Topics

577 AC collector supply not reading zero on display

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.

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
 

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.

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
 

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.

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!


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



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
 

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!




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.



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!


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.

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
 

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
 

I seriously doubt that there is anything wrong with your variac.

If it had a part of the winding that was shorted, it would get hot
and hum and smoke quite spiritedly.

If it had a part of the winding that was open, it would either
put out no voltage, and then at some position the voltage would
suddenly appear, or it would put out normal voltage, and at some
point the voltage would disappear.

The windings are single layer, and spaced. Leakage is highly
unlikely.

-Chuck Harris

DW wrote:

Thanks, the replies are appreciated.

Chuck, you are right, I should measure from the wiper to neutral to get a more appropriate reading.

I have a parts 577 I am thinking about swapping the variacs out of and see what that does if I can't make any progress. I read somewhere on a forum that it is possible to fix a variac with a burnt winding, just solder a bridge to the next winding, but as Chuck mentioned about the wiper essentially shorting the winding where wiper is, that may not work to well when the solder gets hot from the current and fails.



DW
 

Thanks, the replies are appreciated.

Chuck, you are right, I should measure from the wiper to neutral to get a more appropriate reading.

I have a parts 577 I am thinking about swapping the variacs out of and see what that does if I can't make any progress. I read somewhere on a forum that it is possible to fix a variac with a burnt winding, just solder a bridge to the next winding, but as Chuck mentioned about the wiper essentially shorting the winding where wiper is, that may not work to well when the solder gets hot from the current and fails.

Chuck Harris
 

I am pretty sure that chassis ground is not part of the
variac circuit. So measurements to ground are suspect.

Try measuring to power line neutral.

Variac's are funny beasts.

If you think for a couple of seconds about the wiper on
the variac, you will realize that it *has* to short adjacent
turns. Absolutely no choice. Shorting a turn on a transformer
is serious business, as it shoots the current up as high as
it can go, and will burn out the turn....

So, what to do? General Radio came to a solution, they used
a brush that was high enough bulk resistance that having it
short a few turns wouldn't damage the transformer, or the brush.
The equivalent circuit of the brush is sort of like this:

Turn 1 <----R----+
.................|
Turn 2 <----R----+-----> Output from brush
.................|
Turn 3 <----R----+

If the stop on the variac is set so that the brush is parked
over the turns on the variac, the output voltage will never
get closer than one turn from zero.

The proper placement of the stop is so the brush is parked on
a metallic area that is just to the side of the windings.

If your variac is 11V when the knob is in the zero position,
you need to adjust the physical stop so that the brush is not
parked over the windings.

I can't think of any other way the variac could put out voltage
when it is in the zero position.

Also, I'm not believing the schematic I have for the 577's
collector supply. Mine shows C101/R101 as a snubber network
that connects from the wiper of the variac to ground.. there
is L101, but that is immaterial to my problem. My schematic
shows C101 as having a polarity, which cannot be. It has to
be a 3uf non-polar oil, or film capacitor.

-Chuck Harris

DW wrote:

I tried a few things in hopes to resolve the collector supply which seems to be partially on.

I cleaned the contact wiper points as they seemed slightly dirty on the auto transformer and wiper with fine sand paper then wiped it with some cotton swabs tipped in 92% alcohol to clean the surface.

I turned on the instrument and the results are still the same.

I put one probe on ground and the other on R101, which I believe is in circuit with the wiper of the auto transformer T101, I measured the AC voltage. Turned full on it measures line voltage and turned all the way off about 11 VAC, I believe I definitely found where my undesired leakage power is coming from in the collector supply.

Can these auto transformers go bad?

Bob Koller
 

I don't think the variac is bad, it just isn't going all the way to zero. You can also measure the voltage between C-E on the test fixture with the Polarity set to AC.
Have you removed it from the instrument for inspection? It is possible that the carbon brush has worn to the point where it has too wide a contact area to go fully to zero. In that case I guess the variac has "gone bad" You may be able to carefully reshape the brush to a narrower contact area, should be about 1-1.5mm. If not, replacement is the answer.

DW
 

I tried a few things in hopes to resolve the collector supply which seems to be partially on.

I cleaned the contact wiper points as they seemed slightly dirty on the auto transformer and wiper with fine sand paper then wiped it with some cotton swabs tipped in 92% alcohol to clean the surface.

I turned on the instrument and the results are still the same.

I put one probe on ground and the other on R101, which I believe is in circuit with the wiper of the auto transformer T101, I measured the AC voltage. Turned full on it measures line voltage and turned all the way off about 11 VAC, I believe I definitely found where my undesired leakage power is coming from in the collector supply.

Can these auto transformers go bad?

DW
 

Reading the manual I notice the following for the collector supply

120 Volts is fed to T101 autotransformer (Variable collector)

K125 turns T101 on or off

The wiper of T101 goes to what appears to be a filter L101 R101 and C101, and then it goes to the primary of T102

From T102 it appears this is where I get 6.5 25 100 400 and 1600V from for the collector

For 6.5 to 100V rectifier CR107 is used and above that CR103-106 is used

Since different rectifiers are used at higher volts I decided to test my problem at higher voltages, the problem appears to get much worse

At 1600V 8kOhms 5V Division 0 collector volts the line is 4 divisions across which should be 20V, not good

The collector supply appears somewhat simple if i look at certain sections of it. I feel like I simply have a T101 autotransformer issue. The wiper is making contact to the very edge but yet it acts like i have it turned up partially. Maybe a issue with T101 or maybe K125 or perhaps C101 is bad? I feel this problem is occurring at the beginning of the collector supply stage.