Problems with t935a HV supply


sst00940 <b_willemaers@...>
 

Hello.

I've got a problem with a T935a scope. The indicator led on the front panel flashes, which indicates a faulty HV power supply. Thus nothing is displayed.

As a disclaimer, I must say that I've worked with DC voltages above 300V before (for hifi tubes amplifiers) but that I've no experience with such switching power supply. Sadly, this was also my only scope. The only other tool I've got is a DMM.

- The slow blow 60ma HV fuse has blown. Replacing it doesn't fix the issue. Powering the unit for at most a pair of seconds revealed excessive current draw (320ma through r458!) so I quickly shut the scope down.

- Powering the scope in serie with a 20W lightbulb gives me the following readings: http://img512.imageshack.us/img512/9269/t935a.gif

- The low voltage supplies are fine (tested by powering the scope with the HV fuse removed).

- When feeding 8V @15khz into the primary of the HV transformer (through r458), I get the following readings:
6.5VAC developped accros r458.
2.5VAC developped accross the primary.
81VAC appeared on pin 9.
26VAC appeared on pin 10 and 8.
nothing appeared from pin 3 to 6.

- the winding in between pins 3 and 6 is a short for my DMM.

It has been suggested to me that the feedback winding might be shorted. I would really appreciate if anyone could confirm this diagnosis or suggest another reason/fix for this failure. Finding a replacement transformer might be near to impossible I'm afraid.

Thank you in advance for any help,

Ben.


Gilberto Karalus <gkaralus@...>
 

The t922 is basicaly the same, so check
http://www.logwell.com/tech/oscilloscopes/T922R_service_notes.html
for info

On Tue, Sep 1, 2009 at 6:55 PM, sst00940<b_willemaers@hotmail.com> wrote:


Hello.

I've got a problem with a T935a scope. The indicator led on the front panel
flashes, which indicates a faulty HV power supply. Thus nothing is
displayed.

As a disclaimer, I must say that I've worked with DC voltages above 300V
before (for hifi tubes amplifiers) but that I've no experience with such
switching power supply. Sadly, this was also my only scope. The only other
tool I've got is a DMM.

- The slow blow 60ma HV fuse has blown. Replacing it doesn't fix the issue.
Powering the unit for at most a pair of seconds revealed excessive current
draw (320ma through r458!) so I quickly shut the scope down.

- Powering the scope in serie with a 20W lightbulb gives me the following
readings: http://img512.imageshack.us/img512/9269/t935a.gif

- The low voltage supplies are fine (tested by powering the scope with the
HV fuse removed).

- When feeding 8V @15khz into the primary of the HV transformer (through
r458), I get the following readings:
6.5VAC developped accros r458.
2.5VAC developped accross the primary.
81VAC appeared on pin 9.
26VAC appeared on pin 10 and 8.
nothing appeared from pin 3 to 6.

- the winding in between pins 3 and 6 is a short for my DMM.

It has been suggested to me that the feedback winding might be shorted. I
would really appreciate if anyone could confirm this diagnosis or suggest
another reason/fix for this failure. Finding a replacement transformer might
be near to impossible I'm afraid.

Thank you in advance for any help,

Ben.


d.seiter@...
 

Are you sure the flashing LED indicates faulty HV?  On the T912 it indicates something like the input voltage switch is not set correctly (either too high or too low), I think it means greater/less then 10% from optimal(?) 

After you grab the T922 schematic from logwell, read "Tales of Destruction" on the site.  A great history of nitroglycerin and the early oilfields.  Highly recommended!

-Dave

----- Original Message -----
From: "Gilberto Karalus"
To: TekScopes@...
Sent: Tuesday, September 1, 2009 5:47:04 PM GMT -07:00 US/Canada Mountain
Subject: Re: [TekScopes] Problems with t935a HV supply

 

The t922 is basicaly the same, so check
http://www.logwell.com/tech/oscilloscopes/T922R_service_notes.html
for info

