Date   
Re: Isolation???

 

Mike:

As a suggestion:  Use the 7a26 with two x10 probes.  Set the 7a26 with
"DISPLAY MODE" set to "ADD" and with "CH 2 POLARITY" set to "INVERT". 
Connect the two probe ground leads (or optionally only one probe's
ground lead) to the Motorola power supply chassis (I assume this can be
considered as a viable power ground point) and the tip of the channel A
probe to the +41V test point, and channel B probe to the positive DC
output terminal.  This should allow the 7a26 to perform differential
measurements between the two points.  You may have to play with the AC
vs DC input selections on the 7a26 input channels, and/or adjust the
gain control(s) to "non-calibrated" positions for this measurement (to
get the gains of the two channels to accurately match each other, but
the downside of this is that you lose some measure of vertical
calibration accuracy).

I think this will work.  Although I've never done it myself; hopefully
someone more knowledgable than me will chime in if I am way off base.

Mike Dinolfo N4MWP

On 5/23/19 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.

I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
Mike Harmon, WB0LDJ
mharmon at att dot net


Re: Isolation???

 

Hi Mike,
This is where it pays to have a 7A13 Differential Comparator and a P6055 differential probe set if you are going to see some high frequency signals (DC to 110MHz) when you do this.
Alternatively if the signals are under 1MHz the 7A22 differential Amplifier with a P6055 differential probe set.

Both the 7A13 and the 7A22 are must have plugins for any 7000 series scope.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Mike Harmon
Sent: Thursday, May 23, 2019 7:07 PM
Subject: [TekScopes] Isolation???

I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.

I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
Mike Harmon, WB0LDJ
mharmon at att dot net




--
Dennis Tillman W7PF
TekScopes Moderator

Re: Isolation???

Dale H. Cook
 

On 5/23/2019 10:07 PM, Mike Harmon wrote:

Does anyone have a better (safer) way to measure the differential waveform?
The rule of thumb when isolation is required is "Don't isolate the instrument - isolate the DUT." Plug the Motorola supply into an isolation transformer - a true isolation transformer like my RCA WP-26A Isotap which has a 400VA secondary. You want a real isolation transformer that isolates everything from hot, neutral and ground. What you don't want is something labeled an "isolation transformer" intended for EMI filtering that does not provide full isolation.
--
Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcity/radios/

Re: Isolation???

Harvey White
 

On 5/23/2019 10:07 PM, Mike Harmon wrote:
I'm working on an old Motorola power supply. One of the adjustment steps says to connect a scope between the positive DC output terminal and a +41V test point inside the power supply, and look at the waveform. I have a 7904A with 7A26 vertical plugins. The problem is that whichever point I connect the common probe lead to is going to be taken to chassis (earth) ground! I'm not comfortable with lifting the scope chassis above ground by disconnecting the ground wire at the AC input.
What's immediately jumping out at me is that you have a 7904A with a 7A26.  You can put a scope probe on channel 1 to the positive DC output, and the scope probe from channel 2 on the 41v test point.  Set channel 2 to invert, then the display mode on the plugin to "ADD".

The 41 volts should be within the common mode range of the 7A26, but a quick look at the manual will tell you for what ranges.

Very standard way of measuring a differential voltage.

A 7A22 or a 7A13 has a built in differential voltage capability, so you wouldn't necessarily need the DC voltage, but with the 7A26, you should be just fine.

Harvey


I think I have an old A6902 Isolator unit somewhere in the dungeon that someone stole the probes from. As I recall, the only thing in the side bage is the pigtails coming out of the isolator to connect the probes to. Would it make sense to just connect a standard scope probe to the pigtail(s) and feed the isolator to the scope's vertical input?. I guess I'd have to install a BNC female connector on the end of the pigtails. Getting the correct probes for the isolator would cost more than the isolator is worth. I've had it for years and this is the first time I've thought of trying to use it!

Is this a crazy idea? Does anyone have a better (safer) way to measure the differential waveform?

Thanks for the help!!
See above....


