Modifications to 577 Curve Tracer Main Power Supply


Gary Robert Bosworth
 

The main power supply in the 577 Curve Tracer is one of the worst I have ever seen. Does anyone know of suggested modifications to make the power supply behave better???


Rick
 

I have a 577. I haven't had to repair that part yet. What bad about it?


Dave Voorhis
 

Really?

What’s bad about it?

The power supply in my 577 seems to be powering it just fine, and has apparently been doing so since it was made.

On 29 Feb 2020, at 23:56, Gary Robert Bosworth <grbosworth@...> wrote:

The main power supply in the 577 Curve Tracer is one of the worst I have ever seen. Does anyone know of suggested modifications to make the power supply behave better???



Lyle Bickley
 

Hi Gary,

On Sat, 29 Feb 2020 15:56:03 -0800
"Gary Robert Bosworth" <grbosworth@...> wrote:

The main power supply in the 577 Curve Tracer is one of the worst I have
ever seen. Does anyone know of suggested modifications to make the power
supply behave better???
I also have a 577 - and it has always worked flawlessly. What kind of problems
are you having?

Cheers,
Lyle




--
73 NM6Y
Bickley Consulting West Inc.
https://bickleywest.com

"Black holes are where God is dividing by zero"


Gary Robert Bosworth
 

This power supply does not start up correctly. The output is set at 7
Volts both plus and minus. The variable adjustment controls have no
effect. The schematic shows several components in shaded text. There is no
explanation why these components are faded, nor is there an explanation as
to their function. All of the components on the circuit board are the
correct value and are in perfect working order. Does anyone know if the
circuit design went through any changes over the years?

On Sun, Mar 1, 2020 at 7:47 AM Lyle Bickley <lbickley@...>
wrote:

Hi Gary,

On Sat, 29 Feb 2020 15:56:03 -0800
"Gary Robert Bosworth" <grbosworth@...> wrote:

The main power supply in the 577 Curve Tracer is one of the worst I have
ever seen. Does anyone know of suggested modifications to make the power
supply behave better???
I also have a 577 - and it has always worked flawlessly. What kind of
problems
are you having?

Cheers,
Lyle




--
73 NM6Y
Bickley Consulting West Inc.
https://bickleywest.com

"Black holes are where God is dividing by zero"

--
Gary Robert Bosworth
grbosworth@...
Tel: 310-317-2247


 

On Sun, Mar 1, 2020 at 07:02 PM, Gary Robert Bosworth wrote:


The output is set at 7 Volts both plus and minus.


All of the components on the circuit board
are the correct value and are in perfect working order
Hi Gary,
I assume that you're talking about the main power supply's + and - 30 V supplies. You may not like the circuits but a few things about your unit I consider a given:
- It has worked at some stage
- It doesn't work now

If your statement that all components are ok, only wires and traces are left to consider but I've learned never to stick to any assumed certainty.

The circuit isn't the most complicated and well described in the service manual.
Before (or after) studying that, there are several simple things to consider first:

There's one thing that you should check first: Are + and - 40 V unregulated both present? Normally, they'd even be clearly above that value. If not, check rectifiers and bulk caps and their connections.

Next, disconnect the main supply from its loads. I would disconnect all, not just the + and - 30 V parts. The power supply does not get confused without a load. Any change?

The + supply needs the - supply (and +200 V) to work correctly and it mirrors the - 30 V (sign inverted) via R764. IOW, take care of the minus-supply and your problems may be gone.

You may want to check the voltage across R786. If it's anywhere near 0.7 V, the supply is in current limiting mode and your problem is downstream.

This isn't the strangest and most difficult circuit imaginable, keep a cool head and take your time to work systematically.

If no voltage makes sense, consider checking your meter...

Have fun!

Raymond


 

Hi Gary,
Which parts on which schematics are shown in shaded text?

Dennis Tillman W7pF
TekScopes Moderator


 

On Sun, Mar 1, 2020 at 08:53 PM, Dennis Tillman W7PF wrote:


Which parts on which schematics are shown in shaded text?
My service manual (070-1414-00) shows the main power supply on schematics sheet 11. R773 and C773 are shaded.
I made the assumption that Gary is talking about the + and - 30 V supplies, as they are the only + and - supplies in the power supply and adjustable: He mentions both + and - do not respond to adjustment (and are + and - 7V). Hence my response as I made it.