On Tue, Sep 1, 2009 at 6:55 PM, sst00940<b_willemaers@...> wrote:
>
>
> Hello.
>
> I've got a problem with a T935a scope. The indicator led on the front panel
> flashes, which indicates a faulty HV power supply. Thus nothing is
> displayed.
>
> As a disclaimer, I must say that I've worked with DC voltages above 300V
> before (for hifi tubes amplifiers) but that I've no experience with such
> switching power supply. Sadly, this was also my only scope. The only other
> tool I've got is a DMM.
>
> - The slow blow 60ma HV fuse has blown. Replacing it doesn't fix the issue.
> Powering the unit for at most a pair of seconds revealed excessive current
> draw (320ma through r458!) so I quickly shut the scope down.
>
> - Powering the scope in serie with a 20W lightbulb gives me the following
> readings: http://img512.imageshack.us/img512/9269/t935a.gif
>
> - The low voltage supplies are fine (tested by powering the scope with the
> HV fuse removed).
>
> - When feeding 8V @15khz into the primary of the HV transformer (through
> r458), I get the following readings:
> 6.5VAC developped accros r458.
> 2.5VAC developped accross the primary.
> 81VAC appeared on pin 9.
> 26VAC appeared on pin 10 and 8.
> nothing appeared from pin 3 to 6.
>
> - the winding in between pins 3 and 6 is a short for my DMM.
>
> It has been suggested to me that the feedback winding might be shorted. I
> would really appreciate if anyone could confirm this diagnosis or suggest
> another reason/fix for this failure. Finding a replacement transformer might
> be near to impossible I'm afraid.
>
> Thank you in advance for any help,
>
> Ben.
>
>


benoit_willemaers <b_willemaers@...>
 

Thank you for the first comments.

@Gilberto: Here is the relevant information I get from the Logwell website: "If R458 gets very hot or burns out, check C458 for a short, check T460 for a shorted primary, and/or check Q458 for proper operation."

- C458 is ok.
- t460 primary seems ok (test with the 15khz, how crude it is)
- q458 is a TIP150C darlington. With 0.8V on its base, it lets 60ma through. Something is wrong, shouldn't it only turn on with more than 1.6V ? All this is DC of course... If there is high frequency AC voltage there, my DMM won't measure it with any kind of precision.

@Dave: the flashing lamp is triggered by a 10% variation from the unregulated 100V supply (from which the HV is derivated). It can indeed indicate line variation and it is the intended use. But with the fuse blown, the 100V will become 0V. With the fuse on and no limiting, the huge current draw will take the 100V down, below the 10%.


 

Dave,
When I look at your drawing, it looks like you have note 45.6 volts at the 100 volts line ?
Is this a mistake by me
helge


2009/9/2 benoit_willemaers <b_willemaers@...>

Thank you for the first comments.

@Gilberto: Here is the relevant information I get from the Logwell website: "If R458 gets very hot or burns out, check C458 for a short, check T460 for a shorted primary, and/or check Q458 for proper operation."

- C458 is ok.
- t460 primary seems ok (test with the 15khz, how crude it is)
- q458 is a TIP150C darlington. With 0.8V on its base, it lets 60ma through. Something is wrong, shouldn't it only turn on with more than 1.6V ? All this is DC of course... If there is high frequency AC voltage there, my DMM won't measure it with any kind of precision.

@Dave: the flashing lamp is triggered by a 10% variation from the unregulated 100V supply (from which the HV is derivated). It can indeed indicate line variation and it is the intended use. But with the fuse blown, the 100V will become 0V. With the fuse on and no limiting, the huge current draw will take the 100V down, below the 10%.



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benoit_willemaers <b_willemaers@...>
 

The drawing is mine ;) Yes, I have 45.6V instead of 100V because the unit is powered through a 20W lightbulb. As it draws a lot more current than it should, the lightbulb limits the voltage and the current to the unit under test, which avoids blowing it up. All the supplies are thus lower than they should.

Thinking of it, the regulated 100V is taken before the HV fuse (it is just before R458). I'll repost later a picture with voltage readings without the lightbulb and without the HV fuse to see if the operating points of the oscillator are ok.

Ben.

--- In TekScopes@yahoogroups.com, Helge Kyndbo <kyndbo@...> wrote:

Dave,
When I look at your drawing, it looks like you have note 45.6 volts at the
100 volts line ?
Is this a mistake by me
helge


 

I have an old T922 with no HV. T460 winding 4-5 reads 1 ohm and the feedback winding 3-6 reads 0 ohms. Both readings taken with an AVO meter on /100 resistance scale.

I am out of ideas here as all other components in the 15KHz oscillator circuit seem OK.

Can I assume that T460 is defective?

