Topics

577 AC collector supply not reading zero on display

Chuck Harris
 

Wear on the contact brush is something that I have never seen in
manually operated variacs. They would have to be swung back and
forth tens to hundreds of thousands of times to do that.

And, the contact point doesn't really matter much. What matters
is the stop that prevents the brush from moving too far in the CCW
direction. It is fully and easily adjustable.

The stop is a wedge shape piece of sheet metal that has its outer
edges bent up to form the stops. The stop has two center slots
that take a pair of screws. To adjust the position, loosen the
screws, and move the wedge shaped stop in or out from the center
as necessary to get the output to exactly zero.

Out decreases the zero voltage, in increases it.

-Chuck Harris

DW wrote:

What you describe makes sense Chuck. I observe no issues with smoke, hum, and the variac works throughout its full operating range other than not going to zero. Therefore I been convinced the variac as far as the winding is fine.

Having ruled out the variac, Bob made a good point about the carbon brush, I believe he is on to something about the contact point being worn down widening the contact area and potentially causing what I am experiencing. I will try to rework the brush to a point with some sand paper and see if I notice any improvements.

I would like to thank everyone here for their valuable help, I see this being useful for someone with collector supply issues to check the wiper brush!



DW
 

I have managed to sand the carbon brush to more of a appropriate point instead of the entire area of the carbon brush

I noticed no improvement, still 2 divisions at 50mV 6.5V collector setting

Using a non conductive object I slightly pushed the wiper further to the left as it has some easy flex to the wiper metal, I noticed a improvement of about 1.5 divisions at 50mV

I tried lifting the wiper completely off the winding entirely with a plastic can spray straw, that really improved things however I still get 1 division at 10mV, at least it's better than it was. The lowest I can set the volts per divisions is 5mV with horizontal magnification on.

Roy Morgan
 

I have seen Variac brushes that were not point shaped at the contact but rather had s linear shape. The contact end was as long as the brush diameter but narrowed along the sides so as to contact the full length of the flattened portion of one or two windings.

Without examining any of the ones here, I suspect they do contact more than one winding if positioned between two wires in order to create smooth transition from one winding to the next.

I do have one very old GR Variac whose brush is simply round, and is flat on the contact end. It may date from the 40’s or even 30’s.

Roy sends.

On Dec 9, 2019, at 10:57 PM, DW <wilson2115@...> wrote:

What you describe makes sense Chuck. I observe no issues with smoke, hum, and the variac works throughout its full operating range other than not going to zero. Therefore I been convinced the variac as far as the winding is fine.

Having ruled out the variac, Bob made a good point about the carbon brush, I believe he is on to something about the contact point being worn down widening the contact area and potentially causing what I am experiencing. I will try to rework the brush to a point with some sand paper and see if I notice any improvements.

I would like to thank everyone here for their valuable help, I see this being useful for someone with collector supply issues to check the wiper brush!


Chuck Harris
 

There may come a time when you realize that you have made a
critical mistake changing the profile of your carbon brush.

Let's hope not.

Any reason why you didn't simply move the stop?

-Chuck Harris

DW wrote:

I have managed to sand the carbon brush to more of a appropriate point instead of the entire area of the carbon brush

I noticed no improvement, still 2 divisions at 50mV 6.5V collector setting

Using a non conductive object I slightly pushed the wiper further to the left as it has some easy flex to the wiper metal, I noticed a improvement of about 1.5 divisions at 50mV

I tried lifting the wiper completely off the winding entirely with a plastic can spray straw, that really improved things however I still get 1 division at 10mV, at least it's better than it was. The lowest I can set the volts per divisions is 5mV with horizontal magnification on.



Chuck Harris
 

The variac would spit, and spark under load, and the sparking
will cut the sparking wire in half, ruining the variac.

The brush *MUST* contact more than one wire always. That is
crucial to the performance of the variac.

The left hand and right hand stop is fully adjustable to fix
the very problem the OP is complaining of.

-Chuck Harris

Roy Morgan wrote:

I have seen Variac brushes that were not point shaped at the contact but rather had s linear shape. The contact end was as long as the brush diameter but narrowed along the sides so as to contact the full length of the flattened portion of one or two windings.

