Date   
475 Z-Axis Amp Woes

agwnorden@...
 

Dear TekScopes

Could someone a bit more competent than me help with analysing the problem I am having with the Z-Axis Amp on my beloved 475 which has served me incredibly well for over 10 years.

The initial symptom was no trace. All power voltages and ripple were fine.

Using the XY setting to debug, I found the following. A spot will only appear in XY mode when Beam Finder (BF) is activated. BF, as I understand it, overrides the Z-Axis amp and sets the Amp O/P to about 25 Volts (from 6V) which then goes into the DC Restorer. Voltages at TP1364 confirm this works OK. (please see CRT circuit and Z Axis Amp circuits attached; apologies for quality but the best I have). This is also confirmed by simply putting -8V on the junction of R1342 and R1346; the spot reappears. X and Y position controls work fine with BF on.

The operation of the Z-Amp is described as follows in the manual:-

Z-Axis Amplifier
The Z-Axis Amplifier circuit controls the CRT intensity level from several inputs. The effect of these input signals is to either increase or decrease the trace intensity, or to completely blank portions of the display. The input transistor Q1338 is a current-driven, low input impedance amplifier. It provides termination for the input signals as well as isolation between the input signals and the following stages. The current signals from the various control sources are connected to the-emitter of Q1338 and the algebraic sum of the signals determines the collector conduction level.

Q1344, Q1352, Q1354, Q1358, and Q1362 compose a feedback amplifier stage. R 1369 is the feedback element, C 1352 provides high-frequency compensation. Q1344 is an emitter follower that provides drive to the output complementary amplifier made up of Q1352, Q1354, and Q1358. Q1358 is a device with higher frequency characteristics than Q1354 and is used to improve the overall frequency capabilities of the Z-Axis Amplifier. On the fast positive going output signal peaks, Q1358 depletes the charge on C1358. Then, on the negative-going signal peaks, Q1362 is pulsed on to renew the charge on C1358. CR 1367 provides protection to the Z-Axis Amplifier circuitry in the event of short duration arcing in the CRT High-Voltage Power Supplies.

My problem is, I think, that I do not really understand the circuit! When I rotate Beam Intensity I see no change in the voltage on the emitter of Q1338 (even with all other inputs (Q1332 etc.) disconnected. The voltages on Q1338 are Ve -2.498, Vb -1.729 and Vc 0.467 so Vbe is ~0.75 forward biaseed. Perhaps this always pulls Ve down when the Intensity Control is changed? Voltage measurements perhaps of little use in a summing amp like this?

If CR1341 is lifted as per the troubleshooting instructions below I do get the specified change in Vc of Q1338 which swings from +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise

Troubleshooting the Z-AXIS Circuit (CRT Circuit). Set TIME/DIV to 1 ms (lock knobs) and set TRIG MODE to SINGL SWP. Disconnect and lift one end of CR1343. Check the voltage at TP1364 for approximately +6 volts (OK). Press the BEAM FINDER pushbutton and check the voltage at TP1364 for approximately +25 volts (OK). If these voltages cannot be obtained, check the voltage across VR1362 for approximately +6.2 volts. Remove Q1362 from its socket and check the voltage at the base of Q1352 for approximately +1.3 volts. Check the base of Q1344 for approximately +0.6 volt.

If the above-mentioned voltages can be obtained, disconnect and lift one end of CR1341. Check the voltage swing at the collector of Q1338 for approximately +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise. (OK). If these voltages cannot be obtained, check the voltage at the base of Q1338 for –1.75 volts. Remove Q1332 from its socket.
Disconnect J1333 and J1334 (coaxial-type end-lead connectors) from their sockets. Repeat the voltage swing check at the collector of Q1338 as performed previously. Connect J1333, J1334, R1343 and CR1341 to the circuit board. Install Q1362 and Q1332 in their sockets.