--
Regards,
Bernie Murphy


Michael W. Lynch
 

On Sat, May 8, 2021 at 03:09 AM, Ham Radio wrote:


I have an old T922 with no HV. T460 winding 4-5 reads 1 ohm and the feedback
winding 3-6 reads 0 ohms. Both readings taken with an AVO meter on /100
resistance scale.

I am out of ideas here as all other components in the 15KHz oscillator circuit
seem OK.

Can I assume that T460 is defective?
I have a T922 with a good T460, let me check it to confirm the values that I see..

--
Michael Lynch
Dardanelle, AR


satbeginner
 

Hi,

I advise to do a "ringing test" to test the transformer.
It's basically a fast rise, slow repetition square wave connected to a primary winding. (Can be done in circuit)
Use a working scope to see if you see the ringing after the rising flank.

Google is your best friend, I also used it when repairing a 2235.

This thread is about that repair, it also has pictures in an album.

https://groups.io/g/TekScopes/topic/2235_repair_having_no_power/76989243?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,40,76989243

Success,

Leo


 

Leo and. Mike

Thank you both for the quick responses. The Tek 922 is such a nice instrument, I hate to part it out.

Not exactly sure how to inject a square wave pulse into the primary winding — I assume a cap such a 1 uF would be used between the function generator and the primary winding? I assume all other components such as the switching transistor should be disconnected from the primary winding? The switching transistor, a TIP50, tests OK on the transistor tester.

--
Regards,
Bernie Murphy


satbeginner
 

I just used a DC coupled TTL level pulse generator, capable of driving 50 Ohm.
Triggered the scope externally on that pulse, and used a probe to look at the pulse on the primary winding to see the ringing.

Leo


Michael W. Lynch
 

Bernie,

I have a T922 and a T935, both working; nice little basic scopes. I checked my parts mule T460 using a FLUKE Meter and a 4 wire test. The resistances are very low. 3-6 = .033 Ohms 4-5 = 1.096 Ohms, in circuit. The scope i am testing has a known good T460. I am thinking that the ringing test that Leo is suggesting would be a more positive test. I have never done such a test, but this will be a good way to learn about the process for me.

Hope this helps.
--
Michael Lynch
Dardanelle, AR


satbeginner
 

Hi all,

A DC resistance check does not give a valuable result.

E.G. a primary winding only has so many windings, and in a HV transformer could have a quite thick gauge, meaning a low resistance.
If one of these windings has a short to the next one, it would hardly effect the DC resistance of the total winding.

But having a shorted winding when used as an actual AC transformer would be the show stopper.

This is were the ringing test comes in.

A resistance check would only be of basic use to check for a connection between individual windings, like the primary and a secondary winding.
Although no guarantee is given when an ohmmeter says they're isolated, because when used on HV they might still short.

Un saludo,

Leo


 

The ring test with a 10 volt 1000 Hz square wave with a duty cycle of 1% shows ringing on the primary winding of T460 - so that looks good!

No ringing though, on the feedback winding on pins 3 and 6 with the same setup as above. Not sure 🤔 if the ring test would work here as they may be only a few turns on the feedback winding. That winding measures a dead short on the AVO /100 resistance scale but that does not mean the winding is shorted.

I assume that Q444 generates the frequency of the switching pulses for Q458 through the feedback winding.

Not too sure what to try next ...
--
Regards,
Bernie Murphy


satbeginner
 

Hi,

You could try a lower frequency, like 10Hz or so, with a longer 'On' time, to allow the ringing to dampen by itself.
Now it could be stopped by the short 'on' time of the 1% of the 1kHz.

Did you see ringing nicely going down like in one of these pictures?

https://groups.io/g/TekScopes/album?id=254000

This was a bad HV transformer en a good one for a 2235, but the principal is the same.

Good luck,

Leo


-
 

What is the normal operating frequency of that transformer? 1000 Hz test
frequency seems to be on the rather low side. What is the frequency of the
ringing that you're seeing? That's probably the resonant frequency of the
x-former with it's associated circuitry attached and I would expect to see
the X-former normally being operated at about the same frequency. I'm not
familiar with the t935 but IMO you'd probably be better off using a very
low voltage AC signal instead of a square wave to drive the transformer
when testing. Coupling the x-fomer to the signal source through a capacitor
should remove most the higher frequency harmonics from the square wave and
leave you with mostly the base frequency sine wave.

