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  #16  
Old 10-21-2017, 03:32 PM
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I looked up my old thread and it was Bob Andersen who recommended the Vishay metal film resistors and he posted the link below. If he likes them then they must be good. Besides, metal oxide resistors are a bit hard to come by in the values I need.

http://www.mouser.com/catalog/catalogusd/646/817.pdf
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  #17  
Old 10-21-2017, 07:22 PM
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Okay, just a couple more resistor questions; this radio uses mostly 1/2-watt resistors so I have most picked out.

1. Is a 3-watt resistor of about the same physical size as a 1950s 1-watt a suitable replacement?

2. There's a massive 18k 2-watt, ~30mm long carbon comp in there, what can I replace that with?

3. What is the modern equivalent for a 1.6k, 4-watt power resistor?
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  #18  
Old 10-21-2017, 10:06 PM
madlabs madlabs is offline
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https://www.mouser.com/ProductDetail...dji2uRYA%3d%3d

This should work for the 1.6K

https://www.mouser.com/ProductDetail...dji2uRYA%3d%3d

A little smaller than 30mm, 25mm.

Are you using 1 watters for the 1/2 watters?
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  #19  
Old 10-22-2017, 12:56 AM
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Quote:
Originally Posted by madlabs View Post
https://www.mouser.com/ProductDetail...dji2uRYA%3d%3d

This should work for the 1.6K

https://www.mouser.com/ProductDetail...dji2uRYA%3d%3d

A little smaller than 30mm, 25mm.

Are you using 1 watters for the 1/2 watters?
That 1.62 looks good, thanks. Edit: Oh shoot, that's a non-stocked item.

The same link was pasted twice; I've made that mistake many times. Actually it's the 18k 2-watt carbon composition resistor that's about 30mm long; I have no idea what to replace that with. I'm using 2-watt resistors to replace the old 1/2 watters. Bob Andersen said he does this, they're approximately the same physical size.

To sum it all up I plan to use 2-watt metal film resistors in place of the old 1/2 watters and 3-watt to replace the old 1-watters as they're very close in size. I have no idea what I'll do about that 2-watt carbon composition fatty or the 1.6k 4-watt resistor.

I got a close-up of the circuit the 1.6k 4-watt resistor is in, highlighted it and identified the tubes in the area, hopefully that helps.
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File Type: jpg Pye T7103 partial schematic.jpg (83.4 KB, 8 views)

Last edited by Jon A.; 10-22-2017 at 04:39 AM.
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  #20  
Old 10-22-2017, 11:05 AM
madlabs madlabs is offline
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Dang. I thought I checked the non-stock option. Grr. I do see that Mouser has a 10 watt, 1.6 k in stock. If you can do 1.5K Mouser has plenty of in stock options and that is a less than 10% difference. What is the tolerance of the original part?

Here is a 18k, in stock and 25mm.

https://www.mouser.com/ProductDetail...reHfLvJIcaE%3d
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  #21  
Old 10-22-2017, 05:35 PM
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No can do, tolerance is 5% and I wouldn't feel safe going lower. I believe it's a wirewound resistor; I suppose 1.8k 5w will have to do in this case, it's about as close as I can get. As for the 18k resistor I suppose I'll go with the Panasonic 3w metal oxide.

If that's all in order I now just need a source for dial string, 6.5V 0.3A indicator bulbs, tuning condenser grommets and a dial pointer like the one pictured which is the usual grossly overpriced eBay item. The ad says it's 4 3/4" long which seems appropriate for this radio and the carriage looks like the one in the dial stringing instructions.
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File Type: jpg Radio dial pointer.jpg (45.0 KB, 11 views)

Last edited by Jon A.; 10-22-2017 at 07:25 PM.
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  #22  
Old 10-22-2017, 07:26 PM
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It's just a piece of metal. Can't be that hard to make.
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  #23  
Old 10-23-2017, 01:47 AM
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Yeah, some 18 gauge bus wire ought to do it. I just have to finish figuring out the bends needed to mock up a pointer carriage.

Now, is there a one-stop shop for the bulbs, grommets and dial string?
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  #24  
Old 10-23-2017, 01:06 PM
madlabs madlabs is offline
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How about two 3.3k, 3W in parallel? That gives you 1.65k.

https://www.digikey.com/product-deta...W-3BK-ND/36825
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  #25  
Old 10-23-2017, 06:46 PM
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I don't know, wouldn't that cause a drop in available power dissipation? I see they're only 3 watts each, and I just haven't been able to get a grip on series/parallel wiring for resistors.
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  #26  
Old 10-23-2017, 07:05 PM
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2 3W resistors of equal resistance in parallel or series = 1 6W resistor for series it will have twice the rated resistance of each unit, for parallel it will be half the rated resistance.

Power = Voltage * Current In series each unit takes half the voltage of the pair, in parallel each unit takes half the current....This assumes the two resistors are of equal value....If different the math for power ratings gets more complicated.

It may be useful to do some practice ohms law math, and network analysis math to gain a better understanding.

In other news that part that was low stock turned out to be NO-Stock...I had to call Mouser and ask about it for them to send me the rest of my order....Which apparently they were not going to ship till I asked...
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Last edited by Electronic M; 10-24-2017 at 12:35 PM.
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  #27  
Old 10-23-2017, 07:19 PM
madlabs madlabs is offline
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Tom, you got that backwards - Two equal resistors in parallel halves the resistance and doubles the wattage. Two equal resistors in series adds the resistance and the wattage remains the same.

So Jon, no problem with the two resistors in parallel. I would make sure there is a little air gap between them for heat dissipation

Last edited by madlabs; 10-23-2017 at 07:26 PM.
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  #28  
Old 10-23-2017, 07:27 PM
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Got it, thanks.

Any leads on the bulbs, grommets and dial string? Once those are secured I'm all set.
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  #29  
Old 10-23-2017, 08:59 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by madlabs View Post
Two equal resistors in series adds the resistance and the wattage remains the same.
The wattage rating for the pair would double, wouldn't it? For example, with two equal-value half-watters in series, the wattage rating for the pair would be 1 watt, seems like (since the working voltage across each will be halved).

Whereas a pair of half-watters in parallel also has a 1 watt rating because the current through each is halved.
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  #30  
Old 10-23-2017, 10:53 PM
madlabs madlabs is offline
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Nope. With the resistors in series each resistor still has to handle all the current flowing through it. So the wattage rating remains the same. With two equal resistors in parallel each is taking half the current so the wattage rating doubles.
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