Mike Harmon, WB0LDJ
mharmon at att dot net



Re: 465B Attenuator servicing

Redguuz
 

Re: 465B Attenuator servicing

NigelP
 

Thanks for your feedback.

Whilst I admit the specific tin I've just used does only have "Ambersil Electronic Safety Solvent" written on it I have nevertheless used it extensively for years. It has a long list of its suitability regarding electronic circuits written on the can.

However to specifically answer your comment I can assure you that pressurized cans of IPA DO exist and in fact I purchased one just this weekend and it is sitting right in front of me as I write! It's called Pro-Power PPC103 IPA Solvent.

Regards

Nigel

Re: Craigslist- Northern Virginia Tek 545B for sale

Colin Herbert
 

The description says:
"Tektronix Oscilloscope from the 1960s or early 1970s complete with rolling stand and leads. When I plugged it in the lights came on and the screen works. As far as it working this is all I can tell."
So it looks like cart and leads are included.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Mike Dinolfo
Sent: 23 May 2019 23:35
To: TekScopes@groups.io
Subject: [TekScopes] Craigslist- Northern Virginia Tek 545B for sale

Craigslist Northern Virginia (specifically- Lorton VA) advertising a Tektronix 545B with a plugin unit for $100. Possibly in working condition; not clear (to me) if the price includes the scope cart (shown in the Craigslist ad) or the probe(s) (also shown in the ad), but someone here might be interested. I would consider it myself except that I have no room left in my basement, and my back would suffer dearly from moving such a beast. I have no affiliation with the seller: <https://washingtondc.craigslist.org/nva/ele/d/oscilloscope/6893855462.html>

Mike Dinolfo N4MWP

Re: Tektronix 564 WW resistor for -100V supply

Trevor
 

The choice of wire-wound was probably for temperature stability of the -100 volt power supply. Since R617  forms a voltage divider with pot R616 and R618, both also specified as wire-wound, the circuit designer wanted resistors with tracking temperature coefficients. The 1% spec is superfluous. You could use any resistor type, but you would compromise temperature stability if the three resistances don't track. If you use a carbon-composition resistor as a replacement, say 82K, consider changing R618 to carbon-composition as well. Then see how the -100 volt supply behaves as the scope heats up.

An RN65 78.7K metal film resistor is probably a better choice, however.

-Trevor

On Thursday, May 23, 2019, 10:22:08 PM EDT, Brenda via Groups.Io <brendda75=yahoo.com@groups.io> wrote:

HI everyone! I thought I would take my 564 out and see what I possibly do to get the storage section of the CRT working again.  I fired up the scope and only to find that absolutely nothing worked. I measured all the low voltage supplies and they are all severely off.  The -100V was reading -77, and with that, decided to check everything in the -100V circuit.  All of the resistors were still in tolerance except for R617 which is an 80K WW 1W resistor and it was completely open.  The brand of the resistor is Reon. I am wondering if any 80K resistor would work in it's place as long as the wattage is greater than 1W and is within the 1% tolerance spec? Or does it have to be a WW resistor?

Brenda

Re: 465B Attenuator servicing

Colin Herbert
 

OK, so the existence of spray-can IPA is new to me. Perhaps I should see if
I can get it in the UK, but it is cheap enough anyway and I apply it with
cotton-buds, tissues or slivers of paper, whichever is the best way for the
job. (It may be that the propellant is incompatible with the attenuator
substrate, of course).

Perhaps I can point out that the 465B Service Manual (mine covers B060000
and up) states that "carbon-based solvents will damage the boards used for
the attenuators. Apply the isopropyl alcohol with a camel hair brush. Do not
use cotton swabs.." This is at the bottom of page 6-3. I think that Tek
meant "carbon chloride-based solvents", as the isopropyl alcohol molecule
contains three carbon atoms. My 475A Service Manual further states that the
"attenuators use a plastic material (polyphenylene oxide) that is easily
damaged by the use of carbon-based solvents". It goes on to say that
"acetone, benzene, toluene, xylene, petroleum ether, white kerosene, carbon
tetrachloride, methylene chloride, trichloroethane, trichlorotrifluoroethane
(freon-113, -tf, -ta, -tmc) should not be used. As I said before, there are
a number of posts on the subject of cleaning these cam-actuated contacts on
this Forum. The gold-plated leaf contacts are delicate and easily damaged,
too. If you search, you should find the accepted technique - don't forget
that these cam-actuated contacts are common among most, if not all, of the
400-series portable scopes. I'm only trying to help to point you in the
right direction.

