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RCA Victor 813K Restoration

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Forum » Radios and other type of sets (Physics) etc. » MODELS DISPLAYED » RCA Victor 813K Restoration
           
James Hochstetler
 
 
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11.Nov.21 05:38

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I'm restoring an 813K and had a few questions, the cabinet is finished and now working on the electronics. The bezel was my greatest challenge it had shrunk and twisted over time I guess or maybe was left in the sun. I completely re-fabricated one out of wood except for the lower part. Lettering for the knobs was not visible for the two left knobs. Far left is power 2nd from left not sure.So looking for the exact wording for the knobs? Wood working is my forte electronics not so much.

I have replaced all the capacitors and found one electrolyte C56 (20mfd) with positive going to ground. That doesn't seem right to me but it appears to be from the factory. Does that seem right to you?

The schematics that I have do not indicate polarity on any of the electrolytes. C53 (25mfd), C54 (25mfd), C55 & C57 (both 16mfd) were in cans. C53 & C54 had tabs between non-conductive wafers and wires were connected to the tabs. Unfortunately I didn't pay attention to how these wafers were placed, but assume they were to isolate the can from the tab. So my question is this: with only one lead coming out of the top of these cans (which i assume to be +) where does the negative lead go? My assumption is to the tabs? But from inside the can how does the negative lead get to the tabs?

On the top of the drive motor for electronic tuning there is (what appears to be a capacitor) with absolutely no markings on it and I can't find it on the schematic anywhere. but again my electronic knowledge leaves a lot to be desired.

I know this a lot but would appreciate any help anyone can offer.

Jim Hochstetler

361-442-7435

Headline edited and post moved to model

This article was edited 11.Nov.21 10:44 by Bernhard Nagel .

Daniel Consales
Daniel Consales
 
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11.Nov.21 11:58

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Just as an info (not model related):
In general you will find electrolytic capacitors with the positive pole connected to ground when no automatic grid bias is used (which is negative).

And yes there are many (the most) electrolytic capacitors which have the negative pole connected to the can. If there are 2 or 3 wires comming out of the can there are more than one C in it.

Sometimes isolation washers plus a connection washer with a solder lug are used to isolate the can from chassis if neccessary - sometimes simply the mounting is the connection to ground.

In your special case a negative grid bias is generated by the current through R38/R39 to ground. So the voltage at R38 is negative related to ground.

Hope that helps a little :-)

Kind regards, D. Consales

P.S.: in your case it is important that C53 and C54 are NOT connected to ground

 

 

This article was edited 11.Nov.21 12:10 by Daniel Consales .

James Hochstetler
 
 
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11.Nov.21 17:09

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Thank you Daniel that was very informative. What added to my confusion was that the can where C53 was i beleive was replaced for ethetic purposes the replacement had 6 wires coming out the bottom cut off and not connected. If I read the scematic right there should be just one electrolytic capacitor at that location. C53 and C54 both have wafers isolating the can from ground. C54 was not replaced it had two wires connected to the one terminal, one to the react. transformer (L33) and one to the speaker T3 L34 (field). If I were to replace that cap I assume the + end would to those two wires but where would the negative end connect? Same with C53 + end to lug 8 on 5T4 and where do I connect the negative end.

Your PS really hit home because I had connected them both to ground and they both exploded like fire crackers. So again if not to ground where? First I thought that the voltage rating was not high enough on the caps i used (150vdc) but not sure now after you P.S.

Thanks for taking the time to help me

Jim

Daniel Consales
Daniel Consales
 
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11.Nov.21 20:07

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Hello Jim,

if you look at the schematic: C53/C54 (negative side) are both connected to the center tap of the transformer and to one side of R38. Another connection goes to the audio transformer T2 (center tap). Also the negative (!) pole of C56 is connected to that point.Should be no problem to find but I don't own the radio so I can not give any physical hints ...

BTW: I can't believe they exploded just because of the wrong ground connection. If an electrolytic capacitor explodes there are two main reasons: either the voltage was much to high or the polarity was wrong.

The wrong ground connection would have cause strong hum but only slightly higher voltage. If you replace such electrolytics keep in mind that  the the voltage rating has to be high enough - usually higher than the real used voltage.

Correction: the voltage across C53/C54 would not be higher but even lower - so that was not the reason for the explosion. I think your choosen voltage rate (150V) was way to low.

This article was edited 12.Nov.21 11:17 by Daniel Consales .

James Hochstetler
 
 
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13.Nov.21 17:32

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Thanks Daniel,

I replaced C53 & C54 electrolytics with 450v ones except C56 I just turned it around putting positive to ground which originally I thought was wrong. It survived the turn on this time with no fireworks so now just need to check for all the proper voltages as per the schematic.

I really appreciate the help I owe you one

Jim

James Hochstetler
 
 
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20.Nov.21 05:24

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Daniel, an update, after replacing all the the capacitors I checked the voltages and found them to be extremely high compared to the scematics. As an example where it should have been 380vdc it was 560vdc. So I went thru and changed all the resistors (many). and now the 380 reads about 420. So I borrowed a variac and set the input voltage to 115vac (my area is 125vac) the voltage now reads about 396. I need to find a solution to reduce the input voltage.

I'm getting about 5 am stations in very clear however my truck radio gets 10 stations. I also noticed that the 5 stations I'm getting are at the low end of the frequency range 550 to 1100 and nothing above 1100. What does that tell me?

Jim Hochstetler

361-442-7435

Daniel Consales
Daniel Consales
 
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20.Nov.21 11:31

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Hello Jim,

maybe you should use 110VAC for  this set. 420V seems to be quite high.

420V means an ac input of roughly 420V/1,414 ~ 300V. So the mains transformer has a ratio of 1 to 2,6 (300/115). 110 times 2,6 = 286VAC, which would lead to 286 times 1,414  =  404V.

The readings are generally a little higher when measured with modern equipement so that should be OK now.

Regarding to the sensitivity of the set you could try to realign it. But if you are not familar with the align process it can be tricky ;-)

I would consider to leave it alone than or try a better antenna. But also it's possible that your truck radio is  simply better regarding sensitivity ...

Regards, D. Consales.

P.S.: I'm wondering where your voltage values are coming from? I cannot see any voltage value on the schematics which are uploaded here. If you have another (better) schematic consider to upload it

 

 

This article was edited 20.Nov.21 11:58 by Daniel Consales .

James Hochstetler
 
 
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20.Nov.21 16:32

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The voltage procedure calls out 115vac input. With the new resistors and the variac set to 115vac all the readings are only about 5 to 10 volts higher than the schematic.

One thing I was not sure on how to fix is the voltage divider. All the resistance values are very close except one R35. It is supposed to be 7K ohms but it reads 9K ohms.

I agree I'm not going to attempt the alignment procedure for one thing I don't have the right equipment nor the knowledge however the procedure is well documented.

This radio is supposed to have an internal antenna built in however it is not clear to me where. I read that in the documentation somewhere. currently i have no external antenna connected.

I really appreciate your feedback your help and advice has got me to this point.

Jim

 

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Daniel Consales
Daniel Consales
 
D  Articles: 191
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20.Nov.21 17:42
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OK with the voltages - but think of the electrolytic capacitors, can they handle it?

It's not unusual that components change its value over the years or where replaced with what someone had at hand. I would replace the resistor with a 7k (or 6,8k) type

I would say even if there is some internal antenna it can not really replace an external antenna. As a test try some meters of copper wire connected to the antenna post and hanging out the window or so. I think you will be surprised ...

Thanks for the voltages picture - never saw them provided in that way. Most of the time you find these on the (electrical) schematic.

Daniel

  
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