Topics

Tektronix 532 LV rail shorted

randolphbeebe@...
 

Good Morning All,

I have a very clean Tek 532 that has a problem I cannot solve. There is something wrong in the Low Voltage Power Supply specifically the -150V section. The symptoms are as follows:

1.) C640 had blown it's internals out

2.) R100 was shorted and burned

3.) Cannot plug the unit in to check voltages. With a dim bulb tester there is clearly a short somewhere.

I replaced C640 and R100. I checked all associated tubes and caps. No improvement. Clearly I am missing something or the transformer my be shot. Problem is that I do not have the diagnostic skills to go any further. I would like to get a second opinion or three before I sign the death certificate. Any help would be appreciated.

I uploaded some photos and schematics in a folder "Tek 532" I was unable to figure out how to put them in the message.

Thanks,

Randy

John Williams
 

It has been a while since I had to fix a 503-series power supply. But when C640 blows it shorts and blows the 10 ohm fuse resistor. If there are solid state rectifiers one or more of them could also be blown. If there is a tube rectifier you can pull the tube and see if it will power up. There is also an electrolytic on the top by the hv supply which filters the -150. It could also be somewhere else that is shorting the -150. There is a lot of interconnection.

randolphbeebe@...
 

Thanks for the reply John... I will keep looking. I have been working on this on and off for a long time and cannot seem to trace the problem. Pulling the -150V rectifier tubes is a good idea. Worth a try

george gonzalez
 

Those kinds of shorts are hard to find.

If you have an AC current clamp you could try measuring the current in the secondary wires when your light-blob is in the primary. That will help narrow down the shorted circuit.

Without an AC clamp, but with a big variac or big light bulbs, turn the current up to a couple of amps or with a 250 watt bulb, leave it there for five minutes, then unplug the power supply, and very carefully sniff around or feel for hot diodes.

Or as a last resort start unsoldering secondary wires, one at a time, until the short goes away and the primary lamp gets real dim.

Tiresome, but the only way I know of.

The good news is if a capacitor blew, it’s probably not a shorted transformer.

Rajesh VS
 

Have you checked all the electrolytic caps to confirm none are leaking ?
also I guess you meant R110, which is the limiting resistor to the
plug-in. If that is burned, either the plug-in is shorted for -150V or C110
is bad.
After powering off , you can measure resistance to ground from each section
of the power supply output to confirm none are short. Each rail should be
~1k-5k Ohms.

-150V is the reference rail for all LV outputs, so if -150 is dead/bad,
none of the other will come up properly

If you want to isolate the transformer, as John said, you can remove all
5V4G tubes (note their positions to later put them back where they came
from ) and try.
You can measure AC output from the transformer to check for correct
voltages. Remember all heater and +100V rail is still active.
If you want to disconnect +100V section , remove connection to Terminal 14
of the transformer.

I would suggest then start with just -150V and see if its coming up
properly, and then move up one rail at a time, to +100(reconnect
Terminal#14) , +225, +350 by reinstalling the 5V4G tubes.

Also, check the wattage of the "dim" bulb, as these scopes suck a lot of
power.
HTH
rajesh

On Mon, Jan 6, 2020 at 4:11 PM John Williams <books4you@...> wrote:

It has been a while since I had to fix a 503-series power supply. But when
C640 blows it shorts and blows the 10 ohm fuse resistor. If there are solid
state rectifiers one or more of them could also be blown. If there is a
tube rectifier you can pull the tube and see if it will power up. There is
also an electrolytic on the top by the hv supply which filters the -150. It
could also be somewhere else that is shorting the -150. There is a lot of
interconnection.



--
/Rajesh

M M
 

On 01/07/2020 07:35 AM, george gonzalez wrote:>
> Those kinds of shorts are hard to find.

