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I have not had any replies on the TDS644A power supply problem. I have my repair log for the last repair if anyone is interested. It is not a simple power supply.
While troubleshooting I needed my Tek 576 curve tracer to check the TVS and HV transistors, especially the replacement BU508A (the last batch from China were only good to 400v). I had used MJE8501 (pencilled in on a TDS544A schematic) but have not been able to find a parts list to show the correct transistor. HELP!
My curve tracer now does not show any curves. While troubleshooting it suddenly made a sound like a relay chattering and the display went blurry and jumped around. The +100v supply was at 60v. However turning it off and on it ran for a few minutes then did the same thing. I left it off for dinner and when I returned it ran for over an hour while I continued troubleshooting the step generator which does not seem to be getting to the test transistor.
The manual says to check the DAC but does not say where it is.
I am about to start again. Any suggestions?
PeterB

I don't know what shape your 576 is in, but I have found
many with bad 5V filter capacitors. They suddenly go open
circuit. They are the Mallory FP style, and the internal
aluminum strip that connects to the solder lugs etches
open circuit.

The usual symptom is the 576 goes completely goofy. The
relays chatter, the screen goes away, bat bleep crazy.

You can safely parallel a modern capacitor to the terminals
on the FP can and put the capacitors somewhere convenient
while you wait for a better solution.

Worth a look see.

-Chuck Harris

peter bunge wrote:

It may have a PSU problem as Chuck described.
However, if the 100V supply is being loaded, this sounds like the CRT HV transformer problem typical of the older brown encapsulated transformer design. Coupled with your comment of "However turning it off and on it ran for a few minutes then did the same thing"
If your CRT HV transformer is the black silicone encapsulated type, you can disregard, but if not, that would be my very strong suspicion.

Yes, that is a consideration that I try not to think about.
I guess I had better get my transformer business back in order
this winter.

-Chuck Harris

Bob Koller via Groups.Io wrote:

Thanks Bob and Chuck. Unfortunately my 576 has a brown HV transformer, I
just looked.

I also checked C759 in the 5v power supply and I replaced it a few years
ago. My repair log shows how I adapted a more modern capacitor to the metal
can mounting if anyone is interested.

I continued with the Performance/Calibration steps without adjusting
anything. When I got to p5-13 step 14 I could not see the display shown in
Fig5-6 and realized this procedure was not going to be of any further help.

I set the switches to look at an NPN 2N3565 at low current and curves
flashed occasionally on the screen. Turning the Number of Steps knob and
tapping it changed the steps but did not fix the problem. I took to tapping
inside with an insulated rod and found U22 on the Step Gen board was really
sensitive. Re-seating it in the (cheap) socket fixed the problem and now I
get rock solid curves from 1 to 10 as selected.

It has been running for over an hour without problems but I am concerned
about the HV transformer.

Are these HV transformers available anywhere?

If it fails completely should I be looking at the newer 577? I saw someone
started a thread about a comparison but it went nowhere. Is the 577 better
than the 576?



Now I still have the problem with the TDS 644A power supply on the other
thread. I would love to hear comments on this one. Is there a parts list
available that shows the correct transistor for the standby power supply?
Why would a standby power supply run off 408V?

I see there are good comments from Siggi. I will reply on that thread.

I suspect that the problem you originally mentioned is a result of the 100V being loaded by the lossy HV transformer, the symptoms are classic. These transformers are difficult to find, as so many failed.
If your CRT is bright, and the balance of the instrument is in at least good condition, it is worth finding a later s/n unit for parts, or making one out of two if you find a later one with a poor CRT.
The 577 is an option, they are much smaller and lighter, lower peak power, but better small current functionality. The storage version may appeal to some, but the storage CRT can be a problem if you get one with severe differential aging of the storage target, or weak flood guns. The writing gun may still be quite usable.
Personally, I have and use the 576, I guess because I have bought, sold and serviced at least 100 of them.

Peter,

Here are some comments regarding the 576, 577, and TDS644A.

Yes, the brown HV transformer is the bad kind, so it will likely crap out soon. The symptoms are about right.

The 576 v. 577 discussion will likely never end, because they each have their pluses and minuses, and either is a fine CT for most uses. There is no consensus, except that it's nice to have both.

