How do I deal with a failed electronic project?

[Planobilly]

Redraw of the possible new HT power supply circuit.

Also the current HT bridge does not have a blown diode as the ripple is 120Hz.

 

[arydberg]

Have you covered the bottom of the chassis with a shielding material like an aluminum sheet? Also is the scope prorerly grounded?

 

[Svein]

So - what is the diode bridge with the zeners doing? One end is connected to the transformer center tap, the other is floating in the air?

 

[rrogers]

Just a couple of general points: make sure the power supply ground is _between_ the filter caps and the rest of the circuit. Otherwise, the bypassed current will be pumping circuit ground.
Always make sure you know where every return path is for every current you use and make sure the forward and return paths/loops have as small an area as possible.
Pretend that the scope ground is 200V@5amps away from the ground of your power supply and hook the scope ground up so that the current will not go through any circuit ground paths. Unless you have a special scope life will be easier.

 

[Planobilly]

Arydberg...Yes and Yes

Svein... More drawing mistakes...sorry...the zener idea is to stabilize the grid supply.

Rrogers...I assume you mean I should remove the first ground I have on the above schematic. I will have to think about your scope comment. I don't have much experience with a scope to begin with...still learning.

Thanks,

Billy

 

[Svein]

The Zener idea seems more mysterious the longer I look at it. Since I am not quite sure where you have connections in your schematic and where the lines are just crossing without any connection I may be wrong, but:

• If we mentally remove the transformer center tap and everything connected to it, you have a standard bridge rectifier setup, only with serially connected smoothing capacitors. I can live with that.
• Now to the center tap. The way it is connected, it will have a DC potential at the half of B+, including the ripple.And it looks like you try to connect it to half the Z potential though a 100V AC drop...
• The standby switch. When that switch is open, you still have an AC path through the lower half of the transformer. Exactly what happens to B+ is a little unclear, but it will be at least 240V.

So - exactly what are your specifications for the supply?

 

[Mark Harder]

Wow. Cloth covered wires! You ARE striving for authenticity. But there's plastic in there, too. Or, are the cloth-covers original? In which case, I'd consider replacing them, the rubber insulation on those old things oxidizes and cracks, cracks that you can't see under the cloth.

 

[rrogers]

Arydberg...Yes and Yes

Svein... More drawing mistakes...sorry...the zener idea is to stabilize the grid supply.

Rrogers...I assume you mean I should remove the first ground I have on the above schematic. I will have to think about your scope comment. I don't have much experience with a scope to begin with...still learning.

Thanks,

Billy

It's the same thing about currents: the heavy bypass caps should shunt the 60Hz 60/120 Hz currents around the load/signal circuitry. So picture the 60/120 currents going in the top and coming out the bottom of the cap and then going back to the rectifiers and regulators. You want the signal circuitry to "see" the stable cap voltage not the bypassed current. So the grounding of the signal circuitry should be nailed to the bottom of the cap; otherwise it will be exposed to the 60Hz current. Been there done that :) This exposes you to some other problems, like ESD so it's not a cure-all but one factor to balance.
As far as the scope goes:
1) Plug it into the same 115vac wall socket as the circuit. This will go a long way towards mitigation of scope ground interference. There was a story about different instruments plugged into different outlets in the 60's at a hospital, and then the janitor plugging in a vacuum cleaner in the hall; sending a current through the patient's heart. Not a good scene; but memorable. Know where your kids and currents are! In any case, the scope ground will be pumping current out unless it's a very good (and more expensive) design so try to make sure that where you attach the scope ground to your circuit won't interfere with your circuit operation. This can work both ways; there are a couple of tests that can be done to make sure you are seeing the truth on the scope. One important one is to put the scope probe to the point where the scope ground is connected and look at the residual (with your circuit active); this a simple "common mode" test of how much you can believe readings. Unfortunately, it doesn't cover all possible things that can cause false signals.
Try-- http://cp.literature.agilent.com/litweb/pdf/5989-7894EN.pdf
Common mode is item 6.
Agilent has been in business for a number of years and know's what they are talking about. (A few missteps but whose counting?)
The point is: keep a critical attitude about the whole schmear, everything connected to your assembly, when trying to dig out interference in particular 60/120 Hz.

 

[rrogers]

It's the same thing about currents: the heavy bypass caps should shunt the 60Hz 60/120 Hz currents around the load/signal circuitry. So picture the 60/120 currents going in the top and coming out the bottom of the cap and then going back to the rectifiers and regulators. You want the signal circuitry to "see" the stable cap voltage not the bypassed current. So the grounding of the signal circuitry should be nailed to the bottom of the cap; otherwise it will be exposed to the 60Hz current. Been there done that :) This exposes you to some other problems, like ESD so it's not a cure-all but one factor to balance.
As far as the scope goes:
1) Plug it into the same 115vac wall socket as the circuit. This will go a long way towards mitigation of scope ground interference. There was a story about different instruments plugged into different outlets in the 60's at a hospital, and then the janitor plugging in a vacuum cleaner in the hall; sending a current through the patient's heart. Not a good scene; but memorable. Know where your kids and currents are! In any case, the scope ground will be pumping current out unless it's a very good (and more expensive) design so try to make sure that where you attach the scope ground to your circuit won't interfere with your circuit operation. This can work both ways; there are a couple of tests that can be done to make sure you are seeing the truth on the scope. One important one is to put the scope probe to the point where the scope ground is connected and look at the residual (with your circuit active); this a simple "common mode" test of how much you can believe readings. Unfortunately, it doesn't cover all possible things that can cause false signals.
Try-- http://cp.literature.agilent.com/litweb/pdf/5989-7894EN.pdf
Common mode is item 6.
Agilent has been in business for a number of years and know's what they are talking about. (A few missteps but whose counting?)
The point is: keep a critical attitude about the whole schmear, everything connected to your assembly, when trying to dig out interference in particular 60/120 Hz.

Just to emphasize: over 90% of the time hum (60/120 hz) is due to improper grounding. I can give exceptions but they are one or two in 50 years and due to loops in 60/120 Hz lines _around_ the circuitry or poor bypassing.