Date   
Re: 577 AC collector supply not reading zero on display

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!


Re: 577 AC collector supply not reading zero on display

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.

Re: 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!



Re: 577 AC collector supply not reading zero on display

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!

Re: 577 AC collector supply not reading zero on display

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.



Re: 577 AC collector supply not reading zero on display

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.

Re: 577 AC collector supply not reading zero on display (FS: Variac brushes)

Brad Thompson
 

Chuck Harris wrote on 12/9/2019 10:00 AM:<snip>
Variac's are funny beasts.
<much good info snipped>
Hello--
Someone in the group may have an older Superior Electric Variac languishing on
the shelf for want of a replacement brush (online prices for these are scary)..

I'm offering FS the following NOS  replacement brushes for certain models of
Superior Electric's Powerstat (tm) variable-voltage transformers:

One pair (2 pcs.) RB-116 (for all types of models 116, 117, 216, 217 and type 116U-40)    Both for $12.00 plus postage

Three pcs.  RB1126/RB1226 (for all types of models 1126 and 1226) All three for $18.00 plus postage

Bonus: not for variable-voltage transformers, but free to first purchaser of one of the brush sets listed above:

One pair (2 pcs.) Delco-Remy F-700 brush set (for 1946-1962 Mercury  or 1955-1960 Lincoln)

Questions welcomed, PayPal honored

73--

Brad  AA1IP

Re: 1502 TDR project

Dave Casey
 

One thing that has be scratching my head is the 25V/9.7V generated from
the
secondary controlling the oscillator of the primary. Maybe I just haven't
puzzled it long enough, but I can't seem to wrap my head around it.
That's a feedback path for the switcher so it can regulate. The three axial
caps on the back of the board are often bad. Being open or leaky could
interfere with the switcher operating properly; I'd check these first. I
have a similarly broken 1502 that's waiting for some new caps so I can
proceed.

Dave Casey

Re: 7854's for sale

redarlington
 

Do you have any of the newer version 7854? (no banana jacks on back)

-Bob N3XKB

On Sun, Dec 8, 2019 at 8:57 AM demianm_1 via Groups.Io <demianm_1=
yahoo.com@groups.io> wrote:

i have 3 and have decided to move 2 on. They take up space I can no longer
afford to support. The 7854's have no plugins and I'm asking $275 ea w/
carts. I also have a 7613 with a 7L12 that i have not used in a long time.
Also $275 with cart.
I'm looking for someone local (SF Bay Area) since shipping especially with
a cart is prohibitive.
Contact me at demianm_1@... to be sure I see the note.



Re: 1502 TDR project

Mark Pilant
 

Good to know all this information about the batter charging circuit. One
thing I do have going for me is the same modified "battery pack" I'm using
to troubleshoot my 1502 works fine in my 1503. (I probably should have
mentioned this at the beginning :-)

Anyway, back to the DC converter.

One thing that has be scratching my head is the 25V/9.7V generated from the
secondary controlling the oscillator of the primary. Maybe I just haven't
puzzled it long enough, but I can't seem to wrap my head around it.

Really seems like I need to break out my scope and see what is going on with
Q6349 and Q6435.

- Mark N1VQW

Re: 1502 TDR project

Tom Gardner
 

On 09/12/19 17:09, Mark Pilant wrote:
That's insufficient.
Good to know.

You realise that PSU trips if there is a transient overvoltage. At that
point the voltmeter reads "excessively good" :)
I did not.  I just checked, and when powered on, the battery meter reads
about 1/16" past/above the upper portion of the green area.  I'm not sure
if this matches what you mean by "excessively good". :-)
Probably :)


I presume you have read the circuit operation section of the manual?
Yes, although I didn't see anything about overvoltage tripping.
Last paragraph of the "Battery Charger" section indicates VR6134 provides protection in case the battery pack is removed and the line cord is plugged into a line voltage source.

And my foulup: q6547/8/9 are the undervoltage protection.

Re: 1502 TDR project

Mark Pilant
 

That's insufficient.
Good to know.

You realise that PSU trips if there is a transient overvoltage. At that
point the voltmeter reads "excessively good" :)
I did not. I just checked, and when powered on, the battery meter reads
about 1/16" past/above the upper portion of the green area. I'm not sure
if this matches what you mean by "excessively good". :-)

I presume you have read the circuit operation section of the manual?
Yes, although I didn't see anything about overvoltage tripping.

- Mark N1VQW

Re: 1502 TDR project

Tom Gardner
 

On 09/12/19 16:38, Mark Pilant wrote:
Hi Tom.

I haven't tried supplying power directly to the banana plugs, but the
parallel cap/resistor battery substitute (ala TekWiki) seems to work
fine, as the battery meter on the front panel indicates the battery is
"good" and I do see 14.4VDC supplying the DC converter.
That's insufficient.

You realise that PSU trips if there is a transient overvoltage. At that point the voltmeter reads "excessively good" :)

I presume you have read the circuit operation section of the manual?


So it would seem the battery portion of the power supply is working and
I need to determine why the rest of the power supply isn't working.

My general sense is the DC converter is an oscillator driving the T6535
primary to then generate all the secondary voltages needed.  I suspect
the oscillator isn't oscillating (well?), but I'm not sure of the best
way to investigate.  (Not sure how many ways there are to shut it down;
lack of a battery is one.)
One way is the overvoltage detection, via q6547/8/9.