Replacing all parts and confirming initial voltages around Q1338 I see no response to the Intensity Control at Ve of Q1344, the emitter-follower driving the Z Amp. That is surely wrong?

This places the fault around Q1338/1344 but I cannot find a problem. Junctions of the two test OK. The only obvious fault about the board is some discolouration of CR1360 but it still rectifies.

I apologise for the density of this email but any help much appreciated. I have no more hair to tear out.

Anthony M0WWV

2465B vertical shift problem

ppppenguin
 

When I move channels 1 or 2 down the screen the trace is compressed at the bottom. As if the gain near the bottom the screen is about half that at the top. The effect is identical on both channels. Channels 3 and 4 are OK. Channel 2 output is OK. This looks horribly like U400 is in trouble and I've heard these hybrids are unobtanium.

I'd like to look at the differential outputs of U100 and U200 (and U300 for that matter, simply for comparison) where they feed into U400 in case there's a silly amount of common mode offset which might be causing the problem. But I can't see anywhere on the topside that I can probe. Getting at the underside looks tricky.

I've checked the power going to U100/U200. The +5V, +6.2V, -15V, -8V and -5V rails all seem to be within 2% of nominal.

I've looked at this article which isn't 100% relevant but illustrates an interesting fault and showing that U400 isn't always to blame: http://worldphaco.com/uploads/TEKTRONIX_2465B_OSCILLOSCOPE_MAIN_BOARD_INTER-TRACK_LEAKAGE..pdf

Your help, wisdom and advice would be very welcome. Even if it's only to say that U400 has definitely failed and the getting a replacement is well nigh impossible.

2247A PSU Troubleshooting

Nicholas Keller
 

Hello, first post here and first real scope. A tech friend of mine gave me this in broken condition and said I could keep it if I can fix it. So here goes....

It doesn’t boot. Nothing happens when I turn it on.

I have disassembled the unit and visually inspected the PSU board as he said it had been repaired once before and was likely the same fault. He can’t find the service record. I couldn’t tell that anything had been replaced, and couldn’t see any obviously bad parts (swollen, leaking caps, for example).

I reassembled and powered up to test the power headers on the underside of the unit. They are all basically dead, however I did measure some mV on one of the high voltage pins.

I have a PDF of the schematic. I saw mentioned in a previous post about a record of some common issues with this PSU, but I couldn’t find the old post detailing these. Can anyone give me some suggestions before I attempt to trace signals? Is it safe to remove the PSU board and apply power to it even though some connections appear to be via other boards?

Thanks so much,
Nick

TEK 475 Q1478 2n5859

piodelreal@...
 

hello to all, i can´t find the part 2n5859 here in Mexico, this part number is critical in the -15v regulator performance or permits substitution ? any possible subtitutes?

any help will be really appreciated.

best regards.

Pio

Re: Clearing error log in THS720A

 

On Sun, Oct 21, 2018 at 10:29 PM, zenith5106 wrote:


If you connect a PC running a terminal program to the serial port of the THS
you can try sending first PASSWORD PITBULL and then ERRLOG CLEAR.
Thanks a lot, Håkan.
Will try that during the coming week and share result.

Raymond

Re: Clearing error log in THS720A

 

On Sun, Oct 21, 2018 at 01:18 AM, Raymond Domp Frank wrote:


I'm wondering if the error log can be cleared independently
If you connect a PC running a terminal program to the serial port of the THS
you can try sending first PASSWORD PITBULL and then ERRLOG CLEAR.
It doesn't clear any RS232 errors but it resets the numbers of power up and
most likely power on errors (mine has no errors so I can't say for sure).
/Håkan

Re: 7834 High Voltage Board

ef804s tubes
 

Next thing to try is disconnecting the input wire to the x4 voltage multiplier. Then connect the two
coax feeds. If the main supply then comes up OK, the fault is with the multiplier. If it is still in
tick mode it is likely the transformer.

Craig. The old crumbly one that has been on the list for 16 years, to avoid confusion with any other
Craigs

I disconnected the multiplier and the other high voltage secondaries. Still the power supply shuts down.

