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 Post subject: Re: Mica Caps For Hammarlund Source
PostPosted: May Fri 19, 2017 4:42 pm 
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Joined: Jun Wed 13, 2007 4:53 pm
Posts: 207
Don't forget the micamold caps in the tuning unit and a couple of the IF transformers. The IF transformers are relatively easy to take apart to get at the micamolds. The ones in the BC-779 tuning unit require removal of the tuning unit and a side panel to get at the caps. Fortunately, the caps in my SP-400 were relocated so that they were accessible by removing the covers on the tuning unit, so much less disassembly required.

Ed W2EMN


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 Post subject: Re: Mica Caps For Hammarlund Source
PostPosted: May Fri 19, 2017 9:31 pm 
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Joined: Jan Sun 13, 2013 10:38 pm
Posts: 119
Location: Cape Cod, Ma
Ed,

For the tuning unit, are you saying that the micamolds are the AVC caps ?

Marc


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 Post subject: Re: Mica Caps For Hammarlund Source
PostPosted: May Tue 23, 2017 12:43 am 
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Joined: Jun Wed 13, 2007 4:53 pm
Posts: 207
both the AVC bypass and the B+ bypass Micamold caps are in the tuning unit. As I recall, there are 5 in the tuning unit. I learned to solder in tight places replacing bad micamold caps in my BC-779 in the early '60s- at that time I went in through the bottom. Now I'm a believer in pulling the tuning unit and replacing them through the side.

Ed


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 Post subject: Re: Mica Caps For Hammarlund Source
PostPosted: May Tue 23, 2017 12:50 am 
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Joined: Jan Sun 13, 2013 10:38 pm
Posts: 119
Location: Cape Cod, Ma
Ed,

For the Super Pro 400X, the bypass caps are external to the tuning unit. They look like they are mica as they are small and not black.

Marc


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 Post subject: Re: Mica Caps For Hammarlund Source
PostPosted: Jul Wed 05, 2017 6:20 pm 
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Joined: Feb Thu 08, 2007 12:36 am
Posts: 2096
Location: Hillsboro, Oregon
For what it's worth.

I am repairing a National HRO-60 for a friend. He or some predecessor had already replaced all paper caps, some mica's, and most resistors.

I saw some mica caps still in-circuit, and I decided to test the two large-value (0.01uF) parts, because I've seen trouble before in this size. One couples oscillator cathode to mixer grid, the other bypasses AVC inside the antenna section of the tuning cap. (Tough to get to.) They are on-value and quiet, but also leaky, and I replaced them. For coupling, I used an axial-lead polyester, and for AVC I used a short-leaded ceramic disc after choosing one with relatively low DF.

Rsingl says to replace mica only with mica. For these particular applications, do you agree? Why? (If I have to remove the chassis again, I'll need some convincing :))

Dave


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