I'm a little wary just probing around with my scope (7904 with the usual
vertical / horizontal plugins), because of potential ground issues.
Although I do have a Tek 224 I can use that shouldn't have any ground
issues.
If you power off a bench supply, then clipping the scope shield to the chassis is fine.

The 1502's transformer is fully isolated, so**clipping the scope shield to the chassis is fine.


I just checked, and the power supply for the 1503 is the same as the one
for the 1502.  I may be able to open up my 1503 and use its power supply
to troubleshoot the 1502 power supply.
Yes.

Re: 1502 TDR project

Mark Pilant
 

Hi Tom.

I haven't tried supplying power directly to the banana plugs, but the
parallel cap/resistor battery substitute (ala TekWiki) seems to work
fine, as the battery meter on the front panel indicates the battery is
"good" and I do see 14.4VDC supplying the DC converter.

So it would seem the battery portion of the power supply is working and
I need to determine why the rest of the power supply isn't working.

My general sense is the DC converter is an oscillator driving the T6535
primary to then generate all the secondary voltages needed. I suspect
the oscillator isn't oscillating (well?), but I'm not sure of the best
way to investigate. (Not sure how many ways there are to shut it down;
lack of a battery is one.)

I'm a little wary just probing around with my scope (7904 with the usual
vertical / horizontal plugins), because of potential ground issues.
Although I do have a Tek 224 I can use that shouldn't have any ground
issues.

I just checked, and the power supply for the 1503 is the same as the one
for the 1502. I may be able to open up my 1503 and use its power supply
to troubleshoot the 1502 power supply.

- Mark N1VQW

Re: 1502 TDR project

Tom Gardner
 

On 09/12/19 15:18, Mark Pilant wrote:
I've had a 1502 project for a while, and I'm finally getting some time
to work on it.  I've already done the cap/resistor battery "mod" as
suggested on the TekWiki, and I have 14.4 volts on the "BATT" test point
on the power supply board.

However, after that, the power supply doesn't appear to be working so
well.  All the various test points do have a voltage, but they are way
off.  For instance, the +25V test point measures about 3 volts.

I'm trying to wrap my head around how the "DC VOLTAGE CONVERTER" portion
of the supply is supposed to work, but I'm not having much luck. (Maybe
working with software too long :-)

I did check the voltage at Q6345 (it is the same as the "BATT" voltage,
so it would seem the primary of T6535 is OK.

Looking for some guidance.

- Mark  N1VQW
That PSU is, um, subtle. I've had successes and failures with cap/resistor substitutes, and don't trust the technique. Unless the PSU considers the values are /just right/, transient voltages/currents can trip the PSU.

A reliable technique is to put a couple of 4mm banana sockets on the ends of wires attached to a bench PSU set to 12V. ISTR the transient current can be 600mA, but the operating current is much lower. Attach the banana sockets to the connector in the case, ensuring the polarity is correct. That is easier when the case has been removed.

Once you have such a good "battery", you can can ignore the battery charger part of the PSU and proceed with faultfinding the main part of the equipment.

There are fuses on the board; if they have blown then unsurprisingly you will need to find the reason.

Apart from that, faults would normally be faulty electrolytics. I've had a couple on the 25V line (c6341, c6246) spew acid and corrode PCB tracks. Don't look at the prices of direct replacements, unless you have had a stiff drink and are sitting down :) Do look at the diameter of replacements :)

1502 TDR project

Mark Pilant
 

I've had a 1502 project for a while, and I'm finally getting some time
to work on it. I've already done the cap/resistor battery "mod" as
suggested on the TekWiki, and I have 14.4 volts on the "BATT" test point
on the power supply board.

However, after that, the power supply doesn't appear to be working so
well. All the various test points do have a voltage, but they are way
off. For instance, the +25V test point measures about 3 volts.

I'm trying to wrap my head around how the "DC VOLTAGE CONVERTER" portion
of the supply is supposed to work, but I'm not having much luck. (Maybe
working with software too long :-)

I did check the voltage at Q6345 (it is the same as the "BATT" voltage,
so it would seem the primary of T6535 is OK.

Looking for some guidance.

- Mark N1VQW

Re: 577 AC collector supply not reading zero on display

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?

Re: 577 AC collector supply not reading zero on display

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.

Re: DC-508 repair advice

radioconnection@...
 

Figured it out.... pin is only high when "totalize" is selected. Ordering a few replacements to verify the chip has croaked.

Re: repairing a 184

bill K7WXW
 

Eric - that would be great, thank you. The problem? The delay line and the connections of interest are under the countdown board, the one with 23 individual connections. :\ So it's a project: you have to remove at least the three power wires (A, B, and C) and undo six mounting screws to lift the board and take photos. No worries if that's more work than you want to do!

On my unit, someone modified the delay line, removed the chokes, drilled holes in the countdown PCB and pulled the delay line inputs and outputs through the holes and soldered them on top. Why? No idea. I guess they were trying to fix some other problem but the rework is a mess. Without trying to describe it, what I found underneath the countdown board would partially explain why I was seeing the 0.1us edges as crosstalk on the 0.5us and 1ns output lines.

If I can figure out how the chokes/transformers were constructed, and the delay lines were arranged, I will undo the modification, clean up the mess, and replace the components damaged by the rework.