Fred

Re: Output use of PG 508 Pulses

John Ferguson
 

On 10/21/18 1:59 PM, Kurt Rosenfeld wrote:
Connecting the PG508 to the scope and the servo in parallel should be fine. I would connect it to the scope first to get the PWM voltage right, since the PG508 is capable of voltages outside of what most servos are expecting. After you have the PG508's output voltage set, then add the servo. I wouldn't expect the servo to significantly load the 50-ohm output of the PG508. Also beware of driving the servo with a weak power supply. It can make for flaky behavior. (How do I know this?)
U2?

Re: Output use of PG 508 Pulses

Kurt Rosenfeld
 

Connecting the PG508 to the scope and the servo in parallel should be fine. I would connect it to the scope first to get the PWM voltage right, since the PG508 is capable of voltages outside of what most servos are expecting. After you have the PG508's output voltage set, then add the servo. I wouldn't expect the servo to significantly load the 50-ohm output of the PG508. Also beware of driving the servo with a weak power supply. It can make for flaky behavior. (How do I know this?)

Output use of PG 508 Pulses

John Ferguson
 

I bought a PG 508 to use to configure some model airplane servos whose actions are determined by Pulse Width Modulated signals.

I have one question about the CAL pots on some of the controls on the PG 508 panel. If there is any counsel in the manual on how these work, I couldn't find it.  It looks as though turning the knobs fully counterclockwise is the way to get the pulse-width (duration) shown on the panel-mine seems to work this way.  Is this correct?

I don't have a number for the power consumption of the signal input of the servos but suspect it isn't much.

So the second question:

Am I risking damage to either my 2445b or the PG 508 if I connect the output of the PG 508 to the servo and to channel one of the scope?

If this does look risky, I suppose answer is to make a transistor switch to make the the pulse signals going to the servo keeping the PG 508 and Scope on the control side of the transistor.

Is there something dumb in what I have in mind?  If so, what would you recommend?

john

Re: 7834 High Voltage Board

Mark Wendt
 

On 10/21/18 03:34, Craig Sawyers wrote:
Next thing to try is disconnecting the input wire to the x4 voltage multiplier. Then connect the two
coax feeds. If the main supply then comes up OK, the fault is with the multiplier. If it is still in
tick mode it is likely the transformer.

Craig. The old crumbly one that has been on the list for 16 years, to avoid confusion with any other
Craigs
Chuckle.  There's a bunch of us "old crumbly one's" here.  I believe the correct pithy term is curmudgeon.  ;-)


Mark

Re: 7834 High Voltage Board

Craig Sawyers <c.sawyers@...>
 

Next thing to try is disconnecting the input wire to the x4 voltage multiplier. Then connect the two
coax feeds. If the main supply then comes up OK, the fault is with the multiplier. If it is still in
tick mode it is likely the transformer.

Craig. The old crumbly one that has been on the list for 16 years, to avoid confusion with any other
Craigs

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of ef804s tubes
Sent: 21 October 2018 08:19
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7834 High Voltage Board


On 10/16/18 10:55, ef804s tubes wrote:

On Mon, Oct 15, 2018 at 09:09 AM, Craig Sawyers wrote:

What you seem to be saying is that the main power supply works fine when the
coax inputs to T2010 are
not connected, but when you connect them the main supply goes into tick mode?

Is that correct?

Craig

Hi Craig,
that's correct.

Fred

My apologies for not quoting the question

Hi Craig,
that's correct.

Fred

So, what's the verdict?

Fred

Re: 7834 High Voltage Board

ef804s tubes
 

On 10/16/18 10:55, ef804s tubes wrote:

On Mon, Oct 15, 2018 at 09:09 AM, Craig Sawyers wrote:

What you seem to be saying is that the main power supply works fine when the
coax inputs to T2010 are
not connected, but when you connect them the main supply goes into tick mode?