The TDS series scopes, as far as I can tell, all use a similar PS topology, but unfortunately not identical in the details. I had to fix the standby PS in a TDS544A a while back. As I recall, the main supply/preregulator is a power factor correction (PFC) type. In its common form, it is a boost converter, so it is set up to make preregulated DC somewhat above the highest expected peak voltage of the power line - typically 350-375 V for 240 V operation, or doubled 120 V in a dual-range configuration. In a full-range, continuous coverage design, the input line can be anything between the lowest and highest line possible, in any country. I think the ones I have (TDS 544A, 754A, and 820) are full-range, but there may be some that use dual-range. Regardless of this detail, the highest expected rectified raw DC should be somewhat less than 400 V, so somewhere around there is what the PFC converter is designed to put out, and still provide regulation. All lower voltages are simply boosted up to this level, giving very wide coverage.

It's been a while since I studied it, but I think that in standby, the PFC is actually running all the time too, and the standby supply runs from the PFC output. It isn't necessary for the PFC to be running all the time, since when it's off, the rectified DC from the line passes through, but would not be boosted or regulated. This would be OK too, since the standby supply should be able to operate over a wide input range too. Either arrangement can work, but controlling the on and off states would be different.

The standby supply keeps it ready to go, and supplies power for scope-side circuits, including the one-bit memory (I think it's a latching relay) of its last power state. When the power button is pushed, the state is switched to the opposite of whatever it was. In the case of turning the scope on, the PFC converter is activated - if it's not always on anyway - then the main output chopper fires up, converting the 400 VDC to the low voltage secondaries.

Anyway, in standby, you may see the input voltage to the standby converter be something similar to the peak of the input AC, or fixed near 400 V, depending on the implementation details. In the scope on state, this voltage should be around 400 V.

Ed

The manual says to check the DAC but does not say where it is.
I am about to start again. Any suggestions?
PeterB

Peter,

The DAC is Described in the manual on 3-9, it is built from discrete components. My 576 had numerous Axial Electrolytic caps bad as well as the Power Supply filter caps Some of the caps replaced: C759, C294, C698, C 696. The 5 V supply was only putting out 3.6V or so when I got the thing. Of course, nothing worked. I also had several bad transistors as well as a couple of bad J-FETS. In other words, mine was a wreck. Mine suddenly quit working one day due to failed Q800 in the +12.5LVPS. I outlined the numerous repairs on my unit in the "Repairs" section of the 576 page of TEKWiki.


--
Michael Lynch
Dardanelle, AR

Hi Ed; I sent you my repair log and in it you will find that the reason the
first transistor blew a year ago was high ESR in C17.
I had replaced the Q9 and both TVR (VR4 & VR5) and Q9 immediately blew
again. When C17 was replaced (you have to read my notes) Q9 survived for a
year.
I just removed C17 and the ESR is 7 ohms (was less than 1 when replaced). I
should not have installed a used capacitor and obviously need to choose a
NEW replacement more carefully. Any suggestions? Perhaps a tantalum? It is
47 ohms and the voltage cannot go above (CR10 + VR1 + CR11 + CR9 = about
7.8v). So 20 to 35 v.
To check with the boost converter disabled disconnect one end of CR8.
References to the schematic of the TDS 544A with MJE8501 and MJE1320
pencilled in besides Q9.
The TVS are Littlefuse 1.5KE 220CA and 1.5KE250CA available from DigiKey.
BU508AW also available from DigiKey.

On Wed, Aug 21, 2019 at 4:08 PM Ed Breya via Groups.Io <edbreya=
[email protected]> wrote:

Peter,

Here are some comments regarding the 576, 577, and TDS644A.

Yes, the brown HV transformer is the bad kind, so it will likely crap out
soon. The symptoms are about right.

The 576 v. 577 discussion will likely never end, because they each have
their pluses and minuses, and either is a fine CT for most uses. There is
no consensus, except that it's nice to have both.

The TDS series scopes, as far as I can tell, all use a similar PS
topology, but unfortunately not identical in the details. I had to fix the
standby PS in a TDS544A a while back. As I recall, the main
supply/preregulator is a power factor correction (PFC) type. In its common
form, it is a boost converter, so it is set up to make preregulated DC
somewhat above the highest expected peak voltage of the power line -
typically 350-375 V for 240 V operation, or doubled 120 V in a dual-range
configuration. In a full-range, continuous coverage design, the input line
can be anything between the lowest and highest line possible, in any
country. I think the ones I have (TDS 544A, 754A, and 820) are full-range,
but there may be some that use dual-range. Regardless of this detail, the
highest expected rectified raw DC should be somewhat less than 400 V, so
somewhere around there is what the PFC converter is designed to put out,
and still provide regulation. All lower voltages are simply boosted up to
this level, giving very wide coverage.

