Date   
Re: 2235 power supply noise

 

I'm completely at a loss as to how to proceed now. Is the problem elsewhere? Is my replacement
T906 faulty?
Please help!
This was a known problem at the time.
See these articles from the internal Service Info from 85 and 87:

//
RE: Mod #54681

Implemented in Manufacturing:

2213A B016090
22I5A B013550
2235 B015626
2236 B014885

The preregulator and DC-DC converter
frequencies may be harmonically close
enough to produce an audible beat.
This anomaly will only occur after
changing some major power supply
frequency determining part, e.g. T944.
Engineering, changed R919 to a
selectable part, allowing a Technician
to alter the preregulator frequency a
small, but sufficient amount, thereby,
stopping the audible beat.

R919 Nominal value 10Kohm 315-0103-00
Alternate value #1 11Kohm 315-0113-00
Alternate value #2 9.1Kohm 315-0912-00

AND:

2213/2215A /2235/2236 AUDIBLE NOISE
FROM POWER SUPPLY

REF: Mod #61544

S/N: Not available

There have been three "speakers"
identified in the 2200 Series which
emit high-frequency audible sound from
the power supply. Due to the urgency
of this change to the field, only two
of these components have been changed
on this mod. These components and new
Tektronix part numbers are as follows:

T906: Tektronix P/N 120-1439-01
C907: Tektronix P/N 285-1177-01

In a very few instruments T948 was
found to be the third culprit. Chang­ing
this part number will require UL
testing and recertification, so some
testing and evaluation need to be
conducted before the part can be modi­fied.

If an instrument comes into the Ser­vice
Center with this problem try changing
C907 first. If this does not eliminate the
sound, change T905.
//


/Håkan

Re: 2235 power supply noise

n4buq
 

2213/2215A /2235/2236 AUDIBLE NOISE
FROM POWER SUPPLY

REF: Mod #61544

S/N: Not available

There have been three "speakers"
identified in the 2200 Series which
emit high-frequency audible sound from
the power supply. Due to the urgency
of this change to the field, only two
of these components have been changed
on this mod. These components and new
Tektronix part numbers are as follows:

T906: Tektronix P/N 120-1439-01
C907: Tektronix P/N 285-1177-01

In a very few instruments T948 was
found to be the third culprit. Chang­ing
this part number will require UL
testing and recertification, so some
testing and evaluation need to be
conducted before the part can be modi­fied.

If an instrument comes into the Ser­vice
Center with this problem try changing
C907 first. If this does not eliminate the
sound, change T905.
//


/Håkan
T906 / T905? Is that a typo?

Barry - N4BUQ

Re: 2235 power supply noise

 

T906 / T905? Is that a typo?
I just saw that. OCR program error, sorry.
Should be T906.

/Håkan

Re: 2235 power supply noise

mikepickwell@...
 

Many thanks for the helpful replies folks.


I just removed R919 and it measured as almost exactly 10K. I don't have any 9.1K or 11K resistors so a put back in a 10K paralleled with a 1M making 9.09K. This didn't change anything.


I don't know what damping substance Tektronix used but I would think that high temperature RTV sealant would be suitable. Do you think I should give this a try?


Holding the probe near T906 I get a very clear waveform with a period of about 48uS which I believe gives a frequency of about 20.8kHz. Obviously that's not the loud audible frequency I can hear though as 20K is too high. However, half that (i.e. about 10,000Hz) would make sense. I just tried playing about with a test tone generator and I estimate the noise the scope is making is indeed about 10,000Hz


I have a frequency counter on my multimeter. Please can someone tell me where best to connect the leads of the multimeter to measure the frequency the HVO.


Thanks, Mike

Re: 2235 power supply noise

tom jobe
 

Thank you Hakan!
This kind of good information is very rare to see, and hopefully it will
help the current 2235 problem being worked on.
This information will help many folks in the future!
tom jobe...