Is that correct?

Craig

Hi Craig,
that's correct.

Fred

My apologies for not quoting the question

Hi Craig,
that's correct.

Fred

So, what's the verdict?

Fred

Re: Random noise on LV power rails

Harvey White
 

On Sat, 20 Oct 2018 19:00:20 -0700, you wrote:

On Sat, Oct 20, 2018 at 05:56 PM, Harvey White wrote:


On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:
<SNIP>

Ok this is what I have figured out so far. I replaced all the filter caps on
both power supplies. Replaced the bridge rectifiers on the main power supply.
F4009 was blown have no idea how or why. Still has horizontal jitter. Now here
is the kicker. I live in Arizona its always hot so my central air conditioner
is almost always on. The last few days it has been cool enough to have it off
for long cycles. Long story short the jitter happens mostly when the air
conditioning kicks on I measured the voltage drop when the air kicks on its
only about a volt.

Sounds like something is not regulating. What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope. Definitely want to see what
the noise is on the power supplies. It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise
coupling).

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load. It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation. That lets lots of noise get through.

What does the line do when that AC is on? How much noise and what is
the voltage?


Harvey

I have many other scopes that are not affected in the same way. When the air
kicks on I can even hear the fan pitch change. I'm now at a loss in what to
do. Could this have anything to do with the EMI line filter?


No variac :(
I have not looked at the mains with a scope (because I'm scared to do it)
Reasonable enough. As was suggested, a filament transformer or other
transformer designed for AC line use would work well. Even one of
those little AC supply wall warts would be reasonable. Not the best,
but reasonable.


The low/high switch his set to high and the voltage on the line before the air conditioner kicks on is 120-122V and drops to about 119V when its on.
Fairly reasonable....

I plugged the scope into a APC 1500 UPS and that SEEMS to almost totally correct the issue. The issue is so random one time the air will kick on and the scope will display no issues the next time it will. There is no way I have found to reliably induce the symptoms. I think I need a variac.
It could help, but the absolute line voltage may not be the problem.
Noise may be.

You know that UPS supplies can have filtering in them?

You may want to just see if you can find a filtered outlet strip,
borrow it, and see what it does for you.

I have spent 10 hours a day for about a week trying to figure this out.
not surprising, you want a good hard repeatable failure every time.

You don't get them.

Harvey




Re: Random noise on LV power rails

EricJ
 

Oh yes, you do. Every electronics hobbyist should have a variable transformer. They are extremely handy for all sorts of things.
--Eric
Sent from my Samsung Galaxy smartphone.

-------- Original message --------From: "lop pol via Groups.Io" <the_infinite_penguin=yahoo.com@groups.io> Date: 10/20/18 9:00 PM (GMT-06:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Random noise on LV power rails
On Sat, Oct 20, 2018 at 05:56 PM, Harvey White wrote:


On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:
<SNIP>

Ok this is what I have figured out so far. I replaced all the filter caps on
both power supplies. Replaced the bridge rectifiers on the main power supply.
F4009 was blown have no idea how or why. Still has horizontal jitter. Now here
is the kicker. I live in Arizona its always hot so my central air conditioner
is almost always on. The last few days it has been cool enough to have it off
for long cycles. Long story short the jitter happens mostly when the air
conditioning kicks on I measured the voltage drop when the air kicks on its
only about a volt.

Sounds like something is not regulating.  What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope.  Definitely want to see what
the noise is on the power supplies.  It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise
coupling).

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load.  It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation.  That lets lots of noise get through.

What does the line do when that AC is on?  How much noise and what is
the voltage?


Harvey

I have many other scopes that are not affected in the same way. When the air
kicks on I can even hear the fan pitch change. I'm now at a loss in what to
do. Could this have anything to do with the EMI line filter?


