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 Post subject: Mopar 813 question about buffer capacitor
PostPosted: Jul Wed 22, 2009 2:21 am 
Silent Key

Joined: Nov Fri 28, 2008 4:45 pm
Posts: 2330
Location: Near Fargo North Dakota USA
The schematic clearly shows a .0047 capacitor between the output side of the transformer, between pins 1 and 6 of the 6X4 rectifier. The radio plainly does not have it. Nor does it look like someone has removed it. My question is: should I put one in across the 2 pins 1 and 6?


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 Post subject:
PostPosted: Jul Wed 22, 2009 2:43 am 
Member

Joined: Sep Thu 21, 2006 1:14 am
Posts: 134
Location: Youngstown, Ohio
Hello, deltysdal. Yes, this would be called the buffer cap., and is essential to keep from having voltage spikes which can ruin the rectifier tube, and cause arcing of the vibrator points which will shortly destroy the vibrator. Regards, Fred


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 Post subject: Thanks.
PostPosted: Jul Wed 22, 2009 3:07 am 
Silent Key

Joined: Nov Fri 28, 2008 4:45 pm
Posts: 2330
Location: Near Fargo North Dakota USA
I will put one across pins 1 and 6 of the 6X4 (.0047/1600). It could be that someone removed the cap but I do not see any evidence. Thanks. Deltysdal@aol.com


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 Post subject:
PostPosted: Jul Wed 22, 2009 4:17 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 339
Location: North Logan, Utah, USA
I would check the schematic carefully. Some of those Mopars put a 10K resistor in series with the buffer capacitor. They used a unused pin of the 6X4 as a tie point.

Wally

_________________
Wally Gibbons, Wally.gibbons@gmail.com


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 Post subject:
PostPosted: Jul Wed 22, 2009 4:34 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3271
Location: Cedarville OH USA
Also some use between a 0.02 and 0.05 uF buffer from one 6X4 plate pin to ground. Have seen this on some of the early 50s MoPars, I forget which maker does this most often, I have seen Philco and Motorola. These buffers are usually toast and a bad one will take out the power transformer if you keep pushing the radio. The transformers on these MoPars are pretty compact-I've had a number of bad ones. These radios are great perfomrers once recapped and aligned. Oh yes-I'll bet money your audio coupling caps are shot if they are originals. I've seen them leak 35 volts to the 6AQ5 grids and get the tubes hotter than h+++. You'll be amazed how much bad caps in this radio will suck down the B voltages.

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Yes, it plays. No, there was no FM stereo in 1932. Yes, some people still enjoy AM radio.


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 Post subject:
PostPosted: Jul Wed 22, 2009 4:36 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3271
Location: Cedarville OH USA
DELCO made a lot of car radios with a 15K resitor in series with the buffer. See this on 12 volt sets. That resistor will overheat like mad with a shorted buffer. Guess it will act like a fuse after enough abuse :D

_________________
Yes, it plays. No, there was no FM stereo in 1932. Yes, some people still enjoy AM radio.


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 Post subject:
PostPosted: Jul Wed 22, 2009 4:47 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 339
Location: North Logan, Utah, USA
This mopar 603 I'm fussing with had a .2 in the radio for a buffer cap. The 10K resistor was fried. Thus my need for a schematic :).

I found a similiar Mopar and got some idea of the component values from it.

And found the 10K/.006uf buffer combination.

Wally

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Wally Gibbons, Wally.gibbons@gmail.com


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 Post subject: Mopar 813
PostPosted: Jul Wed 22, 2009 5:12 pm 
Silent Key

Joined: Nov Fri 28, 2008 4:45 pm
Posts: 2330
Location: Near Fargo North Dakota USA
I am going to check things on the 6X4 a bit more. Thanks for suggestions. A previous repair person put in two new rectifiers and 2 vibrators in a year an half. But no other additions and may have removed the .0047/1600 buffer.


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 Post subject:
PostPosted: Jul Wed 22, 2009 7:05 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3271
Location: Cedarville OH USA
From what I know of the days of yore in car radio-land, people were constantly buying new vibrators when the radio crapped out, the radio would come back to life and it would be fine.....for a few months. Lots of radios were accessible enough (used to be able to reach up under a car dash and do all kinds of stuff!) to easily get to the vibrators. In fact I think a lot of now-missing car radio back covers were tossed aside to make it easy to switch vibrators out. Most people never took the time (or didn't want to pay for it) to remove the radio to check the buffers-you'll note all vibrator boxes note 'Guarantee void unless buffers are checked". I think vibrators were only $2-3-4 in many places. I once talked to an older gentleman that did car radio repair around Maysville KY in the 40s and 50s. Said he'd sell vibrators all day to people that would come in looking for a quick fix. They'd be back again soon.

Most buffers that go across the 6X4/6X5/0Z4 plates are .003 to .008. From one plate to ground I've seen .02 to around .05 A 0.2 for a buffer sounds high.

A leaky filter/filters will take a quick toll on vibrators too.

_________________
Yes, it plays. No, there was no FM stereo in 1932. Yes, some people still enjoy AM radio.


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