Model 29 Questions - Printable Version

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Model 29 Questions - Eric - 04-17-2013

Gentleman

I have a Philco model 28 cabinet with a 29 chassis. Got the tech data from Chuck Schwark (nice big prints I can read) . Here is some info, most of the chassis looks undisturbed but the filter cap #64 shows to be a three stage 8,8,10 mf now what was in there were two 8's the 10 is missing, also the can itself is mounted on the top side but is insulated( paper wrap) from the clamp holding it in place. There is a lead from the cap case (under chassis) ground that goes to #57

Chuck has marked the polarity for me on the print so my questions are.

1 Could it work with out the 10? Has it been working?

2 In the print the polarity of the 10 the neg is tied to the neg of the two 8's and the pos of the 10 goes to the chassis. How does this work?

http://www.nostalgiaair.org/PagesByModel/181/M0013181.pdf


Want to understand what is going on electrically

Thanks Eric

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RE: Model 29 Questions - Radioroslyn - 04-17-2013

#1 yes but may hum a bit.
#2 The 10uf cap is the filter for the grid bias. True B- is at the center tap of the HV winding of the power transformer. Note there is the resistor tied from B- (the center tap) to the chassis ground. There is a small voltage developed across that resistor. The - side of the voltage is at the center tap and the + is at the chassis. It looks like the 10uf cap is in backwards but it isn't.
Hope this answers your question.
Terry

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RE: Model 29 Questions - BrendaAnnD - 04-17-2013

The 10uf cap is filtering for the bias supply. The negative side goes to common negative (the center tap of the transformer), the positive goes to the chassis, as the chassis is more positive than the common negative line. The set will work without it, but will work much better with it.

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RE: Model 29 Questions - Eric - 04-17-2013

Thanks for the info I intend to put the 10uf in just was wondering why it wasn't and in my limited exp never saw this config before. Just a few more caps and resistors and it will be test time!

Eric

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RE: Model 29 Questions - Eric - 04-19-2013

Ok, new question
#15 Comp Cond OSC L.F. on page 5-13 Fig 4 on the upper tab is a 45k resistor #10(formerly 25k) per a philco change and a lead going to #12 OSC Coil, but on the lower tab which the Fig 4 shows a #66 32k is not there and the tab looks as though there never was.


The schematic shows #15 going to the chassis ground with no resistor (which it is) , and #66 is tied in with #37 and the other end to ground.


Could this be an undocumented change?

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