Raymond


Gary Robert Bosworth
 

Dennis: The parts in faded print are R773 and C773. They are not on the
circuit board.
The circuit design is not well described in the service manual. No normal
voltages and currents are mentioned in the print.
The +40V and -40V unregulated are fine.
ALL of the components measured perfectly good including the rectifier
bridges and filter capacitors.
The power supply behaves the same whether the loading connectors are
attached or not.
All of the other supply voltages are good including +200V, +12V, -12V, and
+5V.
I momentarily disconnected the short-circuit feedback diode CR785 when the
loads were not attached and it had no effect.
The reason why I do not like this design is because of the fact that the
-30V output is used to bias the 6.2V reference diode VR772. This poses a
classic lock-up condition in that if the -30V does not exist, then the
reference diode is never turned on.
I have measured every circuit trace from beginning to end and there are no
micro-cracks to open them up, and there are no solder bridges causing short
circuits.
This is a perplexing problem that I have not seen in 56 years of working in
electronics.

Gary

On Sun, Mar 1, 2020 at 11:53 AM Dennis Tillman W7PF <dennis@...>
wrote:

Hi Gary,
Which parts on which schematics are shown in shaded text?

Dennis Tillman W7pF
TekScopes Moderator



--
Gary Robert Bosworth
grbosworth@...
Tel: 310-317-2247


 

On Sun, Mar 1, 2020 at 11:04 PM, Gary Robert Bosworth wrote:


The reason why I do not like this design is because of the fact that the
-30V output is used to bias the 6.2V reference diode VR772. This poses a
classic lock-up condition in that if the -30V does not exist, then the
reference diode is never turned on.
I think that's incorrect:
Whenever - 40 V is present, Q788 is pulled into conduction by R787 (to gnd), causing the output voltage to move from 0 V towards - 30 V.
The regulating circuit *reduces* that conduction by increasing conduction of Q786.

Raymond


 

...if the -30V does not exist, then the
reference diode is never turned on.
Addition: Strictly speaking, you *are* correct, of course. It's just that the - 30 V *does* exist without the regulation part being functional - or present. In fact, the voltage would probably try to go all the way to - 40 V. The regulation circuitry (mainly VR772, Q772 and Q786) *prevents* that by limiting to - 30 V.

Raymond


 

On Sun, Mar 1, 2020 at 07:02 PM, Gary Robert Bosworth wrote:


This power supply does not start up correctly. The output is set at 7
Volts
- 7 V is just about the voltage at the base of Q772 (and the slider of R775) under normal circumstances...

Raymond


 

On Sun, Mar 1, 2020 at 11:56 PM, Raymond Domp Frank wrote:


- 7 V is just about the voltage at the base of Q772 (and the slider of R775)
under normal circumstances...
... What about R776 or its connection to R775 being open..? I know you checked everything but the mystery remains...

It can't be a shorted R777, because you said that adjusting R775 does nothing. A shorted R775 would give almost - 9 V.

Raymond


Ed Breya
 

Do you have enough access to make live measurements in this section while it's hooked up and powered? If so, you should be able to check some operating voltages and find the problem. Is the -30V zero, or some partial level?

If access during operation is very difficult, a good trick is to pick certain nodes, and tack probing wires onto them, long enough to be brought out into the open. You have to of course be very careful to not short any of these temporary test points.

There is no "lock-up" condition you alluded to - it should start up just fine, unless there's a failure. If VR772 has no current from the -30V output via R772, there should still be plenty (a few mA) to pull it down via R774, into a regulation range where it will produce some output, and then get VR772's operating current to the normal level. This arrangement may be to provide some degree of foldback current limiting, but it's more likely for improvement of line ripple rejection.

The usual suspects in old Tek gear of this vintage, are Ta caps. Only C781 and C772 appear to be candidates for close inspection. First determine what types they are, then act accordingly. If C772 is shorted (or partially shorted), for instance, then VR772's voltage would always be stuck near ground, and the -30V output would only reach a few volts. C781 would of course short the output, if failed short. The next suspect would be Q772 - if it's shorted or breaks down, the output will be cut off completely. And so on down the line.

You may have checked all these parts already, but not necessarily at operating conditions. That's why the best way to diagnose is with actual circuit measurements, directly or via temporary test points. I'd suggest the minimal set would be the bases of Q772 and Q786, which will tell a lot. If temporary TPs are needed, and it's difficult to remove and replace the PS board, then lots of wires will minimize the number of times this is needed. In this case, the BEC of Q772, the BC of Q786, and E of Q788 should be accessed. These should quite quickly and thoroughly show what's going on - it's actually a pretty simple circuit.

One last thing to consider is "mod failure" - if the various parts indicated with value changes or are tacked onto the circuit, look at them closely for proper installation and connections.

Good luck.


 

On Mon, Mar 2, 2020 at 12:04 AM, Raymond Domp Frank wrote:


... What about R776 or its connection to R775 being open..? I know you checked
everything but the mystery remains...
... and it would result in about - 7V (6.2 + 0.7) V at the output.

BTW: Sorry for the concatenation of short posts. I had intended and started to do something completely different but came back to this thread several times because it must be sooo simple that it's not only frustrating Gary.

