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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Feb Sat 15, 2020 2:20 am 
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Location: Wheeling, WV
Well I did a recap and everything seems to be okay! The volume seems to be low, but the speaker is rattling around in the set I will assume it’s just the small size.


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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Feb Sat 15, 2020 3:24 am 
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Location: Ann Arbor, MI
What did you do about that dented cap?

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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Feb Sat 15, 2020 4:00 am 
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Location: Wheeling, WV
I ended up using the dented cap. But I keep seeing a little bit of fuzz on the screen sometimes. Especially when it’s a full grey imagine. You could imagine I didn’t grow up using these sets so I have no idea if that’s normal or if someone of the vhf signal is escaping. I tried to patch that broken wire as best as I could. I think I got it hooked up correctly again :oops:


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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Feb Sat 15, 2020 4:16 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
You are getting to the point now where a known good test signal (aka test pattern) could indicate whether things are as good as they should be.

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It's not the Destination, It's the Journey.


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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Mar Wed 25, 2020 4:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Phoenix, Arizona
As I am sitting around the house bored and low on projects I can't resist yapping here.

We used to plug these old sets in and fix what was wrong and go on with life however now we are re-fixing sets we acquired and fixed years ago.

I can't say in the past that I started with a total re-cap but as I am re-restoring stuff I had not paid attention to.

The paper capacitors fail over time even if the set is off as the paper has acid content and decomposes like old newspaper. You may as well replace them all while you are at it. I use a Tektronix scope made in the late 1960's and it has all original parts and works just fine.

Electrolytic capacitors dry out and loose their forming. I have some sets with original can electrolytic s and they work fine. I bought a big box of new old stock electrolytic s from a going out of business and every cap is essentially open.

There is a company making new Can Electrolytic s and there are combinations that will work for old TV's. I hate soldering in a bunch of tubular capacitors to replace a multi section can, and re-stuffing them is not always possible or productive.

I have had high value resistors be way off value in high voltage circuits.

Tubes on the other hand are usually just fine.

Jim

I always test them by applying voltage slowly and see if they will reform and not short out.


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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Mar Wed 25, 2020 7:42 pm 
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Joined: Jan Wed 17, 2018 8:59 pm
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Location: Wheeling, WV
No worries this set has been completely recapped. It's basically the only repair job I can do fluently now. Still learning the importance diagnosis..... and getting diagnostics tools. Thanks for the help I couldn't have started without the help from here!


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 Post subject: Re: Admiral portable UP9421 help!
PostPosted: Mar Thu 26, 2020 7:18 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 711
Location: Belrose, NSW, Australia
Yep, that looks from the pictures like it's as good as it's going to get. Well done!

The "grey fuzz" might mean the AGC delay to the tuner is not working. I haven't looked at your circuit but there is normally a high value resistor (10 megohms?) from B+ to the tuner AGC line to achieve this, it's VERY common for this part to go open circuit and give you a picture with more snow in it than it should have. Try shorting the tuner AGC line to ground temporarily and see if that improves things. If not, the RF amplifier tube in the tuner might be bad.

AGC delay is a concept that's intended to keep the signal applied to the mixer as high as possible to reduce noise (snow). The idea is that, as the signal level increases, AGC is applied to the IF stages first to reduce the gain and only to the RF amplifier when the signal gets much stronger.

These sets have AC coupled video and so the screen will go grey, instead of black, on dark scenes and the blacks will go blacker than black, losing detail, on bright scenes. This is normal behaviour for this TV and many others of similar vintage. (AC coupled video is one of my pet peeves!). It is possible (but not always easy) to modify the TV so that it retains the correct black level, as was done in the earliest and many later TVs. It does make them more pleasant to watch in my opinion.

This always reminds me of one of my first colour TV customers (PAL, 1974). He was initially very disappointed when watching the cricket that the grass looked light brown instead of the green he was expecting to see. He went to the cricket ground (he lived nearby) the next day and, as he put it to me, he was most surprised to see the grass was the same colour as he saw on his new TV!

Enjoy!!

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