Author Topic: 1962 Magnavox 1ST620A resto or start of working anyway  (Read 359 times)

electra225

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #15 on: October 17, 2018, 04:55:42 PM »
The "original sound" is what, exactly?  The thing is busted or we would not be fixing it/them.  Allen Bradley resistors go along with leaky bumblebees and silver getters.  And cryogenically pure cables.  Silliness.

My problem with carbon comp resistors is that they drift.  That is why we are replacing them.  Why replace them with something else that will drift, when something better is available. 

I have been wrong before.   :-[
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

danrclem

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #16 on: October 19, 2018, 12:31:10 PM »
When I was referring to the original sound it is to me the sound that the engineers intended the amp to have. 

I doubt if I could tell the difference between carbon comp, carbon film and metal film if I heard them back to back to back in the same unit.  I've also read that the the carbon comp resistors have a lot more noise.  I'm not passing this along as fact but just that it's out there.  I've learned a lot by using the internet but there is so much conflicting information that sometimes it's hard to decide what's right and what's wrong. 

Some people pay a premium for NOS PIO caps and swear they sound a lot better but I haven't ever tried any and probably won't due to the price.  Some people won't buy carbon film resistors and some people won't buy metal film resistors.  When I buy caps I try to buy the ones with a higher heat rating and longer service life if they fit my application and they're not overpriced.   

I'm ordering metal film resistors for my Sansui and used metal film in my last Maggie overhaul even though I have a substantial stock of NOS CC resistors that are more than likely military grade and possibly Allen Bradley.  If I knew for sure that they were Allen Bradley I'd probably list them on Ebay.  I have two Toshiba (original or original type) 7591 output tubes and the other two are GE in my Sansui.  I hope they sound the same and they're all good because I'd hate to have to buy some more 'cause they ain't cheap.

I'm not always right and won't ever pretend to be.  I just want to keep learning.


TC Chris

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #17 on: October 19, 2018, 04:53:01 PM »
I've also got a Sansui 1000A and I replaced all the oil caps with film.  They were noted for failures, and on mine, a failed coupling cap had led to a burned-up output tube.  The former owners, the ones who set it at curbside for me to find, had replaced all the tubes, so the tube that burned up was brand new (7591; a tragedy).  I figured that it must have stopped working so they just replaced all the tubes and it promptly failed again.  It's a valuable lesson for all of us.  REplace those caps  before they take out something expensive....

Chris Campbell

danrclem

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #18 on: October 19, 2018, 08:30:00 PM »
A 1000A curb find.  All I can say is wow but I have to admit that I don't have very much in mine either.   :)

What do you think of the 1000A sound?  I didn't even try to fire mine up because I could see that one of the voltage doubler can caps had been leaking.  I'm replacing 33 caps total but that includes the can caps also.  Mine is an early model and has the one pot bias system but I'm ordering parts to convert it to a four pot bias system. 

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #19 on: October 20, 2018, 09:59:23 AM »
The "original sound" is what, exactly?  The thing is busted or we would not be fixing it/them.  Allen Bradley resistors go along with leaky bumblebees and silver getters.  And cryogenically pure cables.  Silliness.

My problem with carbon comp resistors is that they drift.  That is why we are replacing them.  Why replace them with something else that will drift, when something better is available. 

I have been wrong before.   :-[

Ahh, I finally realized that many of the originally used resistors were 20%.  I was making the wrong connection that the carbon based were standard at 5% probably because that is what I saw advertised on just radios.  After looking closely and making a sheet to identify the resistors on my tuner section (because I do not have a SAMS for the tuner) and then re-re-re-reading the resistor guides, I realized that only if a resistor has that fourth or separate band does that indicate a lower % tolerance.  Now understanding that, I note that most of the resistors on the frame don't have a % line and so I now think I understand that those would have been 20%.  I think I now follow your intended meaning a bit more clearly.
I agree, no reason to replace old 20% resistors with new 20% resistors.   At the very least nothing less than 5% and why not just use 1% since they are cheaper and tighter, ie no to little drift.

I also noticed for reference that on the cans as part of the spec description, there is a temp rating specified and on mine that is 85C.  So, at the very least should replace with 85C, but I am also of the thought that 105C would be better and or offer more longevity at least heat breakdown life wise than 85C not that it really matters.  I guess it is like specifying an F or B heat rise on a generator with H class insulation in a sense.  You have a lot more built in margin.
That said, I will be re-purchasing caps at 105C since I had ended up with 85C and one 65C caps originally.  Pricey lesson to re-learn for me but I guess better now than later.  I'll get my new order in here and then go from there.  As always, thanks for all the input!
Dennis H
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my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #20 on: October 20, 2018, 10:14:28 AM »
That and even the resistors with a fourth band , the band is silver so only 10%.  So the original ones are either 10% or 20% and nothing tighter than that if I am now understanding things correctly.
Dennis H
"Don't mind me, I'm just here to break stuff"

19and41

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #21 on: October 20, 2018, 10:50:43 AM »
One can get assortment kits of metal film resistors to put together replacement bins on Ebay for a reasonable price.  I keep mine in Harbor freight compartmented plastic boxes with the values on the back of each compartment in sharpie.
"Any sufficiently advanced technology is indistinguishable from magic."
ó Arthur C. Clarke

TC Chris

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #22 on: October 20, 2018, 12:07:25 PM »
A 1000A curb find.  All I can say is wow but I have to admit that I don't have very much in mine either.   :)

What do you think of the 1000A sound?

My 1000A will likely be a winter project.  The FM stereo switching wasn't working after recapping so I put it away.  Didn't do much listening, and then on bench speakers.  Our dark and indoor season is upon us now. 

Chris Campbell

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #23 on: October 28, 2018, 04:13:13 PM »
I got this multiplexer with an ST659 tuner off ebay for a $15 bid. Iím assuming it should be fine with my ST620 and 8802 amp. This mpx is the 2 tube version. I guess now I can put an order in for caps and stuff.  Th last pic is both tuner amps for comparison . I alwaya am a sucker for parts.
Dennis H
"Don't mind me, I'm just here to break stuff"

electra225

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #24 on: October 28, 2018, 06:57:22 PM »
That should work just fine.  Check the Molex connector to be sure they are the same.  The schematic for your MPX is in the Downloads section at the top of this page.  Print that out and go through the Multiplex adapter.  It has a 30uf electrolytic and some coupling caps that should be changed.  Then make sure the cables are good, and adjust the input from the MPX to tuner to be sure it is a tad higher than when on regular FM.  I have never encountered a bad tube in an MPX, but it is always good to run them on a tube checker, or substitute them temporarily, to be sure.  Good luck.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.