Author Topic: Magnavox console / amp repair  (Read 5131 times)

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #30 on: August 02, 2016, 07:17:42 PM »
Well the parts I ordered are starting to show up and I am looking at how to do it. I have a pretty good understanding based on the pictures you posted about how to replace that can capacitor with the exception of one really stupid question. On the side of the can it has the list of capacitors along with symbols that match up to them, then on the bottom each of the four pins coming out has a symbol next to it that matches to the ones on the side. I figure it's safe to assume that the symbols match up which capacitor goes to which pin, but capacitors obviously have two wires going to them, a positive and a negative. I assume that the positive lead of the capacitors go to the pin with the correct symbol and then the negative lead gets connected to the chassis of the amp, but I want to verify this before I do anything wrong.

electra225

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Re: Magnavox console / amp repair
« Reply #31 on: August 02, 2016, 08:15:50 PM »
On the original can cap, the can is negative.  You have the rest correct as far as where the positives go.  You can put a terminal to chassis for the negatives, or just use the mounting tabs for the can for the negative connection.  I hope this helps more than confuses......
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Motorola Minion

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Re: Magnavox console / amp repair
« Reply #32 on: August 03, 2016, 11:41:11 AM »
Tube amplifiers and tuners always have positive voltages so the can capacitor terminals follow as stated in the previous post.

Please note that most of the Magnavox and Zenith Early Solid State consoles have a negative power supply and the can is often the "common POSITIVE" for terminals of all its individual capacitors. A 1968 Zenith Im working on has its common positive as the "_" terminal, not the can, triangle, square or half-circle terminals. ???
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Dave

voxACthirtee

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Re: Magnavox console / amp repair
« Reply #33 on: August 03, 2016, 12:51:14 PM »
Tube amplifiers and tuners always have positive voltages so the can capacitor terminals follow as stated in the previous post.

It might be nit picking, but its probably good to note to someone learning....PtrkLnk's
amp is all positive, but ALL tube amps are not all Positive, or ONLY all positive.
There are a lot of tube amps that will have a negative bias supply
if the power tubes are fixed bias (NON cathode bias)

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #34 on: August 03, 2016, 10:42:28 PM »
Just to verify that I am understanding this correctly, the positive lead of each cap gets soldered to the lead with the matching symbol then all of the negative leads for the capacitors get soldered to either the metal can or the amp chassis?

Consoleman

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Re: Magnavox console / amp repair
« Reply #35 on: August 04, 2016, 06:21:57 AM »
Yes.
Mark

Motorola Minion

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Re: Magnavox console / amp repair
« Reply #36 on: August 04, 2016, 10:39:51 AM »
Good luck to you. These amps are pure pleasure to restore with no odd features 8)
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Dave

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #37 on: August 06, 2016, 01:46:50 PM »
Well I am working on getting those caps replaced and I have another question, specifically about the polarity of the coupling capacitors. The original coupling capacitors have a black stripe on one side, which I presume means that side is the negative and the other side is positive. Is this correct.?

Also I am looking at the new coupling capacitors that I got from Sal and they don't seem to have any markings for the polarity on them.  They have the values printed on the side but there is nothing else on them and both leads are the same length.

electra225

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Re: Magnavox console / amp repair
« Reply #38 on: August 06, 2016, 04:32:29 PM »
Polarity on coupling caps is not important.  The black stripe on the original caps is indicating negative.  Sal's caps are not marked with polarity due to the fact that polarity is not important.  Polarity on electrolytic caps is important due to the fact that they are in the power supply.  Those MUST be connected correctly.  Confusing, huh?   ???

It might help to remember that a capacitor will only allow AC (alternating current) to pass.  Filter caps pass the remaining AC from the rectifier (ripple) to ground to eliminate AC hum in the power supply.  The "signal" or what you want to hear from a record or radio, is also AC.  Coupling caps are used to allow the AC signal to pass, and to block high DC potential from the signal.  They "couple" or allow the AC signal to pass from one stage to the next.  When a capacitor becomes "leaky" (why we replace them) they allow the high DC potential from the plate to be introduced to the grid of the following stage.  If the grid is driven sufficiently positive, it can cause distortion or even make the stage totally inoperative, given the right circumstances.  Damage to certain other components are a possibility as well.  Hope this helps.
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PtrkLnk