(rest of that posting trimmed for brevity)

Back when I worked for NASA-JPL in the 1970s we had another way
to find shorted power supplies... Our group of 26 techs was
responsible for the 24x7x366 maintenance of over 5,000 pieces of
equipment from over 300 manufacturers. One piece was a custom
designed and one-off build that had over 950 TTL chips and a
bypass cap for every two chips... and multiple 30 amp supplies
at 5 volts... and Murphy's Law says that the manufacturer had
a bad batch of bypass caps that would randomly short.

Try and locate a micro-ohmmeter.

With that piece of gear you could "walk" down the 5 volt buss
and watch the meter reading get lower and lower until you were
less than 3 inches from the bad cap. Swap that one cap and
the equipment was back in operation (until the next one
shorted a month or two later).

Back in the 70s we borrowed a micro-ohmmeter from the NASA tech
equipment pool...
I do not remember who made it or how much it cost... but 50 years
later there is probably a less expensive and more readily
available one...

Mike Morris WA6ILQ

Paul Amaranth
 

Actually, a milliohm meter is often sufficient. I've used an
HP3456a to track down shorts.

My currently favorite way to track down a shorted components is
to use an HP547a current probe in conjunction with a pulser
(HP 546a, although you can use a pulse generator as well). You
use the probe to follow the path of the current and when it
goes through a component you've found the problem. Generally
takes me longer to set things up than to find the bad part.

People must have figured that out; those probes used to be
reasonably priced.

If you like the ohm meter way, you can find audio output
milliohm meters that make tracing shorts pretty easy.

Paul

On Tue, Jan 07, 2020 at 06:40:45PM +0000, M M wrote:
On 01/07/2020 07:35 AM, george gonzalez wrote:>
> Those kinds of shorts are hard to find.

(rest of that posting trimmed for brevity)

Back when I worked for NASA-JPL in the 1970s we had another way
to find shorted power supplies... Our group of 26 techs was
responsible for the 24x7x366 maintenance of over 5,000 pieces of
equipment from over 300 manufacturers. One piece was a custom
designed and one-off build that had over 950 TTL chips and a
bypass cap for every two chips... and multiple 30 amp supplies
at 5 volts... and Murphy's Law says that the manufacturer had
a bad batch of bypass caps that would randomly short.

Try and locate a micro-ohmmeter.

With that piece of gear you could "walk" down the 5 volt buss
and watch the meter reading get lower and lower until you were
less than 3 inches from the bad cap. Swap that one cap and
the equipment was back in operation (until the next one
shorted a month or two later).

Back in the 70s we borrowed a micro-ohmmeter from the NASA tech
equipment pool...
I do not remember who made it or how much it cost... but 50 years
later there is probably a less expensive and more readily
available one...

Mike Morris WA6ILQ






!DSPAM:5e14d0c3307671733312476!
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix & Windows

Harvey White
 

The problem I see with a micro-ohmmeter, and indeed, any low resistance measurement, is that it needs to be done in Kelvin mode.  If you can do that, then that's ok.

I prefer a constant current power supply, set to say, 100 ma or so, and limited to the voltage expected on the bus.  The contact resistance of the meter probes is no longer a factor.  What you're looking for is the voltage drop across each part.  The lowest one is the suspect.  Anything with the same reading that is downstream of the part (you do need to know the topology) that has the same reading isn't the suspect.

You can also use one of HP's current probes if you can pulse the power supply under voltage/current conditions.

Harvey

On 1/7/2020 1:40 PM, M M wrote:
On 01/07/2020 07:35 AM, george gonzalez wrote:>
> Those kinds of shorts are hard to find.

(rest of that posting trimmed for brevity)

Back when I worked for NASA-JPL in the 1970s we had another way
to find shorted power supplies... Our group of 26 techs was
responsible for the 24x7x366 maintenance of over 5,000 pieces of
equipment from over 300 manufacturers. One piece was a custom
designed and one-off build that had over 950 TTL chips and a
bypass cap for every two chips... and multiple 30 amp supplies
at 5 volts... and Murphy's Law says that the manufacturer had
a bad batch of bypass caps that would randomly short.