On Sun, May 9, 2021 at 11:03 AM Ham Radio <bernard.murphy@rogers.com> wrote:

The ring test with a 10 volt 1000 Hz square wave with a duty cycle of 1%
shows ringing on the primary winding of T460 - so that looks good!

No ringing though, on the feedback winding on pins 3 and 6 with the same
setup as above. Not sure 🤔 if the ring test would work here as they may
be only a few turns on the feedback winding. That winding measures a dead
short on the AVO /100 resistance scale but that does not mean the winding
is shorted.

I assume that Q444 generates the frequency of the switching pulses for
Q458 through the feedback winding.

Not too sure what to try next ...
--
Regards,
Bernie Murphy






-
 

Leo,

I see that you took the x-former in your pictures apart. Did you ever
determine what the internal failure mode was?

General comment. As Leo's pictures show, transformer failures can be
tough to diagnose since they frequently still have an output but just not
the correct output. And if you don't know exactly what the output waveform
is supposed to look like, or how long the output signal is supposed to
ring, then you could overlook the failure. This type of mis-diagnoses was
common when automobiles were still using a Kettering ignition system.

On Sun, May 9, 2021 at 11:22 AM satbeginner <castellcorunas@gmail.com>
wrote:

Hi,

You could try a lower frequency, like 10Hz or so, with a longer 'On' time,
to allow the ringing to dampen by itself.
Now it could be stopped by the short 'on' time of the 1% of the 1kHz.

Did you see ringing nicely going down like in one of these pictures?

https://groups.io/g/TekScopes/album?id=254000

This was a bad HV transformer en a good one for a 2235, but the principal
is the same.

Good luck,

Leo






satbeginner
 

Hi,

No, once I confirmed by the ringing test it was bad, my guess would be the HV winding.

I tried to take it apart to see if a (partial) rewind would be possible, but it was impregnated solid, so a rewind was a no-go.

The new one performed perfectly, so I added the fan ( as in many 22xx scopes) and performed a full calibration.

Leo


Harvey White
 

What you're doing, I think, is to shock excite the winding. Ideally, you'd use a unit impulse, but a short pulse may work just as well.  The frequency of the waveform simply allows the system to ring until it naturally damps.  Too fast and you'll not see the whole pulse train.  The AC signal will likely give you the ratios, but not an idea of loading/shorts.  For this test, you actually do want those high frequency harmonics so the winding is shock excited.

Some of the Sencore capacitor testers also have a ringing test available for inductors.

Harvey

On 5/9/2021 11:22 AM, - wrote:
What is the normal operating frequency of that transformer? 1000 Hz test
frequency seems to be on the rather low side. What is the frequency of the
ringing that you're seeing? That's probably the resonant frequency of the
x-former with it's associated circuitry attached and I would expect to see
the X-former normally being operated at about the same frequency. I'm not
familiar with the t935 but IMO you'd probably be better off using a very
low voltage AC signal instead of a square wave to drive the transformer
when testing. Coupling the x-fomer to the signal source through a capacitor
should remove most the higher frequency harmonics from the square wave and
leave you with mostly the base frequency sine wave.

On Sun, May 9, 2021 at 11:03 AM Ham Radio <bernard.murphy@rogers.com> wrote:

The ring test with a 10 volt 1000 Hz square wave with a duty cycle of 1%
shows ringing on the primary winding of T460 - so that looks good!

No ringing though, on the feedback winding on pins 3 and 6 with the same
setup as above. Not sure 🤔 if the ring test would work here as they may
be only a few turns on the feedback winding. That winding measures a dead
short on the AVO /100 resistance scale but that does not mean the winding
is shorted.

I assume that Q444 generates the frequency of the switching pulses for
Q458 through the feedback winding.

Not too sure what to try next ...
--
Regards,
Bernie Murphy








 

Thank you to all that replied. The resistance checks validate what Micheal reported.

My ringing tests on the primary indicate a lot of damping and not what Leo eluded in the posted pictures.

The ringing test on the feedback winding, indicate a much lower amplitude but the ringing is there.

The test were done with a 1000 Hz square wave with a duty cycle of 1% and an amplitude of 10 volts.

I have posted two scope pictures in the VE3FWF folder in the Photos section showing the results of these tests.

So I guess the HV transformer defective and the T922 becomes a parts mule ...






--
Regards,
Bernie Murphy