On 10/20/2017 6:30 AM, hahi@... [TekScopes] wrote:

I'm completely at a loss as to how to proceed now. Is the problem
elsewhere? Is my replacement
T906 faulty?
Please help!
This was a known problem at the time.
See these articles from the internal Service Info from 85 and 87:

//
RE: Mod #54681

Implemented in Manufacturing:

2213A B016090
22I5A B013550
2235 B015626
2236 B014885

The preregulator and DC-DC converter
frequencies may be harmonically close
enough to produce an audible beat.
This anomaly will only occur after
changing some major power supply
frequency determining part, e.g. T944.
Engineering, changed R919 to a
selectable part, allowing a Technician
to alter the preregulator frequency a
small, but sufficient amount, thereby,
stopping the audible beat.

R919 Nominal value 10Kohm 315-0103-00
Alternate value #1 11Kohm 315-0113-00
Alternate value #2 9.1Kohm 315-0912-00

AND:

2213/2215A /2235/2236 AUDIBLE NOISE
FROM POWER SUPPLY

REF: Mod #61544

S/N: Not available

There have been three "speakers"
identified in the 2200 Series which
emit high-frequency audible sound from
the power supply. Due to the urgency
of this change to the field, only two
of these components have been changed
on this mod. These components and new
Tektronix part numbers are as follows:

T906: Tektronix P/N 120-1439-01
C907: Tektronix P/N 285-1177-01

In a very few instruments T948 was
found to be the third culprit. Chang­ing
this part number will require UL
testing and recertification, so some
testing and evaluation need to be
conducted before the part can be modi­fied.

If an instrument comes into the Ser­vice
Center with this problem try changing
C907 first. If this does not eliminate the
sound, change T905.
//


/Håkan






[Non-text portions of this message have been removed]

Re: 2235 power supply noise

Ed Breya
 

See message #76038. Ed

Re: 2235 power supply noise

mikepickwell@...
 

Thank you Ed.


I'll fill the hole with silicone and experiment with different values at R919.


Cheers, Mike

7A26 caps

 

I have several 7A26 that had been sitting unused for a long time. I thought I'd check their
status and calibrate them. I was surprised to see so many bad variable caps in the step response adjustments. I didn't count but it was probably at least ten total out of four plugins. They are the
small round ones with 5mm spacing and all are the same brand. They are marked with AJ plus
the capacitance range and could also be identified by one of the mounting pins is soldered to the
side of the cap. The problem is that they seem to completely open i.e. no capacitance.
Does anyone else have the same experience ?

There is also another brand of similar caps which rarely cause problems.

/Håkan

Re: 2235 power supply noise

tom jobe
 

Wouldn't it be good to try changing C907 as the document suggests?
It is directly connected to T906 and it was given as a possible problem
in Hakan's document, plus you have already had CR907 looking a bit toasty.
I think a change in R919 just shuffles the operating frequency of
U930/Q9070 around, and that is already up in the 20k to 24k range
somewhere, all of which might be above your hearing ability?
I wonder if any of the more knowledgeable members would have an opinion
about possibly changing the value of C907 as well?
tom jobe...
PS I wish I could be more helpful, but I could have the same exact
problem on some of my 22xx's and just not be able to hear it.



On 10/20/2017 11:07 AM, mikepickwell@... [TekScopes] wrote:

Thank you Ed.


I'll fill the hole with silicone and experiment with different values
at R919.


Cheers, Mike


[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]

Re: 2235 power supply noise

tom jobe
 

Thanks Ed for that message number from 2012!
That leads us to some other comments in the same thread.
I have not run into this problem on any 22xx's, but I'm sure it is just
because my hearing is gone in the higher frequency ranges.
I'm glad to good to know about this problem, and I hope someone comes up
with a simple way to 'see' it with an oscilloscope for us old folks.
tom jobe...