Without examining any of the ones here, I suspect they do contact more than one winding if positioned between two wires in order to create smooth transition from one winding to the next.

I do have one very old GR Variac whose brush is simply round, and is flat on the contact end. It may date from the 40’s or even 30’s.

Roy sends.

On Dec 9, 2019, at 10:57 PM, DW <wilson2115@...> wrote:

What you describe makes sense Chuck. I observe no issues with smoke, hum, and the variac works throughout its full operating range other than not going to zero. Therefore I been convinced the variac as far as the winding is fine.

Having ruled out the variac, Bob made a good point about the carbon brush, I believe he is on to something about the contact point being worn down widening the contact area and potentially causing what I am experiencing. I will try to rework the brush to a point with some sand paper and see if I notice any improvements.

I would like to thank everyone here for their valuable help, I see this being useful for someone with collector supply issues to check the wiper brush!




DW
 

As suggested and as I discovered with delight the stop tab is adjustable and moved it further down, that made a big difference, more so then bending it, now I am at about 1.6 divisions at 5mV a division I will attempt to get it to 0 now

Chuck Harris
 

In your zeal to get perfection, be sure that you don't let the
brush fall off of the winding when in the most CCW position.

It would not go well for the winding, in the long run.

And note that you will never be able to get exactly zero. This
is because there will always be a fraction of a turn even if
the brush only touches the very last winding.

-Chuck Harris

DW wrote:

As suggested and as I discovered with delight the stop tab is adjustable and moved it further down, that made a big difference, more so then bending it, now I am at about 1.6 divisions at 5mV a division I will attempt to get it to 0 now



k1ggi
 

Google US patent 2,009,013.
It was a case study in a course in an engineering curriculum, taught by R.H.
Rines, son of the patent attorney.
Yes, Rines of Nessie fame.
Ed, k1ggi

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roy
Morgan
Sent: Tuesday, December 10, 2019 3:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 577 AC collector supply not reading zero on display

I have seen Variac brushes that were not point shaped at the contact but
rather had s linear shape. The contact end was as long as the brush diameter
but narrowed along the sides so as to contact the full length of the
flattened portion of one or two windings.

Without examining any of the ones here, I suspect they do contact more than
one winding if positioned between two wires in order to create smooth
transition from one winding to the next.

I do have one very old GR Variac whose brush is simply round, and is flat on
the contact end. It may date from the 40's or even 30's.

Roy sends.

On Dec 9, 2019, at 10:57 PM, DW <wilson2115@...> wrote:

What you describe makes sense Chuck. I observe no issues with smoke, hum,
and the variac works throughout its full operating range other than not
going to zero. Therefore I been convinced the variac as far as the winding
is fine.

Having ruled out the variac, Bob made a good point about the carbon brush,
I believe he is on to something about the contact point being worn down
widening the contact area and potentially causing what I am experiencing. I
will try to rework the brush to a point with some sand paper and see if I
notice any improvements.

I would like to thank everyone here for their valuable help, I see this
being useful for someone with collector supply issues to check the wiper
brush!


DW
 

I should note the feedback here has been noted and considered so far here.

I have avoided shaping the brush to a sharp point as I was observing how the other variac had a point but it was blunt on the end so I worked to mimic that, somewhat by slightly giving a flat roll off on on just the two ends.

Vertical lines represent contact area to windings.
---||||----------- <Winding
| |
|_ _ | < Brush

| |
| | <Top view of contact area of brush
| | winding aligns perpendicular to this


These adjustments I am making are only brief and temporary, I am just experimenting to see how I can get the collector supply to 0 volts.

Obviously as noted I want to make absolutely sure the carbon brush is making a wide enough contact patch to prolong the life of the variac for the long term.

Also the brush was running off the winding on to a metalic platform. I observed the control slightly sticks as the brush has to overcome the lip of the first winding, as Chuck noted and as I obviously noticed, I don't want this to occur to avoid premature failure. I will readjust approximately.

Chuck Harris
 

The first turn on a variac, and the last turn on a variac are both
very vulnerable to damage... physical and electrical.

