Author Topic: 1962 Magnavox 1ST620A resto or start of working anyway  (Read 358 times)

my junk is stuff

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1962 Magnavox 1ST620A resto or start of working anyway
« on: October 16, 2018, 09:01:04 AM »
I got this back in June but am just now looking at it again. Itís got the 8802 amp which I was able to download the schematic for from this forum! I havenít been so lucky on the tuner preamp though. Its a 59 38 00. Iíll post some pictures. Even so I should be able to locat and ID the caps to replace . What else besides the caps to start with?
Dennis H
"Don't mind me, I'm just here to break stuff"

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #1 on: October 16, 2018, 09:03:17 AM »
So far I have purchased the power amp caps. Have not checked any tubes. Did not power up. She said it hummed... so caps first. Should I add a fuse ?
Dennis H
"Don't mind me, I'm just here to break stuff"

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #2 on: October 16, 2018, 09:06:54 AM »
Here are a couple of shots from when I brought it home
Dennis H
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electra225

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #3 on: October 16, 2018, 09:33:29 AM »
You should replace coupling caps, electrolytic caps.  You should check every resistor and replace any that are more than 10% off value.  You should replace the cathode bypass capacitor with a slightly larger capacity, not more than 50uf @ 50 volts.  You should also change the cathode bias resistor.  Those are known to be burned internally and can cause a hum that can be hard to find.  This condition puts the audio output transformer at risk.  There is a double electrolytic cap in the cathode circuit in the tuner that is often overlooked.  It needs to be replaced.  I prefer to re-stuff the e-cap cans on Magnavox amplifiers, but you can use terminal strips if you are careful to isolate it from the hum control.  The hum control works in the bias circuit of the output tube cathodes, so the bias voltage needs to be checked after you have replaced components.  The 220K ohm grid resistors in Magnavox amps are frequently found drifted high.  I just replace them, many times without bothering to check them.  If you install a fuse in the amp, mount it close to the transformer, away from any audio wires.  Don't forget about the crossover caps for the little speakers.  I typically find they are not good anymore.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #4 on: October 16, 2018, 11:42:46 AM »
Ok, sounds good.  Yes I did get the crossover caps for the speakers, looks like they are listed as C2 and C3, so good there.  The one fixed cap I didn't locate yet was C4 shown as a 100 N150 Rating, crossing to a  Sprague 10TCP-T10.  I didn't look to hard, I think the wife interrupted me so I'll get back and take a look.  Cathode bypass cap and bias resister are in the tuner section which I don't have a diagram for yet.  I'll take a closer look at that and report back.  I think I can maybe put 2 of the four electrolytic caps back in the can and then just mount the others underneath. I've done this once before, but it's been a few years and I only replaced the caps that time.  This time I'd like to be more thorough.  Thanks for the quick reply!
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 #5 on: October 16, 2018, 02:42:31 PM »
The cathode bias resistor and the bypass cap are in the amplifier.  There is a dual bypass cap in the tuner.  The bias resistor should be somewhere around 100 ohms at 5 watts and the bypass cap should be like a 25uf at 50 volts originally.  I like to increase the value of that cap in the effort to enhance bass.  Be careful with the values of the crossover caps, lest you get weak treble and "muddy" sounding output.  You need those caps correctly valued to make the treble response bright enough.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #6 on: October 16, 2018, 03:15:22 PM »
Ok, I think I found them in the diagram, looks like the CAP is  item C1-D 30mfd @ 25V and the resister is 180 Ohms at 5W.  I did already get 30mfd 150V to replace the cap, but if it's a good idea to up the mfd, I would do that with no issue.  How much should I bump the cap up?

The cross overs were 16 mfd @ 25VNP which I was able to get but I got them at a higher voltage assuming that would be ok.  That said, I am not sure the distinction of the "VNP" versus just 25 volts. 
Dennis H
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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #7 on: October 16, 2018, 04:23:36 PM »
Here are the caps I have so far. Actually now thinking I should have gotten 105C do you think this will be an issue? Itís not clear if a specific temp rating is required.
Also, are the Orange drops correct to replace these paper tube caps? Both are .047 but my Orange drops are 600v as you can see
Dennis H
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danrclem

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #8 on: October 16, 2018, 07:54:33 PM »
Those 65 C caps should be ok.  That's 149 F.  I like to get at least 85 C caps but that's just me.  There should be plenty of ventilation and they're not sitting right next to any tubes.

