[AMRadio] [AM Radio] Something strange


Jim Wilhite w5jo at brightok.net
Wed Nov 19 19:05:03 EST 2014


You guys have some good thoughts on this.  A couple of things I did not 
mention.  The transformer output is rectified by a 5U4 and has a 50 K 
bleeder resistor across the output.  The CT is lifted above ground to kill 
the voltage in standby.  Everything is standard 1955 and should not have 
this trouble.  This circuit is a choke input with a single capacitor filter 
with the bleeder across it, but the bias is cap, resistor, cap.  This full 
wave low voltage is really a mystery to me, and given the failure of another 
cap of different value bought at the same time makes me wonder if there was 
a bad run and I managed to get two of them.

The bias circuit is 110 volts rectified by a Selenium (replaced by a silicon 
diode, 1N4007) and previous sets like this that I restored never had a 
problem with them.  So I have different caps in it now and will try it.  I 
have checked the choke and diodes along with associated tubes and other 
parts and found nothing that should cause the trouble.  The transformer is a 
recent rewind with a 125 volt primary  (Don't ask how that happens to be).

Thanks all and I will watch it out of the case for a number of hours and if 
nothing happens, it will go into the case.  If something does, I will put a 
100 ohm resistor in the appropriate place and hope that does it.  Larry, 
these Sprague were marked Made in the US.  Motorola used to buy offshore 
parts and assemble pagers in this country so they could say Made in the US, 
I don't know if that is what happened in this case or not, but nothing would 
surprise me.

Thanks guys,

Jim
W5JO

P.S. Pardon me Larry for sending this to you two times.





-----Original Message----- 


On 11/18/14 8:56 PM, CL in NC via AMRadio wrote:
> By any chance where the the first filter cap  after the rectifier? Could 
> be a peak to peak ripple issue
>
>
Could be.

I had a recurring problem with electrolytics repeatedly failing in a the
low voltage supply in my Viking Ranger. The power supply was not stock,
because the original power transformer had failed (I never owned the
Ranger when the original power xfmr was present, so I've never ieven
seen it.) Instead the power transformer was from an old tube-typed TV,
with a silicon diode bridge around the HV secondary and low-voltage B+
taken from the center-tap of the winding. The high-voltage B+ went to a
choke-input filter, but the low-voltage B+ was capacitor-input - simply
a 450WVDC electrolytic in the 40-80uF range from the HV center-tap to
ground. (I used several capacitors of different values in this range as
they kept failing, and I replaced them with whatever I had on hand. The
original low-voltage filter choke was also missing from the rig.) The
failure mode was always open, or high series resistance. I finally
eliminated the problem by installing a replacement choke from the
junkbox to make the low-voltage B+ supply filter choke-input, like the
rig originally used, and like the high-voltage B+ supply. This was
probably over 10 years ago, and I have not had a failure since. I had to
conclude it was a peak voltage or peak current problem killing the
capacitors. I had used many different brands of capacitors, both modern
and NOS, all failed eventually before I added the choke.

Also, I have had experiences of modern off-shore manufactured
electrolytics overheating and letting their smoke out, sometimes
spectacularly, when used as replacements on the input side of a
conventional pi capacitor-input filter in high-voltage supplies. I
started using Nichicons and have not experienced this since. YMMV.

73,
-Larry/NE1S


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