It's been a while since I studied it, but I think that in standby, the PFC
is actually running all the time too, and the standby supply runs from the
PFC output. It isn't necessary for the PFC to be running all the time,
since when it's off, the rectified DC from the line passes through, but
would not be boosted or regulated. This would be OK too, since the standby
supply should be able to operate over a wide input range too. Either
arrangement can work, but controlling the on and off states would be
different.

The standby supply keeps it ready to go, and supplies power for scope-side
circuits, including the one-bit memory (I think it's a latching relay) of
its last power state. When the power button is pushed, the state is
switched to the opposite of whatever it was. In the case of turning the
scope on, the PFC converter is activated - if it's not always on anyway -
then the main output chopper fires up, converting the 400 VDC to the low
voltage secondaries.

Anyway, in standby, you may see the input voltage to the standby converter
be something similar to the peak of the input AC, or fixed near 400 V,
depending on the implementation details. In the scope on state, this
voltage should be around 400 V.

Ed

OK that was a mix up. Two threads crossed and this was meant for the
TDS644A thread. And of course the capacitor was 47 uF not 47 ohms although
the ESR is 7 ohms on the one removed.

On Wed, Aug 21, 2019 at 8:09 PM peter bunge via Groups.Io <bunge.pjp=
[email protected]> wrote:

Hi Ed; I sent you my repair log and in it you will find that the reason the
first transistor blew a year ago was high ESR in C17.
I had replaced the Q9 and both TVR (VR4 & VR5) and Q9 immediately blew
again. When C17 was replaced (you have to read my notes) Q9 survived for a
year.
I just removed C17 and the ESR is 7 ohms (was less than 1 when replaced). I
should not have installed a used capacitor and obviously need to choose a
NEW replacement more carefully. Any suggestions? Perhaps a tantalum? It is
47 ohms and the voltage cannot go above (CR10 + VR1 + CR11 + CR9 = about
7.8v). So 20 to 35 v.
To check with the boost converter disabled disconnect one end of CR8.
References to the schematic of the TDS 544A with MJE8501 and MJE1320
pencilled in besides Q9.
The TVS are Littlefuse 1.5KE 220CA and 1.5KE250CA available from DigiKey.
BU508AW also available from DigiKey.

On Wed, Aug 21, 2019 at 4:08 PM Ed Breya via Groups.Io <edbreya=
[email protected]> wrote:

Peter,

Here are some comments regarding the 576, 577, and TDS644A.

Yes, the brown HV transformer is the bad kind, so it will likely crap out
soon. The symptoms are about right.

The 576 v. 577 discussion will likely never end, because they each have
their pluses and minuses, and either is a fine CT for most uses. There is
no consensus, except that it's nice to have both.

The TDS series scopes, as far as I can tell, all use a similar PS
topology, but unfortunately not identical in the details. I had to fix

the

standby PS in a TDS544A a while back. As I recall, the main
supply/preregulator is a power factor correction (PFC) type. In its

common

form, it is a boost converter, so it is set up to make preregulated DC
somewhat above the highest expected peak voltage of the power line -
typically 350-375 V for 240 V operation, or doubled 120 V in a dual-range
configuration. In a full-range, continuous coverage design, the input

line

can be anything between the lowest and highest line possible, in any
country. I think the ones I have (TDS 544A, 754A, and 820) are

full-range,

but there may be some that use dual-range. Regardless of this detail, the
highest expected rectified raw DC should be somewhat less than 400 V, so
somewhere around there is what the PFC converter is designed to put out,
and still provide regulation. All lower voltages are simply boosted up to
this level, giving very wide coverage.

It's been a while since I studied it, but I think that in standby, the

PFC

is actually running all the time too, and the standby supply runs from

the

PFC output. It isn't necessary for the PFC to be running all the time,
since when it's off, the rectified DC from the line passes through, but
would not be boosted or regulated. This would be OK too, since the

standby

supply should be able to operate over a wide input range too. Either
arrangement can work, but controlling the on and off states would be
different.

The standby supply keeps it ready to go, and supplies power for

scope-side

circuits, including the one-bit memory (I think it's a latching relay) of
its last power state. When the power button is pushed, the state is
switched to the opposite of whatever it was. In the case of turning the
scope on, the PFC converter is activated - if it's not always on anyway -
then the main output chopper fires up, converting the 400 VDC to the low
voltage secondaries.

Anyway, in standby, you may see the input voltage to the standby

converter

be something similar to the peak of the input AC, or fixed near 400 V,
depending on the implementation details. In the scope on state, this
voltage should be around 400 V.

Ed