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Thread: 1964 Magnavox Micromatic ISC601

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    1964 Magnavox Micromatic ISC601

    Haven't located a schematic for this yet, does anyone have one??, unit works mechanically ,has a static sound in the background at idle and in use, reminds me of the static on old tv's when they went off the air, or no signal to that channel , talking back in the early 60's . It does have decent audio ,just the annoying background static
    I'm thinking caps, where do I start ?? Appears to have a main filter cap, are the caps at the speakers cross overs?? Here's some pics that might help ,don't mind the time traveling.
    100_2549.jpg100_2560.jpg100_2557.jpg

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    Not saying it's impossible, but I've never experienced static noise caused by caps. I would think semi-conductors the number one suspect, then maybe those RC packs, or the carbon comp. resistors. Do you have freeze spray?
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    Quote Originally Posted by g1 View Post
    Not saying it's impossible, but I've never experienced static noise caused by caps. I would think semi-conductors the number one suspect, then maybe those RC packs, or the carbon comp. resistors. Do you have freeze spray?
    Don't have any freeze spray , but I will have . I couldn't get the unit here today because of weather, I did get some more info, static is more prominent to the right side , and has a slight increase with volume from 1/4 volume on up . thanks for the reply reason I suspected a cap if I zoom in on 6332 next to the transformer, I think I'm seeing leakage on the banded end
    Last edited by shortcircuit; 04-18-2018 at 10:42 PM. Reason: +

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    Quote Originally Posted by g1 View Post
    Not saying it's impossible, but I've never experienced static noise caused by caps. I would think semi-conductors the number one suspect, then maybe those RC packs, or the carbon comp. resistors. Do you have freeze spray?
    can I use inverted can of air ??

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    I think that will work, yes.
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    Quote Originally Posted by g1 View Post
    I think that will work, yes.
    have isolated the problem to channel1 , had a slight increase in static with freeze on Q1, Q2 which would be good culprits, vfo2744b, efv2744c, going to replace with NTE102 . The resistors are almost spot on ,all the caps are shot, a question on the filter cap
    100_2569.jpg 1000vdc/20uf 500vdc/20uf ?? isn't 1000vdc a little extreme ?? or was this just the norm back then ?? can I use a 500vdc 32uf/32uf ??

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    I believe that cap labeling indicates 1000uf and 500uf both @ 20VDC. Sure is big for that voltage rating, but uf's were harder to come by in '64 I'd suggest measuring the voltage on the cap just to be sure a modern part rated below 50V would be suitable.
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    Quote Originally Posted by Chuck H View Post
    I believe that cap labeling indicates 1000uf and 500uf both @ 20VDC. Sure is big for that voltage rating, but uf's were harder to come by in '64 I'd suggest measuring the voltage on the cap just to be sure a modern part rated below 50V would be suitable.
    thanks,that's the way I read it also, I was wondering if it was marked backwards, they did have human error in the 60's didn't they??

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    It's marked correctly, Voltage under the VDC column (20 for both halves). It's rare to see caps over 100V unless in tube gear. Even in modern caps, I don't think you would find 20uF in 1000V, they would use two 40uF 500V in series if they needed 1000V rating.
    Even with higher supply rails from the modern line voltages, 35V caps will probably be enough there, but like Chuck said, check them anyway.
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    Quote Originally Posted by g1 View Post
    It's marked correctly, Voltage under the VDC column (20 for both halves). It's rare to see caps over 100V unless in tube gear. Even in modern caps, I don't think you would find 20uF in 1000V, they would use two 40uF 500V in series if they needed 1000V rating.
    Even with higher supply rails from the modern line voltages, 35V caps will probably be enough there, but like Chuck said, check them anyway.
    Thanks for the reply and your knowledge and advice , I knew I read it right ,I guess I just didn't want to believe it . I will check voltages ,will probably go with 50v caps , same $$ if not cheaper. I'll be back when the order shows.

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    Quote Originally Posted by g1 View Post
    It's marked correctly, Voltage under the VDC column (20 for both halves). It's rare to see caps over 100V unless in tube gear. Even in modern caps, I don't think you would find 20uF in 1000V, they would use two 40uF 500V in series if they needed 1000V rating.
    Even with higher supply rails from the modern line voltages, 35V caps will probably be enough there, but like Chuck said, check them anyway.
    What have I done wrong ?? I Couldn't get a replacement can so I took a 50v 1000uf and a 50v 500uf , here's a pic of the original and the way I have it . It works great, sounds great, but the caps get HOT. I know it's more than ugly,but I didn't want to do anything picky perfect until I knew it worked.
    100_2572-2-.jpg100_2591.jpg
    Last edited by shortcircuit; 05-04-2018 at 08:03 PM.

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    Well, sorry about the error but looking closer, the original can says "common pos can", something I've not seen before but means positive is grounded and supply will be a negative voltage. Checking voltage on original cap would have shown this.
    I don't know if those new caps would be damaged yet, you can try reversing them, see if there is bad hum or if they still get hot.
    They are cheap enough, probably best to just replace them. Positive end to ground for both.
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    Quote Originally Posted by g1 View Post
    Well, sorry about the error but looking closer, the original can says "common pos can", something I've not seen before but means positive is grounded and supply will be a negative voltage. Checking voltage on original cap would have shown this.
    I don't know if those new caps would be damaged yet, you can try reversing them, see if there is bad hum or if they still get hot.
    They are cheap enough, probably best to just replace them. Positive end to ground for both.
    Lesson learned and will replace both, a note to all getting advice from the forum , do as your ADVISED, post #7 #9 ( check voltages ) I just assumed 50v would do the job , 30 seconds and a meter would have avoided this error. I do want to thank g1 for pointing me in the right direction to begin with , Q1 was the fix to the initial static noise complaint. Thanks to all
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    Quote Originally Posted by g1 View Post
    ... looking closer, the original can says "common pos can", something I've not seen before but means positive is grounded and supply will be a negative voltage...
    Interesting, I don't recall ever seeing that in a multi section can cap either. Now that additional gut shots have been posted we can see another clue that the power supply produces a negative DC voltage. The clue is the orientation if those pretty gold Top Hat style rectifier diodes. Note that the power transformer winding is feeding the cathode end of the diodes. If a shotgun parts replacement approach had been attempted, which we frown upon in most cases, it would have been easy to assume that the rectifier diodes belonged the other way around. I can imagine the confusion that would have ensued. It's great that g1 figured out the situation in short order.

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    Couldn't see looking on my phone but 1964 probably has all PNP germanium outputs. It was difficult to make an NPN power transistor. So the power supply will be designed around that.
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    This was one of those where you really suffer for not having a schematic.
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