On 10/20/2017 10:26 AM, edbreya@... [TekScopes] wrote:

See message #76038. Ed

[Non-text portions of this message have been removed]

Re: 2235 power supply noise

k1ggi
 

If I am following, the OP tried a 1% mod to R919 without success.
Would be interesting to see about the recommended 10% (parallel with 100k).
Ed, k1ggi

________________________________________
From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, October 20, 2017 9:30 AM
To: TekScopes@...
Subject: Re: [TekScopes] 2235 power supply noise

 
I'm completely at a loss as to how to proceed now. Is the problem
elsewhere? Is my replacement
T906 faulty?
Please help!
This was a known problem at the time.
See these articles from the internal Service Info from 85 and 87:

//
RE: Mod #54681

Implemented in Manufacturing:

2213A B016090
22I5A B013550
2235 B015626
2236 B014885

The preregulator and DC-DC converter
frequencies may be harmonically close
enough to produce an audible beat.
This anomaly will only occur after
changing some major power supply
frequency determining part, e.g. T944.
Engineering, changed R919 to a
selectable part, allowing a Technician
to alter the preregulator frequency a
small, but sufficient amount, thereby,
stopping the audible beat.

R919 Nominal value 10Kohm 315-0103-00
Alternate value #1 11Kohm 315-0113-00
Alternate value #2 9.1Kohm 315-0912-00

AND:

2213/2215A /2235/2236 AUDIBLE NOISE
FROM POWER SUPPLY

REF: Mod #61544

S/N: Not available

There have been three "speakers"
identified in the 2200 Series which
emit high-frequency audible sound from
the power supply. Due to the urgency
of this change to the field, only two
of these components have been changed
on this mod. These components and new
Tektronix part numbers are as follows:

T906: Tektronix P/N 120-1439-01
C907: Tektronix P/N 285-1177-01

In a very few instruments T948 was
found to be the third culprit. Changing
this part number will require UL
testing and recertification, so some
testing and evaluation need to be
conducted before the part can be modified.

If an instrument comes into the Service
Center with this problem try changing
C907 first. If this does not eliminate the
sound, change T905.
//


/Håkan


[Non-text portions of this message have been removed]

Re: 2235 power supply noise

Ed Breya
 

Tom, you can probably rig something up to see the sounds with a plastic tube for probing around the circuits, with a small electret or whatever type microphone shoved into the other end of the tube, The tube will safely convey some of the sound away from the hot circuits and strong electrical interference, to the mic, where it can be amplified up to observe. It may also need a bunch of low-pass filtering and shielding to cut out the stuff above audio. The electrial interference may be quite strong even some distance away - maybe stronger than the mic signal.

The kind of mics I picture are those little Al cylinder types that are in almost everything that has voice/sound input - especially phones. It's the same setup as the old mechanic's trick, except that instead of the tube going to your ear, it goes to a mic. It definitely won't be precise, but may be OK for locating stuff, and relative measurements.

You can also build or get ultrasonic receivers/downconverters to see/hear the sound above audio, emanating from SMPSs and other stuff.

Ed

Re: 2235 power supply noise

Vincent Trouilliez
 

Keeps us posted Mike ! I wanna know the outcome :-)

Yep thanks to Hakan for the great info !

I do have a 2232 scope with the same problem and similar SMPS design as far as I understand. All the component designators I read above surely ring a bell, at the very least...
Luckily for me, in my case, that ear breaking screaming sound progressively goes away after 5 to 10 minutes as the scope warms up. Still, would love to eliminate this noise completely if at all possible.

A few months ago the SMPS blew and I had to replace half of it. Had to replace T906 because it was burnt/open circuit when the FET died and dumped the mains into it (and the PWM chip and other stuff as well), poor little thing. Anyway, just for the record, I replaced T906 with one that had the white gunk in the middle, and it still does that hight pitched horrible noise, so this might not be a magical stick.


Anyway, watching this thread with great interest, so keep us informed of your progress Mike ! :-)


Regards,


Vincent Trouilliez
Frog Land

Re: 2235 power supply noise

Vincent Trouilliez
 

Just looked at the parts list for my 2232, on this model they allow to move even further away from the nominal 10K. You can go up to 12K rather than 11K on the other models.

I guess it would be easier to just wire a little trimmer in parallel with R919 so you can adjust it finely and in a continuous manner, and quickly and easily figure out how it affects the noise, if it does, and easily figure out the optimal resistor value. It's only running at 60kHz or so, so I guess the parasitics of this arrangement would not matter a bit, would they.