Try and locate a micro-ohmmeter.

With that piece of gear you could "walk" down the 5 volt buss
and watch the meter reading get lower and lower until you were
less than 3 inches from the bad cap. Swap that one cap and
the equipment was back in operation (until the next one
shorted a month or two later).

Back in the 70s we borrowed a micro-ohmmeter from the NASA tech
equipment pool...
I do not remember who made it or how much it cost... but 50 years
later there is probably a less expensive and more readily
available one...

Mike Morris WA6ILQ


randolphbeebe@...
 

Thanks for the tips! In my original message I typed the resistor that is getting hot as R100, it is actually R110 as was correctly pointed out. I do not understand why it does get hot...the first thing I did was replace R110 and C110 and check the plug-in. R110 still gets very hot. This will be a good learning experience, currently looking for an AC current clamp. I did check all the electrolytics and replaced as necessary but will definitely re-check them. The only bad cap if I recall was C640. The bad thing here is I had this scope for over a year know and am trying to remember what I did and did not check.

One thing I did try earlier was to disconnect tap 16 from the transformer and there was no improvement...next I will try tap 17 maybe.

Rajesh VS
 

You can remove R110, and the plug-in
Then try powering up the scope and check if the -150V rail is stable.

On Wed, Jan 8, 2020 at 8:14 AM <randolphbeebe@...> wrote:

Thanks for the tips! In my original message I typed the resistor that is
getting hot as R100, it is actually R110 as was correctly pointed out. I
do not understand why it does get hot...the first thing I did was replace
R110 and C110 and check the plug-in. R110 still gets very hot. This will
be a good learning experience, currently looking for an AC current clamp.
I did check all the electrolytics and replaced as necessary but will
definitely re-check them. The only bad cap if I recall was C640. The bad
thing here is I had this scope for over a year know and am trying to
remember what I did and did not check.

One thing I did try earlier was to disconnect tap 16 from the transformer
and there was no improvement...next I will try tap 17 maybe.




--
/Rajesh

randolphbeebe@...
 

Tonight I removed all the rectifier tubes including the 6080 series regulator tube. Then I lifted a lead on R110. When the scope was powered up there was still an excessive current draw and C640 (the new, replaced one) started to cook and get very hot including a little bit of a smoke show. This was through the dim bulb tester and a variac with a maximum voltage of 39V AC. I also checked my work and no capacitors were installed incorrectly.

Then I lifted the lead from Tap 17 and for the first time the fan started to turn but when I reconnected R110 that still heated up right away when I re-installed the rectifier tubes.

One interested observation; The voltage from tap 16 is 70-90V AC but no DC volts... that does not seem correct?

Rajesh VS
 

You can keep the R110 disconnected until the problem is resolved.
was V606 and V605 in place when the cap cooked ? If it was in place, may be
remove them, power up and see if the cap is cooking, check the voltage
across the cap, both AC and DC volts.
while the rectifier tubes is out, check filament voltage (AC) between pin
2-8 of V606. I hope the tubes are tested, and good, may be to confirm you
can try a pair from another rail.
reinstall V606 and V605 and check the DC voltage across the cap to see
if its matching the polarity of the capacitor ( +ve of the cap to ground
and -ve to -150V rail)

On Wed, Jan 8, 2020 at 6:05 PM <randolphbeebe@...> wrote:

Tonight I removed all the rectifier tubes including the 6080 series
regulator tube. Then I lifted a lead on R110. When the scope was powered
up there was still an excessive current draw and C640 (the new, replaced
one) started to cook and get very hot including a little bit of a smoke
show. This was through the dim bulb tester and a variac with a maximum
voltage of 39V AC. I also checked my work and no capacitors were installed
incorrectly.

