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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Sat 28, 2020 10:30 pm 
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Joined: Apr Wed 03, 2013 2:38 pm
Posts: 98
We might be able to bypass some steps here;
the unbanded end of CR1 is connected to power switch and then on to
pin 3 of phono ac socket but there is no meter deflection from above points to
any neg point of C19a thru d
just wanted to mention again that the wires that were connected to the
multi cap neg cap tab are now soldered together but I'm wondering now
if there is a wire missing from cr1 to that tie point


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Sat 28, 2020 10:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4391
Location: Boston, MA USA
That's exactly what I'm thinking.

-David


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Sun 29, 2020 2:31 am 
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Joined: Apr Wed 03, 2013 2:38 pm
Posts: 98
So taking all the liability out of this lol do you think I could put a connecting wire
from unbanded end of cr1 to the tie points that include the neg side of C19a thru C19d ??


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Sun 29, 2020 3:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4391
Location: Boston, MA USA
Yes, you absolutely need that. How was it wired to the original selenium rectifier? You should duplicate that.

-David


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Sun 29, 2020 10:16 pm 
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Joined: Apr Wed 03, 2013 2:38 pm
Posts: 98
There was a wire that went from the selenium rectifier neg to C20 tab that was mounted to the chassis

UPdate;

I put a connecting wire from CR1 unbanded side to the B-
Applied power and it had a loud high pitch sound at low and high volume
I then checked voltage at the pos schematic points again as follows
C19a=160v C19b=182v C19c=203v C19d=145v


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Mar Mon 30, 2020 3:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4391
Location: Boston, MA USA
The high pitch sound is called oscillation, cause by unwanted feedback in the amplifier. When you say you get it at high and low volume, do you mean that the sound is the same loudness regardless of the position of the volume control?

Check your voltage measurements again, the values you posted don't make sense. The voltage at C19a must be higher than at C19c.

When you replaced the capacitors, did you happen to move any wires or components around? Please make sure to place them back where they were. High gain circuits often require specific component placement, called lead dress, to prevent oscillation.

The good news -- the fact that the amplifier is oscillating actually means that it is working.

-David


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Apr Sat 04, 2020 4:36 am 
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Joined: Oct Wed 25, 2017 2:50 pm
Posts: 12
dberman51 wrote:
I would use the resistor values in the schematic. You have plenty of margin in the capacitor voltage rating to accommodate a few more volts of B+.


Selenium rectifiers have a much bigger voltage drop than silicon types. When you rectify 110 mains and then double it your above 300 VDC . Two silicon diodes might account for 1.4 volts of drop but there is a drop of nearly 100 VDC between R25, CR2, C20 and C19A (from the schematic). R25 would have to be made larger or your going to boil the 50L6's. What diodes are you using. They look like 1N100X's but I can't see exactly from the picture. They should have a reverse voltage rating of at least 400VDC or better. I would say a 1N1005. Anything less will break down like a 1N1003 or 1N1002 . Even a 1004 is risky. Your approach of starting with a larger R25 is a prudent approach and then either through calculation or trial and error lower R25 until 240VDC at C19A is achieved. Too low a voltage won't do any damage but to high and you run the risk of something blowing up or you getting hurt. I would not be counting on margins when dealing with a 70 year old electrolytic. The correct method is to bring the mains up slowly with a variac while monitoring the key voltages. I suspect the original selenium rectifier failed because something else is wrong. Also make sure you have a 12AT7 and not an 12AX7. The AX has a higher gain and could be the cause of the oscillation. A lot of Radio Shack salesman claimed a 12AX7 was a direct substitute for a 12AT7. The same salesman would sell a 3 amp fuse to replace a 2 amp and claim there was no difference.


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 Post subject: Re: Philco D-1347 Selenium Replacement Problem
PostPosted: Apr Sat 04, 2020 4:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4391
Location: Boston, MA USA
@robertjt:

1) Please read the entire thread. You will see that the electrolytic capacitors are new.

2) The forward voltage drop of a NEW selenium rectifier is approx 7 volts. So the difference in forward voltage drop between selenium and silicon rectifiers is all of 6 volts. Hardly a big deal. No need to increase series resistance beyond the design value.

3) Selenium rectifiers don't need anything to take them out. As they age, their internal resistance increases so forward voltage drop increases to 30 volts or more, reducing B+ significantly. This is the most common failure mode. They can also fail dramatically (and in a very smelly fashion) by over-current due to leaky filter capacitors. I have never experienced this but others on this forum have.

4) I never recommend that phonographs be brought up on a Variac. Unless the motor can be disconnected (sometimes not possible) it will overheat and can be destroyed by operating at voltage lower than rated. Some members of this forum have experienced this.

-David


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