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Voltage on the chassis Philco 38-8 - Printable Version

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RE: Voltage on the chassis Philco 38-8 - morzh - 03-19-2016

Here's another experiment once you are done with the first part: measure (you are measuring from Earth in the outlet, right?) with the plug inverted 180 degrees (small neutral, wide hot) and see if this changes the reading.
Again, without the caps.

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-19-2016

OK, I took the caps out of the circuit and this is what I found..

Hot on the switch, switch off, No voltage,.. with the switch on, 35volts.

Neutral on the switch, switch off, 24volts,.. with the switch on, 64volts.

Measurements were taken between the chassis and outlet ground..

Stan

RE: Voltage on the chassis Philco 38-8 - morzh - 03-20-2016

Looks benign to me.

Just for giggles 1) put 10K resistor across the voltmeter when measuring with neutral on the switch, see if there is still voltage left. 2) same arrangement, measure current between Gnd and chassis. Should be extremely small, in few uA.

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-20-2016

Did some checking later last night and it appeared to be the transformer, even though I had already checked it for resistance to the chassis.. I just knew it was the transformer leaking.. I isolated the transformer from the chassis this morning..
I still get AC voltage..
It does seem like I'm chasing something that really isn't there.. the more I test, the less sense it makes.. The radio seems to play ok,
voltages at the tubes seem to be in line.. I think I will do the alignment and call it done.. I do have a little hum now but I hope that will go away once the y-caps are back in..

RE: Voltage on the chassis Philco 38-8 - morzh - 03-20-2016

Well yes it is the xfmr, there is no other ways to leak.

Again, it is probably benign and is what most radios will have.
I recommended measuring the current just to make sure it is small.
Wouldn't hurt to know it for a fact.

There are two paths to induce the voltage through the transformer: 1) via the capacitance of the primary to the shell which is connected to chassis, and through the capacitance to the secondaries, which then through the capacitors or simply through grounded filaments bring it there.

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-20-2016

Interesting development,.. initially I had to replorseace my bias resistor with a terminal strip and individual resistors..
That hum I mentioned earlier only got worse when I put everything back together.. so I started probing around with a small screw driver.. I found that probing around my bias resistor replacement would change the hum, sometimes better, sometimes worse.. then I noticed the screw that was holding the terminal strip was loose,.. I had a loose ground..
That killed the hum.

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-20-2016

Everything back in its place, shows 55volts.. My meter shows no current.. I'm sure there is something there, my meter just won't show it.. interestingly though, that hum got worse.. I took a small screw driver and probed around until i found what changed the hum..
Originally I had to replace the bias resistor with a terminal strip and resistors.. the screw that was holding the strip, and make up the ground was loose..
Tightening that screw killed the hum..

RE: Voltage on the chassis Philco 38-8 - morzh - 03-20-2016

Well if it shows no current (AC setting) on ma setting then there's nothing to worry about.

Voltmeter is very high impedance so you will see the voltage even if it is very high impedance source. Any sensible load just brings it down to zero.

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-21-2016

Hey, I'm still looking over this radio (38-8, making sure I have everything in order..
Will someone confirm the polarity of cap #45..

RE: Voltage on the chassis Philco 38-8 - 462ron - 03-21-2016

Correct!

Ron

RE: Voltage on the chassis Philco 38-8 - Stan the Man - 03-21-2016

Thanks Ron,.. Was afraid I had stuffed the can backwards..

Stan