You could perhaps look at:

https://groups.io/g/TekScopes/message/144408?p=,,,20,0,0,0::Created,,cam+swi
tch+cleaning,20,2,0,11350196

(I hope the URL comes out ok).

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of NigelP
Sent: 24 May 2019 10:02
To: TekScopes@groups.io
Subject: Re: [TekScopes] 465B Attenuator servicing

Thanks for your feedback.

Whilst I admit the specific tin I've just used does only have "Ambersil
Electronic Safety Solvent" written on it I have nevertheless used it
extensively for years. It has a long list of its suitability regarding
electronic circuits written on the can.

However to specifically answer your comment I can assure you that
pressurized cans of IPA DO exist and in fact I purchased one just this
weekend and it is sitting right in front of me as I write! It's called
Pro-Power PPC103 IPA Solvent.

Regards

Nigel

Re: 465B Attenuator servicing

Chuck Harris
 

Easy and cheap enough to get a pump sprayer bottle
from the store, and fill it with IPA. That is what
I do when I need to flood a circuit/switch/keyboard
with IPA.

-Chuck Harris

Colin Herbert via Groups.Io wrote:

OK, so the existence of spray-can IPA is new to me. Perhaps I should see if
I can get it in the UK, but it is cheap enough anyway and I apply it with
cotton-buds, tissues or slivers of paper, whichever is the best way for the
job. (It may be that the propellant is incompatible with the attenuator
substrate, of course).

Perhaps I can point out that the 465B Service Manual (mine covers B060000
and up) states that "carbon-based solvents will damage the boards used for
the attenuators. Apply the isopropyl alcohol with a camel hair brush. Do not
use cotton swabs.." This is at the bottom of page 6-3. I think that Tek
meant "carbon chloride-based solvents", as the isopropyl alcohol molecule
contains three carbon atoms. My 475A Service Manual further states that the
"attenuators use a plastic material (polyphenylene oxide) that is easily
damaged by the use of carbon-based solvents". It goes on to say that
"acetone, benzene, toluene, xylene, petroleum ether, white kerosene, carbon
tetrachloride, methylene chloride, trichloroethane, trichlorotrifluoroethane
(freon-113, -tf, -ta, -tmc) should not be used. As I said before, there are
a number of posts on the subject of cleaning these cam-actuated contacts on
this Forum. The gold-plated leaf contacts are delicate and easily damaged,
too. If you search, you should find the accepted technique - don't forget
that these cam-actuated contacts are common among most, if not all, of the
400-series portable scopes. I'm only trying to help to point you in the
right direction.

You could perhaps look at:

https://groups.io/g/TekScopes/message/144408?p=,,,20,0,0,0::Created,,cam+swi
tch+cleaning,20,2,0,11350196

(I hope the URL comes out ok).

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of NigelP
Sent: 24 May 2019 10:02
To: TekScopes@groups.io
Subject: Re: [TekScopes] 465B Attenuator servicing

Thanks for your feedback.

Whilst I admit the specific tin I've just used does only have "Ambersil
Electronic Safety Solvent" written on it I have nevertheless used it
extensively for years. It has a long list of its suitability regarding
electronic circuits written on the can.

However to specifically answer your comment I can assure you that
pressurized cans of IPA DO exist and in fact I purchased one just this
weekend and it is sitting right in front of me as I write! It's called
Pro-Power PPC103 IPA Solvent.

