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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 10:59 am 
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Audioman wrote:
Hi all. Changing to a full wave bridge has two advantages. First the hum level will be easier to filter given the same size capacitors and secondly the power transformer will run cooler.
Bob

Why would the transformer run cooler? The peak current is higher, but the average is the same.

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 7:38 pm 
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Because half wave rectifiers dc component tend to saturate the core. A bridge does not put dc through the transformer.


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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 7:58 pm 
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Audioman wrote:
Because half wave rectifiers dc component tend to saturate the core. A bridge does not put dc through the transformer.

I'm not sure I follow that. But maybe you meant more than I understood.

Here's more detail and theory:
https://electronics.stackexchange.com/q ... e-rectifie

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Last edited by Pbpix on Mar Sat 31, 2018 8:07 pm, edited 1 time in total.

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 8:02 pm 
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Thanks to the advice, I now have a vintage looking top chassis. I should have thought of that. Vintage is my middle name.
It did not go with out some work though. Thanks for the tip....
Attachment:
20180331_114136.jpg
20180331_114136.jpg [ 127.55 KiB | Viewed 1162 times ]

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 8:13 pm 
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Why are there two old caps sideways on top? Is the paper covered one re-stuffed?

Is that rusted tubular thing on the left a tube shield?

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 8:29 pm 
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Pbpix wrote:
Why are there two old caps sideways on top? Is the paper covered one re-stuffed?

Is that rusted tubular thing on the left a tube shield?


Yes, its a tube shield. I will try to clean it up. All FM4's I have seen have the same rust.

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 8:41 pm 
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john8750 wrote:
Pbpix wrote:
Why are there two old caps sideways on top? Is the paper covered one re-stuffed?

Is that rusted tubular thing on the left a tube shield?


Yes, its a tube shield. I will try to clean it up. All FM4's I have seen have the same rust.

I guess because of horizontal mounting. Most vertical mounted tubes allow excess heat to escape through the top whereas this is cooking that metal and when it cools it condenses moisture from the air causing the rust?

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 8:50 pm 
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Pbpix wrote:
john8750 wrote:
Pbpix wrote:
Why are there two old caps sideways on top? Is the paper covered one re-stuffed?

Is that rusted tubular thing on the left a tube shield?


Yes, its a tube shield. I will try to clean it up. All FM4's I have seen have the same rust.

I guess because of horizontal mounting. Most vertical mounted tubes allow excess heat to escape through the top whereas this is cooking that metal and when it cools it condenses moisture from the air causing the rust?



Very logical Peter. I was thinking it was the type of metal they used. I like your theory better.

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 9:16 pm 
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john8750 wrote:
...I was thinking it was the type of metal they used. ...

Could be a combination.

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 Post subject: Re: Heathkit FM4
PostPosted: Mar Sat 31, 2018 11:14 pm 
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Sol wrote:
john8750 wrote:
...I was thinking it was the type of metal they used. ...

Could be a combination.



Thanks. I like your theory very much. Probably the best yet.

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 Post subject: Re: Heathkit FM4
PostPosted: Apr Sun 01, 2018 11:10 pm 
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I am waiting on the new dial cord now. I need to disassemble the front, because there are some loose nuts. I cant reach them. One is for the shaft that holds the tuning cap shaft. It could be interfering with the dial cord operation. And then the whole tuning mechanism needs to be aligned. The dial light is shot. I wont replace it because the unit will not be in an enclosure, and I can see it clearly where it will sit.

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 Post subject: Re: Heathkit FM4
PostPosted: Apr Wed 04, 2018 11:34 pm 
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john8750 wrote:
Pbpix wrote:
... Is that rusted tubular thing on the left a tube shield?
Yes, its a tube shield. I will try to clean it up. All FM4's I have seen have the same rust.
On my two FM4s (as acquired), one tube shield was quite clean, but the other was so rusty it was hard to get it off. Tube shield was mostly OK. Scrubbed up to bare metal it went back on easily and is still clean a few years later!
Cheers,
Roger

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 Post subject: Re: Heathkit FM4
PostPosted: Apr Wed 04, 2018 11:53 pm 
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Pbpix wrote:
Why are there two old caps sideways on top? Is the paper covered one re-stuffed?

Those are the original parts, for show off. They are not connected. The new parts are connected under the chassis.
Is that rusted tubular thing on the left a tube shield?

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 Post subject: Re: Heathkit FM4
PostPosted: Apr Wed 04, 2018 11:58 pm 
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engineer wrote:
john8750 wrote:
Pbpix wrote:
... Is that rusted tubular thing on the left a tube shield?
Yes, its a tube shield. I will try to clean it up. All FM4's I have seen have the same rust.
On my two FM4s (as acquired), one tube shield was quite clean, but the other was so rusty it was hard to get it off. Tube shield was mostly OK. Scrubbed up to bare metal it went back on easily and is still clean a few years later!
Cheers,
Roger



My tube shield don't want to come off either. I thought maybe it was a press fit.

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 Post subject: Re: Heathkit FM4
PostPosted: Jul Wed 04, 2018 7:12 pm 
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I will be back to this project later. Right now have another amp project, Williamson, to work on. Check it out.
Flip has it all under control.
viewtopic.php?f=12&t=338702

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