Raymond


 

On Mon, Mar 2, 2020 at 12:08 AM, Ed Breya wrote:


Is the -30V zero, or some partial level?

Gary mentioned - 7 V.


There is no "lock-up" condition you alluded to - it should start up just fine,
unless there's a failure. If VR772 has no current from the -30V output via
R772, there should still be plenty (a few mA) to pull it down via R774, into a
regulation range where it will produce some output, and then get VR772's
operating current to the normal level. This arrangement may be to provide some
degree of foldback current limiting, but it's more likely for improvement of
line ripple rejection.
I gave my opinion on this a few posts ago. If I'm not mistaken, Q788 is in full conduction by R787 pulling to gnd, *unless throttled* by the regulating circuits.

Raymond


Ed Breya
 

OK, so with partial output to -7V, then it's likely the voltage on VR772 is stuck way too low (failed or dumped by C772), or Q772 is breaking down or leaking. If Q772 went open, it would go open loop overall, and the no-load output would be near the 40V raw supply. There are a number of ways it could have failed and makes -7V output, but I'd suspect the "772" parts at this point.

Yes of course Raymond, the output transistor would be turned on fully if it went open loop. I'm presuming it's still closed loop, with the failure limiting how high it can go. This appears to be the better kind of failure - not enough output rather than way too much, which could do some damage.

Ed


 

Gary,
I believe this is easily explained by looking back at the parts list where it says R773 and C773 were discontinued at Serial Number B049999.

There is a discrepancy between this PDF of the manual I have and a later paper original manual I have. The earlier PDF says it is:
070-1414-00, First Printing JUN 1973, Revised DEC 1981. On Schematic 12 it says REV E, MAR, 1978. In this PDF R773 and C773 are visible. They are a little fainter than the other parts around them but that appears to be a side effect of the scanner that was used to convert the manual into a PDF.

In the original manual I have which is more recent it says:
070-1414-00, First Printing JUN 1973, Revised SEP 1986. On Schematic 12 it says REV JAN 1985. In this manual R773 and C773 are drawn in faintly. That is not an accident. It appears deliberate.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Gary Robert Bosworth
Sent: Sunday, March 01, 2020 2:04 PM
To: TekScopes@groups io <TekScopes@groups.io>
Subject: Re: [TekScopes] Modifications to 577 Curve Tracer Main Power Supply

Dennis: The parts in faded print are R773 and C773. They are not on the circuit board.
The circuit design is not well described in the service manual. No normal voltages and currents are mentioned in the print.
The +40V and -40V unregulated are fine.
ALL of the components measured perfectly good including the rectifier bridges and filter capacitors.
The power supply behaves the same whether the loading connectors are attached or not.
All of the other supply voltages are good including +200V, +12V, -12V, and
+5V.
I momentarily disconnected the short-circuit feedback diode CR785 when the loads were not attached and it had no effect.
The reason why I do not like this design is because of the fact that the -30V output is used to bias the 6.2V reference diode VR772. This poses a classic lock-up condition in that if the -30V does not exist, then the reference diode is never turned on.
I have measured every circuit trace from beginning to end and there are no micro-cracks to open them up, and there are no solder bridges causing short circuits.
This is a perplexing problem that I have not seen in 56 years of working in electronics.

Gary

On Sun, Mar 1, 2020 at 11:53 AM Dennis Tillman W7PF <dennis@...>
wrote:

Hi Gary,
Which parts on which schematics are shown in shaded text?

Dennis Tillman W7pF
TekScopes Moderator



--
Gary Robert Bosworth
grbosworth@...
Tel: 310-317-2247





--
Dennis Tillman W7PF
TekScopes Moderator


 

On Mon, Mar 2, 2020 at 01:08 AM, Ed Breya wrote:


OK, so with partial output to -7V, then it's likely the voltage on VR772 is
stuck way too low (failed or dumped by C772), or Q772 is breaking down or
leaking. If Q772 went open, it would go open loop overall, and the no-load
output would be near the 40V raw supply. There are a number of ways it could
have failed and makes -7V output, but I'd suspect the "772" parts at this
point.
Ed,
What about my suspicion of R776 or its connection to R775 being open? It would result in about - 7V (6.2 + 0.7) V at the output, which Gary has observed.

Raymond


Ed Breya
 

Raymond wrote: "What about my suspicion of R776 or its connection to R775 being open? It would result in about - 7V (6.2 + 0.7) V at the output, which Gary has observed."

Yup, that would do it too - the R or the end of the pot could be open. But, considering that all of the parts were checked, it's more likely that any resistance and connection measurements would still be valid, while those made on caps and transistors may not reflect how they behave in actual operation.

Ed