Regards

Nigel










Re: Tektronix 564 WW resistor for -100V supply

Dave Hills
 

I would use a low Tempco metal film, say 15 to 50ppm. The only reason I can think of using WW in that location is that carbon comp and WW were the primary choices when that scope was designed, and carbon would have caused the supply output to drift all over the place as the instrument warmed up. The 80k value may be difficult to find, but a modern standard value of 80.6k or 78.7k will work fine as would a series combination of 49.9k and 30.1k. You might consider replacing R618, 10k with the same type so as to match the Tempco and minimize the warm up drift.

Dave

On Thu, May 23, 2019 at 07:22 PM, Brenda wrote:


HI everyone! I thought I would take my 564 out and see what I possibly do to
get the storage section of the CRT working again. I fired up the scope and
only to find that absolutely nothing worked. I measured all the low voltage
supplies and they are all severely off. The -100V was reading -77, and with
that, decided to check everything in the -100V circuit. All of the resistors
were still in tolerance except for R617 which is an 80K WW 1W resistor and it
was completely open. The brand of the resistor is Reon. I am wondering if any
80K resistor would work in it's place as long as the wattage is greater than
1W and is within the 1% tolerance spec? Or does it have to be a WW resistor?

Brenda

Re: 7904 excessive shadow/flare

unclebanjoman
 

O.K.! Thanks to everyone for the comprehensive answers, very instructive to me!

Undoubtedly the driving circuits of the 7904 and 7804 CRTs are completely different and this could partly explain the different behavior regarding the halo / flare effect.
Over time I will get used to the remarkable brilliance of my new 7904 (and his associated halo /flares) which, moreover, is really in excellent condition and after careful calibration it matches perfectly the manual's specifications.

My 7904 has serial number B269718; reading the manufacture date of a pair of TI SNxxxx ICs, it should be from 1979.

If another 7904 to be repaired happens to me, I will be curious to observe the halo / flare effect, whether it is identical or not. I will let you know if the case.

Thanks again,
Max

Re: 7B92A time-base plug-in odd behaviour

Colin Herbert
 

Thanks for the info, guys. I have now located where these switches go, but I am unclear as to what their functions are. Perhaps I don't really need to know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the behaviour before warm-up. Once the scope has warmed-up, the time-base is still showing some instability which can be induced/stopped by wiggling the time-base "Time/Div" knob. I am now thinking that it might be caused by poor contact on one or more of the cam-switches. This looks like it will involve taking the cam-switch assembly out and doing the "paper-soaked-in-IPA" procedure. As there are about 50 of these, has anyone any idea which might be the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single wire to P911 on the interface board. P911 is to be found on schematic <4>, 'Delaying Sweep', near the top right >corner. It is marked as going to R334 on schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic', schematic <7> is marked with 'pull for alt, push for delay' and is surely the >second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel' that activates the microswitch didn't seem to be an entirely tight fit on the shaft but settled down happily after a few >iterations. I hadn't noticed any odd behaviour when it is operating in the mainframe.
Regards,
Roger

Re: 7B92A time-base plug-in odd behaviour

Albert Otten
 

Hi Colin,

Since most of the problem disappears after warming up I guess it's more like a semiconductor issue.
Do you have the model with "Alt Off" at the ccw position of the TRACE SEP knob, S/N B06-up? There are several ICs involved in processing the Alt Off position, like U635<5> and U856<7>.

Albert

On Fri, May 24, 2019 at 03:24 PM, Colin Herbert wrote:


Thanks for the info, guys. I have now located where these switches go, but I
am unclear as to what their functions are. Perhaps I don't really need to
know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the
behaviour before warm-up. Once the scope has warmed-up, the time-base is still
showing some instability which can be induced/stopped by wiggling the
time-base "Time/Div" knob. I am now thinking that it might be caused by poor
contact on one or more of the cam-switches. This looks like it will involve
taking the cam-switch assembly out and doing the "paper-soaked-in-IPA"
procedure. As there are about 50 of these, has anyone any idea which might be
the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger
Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single
wire to P911 on the interface board. P911 is to be found on schematic <4>,
'Delaying Sweep', near the top right >corner. It is marked as going to R334 on
schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic',
schematic <7> is marked with 'pull for alt, push for delay' and is surely the
second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel'
that activates the microswitch didn't seem to be an entirely tight fit on the
shaft but settled down happily after a few >iterations. I hadn't noticed any
odd behaviour when it is operating in the mainframe.

