Rookie mistake

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Rookie mistake

Post by Ben Delk on Tue May 08, 2012 5:47 pm

So I found deal on an RCA Senior WV-98C. To be expected with any "deal" it needs a little work.
I removed the soldered in dead battery, installed a snap in battery cradle, replace the selenium rcfi with a 1N4005 diode, replace the 10@200 electr with 10@450 cap. The scale needle was sticking about a quater way from the left and just hang there until I bump the meter and then it would drop make to the left. I pulled the needle zero adjust cap and tried the lateral adjustment with no success so I started disassembling the meter. Removed the needle\scale assembly and discovered the the lateral adjustment had been push down on rahter than moved side to side and had come loose on one side. Popped it back into place, the needle freed up and said ok, I'm home free. Enter the rookie......somewhere in the process what appears to be a copper grounding strip about 1/2 wide and 1 to 1 1/4 inch long fell out of some place I did not see but it was on the workbench right where I was so intently disassembling this new found toy. I don't have picture today but I can post one if needed, I just thought maybe someone else had made the same mistake or happen to know where it goes.

Thanks,
Ben
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Re: Rookie mistake

Post by willy3486 on Thu May 10, 2012 9:55 pm

You can go to http://bama.edebris.com/manuals/rca/wv98c It has 3 files there and the 6 meg file has a diagram in it. You should be able to find where it goes from that.

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Re: Rookie mistake

Post by Ben Delk on Fri May 11, 2012 8:04 am

Thanks but I have this already. The piece that fell out is not part of the curcuit board it is a separate copper strip. There is an elongated hole on one end fill certain it goes where one of the case or meter scale assembly attaches, just not sure which one. Darnest thing....just never saw it fall out. I've been watching guys rebuild them on youtube to see if I can catch a glimbse of the part and have not seen it so far. You know it is bad enough when you get something someone else has worked on and you have to guess where things go but when you do it to yourself.....What a bone head..........Thanks again........
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Re: Rookie mistake

Post by Resistance is Futile on Fri May 11, 2012 2:23 pm

I have not seen the inside of one but a speculation only. The strap may have been from the jack ground to either the case or ground on the circuit card?

Did you put in a dropping resistor in the B+ to compensate for the diode you replaced?
The selenium diode has about a 60volt drop or so depending on number of stacks (plates) and the silicon diode has only a .6 volt drop.
I could be way off as the article I will refer you to below states about a 200 ohm 2-5 watt resistor on the anode.
--------------------------
" My rule-of-thumb is to size the resistor to drop about 10 volts, simulating the drop of the
old selenium rectifier. If the equipment requires 50 milliamperes, we can use Ohmís Law to
calculate:
R = Vdrop/IDC = 10 V/0.050 A = 200 ohms
This is only a rule-of-thumb. If you want to duplicate the exact original output voltage, start
with a resistor of about 100 ohms and measure the output voltage with the equipment operating.
Adjust the resistor value up or down until you achieve the voltage specified by the manufacturer
of the equipment. Typical values will be from 20 to 200 ohms. The resistor has to dissipate
some power, which means it will generate heat, so be sure to use a resistor rated for the
power. In the example above, the approximate power required will be:
P = V2/R = 102/200 = 0.5 watts
Donít use a half-watt resistor! A half-watt resistor gets very hot when dissipating a half-watt.
I would recommend at least 100% to 200% safety factor. In fact, 5 watt resistors are probably
easier to find than 2 watt types. If your current calculation is different, use a resistor rated
for at least twice the power calculated. Resistors are cheap, so donít cut corners.
Youíll undoubtedly also be replacing the filter capacitorsóstandard practice in most
restorations. In AC-DC power supplies, make sure that your new filter capacitors are rated for
at least 170 Volts. Old equipment might have 150 V caps, but use the higher voltage rated capacitors
when renovating".


Click Here

----------------------------------------------------------------------------------
note: this may be wrong
The schematic states its diode is30 Ma and the DC voltage is +88 volts so the dropping resistor should be E/I or 88v/30ma (88v/0.03amps)= 2.93kohms I would use a pot and adjust it as you read the B+ voltage to 88volts DC.

88v/200ohms - .44watts


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Re: Rookie mistake

Post by Ben Delk on Fri May 11, 2012 4:26 pm

No I did not put in the dropping resistor. I have read article leading both ways so I thought I would try without. I left it connected and turn on anthe 10\450V cap had started swelling after only an hour so I dropping resistor is next. Thank you very much for the calulation on the size. So will the dropping resistor go in series after the rectificer in this case?
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Re: Rookie mistake

Post by Resistance is Futile on Fri May 11, 2012 4:44 pm

Between the rectifier and the first filter cap (so it will be in series with the Diode).
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Re: Rookie mistake

Post by Ben Delk on Fri May 11, 2012 5:26 pm

Man.....this is a hot topic among the realist and the purist, Ha! Once agin , you are a gentleman and a scholar ole' great one.

Thanks
Ben
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Re: Rookie mistake

Post by Resistance is Futile on Fri May 11, 2012 11:45 pm

As goofy would say AWR GORSH yuk yuk Embarassed

Pleased to be of assistance.
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