Regards,


Vincent Trouilliez

DM5010 error 318

Harvey White
 

When calibrating a DM5010, I get error 318, bad ohms calibration
constant. This happens even when I am attempting to zero the reading.
It doesn't happen on some ranges, but does on others.

Not quite sure what to do about it. I have all the stuff needed to
calibrate it otherwise, I think.

I have three to work on, one with this problem, one that seems to want
to reboot and never make it out of power on test, and I'm not sure of
the last one.

The reboot one I can check the signatures, the one I'm not sure of I
can test further, but the 318 error? Not sure where that's coming
from, is it not storing the cal data properly (ram error?) or is it
the ohms converter?

I've replaced the batteries in two of them with li batteries, diode in
series, very light drain. All three had some battery leakage and
damage, though.

Harvey

7904 and 7A19 attenuator and vertical scale woes

Evan Geller
 

Hey everyone!


I've run into a few problems with my newly acquired 7904. The first seems to be an intermittent connection underneath S135. I've scoped a test signal up to J12 and everything seems happy... and specifically touching the coupler on S135 makes the signal jump. Any recommendations for cleaning under this switch without damaging anything?


My second question pertains to the on screen display. It seems to have walked off the screen a bit! My first thinking was that the CRT just wound up vertically scaled a bit... but hooking the 7A19 up to the voltage calibration shows that the voltages line up with the lines on the graticule. Is there somewhere that moves the text around on screen? If not, does this just mean that the vertical scale is off in the mainframe and someone decided to compensate by rescaling by rescaling the 7A19? That would be pretty unfortunate. I don't have an extender so I can't trivially pick off the voltages from the card. Maybe some of you have a trick for this that's more elegant than soldering kynar wire to the board and scoping that.


Best,
Evan

Re: 7904 and 7A19 attenuator and vertical scale woes

Evan Geller
 

Apologies, the first paragraph refers specifically to the 7A12 module.

Re: 7904 and 7A19 attenuator and vertical scale woes

Roger Evans
 

Evan,

You can find the manuals online at w140.com/tekwiki or Google for 'BAMA manuals'. Artek Manuals have very high quality scans at modest prices. The 7904 and 7904A are very different.

There are separate adjustments for centering the character readout and the vertical signal. If you want to check the centering of the mainframe vertical amplifier without any special equipment just remove the vertical plugins and turn the scope on with the timebase set to auto trigger.

If you don't have plugin extenders you can get some access to the left side of the vertical plugins and the right side of the horizontal plugins by removing the main side panels, but you need to take extra care to avoid shorts to the metalwork.

Regards,

Roger

Re: 2235 power supply noise

mikepickwell@...
 

Hi everyone,


C907 is a 1uF 400v film. I hadn't initially considered changing it as since it won't be prone to aging in the same way as the electrolytics. It won't cost much to give it a try though.


The squealing noise is definitely more in the region of 10,000Hz rather than 20,000Hz, I don't think I'd be able to hear much at a frequency as high as 20kHz.


I've tried slightly lowering the value of R919 but I haven't tried raising it yet. I'll give it a try and report back.

Re: 7A26 caps

Göran Krusell
 

Hi Håkan,



Yes, I experienced this problem when I calibrated a number of 7A26´s a few
years ago. I can identify three different round capacitors. I can read the
capacitance range but not the letters AJ. I did not exchange them instead I
settled with not getting the last ns out of the pulse shape. I was
surprised that Tek would chose such low quality components in this
amplifier. And I get the impression that the amplifier was fairly expensive
to build due to a lot of manual mounting. This amplifier was built in a
number of 120 000 units so there was reasons to optimize its design, and
perhaps they also did. But what I found even more disturbing was that the
variable resistors in many cases had very large or unnecessarily large
adjustment ranges.



I usually use my 7A18 instead, I find it more stable, lower drift – or is
it an illusion…



Göran


[Non-text portions of this message have been removed]