[Sourbone]

Hello,

I've been working on a model 650 with help here and there from the phorum. Last night I finished recapping, and played the radio with success for about an hour while dxing and listening. Today after replacing the shadow meter pilot light and cleaning the chassis, the 8mfd cap that replaces one of a dual in can 77 blew out when I fired it up. I've gone over everything looking for a bad solder joint, broken connection, shorted connection. I replaced the cap, and it blew out again. I'm not finding my error. Maybe the experts have an idea of what would cause this after it working okay. 

Thanks, 

Dave

[Mondial]

Is it the 8 uF connected to the filament of the 80 rectifier or the one after the filter choke? I am assuming that the cap that blew was installed with the proper polarity.

The first 8 uF connected to the 80 sees a lot of ripple current and if it is not a high quality cap it may overheat and blow out. This radio has a high DC current draw and a poor grade cap may not be able to handle the load.

[Sourbone]

It is cap 77 at 8mfd connected negative side to the power transformer center tap and positive side to the field coil, .3mfd cap and filter choke. It's MIEC cap. Thanks for your help. 

[Mondial]

What is the voltage rating of the problem cap? Was it getting hot before failing?  I have used some of the MIEC 10 uF  500V caps as filters with no problems.

It is odd that the cap 77 on the schematic is blowing, as it is under no unusual stress. Its typically the first cap after the rectifier that goes. If it was installed correctly and the radio was playing normally for a hour, there is very little else in the circuit that could cause it to blow. Provided the cap was rated at 450 VDC or more, my guess is that you may have gotten a bad batch of caps and I would try a different replacement.

[morzh]

I think the explanation might be simple. 77 is that plus to chassis cap, often gets miswired.
Considering its voltage rating needs to be under 100V.

Oh....and it sees virtually no ripple current to speak of.

[Sourbone]

You have the right cap identified Cap 78. It is the 8mfd connected to the rectifier. I put in a new cap with the same specs 8mfd for 450 volts negative to center tap, positive to rectifier, filter choke, and cap 80. It blew again. I can't think why anything is different. I was working pretty well. I took out the pilot light that I installed earlier and looked everything over for screw ups. Thanks for you advice. 

Dave

[morzh]

That one.
OK, minus on the right, plus on the left? (Centertap and choke respectively?)

Have you been able to see the voltage across it?

Stupid question: is the choke 79 open?

[Sourbone]

Okay thanks for your help. I soldered in a new cap and here's how 77 and 78 are connected.

77
10mfd Negative lead to center tap/positive lead to ground

77
8mfd Negative lead to center tap/positive lead to field coil, .3mfd cap, filter choke

78

8mfd Negative lead to center tap/positive lead to .3mfd cap, filter choke, recitifier.

The filter choke measure properly. No lie it worked well last night.

Thanks again.

[Mondial]

What is the voltage reading across cap 78? Can you measure it before it blows up?

[morzh]

(By chance rect tube got bad....can't quite figer in what way to do that).

See if you can measure the voltage across it.

[Sourbone]

The 80 tests great. I'll give it another shot. 

[morzh]

Well...I say measure the voltage WHILE it warms up.

How long does it take to blow up? Seconds?minutes? If minutes, it is possible to measure the voltage.

[Sourbone]

Thanks for your help. The 650 works great. I don't know what I did to screw it up, but I got it back together with your help. I found I had a broken lead on the other 77 cap. I don't know that that would cause the problem, but just reconfigured it all. I measured the voltage on the 78 cap at 110 v on the variac it measures 340v. Thanks for your patience. 

Dave

[morzh]

I'm not sure how the other 77 (the one I hough of first mistakenly, the centertap to GND) being disconnected could've done that, as the voltage is still filtered some and even if not....anyways all is well that ends well.

[Radioroslyn]

I'm no expert but what could cause this problem is one of two things.
If you had a bad connection at the filter choke or open filter choke it
wouldn't load the power supply and #78 would just keep charging up more
and more till it passes the maximum voltage rating and it blows it's lid!!!
Or
If 78 was installed backwards however the set would not have played
at all. Shortly after snapping in on 78 would look like a dead short and
would heat up and blow it's lid. A good chance that it would take the
80 tube with it. If left on for a length of time the shorted 80 tube could
take the power transformer too.

This is the same sort of scenario you can have when you try out a set that
have the original filter caps in it. Although the original aren't in backwards they
can over a long time develop a low resistance.

Terry