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577 AC collector supply not reading zero on display

DW
 

With the collector supply set to AC, the collector voltage set to 0 volts, and particularly with a lower volts / division selection (1V, 0.5V) I notice a horizontal line about 1 division wide appearing across the the display as if the collector supply isn't quite going to zero. Is there an adjustment to compensate for this? The line becomes a dot in either + or - collector polarity. The line becomes longer in length when selecting higher collector voltages in AC.

Thanks

Bob Koller
 

Sometimes the carbon brush on the varia does not go all the way to zero volts. This can be corrected by carefully bending the wiper stop tab slightly. Be careful not to allow the carbon brush to drop off the winding completely.

 

Hi DW,
After fixing this problem (others have suggested how already) you will discover that the trace dims considerably as soon as you get very close to 0V. This is not a bug. This is an important feature added to the 577 (it's not on any earlier model curve tracers) to prevent phosphor burn from occurring at the lower left (or upper right) corners of the CRT.
Dennis Tillman W7PF

-----Original Message-----
From: DW
Sent: Friday, November 29, 2019 7:16 PM
Subject: [TekScopes] 577 AC collector supply not reading zero on display

With the collector supply set to AC, the collector voltage set to 0 volts, and particularly with a lower volts / division selection (1V, 0.5V) I notice a horizontal line about 1 division wide appearing across the the display as if the collector supply isn't quite going to zero. Is there an adjustment to compensate for this? The line becomes a dot in either + or - collector polarity. The line becomes longer in length when selecting higher collector voltages in AC.

Thanks




--
Dennis Tillman W7PF
TekScopes Moderator

DW
 

Thanks for the replies,

I will take a look at the collector variac transformer wiper as suggested, and thanks for the heads up about the display dimming

NigelP
 

If I'm not mistaken this is the looping issue due to Miller effect in the DUT (I'm assuming you actually have a DUT under measurement)? There is an anti-looping adjustment but it doesn't always cancel out in my experience; left-hand front-panel knob Looping Compensation. Might be wrong about that though.

Alternatively check out the HV supply system; it's a bit of a beast to understand what with all the reversible floating supplies and current and voltage sense networks! I spent many hours trying to fathom out a problem around the machine only to discover I had inadvertently replaced a 2-way connector the wrong way round!

Regards

Nigel

DW
 

I found the stop tab on the variac control for the collector. I carefully bent it slightly to get the wiper to come more on the edge without falling off.

I believe I notice improvement from this, now the collector in AC isn't quite as long as it was but it is still there. At 50mV horizontal volts at 6.5 collector volts with the variable collector all the way at 0 the line is 2 divisions long and has a strange flickering pattern.

DW
 

I should add there is no device under test, the test fixture is under no load. If I short the emitter to the collector at 0 collector voltage the line goes to a dot on the display.

 

Hi DW,
It is unfortunate that you still have the two division long trace at the collector 6.5V range with the Horizontal set to 50mV/Div. I checked my 577 and it goes all the way to zero under the same conditions.
You should go to the Check and Adjustment Procedure in the Service Manual (section 5) and perform Performance Check / Adjustment Procedures 5, 6, 7, 8 on pages 5-6 through 5-8. Those adjustment / calibration steps will calibrate every aspect of the horizontal section of the 577. One of those steps will result in your adjusting the collector voltage to eliminate the 2 division long collector line you are seeing.

I do see a faint flickering pattern which shows up as a trace about 1 horizontal division long which happens 1 to 2 times per second. I never noticed it before because it is faint and infrequent. I don't think it is important enough to be concerned with. I'm not concerned with it on my 577.

Dennis Tillman W7PF

-----Original Message-----
From: DW
Sent: Sunday, December 01, 2019 11:28 AM
Subject: Re: [TekScopes] 577 AC collector supply not reading zero on display

I found the stop tab on the variac control for the collector. I carefully bent it slightly to get the wiper to come more on the edge without falling off.

I believe I notice improvement from this, now the collector in AC isn't quite as long as it was but it is still there. At 50mV horizontal volts at 6.5 collector volts with the variable collector all the way at 0 the line is 2 divisions long and has a strange flickering pattern.




--
Dennis Tillman W7PF
TekScopes Moderator

DW
 

I took a look at the service manual on page 5-6

On step 6, check horizontal gain.

I connected a voltmeter to E Sense and Base and got -95mV at 0.0 on the offset multi
When I turned the offset multi so I got -595mV it seems the manual indicates the spot is supposed to move somewhat but I didn't notice any movement of the spot, I believe this is a good indication?

I should note I did not see no spot but a flickering line 2 divisions long, I thought I could tell if there is a deviation by looking at the ends of the line and noting the new relative location, I noted no change of position.

 

Hi DW,
Unfortunately I don't know if that is good or bad.
<SNIP>
I connected a voltmeter to E Sense and Base and got -95mV at 0.0 on the offset multi When I turned the offset multi so I got -595mV it seems the manual indicates the spot is supposed to move somewhat but I didn't notice any movement of the spot, I believe this is a good indication?

