C79 C82. I only have Panasonic FMs on hand. I didn't want to order anything until I had a full parts list.

It's a long shot, but I am going to replace the isolator under Q22.


Update in a few minutes.


Thanks.

[petehall347]

Re: 1973 Magnavox. One dead channel.

try without loads . and measure centre voltage .

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

New isolator didn't help, removing the speaker load didn't help.

[budwich]

Re: 1973 Magnavox. One dead channel.

quote: "Voltages at all the test points in range?"...
I don't think that you answered this question. Have you checked the voltages shown along the circuit at the various points.
another question: what are the q22/21 transistors? How are you testing them... with a checker or ddm?

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

quote: "Voltages at all the test points in range?"...
I don't think that you answered this question. Have you checked the voltages shown along the circuit at the various points.
another question: what are the q22/21 transistors? How are you testing them... with a checker or ddm?

Q21= 111N4B (replacement would be a NTE130)
Q22= 111P3B (replacement would be a NTE219)

I use the diode test on my Fluke 88V. I also double check everything using a Proster BM4070.


So, with R101 and R107 open (or removed), the readings in the right channel are not right, as you would expect. These are the reading I got when troubleshooting before removing/replacing any parts:

Q22:
C=48v
E=44v
B=44v

Q21:
C=0v
E=0v
B=44.3v


On the left channel(see new attachment),

Q16
C=48.2v
E=22.3v
B=22.9

Q17
C=0
E=22.3
B=22.1


Now, If I install new parts for R101 and R107,

Q22
C=30V (the input voltage from the PSU, should be 48)
E=4.4 (should be 22ish)
B=5V (should be 22ish)

Q21
C=0v
E=.39v
B=5v

Q16 (left channel)
C=30v
E=16v (the voltage starts at 12, then rises up to 15 in about 5 seconds)
B=16v (the voltage starts at 12, then rises up to 15 in about 5 seconds)


Q17 (left channel)
C=0
E=15 (the voltage starts at 12, then rises up to 15 in about 5 seconds)
B=15 (the voltage starts at 12, then rises up to 15 in about 5 seconds)


Just to help a bit, I attached the whole AF/Output for both channels.

[R_J]

Re: 1973 Magnavox. One dead channel.

Have you checked R99, R100 & R112? Also Q18, Q19 & Q20

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

I have tested Q18, Q19 & Q20

R100=122.8ohm
R99=128.1ohm
R112=22ohm

[R_J]

Re: 1973 Magnavox. One dead channel.

Q21= 111N4B (replacement would be a NTE130)
Q22= 111P3B (replacement would be a NTE219)

^Is this how you installed the outputs? ^

Q21 is a PNP transistor (nte219)
Q22 is a NPN transistor (nte130)
I suspect tor transistors are in backwards

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Let me take a closer look at this. I gave you the part numbers based off the parts list, not based off the unit itself.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Yes, this must be an error in the parts list.

As-installed:
Q22 and Q16 are the same parts
Q21 and Q17 are the same parts.


This would look like an error in the parts list, correct? I have noticed some other errors. For example, the parts list has 35v caps where 50v caps are installed from the factory.


The parts list has Q22 and Q16 as 111P3B

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Q21 must be PNP. If C is 0V, B is 5V and E is .31V this looks like saturation and is a problem. This does not make sense as the transistor should be cutoff, seems it either has an E-C short or you have an NPN transistor installed here.

But according to the data presented, this is impossible that it had the wrong parts installed at the factory...or there's some history not here? Where are the transistors installed? Chassis? Any way to have an erroneous identification of the two transistors?

Having 50V caps installed instead of 35V caps isn't of concern, it will work just fine. Swapping a PNP for NPN, that's a problem.

Yay Q16=Q22=NPN=111N3B=NTE130=our favorite 2N3055
Q17=Q21=PNP=111P3B=NTE219=MJ2955

[budwich]

Re: 1973 Magnavox. One dead channel.

Yes, this must be an error in the parts list.

As-installed:
Q22 and Q16 are the same parts
Q21 and Q17 are the same parts.


This would look like an error in the parts list, correct? I have noticed some other errors. For example, the parts list has 35v caps where 50v caps are installed from the factory.


The parts list has Q22 and Q16 as 111P3B

look and see what is installed in the working left channel to confirm the correct component... but you need to follow the schematic orientation as opposed to parts list.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

One thing to try, with that Q21/PNP removed, does Q22/NPN and resistors still get hot?