The 25VNP caps are a non polar or bi polar electrolytic. 

electra225

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #9 on: October 16, 2018, 11:59:22 PM »
There is nothing to be gained by using a 150 volt cap for a bypass cap.  I would use something like 50uf @ 50 volts for bypass, that would be plenty.  Going higher negates any benefit.  I don't like orange drops for Magnavox amps, but that is just me.  I like to use orange drops in TV chassis, in higher voltages.  My personal preference is the regular caps that Sal sells.  Cheap and effective.  Using orange drops is spending money unnecessarily, but perhaps you are not as cheap as I am.  Orange drops won't hurt anything, if that is your question.  Replace the bias resistor with one as close to exact value as you can get.  I try to get higher temp caps if they are not a lot more money.  That thing will get warm enough to bake bread in the changer compartment in any event.  You can fry eggs on the power transformer after it runs all day.  That is the nature of the beast.  Better cabinet ventilation helps.  The transformers are rated 105-125 volts AC, so modern line voltage is not normally a problem for them.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #10 on: October 17, 2018, 08:03:04 AM »
Thanks guys,  I'm putting together an order for some resistors and caps.  I was thinking of getting the carbon composition type since they seem to be what was originally used.  I wish I had done a little more homework be fore I ordered the stuff I currently have.  I did spend a bit more than I should have and now will be not using some of it.  Learning experience I guess.  Not the end of the world.  I do also feel more comfortable with the 105C caps.  I unknowingly (then anyway) replaced a 105C cap with an 85C cap on a tankless hot water heater years ago.  It failed in short order and so since then, I have tried to pay more attention to such details.  It's funny, being an ME in real life, mechanical systems just flow to me like candy.  Electrical stuff, I have to think about a bit.  And well, controls logic, I just down right hate!.  I can do it, just don't like it.  I have to pay more attention and focus which is something I have trouble with outside my wheelhouse.  That's enough TMI BS though, I'll see what I can come up with here.  I am assuming the 105C caps will be dimensionally larger than the 65C and 85C caps I currently have for the same uf, is that correct?

PS, side note, I did realize that I do have an EV26 cartridge on my tone arm.  I'm not sure if that came originally, but it is there now and looks to be in good condition.  I have a feeling this is good at some level after seeing the EV26 being suggested as a very good or maybe the best cartridge to use on these old Magnavox's...  I guess I am just glad I don't have to also buy one since they have become unobtanium like my old 911 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 #11 on: October 17, 2018, 09:27:38 AM »
I like using 1% film resistors.  5% is not bad, either.  Why use something that can be 20% off value?  If they wanted something off value used in the circuit, why would they give it an exact value? 

You can't beat an EV 26 when it comes to a ceramic cartridge and Magnavox.  That setup was King Of The Hill in its day.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #12 on: October 17, 2018, 10:09:20 AM »
So the Carbon compound ones say 5%, which ones are 20%?  I see the film ones as well,  nice that they are cheaper.  Again, my only reason for thinking of using the carbon compound ones was to keep the "look".  Granted, they are underneath and it really won't matter....  Good points you make though! :)
Dennis H
"Don't mind me, I'm just here to break stuff"

danrclem

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #13 on: October 17, 2018, 11:03:05 AM »
Some people say that carbon comp resistors should be used to maintain the original sound.  I have no idea if that's true or not but I usually order film caps too.  Some of the Allen Bradley NOS carbon comp resistors sell for good money though.

I'm working on a Sansui 1000A receiver.  One of the can caps is going to be hard to fit four caps in it so I thought about using a prebuilt can from Tubes And More but when I saw that it had a rating of only 55 C I decided against it even if I have to place one caps on the outside.  The 1000A is known for running extremely hot but many others have said they have used the can rated for 55 C with only 1000hrs of service life and had good luck with it.

my junk is stuff

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Re: 1962 Magnavox 1ST620A resto or start of working anyway
« Reply #14 on: October 17, 2018, 11:25:10 AM »
I've seen some clean under mountings using plexiglass or fibre board with brackets riveted to the base from the underside.  They can have the caps sandwiched between the panels and thus supported and insulated from everything else.  On my 8802, I think there would be room to do that so it is an option.

Funny you mention the idea of sound related to the use of the carbon comp resistors versus the newer film caps.  That would be a real science project!  Interesting though.

Like everything, it starts simple and elevates from there.  The more you consider, the more you must take into account and then make decisions based on that new knowledge or understanding.

I also remembered that my Simpson meter is OOC due to being a dummy and setting something on the glass face which broke the glass and tweaked the needle.  I just need to bear down and get some parts for it and get it back working, I love that meter!
Dennis H
"Don't mind me, I'm just here to break stuff"