Then I lifted the lead from Tap 17 and for the first time the fan started
to turn but when I reconnected R110 that still heated up right away when I
re-installed the rectifier tubes.

One interested observation; The voltage from tap 16 is 70-90V AC but no DC
volts... that does not seem correct?



--
/Rajesh

randolphbeebe@...
 

I have been away from my computer for a few days and finally have been able to try Rajesh's diagnostic suggestions. All the tubes were previously checked but I re-checked all the rectifier tubes and they were all good with no shorts. With all the tubes in place C640 and R110 were getting hot when the scope is powered up with a current limiter.

-I removed V605 and V606 from the sockets and powered it up. C640 no longer gets hot but R110 still does. R110 gets hot so quickly that if I left the power switch on for more than 90 seconds I am certain it would burn in half . This is without a plug-in installed

-Voltage across C640 measures 0V DC and around 80V AC which is about the same whether or not V605-V606 are installed. (Measured when C110 was disconnected to remove ground and short on R110, C110 is a new cap and still tests good)

-Filament voltage measures 2.3V AC across Pins 2 and 8 of 606

Once again This is through a Variac with 90V selected. When the scope is turned on this value reduces to 50V.

It was suggested that perhaps C110 was installed backwards, the schematic indicates the (-) lead to ground. I tested it when C110 was installed in both directions and the results are the same either way.

The pertinent schematic photos are under the Tek 532 folder

Thanks Again Guys

John Williams
 

Hi Randolph. I have been watching you thrash around like a fish on a hook for a while, but you seem to have knowledge of troubleshooting. So I will try to help a bit. You will already know some of this so please don’t be offended if I cover that stuff.

As you are well aware, what causes a component to heat up is current flowing through it. If the resistor heats, there is an ac or dc current from somewhere flowing through it so there must be voltage across it. Same thing for the cap. But the cap can’t pass dc normally, so the current must be ac. This is confirmed by the reversing of it. If there was dc across it, putting it in backwards would blow it all to hell.

Now you can confirm this by disconnecting one lead to the cap and see what happens. If the cap is at fault, things should settle down a bit.

So if you remove the 150 supply rectifiers, then any current heating those components has to be coming from somewhere else. Barring an actual short somewhere, it would have to be coming from one of the other power supplies. You can find out by pulling all the rectifiers and then replacing them one at a time. Once you confirm this, you may be able to confirm if the problem is in the 150 or one of the other ones.

The 500 series power supplies are all basically the same. You can study how it works in the circuit description section of the manual. They are all interconnected so a fault in one can easily show up in another or cause damage in another. Go back to the basics, that is the transformer and work from there.

But you knew all that, right? It is all about being methodical. Anything can be broken, and anything can be fixed. Good luck don’t give up.

John

Rajesh VS
 

R110 is feeding -150V to the Plug-in.
There should not be any current through it without a plug-in inserted.
(Assuming C110 is good).
C110's +ve should go to ground and -ve to R110 as its a -ve power supply.
It is an error in the schematic. Ensure it is in the correct polarity else
it can load R110, Same for C640.

I would also suggest that you visually inspect the plug-in connector and
ceramic strips for any shorted leads.

Regards,
Rajesh

On Mon, Jan 13, 2020 at 3:24 PM John Williams <books4you@...> wrote:

Hi Randolph. I have been watching you thrash around like a fish on a hook
for a while, but you seem to have knowledge of troubleshooting. So I will
try to help a bit. You will already know some of this so please don’t be
offended if I cover that stuff.

As you are well aware, what causes a component to heat up is current
flowing through it. If the resistor heats, there is an ac or dc current
from somewhere flowing through it so there must be voltage across it. Same
thing for the cap. But the cap can’t pass dc normally, so the current must
be ac. This is confirmed by the reversing of it. If there was dc across it,
putting it in backwards would blow it all to hell.

Now you can confirm this by disconnecting one lead to the cap and see what
happens. If the cap is at fault, things should settle down a bit.