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Re: Magnavox console / amp repair
« Reply #39 on: August 06, 2016, 04:49:41 PM »
Well things are getting interesting now. I got it all put together and put it back in and it started making that same sound and the same tube refused to light up along with the one next to it of the same type. A bit of smoke came out from under it as well. I took the amp back out and looked and I could see some black burn marks on the edge of the pot marked as R2 hum on the schematic. I took it off and took it apart and found that part of it had been blown apart by having clearly way to much electricity going through it. I looked back at the original pictures I took of the amp before I took it apart and found that it had those burn marks although less severe around the edge before so I suspect that is part of what caused the original issue. I figured it was just a bad pot that was internally shorted or something and I actually had a spare pot of the same value and rating so I put that in there and tried it again. The new one also exploded and let smoke out of it and taking the new one apart showed that the same part of the ring on the inside had been blown apart. Any idea what would be causing this issue?

electra225

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Re: Magnavox console / amp repair
« Reply #40 on: August 06, 2016, 06:58:55 PM »
Did you replace the cathode by-pass cap?  Is it correctly wired?  Is it of equal or greater capacity and of equal or greater voltage rating as the original?  Did you check the cathode by-pass resistor, R-12?  I have actually come across three cathode by-pass resistors that were burned in two.  I'd guess you have too much current in the cathode circuit someplace.  Did you replace all four 6BQ5 output tubes?  Do not test.  Replace.  There could be a short in one of these tubes that caused the problem in the first place and continue to cause trouble after you have replaced caps.  Did you check and replace all the 220K grid resistors?  If these have drifted high, they will cause problems.

Smoke is NEVER good.  You should not power this thing up again until you figure out what ails it.

You might also consider running resistance tests per the values given in the schematic.  Such tests are designed to help narrow down instances of over-current in a circuit.  These amplifiers are not hard to service.  You merely have the original problem still existing.  You need to look at any component in the amplifier, particularly in the cathode circuit, that you have not replaced.  And make sure what you have replaced is correctly installed.  My belief is that you are probably good there.  The problem has not changed.  The hum balance pot is 750 ohms.  It is likely the original is still good.  I have never seen one go bad.
If it ain't broke, call me.  I can break it....

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #41 on: August 06, 2016, 07:06:51 PM »
I replaced the cathode bypass cap with one higher then the original rated value so that should be fine. I suspect this was the original cause of the failure in the first place so I don't think the repairs I made caused it, although they certainly didn't do anything to fix it. I found a matched pair of 6BQ5 tubes and replaced the one that was not lighting up along with its partner, although they still didn't light up when I tested it. I didn't test the cathode bypass resistor although they were not visually damaged. I will test it tomorrow. Any other ideas on why that circuit would have too much current flowing through it?

electra225

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Re: Magnavox console / amp repair
« Reply #42 on: August 06, 2016, 07:22:32 PM »
Do not rely upon your not being able to see a particular tube light.  Measure the voltages on the various tube elements and be sure.  Although I do not recommend using used output tubes in Magnavox amplifiers, it appears you are having the same issues using "different" than original tubes.  The cathode by-pass cap was not the cause of the original problem.  You changed that cap and it should have fixed it if it was the problem. 

You can't always tell if the cathode by-pass resistor is bad by merely looking at it.  Two of the ones I mentioned burned internally, one was burned completely in two.  Take the resistor out of the chassis and wiggle on the wires going into it.  Then measure the resistance with it out of the chassis.  It should be 100 ohms, pretty much on the dime.  Its value is fairly critical for proper bias.  Confirm that you have the correct voltages on the screen and plates of the 6BQ5's. 
If it ain't broke, call me.  I can break it....

electra225

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Re: Magnavox console / amp repair
« Reply #43 on: August 06, 2016, 09:38:53 PM »
I found some pictures I took of my Imperial amp chassis.  This is the first time I ran across a bad cathode by-pass resistor.  It looks okay in the first picture, if you discount all the black marks under and around it.  ( In the lower LH corner of the chassis). Subsequent pictures show how deteriorated it actually was.  Now I replace these resistors as a matter of course.  A shorted 6V6 did the damage.  This is why I typically replace all the output tubes with new, known good tubes when I see this type of damage.

If it ain't broke, call me.  I can break it....

PtrkLnk

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Re: Magnavox console / amp repair
« Reply #44 on: August 06, 2016, 09:54:15 PM »
Well I don't have the amp with me at the moment but I looked back through my pictures of it on my phone and I noticed there is a black mark on the amp chassis under the bypass resistor along with some white stuff around the black mark. I will take a look at that tomorrow and see what it measures at.