No variac :(
I have not looked at the mains with a scope (because I'm scared to do it)
The low/high switch his set to high and the voltage on the line before the air conditioner kicks on is 120-122V and drops to about 119V when its on. I plugged the scope into a  APC 1500 UPS and that SEEMS to almost totally correct the issue. The issue is so random one time the air will kick on and the scope will display no issues the next time it will. There is no way I have found to reliably induce the symptoms. I think I need a variac. I have spent 10 hours a day for about a week trying to figure this out.

A (safer) way to look at the mains.....

Dale Chayes
 

Noise on the “mains” are often an issue on modern “all electric” ships where I have spent way too much time and with the advent of so many poorly designed switching power supplies and drive systems they can appear anywhere.

The THD from those switchers is a continuing to grow issue. It can cause all kinds of obscure behaviors.

When I have to do this, I used to start with a 10:1 transformer - to get down to 12 (or 22, or 44 or 60) volts AC in front of my scope probe, even though I usually carry a high voltage probe for troubleshooting sonar system power supplies.

Depending on the transformer it will/may have some impact, on the signal but it’s a decent place to start.

I eventually graduated to a Fluke 43b

As always, be thoughtful and careful when poking around any high voltage stuff.

-Dale

On Oct 20, 2018, at 18:00 , lop pol via Groups.Io <the_infinite_penguin=yahoo.com@groups.io> wrote:

On Sat, Oct 20, 2018 at 05:56 PM, Harvey White wrote:


On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:
<SNIP>

Ok this is what I have figured out so far. I replaced all the filter caps on
both power supplies. Replaced the bridge rectifiers on the main power supply.
F4009 was blown have no idea how or why. Still has horizontal jitter. Now here
is the kicker. I live in Arizona its always hot so my central air conditioner
is almost always on. The last few days it has been cool enough to have it off
for long cycles. Long story short the jitter happens mostly when the air
conditioning kicks on I measured the voltage drop when the air kicks on its
only about a volt.

Sounds like something is not regulating. What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope. Definitely want to see what
the noise is on the power supplies. It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise
coupling).

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load. It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation. That lets lots of noise get through.

What does the line do when that AC is on? How much noise and what is
the voltage?


Harvey

I have many other scopes that are not affected in the same way. When the air
kicks on I can even hear the fan pitch change. I'm now at a loss in what to
do. Could this have anything to do with the EMI line filter?


No variac :(
I have not looked at the mains with a scope (because I'm scared to do it)
The low/high switch his set to high and the voltage on the line before the air conditioner kicks on is 120-122V and drops to about 119V when its on. I plugged the scope into a APC 1500 UPS and that SEEMS to almost totally correct the issue. The issue is so random one time the air will kick on and the scope will display no issues the next time it will. There is no way I have found to reliably induce the symptoms. I think I need a variac. I have spent 10 hours a day for about a week trying to figure this out.



Re: Random noise on LV power rails

Brendan
 

On Sat, Oct 20, 2018 at 05:56 PM, Harvey White wrote:


On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:
<SNIP>

Ok this is what I have figured out so far. I replaced all the filter caps on
both power supplies. Replaced the bridge rectifiers on the main power supply.
F4009 was blown have no idea how or why. Still has horizontal jitter. Now here
is the kicker. I live in Arizona its always hot so my central air conditioner
is almost always on. The last few days it has been cool enough to have it off
for long cycles. Long story short the jitter happens mostly when the air
conditioning kicks on I measured the voltage drop when the air kicks on its
only about a volt.

Sounds like something is not regulating. What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope. Definitely want to see what
the noise is on the power supplies. It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise
coupling).

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load. It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation. That lets lots of noise get through.

What does the line do when that AC is on? How much noise and what is
the voltage?


Harvey

I have many other scopes that are not affected in the same way. When the air
kicks on I can even hear the fan pitch change. I'm now at a loss in what to
do. Could this have anything to do with the EMI line filter?