So if you remove the 150 supply rectifiers, then any current heating those
components has to be coming from somewhere else. Barring an actual short
somewhere, it would have to be coming from one of the other power supplies.
You can find out by pulling all the rectifiers and then replacing them one
at a time. Once you confirm this, you may be able to confirm if the problem
is in the 150 or one of the other ones.

The 500 series power supplies are all basically the same. You can study
how it works in the circuit description section of the manual. They are all
interconnected so a fault in one can easily show up in another or cause
damage in another. Go back to the basics, that is the transformer and work
from there.

But you knew all that, right? It is all about being methodical. Anything
can be broken, and anything can be fixed. Good luck don’t give up.

John



--
/Rajesh

randolphbeebe@...
 

I am certainly thrashing around on the hook with this one! You got me thinking however about isolating the problem power source and when I test the resistance between ground and the the (-) side of C640 it shows a dead short. This is also true with Pins 16 and 17 disconnected from the transformer. I also checked the plug-in connector pin 19 as Rajesh suggested and found 100 ohms which is what I would suspect to see through R110 and some type of short on the other side. The connector is in good condition. All the ceramic strips look good too.

When I pull V605 and V606 C640 does not cook.

I did correct C110 and connected the (+) to ground as was suggested. The cap is new but I re-checked that and it checks good.

I am thinking the problem is directly associated with the -150V supply but now the scope is is worse off than it was earlier.. just a dead short. I am pretty gentle with stuff and check my work so the only thing I can think of is something is shorted or damaged under the sockets of V605 and 606. These are mounted directly on top of the power transformer and maybe the act of removing and replacing rectifier tubes damaged something. Or possibly repeatedly stressing the new C640 shorted it out.

Anyway I will pop the top off the transformer and check the socket wiring but for now I need a break from this thing. I shouldn't work when I am frustrated...

Cheers,

Randy

John Williams
 

Good work Randy. You are making progress. Troubleshooting is as much about eliminating suspects as it is about finding the real cause. I have seen more scopes than I care to remember where a former owner has made a terrible mess trying to haphazardly find the problem. Kinda like some auto shops, if you get my meaning. Take a break that usually helps.

John Williams
 

This probably is a stupid question, but have you checked that the cans of the electrolytics are properly insulated from ground with the fibre wafers underneath and the mounting screws not grounding? I thought you had.

Rajesh VS
 

Good call John, also Randy, if you see short across C640, it is potentially
on the output side of the power supply.
If there is any short around V605 and V606, they will cook themselves,
nothing will reach C640. But as always, check everything.
Also, as part of troubleshooting, you can temporarily remove|disconnect
C640 and try power up. It wont hurt the scope(it will not behave well
though), but you can confirm if the short goes away.


Totally understand the frustration and we all been through that many times.
Since you are taking a break, there is an excellent book from Tek Concept
series in Tekwiki, explaining these power supplies in detail
its @ http://w140.com/tekwiki/images/a/a1/062-0888-01.pdf
These things are engineered and built so well, in most cases you can bring
them back to life.
Good luck.

On Tue, Jan 14, 2020 at 10:01 PM John Williams <books4you@...> wrote:

This probably is a stupid question, but have you checked that the cans of
the electrolytics are properly insulated from ground with the fibre wafers
underneath and the mounting screws not grounding? I thought you had.



--
/Rajesh

randolphbeebe@...
 

Thanks Gentlemen, I will put it back on my bench and follow your suggestions. Also, I downloaded the "Circuit Concepts" Tekwiki publication and will read that as well as the 532 circuit description in the manual as John suggested. I am in the process of putting together an electrical work bench as a hobby and can navigate simpler circuits but the fact is I do not quite understand complex interrelationships of the 532 power supply.

The thing that caused me to suspect the rectifier tube sockets is that now there is no voltage reaching C640 at all. So I will check them as well.

Best,

Randy