QUESTION:
What happens if you turn the collector voltage to zero and then switch the Collector Supply Polarity from NPN to +DC (or from PNP to -DC)? Does the beam stay at 2 divisions? It will no longer be a line since you are not sweeping the collector voltage. Instead it will be a dot. The question is does it stay at the 2 division point or does it return to zero when you do this. The answer may provide a clue to what is causing this.

Dennis Tillman W7PF

-----Original Message-----
From: DW
Sent: Sunday, December 01, 2019 3:24 PM
Subject: Re: [TekScopes] 577 AC collector supply not reading zero on display

I took a look at the service manual on page 5-6
On step 6, check horizontal gain.
I connected a voltmeter to E Sense and Base and got -95mV at 0.0 on the offset multi When I turned the offset multi so I got -595mV it seems the manual indicates the spot is supposed to move somewhat but I didn't notice any movement of the spot, I believe this is a good indication?

I should note I did not see no spot but a flickering line 2 divisions long, I thought I could tell if there is a deviation by looking at the ends of the line and noting the new relative location, I noted no change of position.




--
Dennis Tillman W7PF
TekScopes Moderator

DW
 

Thanks for the reply

With the collector in AC on 6.5V with 2k series resistor, horizontal devision set to 50mV...
A flickering line appears 2 divisions in length
I find if I increase the D1 brightness (Turn the D1 brightness fully clockwise) the flickering pattern goes away into a solid green light which is interesting to note
I plugged a voltmeter into the Emitter and collector terminals and observe 38mV of AC voltage
I short the emitter and collector terminals and find the 2 division line shrinks to a dot and the voltmeter reads about 0mV AC
Obviously some small amount of AC power is getting through and measured on the display and voltmeter

When I put the collector in DC- PNP the line shifts to the upper right corner of the display as a dot with the voltmeter measuring about 9mV AC, barely visible on the display
When the LEFT RIGHT selector switch is switched to neutral position I observe 500mV AC, strange
When I put the collector in DC+ NPN the line shifts to the lower left corner of the display as a dot with the voltmeter measuring about 9mV AC, again barely visible on the display

I find if I select any position other than AC on the collector the display shots a dot.

In the following test I put the collector to 25 volts 2kohm in DC- and DC+ at 0 collector voltage 50mV Division and measure the emitter to collector voltage of any slight DC voltage
(DC+ -0.4mV) (DC- 0.3mV)
Interestingly when performing this test I found if I move the series resistor control above 2kohm the dot starts to become the flickering line I observe before in AC but now it is in DC+ or DC-, If I set the series resistance lower about 2k or below the flickering line becomes the dot.

In AC 25V 50mV Division, changing the series resistance has no effect other than just displaying a line that is 7 divisions long which is pretty bad!

 

Hi DW,
If the 577 is showing 2 horizontal divisions at 50mV/Div that implies there should be 100mV p-p between emitter and collector banana jacks. If that voltage is a really there then we can measure a current using a 100 ohm resistor.

Put the resistor between the emitter and collector in the left set of banana jacks and flip the left / right switch to the left. Set the series resistor to its smallest value of 0.12 ohms. You should get 1ma of current from 100mV through a 100 ohm resistor. If you set the vertical knob to 0.2ma/ Div you should get a DIAGONAL LINE that goes from 0V and 0ma (the lower left corner) to 2 divisions (100mV) horizontal and 5 divisions (1mA) vertical. If you switch the collector supply from NPN to PNP the diagonal line will move to the upper right corner.

Dennis Tillman W7PF

-----Original Message-----
From: DW
Sent: Sunday, December 01, 2019 6:17 PM
Subject: Re: [TekScopes] 577 AC collector supply not reading zero on display

<SNIP>
With the collector in AC on 6.5V with 2k series resistor, horizontal devision set to 50mV...
A flickering line appears 2 divisions in length I find if I increase the D1 brightness (Turn the D1 brightness fully clockwise) the flickering pattern goes away into a solid green light which is interesting to note I plugged a voltmeter into the Emitter and collector terminals and observe 38mV of AC voltage I short the emitter and collector terminals and find the 2 division line shrinks to a dot and the voltmeter reads about 0mV AC Obviously some small amount of AC power is getting through and measured on the display and voltmeter




--
Dennis Tillman W7PF
TekScopes Moderator

DW
 

Max peak volts 6.5
Series resistors 0.12
Horzi V Division 50mV
Vert I Division 0.2mA
Collector voltage 0V
100 ohm Resistor across Collector Emitter

With collector set to AC and switch set to neutral a solid horizontal line appears across 2 divisions.

With the selector switch set to left a 50 deg vertical line appears approximately 3 divisions vertically

In any position but AC for the collector polarity there is a displayed dot.

It appears there is a small residual power flowing through the collector supply when set to AC, about 0.5mA of current

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.

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?

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.

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?

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 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
 

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!