No variac :(
I have not looked at the mains with a scope (because I'm scared to do it)
The low/high switch his set to high and the voltage on the line before the air conditioner kicks on is 120-122V and drops to about 119V when its on. I plugged the scope into a APC 1500 UPS and that SEEMS to almost totally correct the issue. The issue is so random one time the air will kick on and the scope will display no issues the next time it will. There is no way I have found to reliably induce the symptoms. I think I need a variac. I have spent 10 hours a day for about a week trying to figure this out.

Re: Random noise on LV power rails

Harvey White
 

On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:
<SNIP>

Ok this is what I have figured out so far. I replaced all the filter caps on both power supplies. Replaced the bridge rectifiers on the main power supply. F4009 was blown have no idea how or why. Still has horizontal jitter. Now here is the kicker. I live in Arizona its always hot so my central air conditioner is almost always on. The last few days it has been cool enough to have it off for long cycles. Long story short the jitter happens mostly when the air conditioning kicks on I measured the voltage drop when the air kicks on its only about a volt.
Sounds like something is not regulating. What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope. Definitely want to see what
the noise is on the power supplies. It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise
coupling).

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load. It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation. That lets lots of noise get through.

What does the line do when that AC is on? How much noise and what is
the voltage?


Harvey

I have many other scopes that are not affected in the same way. When the air kicks on I can even hear the fan pitch change. I'm now at a loss in what to do. Could this have anything to do with the EMI line filter?


Re: Random noise on LV power rails

Brendan
 

On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:


On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:


While trying to figure out what is going on with my 468 I decided to monitor
the -8V and 15V supply DC coupled. When the trace starts to jitter there is
noise on both of them. Not ripple, it almost looks like a digital signal. What
would cause that? This has been a pretty cool trouble shooting experience.

Just loud thinking...when jitter starts in some circuit, sudden current draw
changes may cause sudden voltage changes, which may appear as noise. What
are
the amplitudes of these noises on power rails? You may also want to check
ripple specifications and bypass capacitors. My two cents on this problem,
apart from the mention of some failing pots earlier...

Shailendra
I ordered new filter caps today. I'm unable to get any capacitance reading
from C926 15V unregulated line. I'm hoping that will cure the issues. The caps
will be here Friday. I already have the old caps out, while I was in there I
replaced all the bridge rectifiers also. I replaced all the small caps in the
storage power supply today and will do the big caps when they come. Figured I
may as well give it the full treatment.
Ok this is what I have figured out so far. I replaced all the filter caps on both power supplies. Replaced the bridge rectifiers on the main power supply. F4009 was blown have no idea how or why. Still has horizontal jitter. Now here is the kicker. I live in Arizona its always hot so my central air conditioner is almost always on. The last few days it has been cool enough to have it off for long cycles. Long story short the jitter happens mostly when the air conditioning kicks on I measured the voltage drop when the air kicks on its only about a volt. I have many other scopes that are not affected in the same way. When the air kicks on I can even hear the fan pitch change. I'm now at a loss in what to do. Could this have anything to do with the EMI line filter?

Clearing error log in THS720A

 

The recent topic about the THS720/THS720A encourages me to submit an issue that I've had with my THS720A for some time.
The instrument is in pristine condition, both cosmetically and technically. Unfortunately, some time ago I could not resist the temptation to "upgrade" it to a THS730A by moving a few resistors, as described somewhere on the 'net.
Basically, it sort-of worked (both bandwidth and samples/sec) but it slowly filled the error log so I restored it to its original state as a THS720A.
Since then, it has been fine except for the old entries in the error log, which I haven't been able to clear. It's ugly but not a real problem of course.

I'm wondering if the error log can be cleared independently, i.e. without calibrating first (it doesn't need any adjustments) and if so, how, and if not, is calibration of the 'scope-part i.e. leaving the DVM as-is possible and sufficient? I don't have any of the special DC voltage generators needed and I'm not looking forward to set the voltages ad hoc on a normal voltage source.

Thanks in advance for any info!

Raymond