Regards,
Roger



Re: Latest firmware for TDS210 and TDS2CM?

Jared Cabot
 

Hmm, good info. I think I probably have a late firmware then, it had the earthing recall fix already done too, so it was post recall as well.

I guess I'll leave this as it is unless something turns up by chance, no point poking the bear without all the stuff needed seeing as it isn't a simple thing to upgrade without the right equipment.

497P owners-- bad RADIAL lead through-hole tantalum caps!

radioconnection@...
 

During trouble shooting, I've found several bad 50 uF at 25 VDC radial lead (not SMD!) on the first few boards I pulled for inspection.
All of those caps showed severe leakage and many had open leads due to corrosion. If you own a 497P it might be worth inspecting
the first few plug in boards (LO driver, Preselector driver, etc.) to see if your version has these caps (photo posted in library.)
Serial number of unit with bad caps is B020199 version 9.7--late production run. I

Re: 497P owners-- bad RADIAL lead through-hole tantalum caps!

Chuck Harris
 

That type of capacitor is a wet slug tantalum capacitor, and
it uses sulfuric acid as its electrolyte. Over time, the
positive lead gets etched away, and the acid leaks out. Lots
are starting to fail these days... more will in the future.

Wet slug tantalum capacitors are used because they have very
high voltage ratings for their size... much higher than dry
slug tantalum capacitors. They also can be very small in
size for their voltage and capacitance. You will often find
them in sub microfarad values, at relatively high voltages,
so don't be fooled.

You can recognize this type of capacitor, as the metal case is
silver plated, with usually a mossy silver color, and the positive
terminal will have a weld, or a ball on it right next to the case
insulator. The case insulator is usually teflon. The weld/ball
joint is there to separate the internal tantalum terminal from
the more easily soldered tinned copper, or copper clad steel, wire.

There is another type of capacitor that looks similar to the
wet tantalum, but is actually a hermetically sealed dry tantalum
slug capacitor. This type has a steel colored case, a glass
insulator that is soldered into the tinned steel case, and
may or may not have a ball on the positive wire. The clue is
the glass insulator, and the color of the case.

Hermetically sealed dry slug tantalum capacitors are space rated,
and will last about forever, if you don't reverse their polarity,
or exceed their rated voltage rating. I don't believe I have
ever found one to be bad in 40 years of messing with this stuff.

One you should consider replacing (wet slug), the other you should
keep (hermetic dry slug).

-Chuck Harris

radioconnection@... wrote:

During trouble shooting, I've found several bad 50 uF at 25 VDC radial lead (not SMD!) on the first few boards I pulled for inspection.
All of those caps showed severe leakage and many had open leads due to corrosion. If you own a 497P it might be worth inspecting
the first few plug in boards (LO driver, Preselector driver, etc.) to see if your version has these caps (photo posted in library.)
Serial number of unit with bad caps is B020199 version 9.7--late production run. I



Re: 7B92A time-base plug-in odd behaviour

Colin Herbert
 

Not totally the case, Albert. As I said, the instability can be present after warm-up and can be stopped or induced by "wiggling" the Time/Div knob. Specifically, if the delaying or delayed time-base is showing (i.e. the Time/Div knob is "IN") and the Time/Div knob is pushed in a little more firmly, the time-base stops and a vertical line or dot (depending on the Y input) at the left-hand side is shown.

Yes, It is the model with the "Alt Off" position on the Trace Sep knob; the S/N is B096566.

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Albert Otten
Sent: 24 May 2019 15:31
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour

Hi Colin,

Since most of the problem disappears after warming up I guess it's more like a semiconductor issue.
Do you have the model with "Alt Off" at the ccw position of the TRACE SEP knob, S/N B06-up? There are several ICs involved in processing the Alt Off position, like U635<5> and U856<7>.