When properly adjusted, the brush should fully cover (and connect to)
the first through about the 5th turn, and about the 5th to last through
the last when at full scale.

The brush is highly resistive to avoid shorting out the transformer
when it straddles turns. If you were to sharpen the brush to a point,
so that it covered only a single turn, it would burn right through
that turn when operated anywhere near its full current rating.

A pointed carbon brush, pulling very little current will heat to
incandescence long before the skinny little copper wires used in this
variac get to their rated current.

[I used to engrave tools using a pointy carbon rod and a 2V transformer.]

The variac's design requires that the carbon brush be covering 5 or 6
turns, and resistively combining each turn's contribution, to achieve
its full current rating.

This is also required to give the variac seamlessly smooth changes in
output voltage as the voltage control knob is adjusted.

Historically, the variac was invented as a replacement for huge highly
lossy carbon pile rheostats that were used in stage lighting. It was
a real boon, as a variac could be very small, and not get hot... unlike
the rheostats that were being used.

It allowed the light's brightness to be changed smoothly, without jumps.

The variac wasn't meant to go all the way down to zero, only all the
way down to where the flood light is dark.. a couple of volts here or
there didn't matter.

The Tek curve tracer takes this imperfect lamp dimmer device, and
multiplies its output voltages by the step up/down transformer to create
a collector supply that ranges from 0-6.5V all the way up to 0-1500V.

Tektronix never expected that you would use the 50mv per division scale
on the 1500V range. They expected that if you wanted to measure such
a small range of voltages, you would use the 6.5V range.

The only reason you can even see this flaw in your variac is because
it was too expensive to build interlocks to prevent every possible
undesirable setting. They instead concentrated on some of the unsafe
settings.

Are you sure that what you are asking your 577 to do is reasonable?

I would hate to see you do any more damage to your curve tracer.

-Chuck Harris

DW wrote:

I should note the feedback here has been noted and considered so far here.

I have avoided shaping the brush to a sharp point as I was observing how the other variac had a point but it was blunt on the end so I worked to mimic that, somewhat by slightly giving a flat roll off on on just the two ends.

Vertical lines represent contact area to windings.
---||||----------- <Winding
| |
|_ _ | < Brush

| |
| | <Top view of contact area of brush
| | winding aligns perpendicular to this


These adjustments I am making are only brief and temporary, I am just experimenting to see how I can get the collector supply to 0 volts.

Obviously as noted I want to make absolutely sure the carbon brush is making a wide enough contact patch to prolong the life of the variac for the long term.

Also the brush was running off the winding on to a metalic platform. I observed the control slightly sticks as the brush has to overcome the lip of the first winding, as Chuck noted and as I obviously noticed, I don't want this to occur to avoid premature failure. I will readjust approximately.

DW
 

You bring a valid point Chuck, I obviously don't want to do more harm than good.

As of wring this I am defaulting the variac brush to its previous state and adjusting the tab stop so the brush sits on the windings. I am making sure the brush will make a contact with a appropriately wider patch on the windings so as to distribute the power in a way that doesn't cause premature wear.

When I am at 5mV a division that is at the 6.5V setting, not 1600V

At this point I will perform some final touches on this curve tracer, after everything is appropriately set I will put the covers back and and leave it alone. Like you mention I don't want to break anything, leave well enough alone. Thanks for all your helpful advise by the way, I learned some things.

DW
 

All done

I used some sand paper and pressed on the variac windings I carefully slid the carbon brush across it to make sure there is even and complete contact patch on the windings.

The stop is adjusted so the brush is setting similar as before, safely on the windings.

I now get at 6.5V and at 50mV a division a 2 division line.

My work is officially done with the variac and everything still works, everything together, covers back on

I should add when i had the wiper parked so far back previously to minimize the collector supply issue, parked off the winding on the plate, I noticed the transition being non linear. Slightly turning the control the voltage would suddenly instantly increase instead of being smooth

DW
 

I would like to thank everyone here for their help and especially advice. In this case we noticed sometimes it is good to let things be and I have personal experience before from this. Given this advice I immediately up on that option.