Albert

On Fri, May 24, 2019 at 03:24 PM, Colin Herbert wrote:


Thanks for the info, guys. I have now located where these switches go, but I
am unclear as to what their functions are. Perhaps I don't really need to
know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the
behaviour before warm-up. Once the scope has warmed-up, the time-base is still
showing some instability which can be induced/stopped by wiggling the
time-base "Time/Div" knob. I am now thinking that it might be caused by poor
contact on one or more of the cam-switches. This looks like it will involve
taking the cam-switch assembly out and doing the "paper-soaked-in-IPA"
procedure. As there are about 50 of these, has anyone any idea which might be
the dodgy one(s)?

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Roger
Evans via Groups.Io
Sent: 22 May 2019 16:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single
wire to P911 on the interface board. P911 is to be found on schematic <4>,
'Delaying Sweep', near the top right >corner. It is marked as going to R334 on
schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic',
schematic <7> is marked with 'pull for alt, push for delay' and is surely the
second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel'
that activates the microswitch didn't seem to be an entirely tight fit on the
shaft but settled down happily after a few >iterations. I hadn't noticed any
odd behaviour when it is operating in the mainframe.

Regards,
Roger



Re: Isolation???

dnmeeks
 

Using the two-probe method that Mike suggested is the usual way of making a
differential measurement (when expensive diff probes are not available, or
when the voltages are too high for them).

One thing I wanted to add, is you can simply connect the probes' grounds
together WITHOUT connecting them to chassis ground or anything else. That is
also a common technique.

Good luck -

Dan

Re: 497P owners-- bad RADIAL lead through-hole tantalum caps!

Miguel Work
 

https://nepp.nasa.gov/files/29192/NEPP-TR-2018-Teverovsky-T22-Capacitors-TN52048.pdf

-----Mensaje original-----
De: TekScopes@groups.io [mailto:TekScopes@groups.io] En nombre de Chuck Harris
Enviado el: viernes, 24 de mayo de 2019 17:45
Para: TekScopes@groups.io
Asunto: Re: [TekScopes] 497P owners-- bad RADIAL lead through-hole tantalum caps!

That type of capacitor is a wet slug tantalum capacitor, and it uses sulfuric acid as its electrolyte. Over time, the positive lead gets etched away, and the acid leaks out. Lots are starting to fail these days... more will in the future.

Wet slug tantalum capacitors are used because they have very high voltage ratings for their size... much higher than dry slug tantalum capacitors. They also can be very small in size for their voltage and capacitance. You will often find them in sub microfarad values, at relatively high voltages, so don't be fooled.

You can recognize this type of capacitor, as the metal case is silver plated, with usually a mossy silver color, and the positive terminal will have a weld, or a ball on it right next to the case insulator. The case insulator is usually teflon. The weld/ball joint is there to separate the internal tantalum terminal from the more easily soldered tinned copper, or copper clad steel, wire.

There is another type of capacitor that looks similar to the wet tantalum, but is actually a hermetically sealed dry tantalum slug capacitor. This type has a steel colored case, a glass insulator that is soldered into the tinned steel case, and may or may not have a ball on the positive wire. The clue is the glass insulator, and the color of the case.

Hermetically sealed dry slug tantalum capacitors are space rated, and will last about forever, if you don't reverse their polarity, or exceed their rated voltage rating. I don't believe I have ever found one to be bad in 40 years of messing with this stuff.

One you should consider replacing (wet slug), the other you should keep (hermetic dry slug).

-Chuck Harris

radioconnection@... wrote:

During trouble shooting, I've found several bad 50 uF at 25 VDC radial lead (not SMD!) on the first few boards I pulled for inspection.
All of those caps showed severe leakage and many had open leads due to
corrosion. If you own a 497P it might be worth inspecting the first
few plug in boards (LO driver, Preselector driver, etc.) to see if
your version has these caps (photo posted in library.) Serial number
of unit with bad caps is B020199 version 9.7--late production run. I