Flyback Testing w/'scope

[Al]

Hi, Many years ago I was involved in TV repair and remember using a
method of 'Ring' testing a flyback using nothing more than a cap and a
scope. It was from a UK Practical Electronics article and it worked
very reliably to find shorted turns. I seem to remember it using the
horiz flyback pulse from the scope to inject a pulse and the rate of
decay was seen on the scope in the form or decaying oscillations, if
the txmr was shorted the decay was none linear and very short.
Anyone seen or using this method as I would like to get the set up and
use it again.
Al

 

[Phil Allison]

"Al" <intel@mymts.net>

Hi, Many years ago I was involved in TV repair and remember using a
method of 'Ring' testing a flyback using nothing more than a cap and a
scope. It was from a UK Practical Electronics article and it worked
very reliably to find shorted turns. I seem to remember it using the
horiz flyback pulse from the scope to inject a pulse .....

** Its a long time since all CROs had any external connection to the
horizontal time base oscillator *output*. Modern CROs have quite elaborate
triggering facilities - ie the waveform triggers the time base not the
other way around.



............. Phil

 

[John Crighton]

On Sun, 18 Apr 2004 10:29:35 +1000, "Phil Allison"
<philallison@optusnet.com.au> wrote:

"Al" <intel@mymts.net

Hi, Many years ago I was involved in TV repair and remember using a
method of 'Ring' testing a flyback using nothing more than a cap and a
scope. It was from a UK Practical Electronics article and it worked
very reliably to find shorted turns. I seem to remember it using the
horiz flyback pulse from the scope to inject a pulse .....



** Its a long time since all CROs had any external connection to the
horizontal time base oscillator *output*. Modern CROs have quite elaborate
triggering facilities - ie the waveform triggers the time base not the
other way around.



............ Phil

Hello Phil,
speaking of CROs, your own little BWD 820 has a time base output.
Any chance of a photo copy of your service manual or at least the
circuit diagram? I will pay all expenses of course.

I found the x input facility quite handy to make those LazyJew
patterns to indirectly measure the frequency of a short burst of
oscillations such as from an echo sounder output. Note the
pattern on the CRO screen then feed in a continuous external
oscillator to make the same pattern and then measure the external
oscillator with a frequency counter.

Getting back to Al's enquiry, I did read about that particular
method of testing the flyback transformers somewhere on the
internet. I seem to remember Bob Parker having made something
like what Al is looking for also. I am probably wrong but Bob can
put me right.

Regards,
John Crighton
Hornsby

 

[Phil Allison]

"John Crighton" <
"Phil Allison"

** Its a long time since all CROs had any external connection to the
horizontal time base oscillator *output*. Modern CROs have quite
elaborate
triggering facilities - ie the waveform triggers the time base not the
other way around.



............ Phil


Hello Phil,
speaking of CROs, your own little BWD 820 has a time base output.

** Sorry John, but I have never owned a BWD 820.

I own a BWD 821 and a BWD 824a, plus two others that are rather basic
including the May 1966 RTV&H 2 MHz all valve CRO which I built while at
high school. Only the valve CRO has accessible time base (sweep) output.

Getting back to Al's enquiry, I did read about that particular
method of testing the flyback transformers somewhere on the
internet.

** The idea never caught on - TV techs have a massive aversion to using
CROs or scientific methods.

I seem to remember Bob Parker having made something
like what Al is looking for also. I am probably wrong but Bob can
put me right.

See: http://members.ozemail.com.au/~bobpar/fbt.htm for Bob's LOT
tester.




.............. Phil

 

[John Crighton]

On Sun, 18 Apr 2004 12:18:31 +1000, "Phil Allison"
<philallison@optusnet.com.au> wrote:

** Sorry John, but I have never owned a BWD 820.

I own a BWD 821 and a BWD 824a, plus two others that are rather basic
including the May 1966 RTV&H 2 MHz all valve CRO which I built while at
high school. Only the valve CRO has accessible time base (sweep) output.





Getting back to Al's enquiry, I did read about that particular
method of testing the flyback transformers somewhere on the
internet.


** The idea never caught on - TV techs have a massive aversion to using
CROs or scientific methods.



I seem to remember Bob Parker having made something
like what Al is looking for also. I am probably wrong but Bob can
put me right.



See: http://members.ozemail.com.au/~bobpar/fbt.htm for Bob's LOT
tester.




............. Phil

Hello Phil,
there you go, I was out by one digit. 821 not 820.
Maybe I am getting you confused with Trevor. My memory
is playing tricks again.

Getting back to the flyback transformer tester.
That tester by Bob Parker looks good. All the info is there for Al
to build the thing from scratch if kits are no longer avaiable.
Al, you probably could add monitor points to connect your CRO
to observe the ringing if you wanted to.

Regards,
John Crighton
Hornsby

 

[Morris Odell]

Al <intel@mymts.net> wrote:

Hi, Many years ago I was involved in TV repair and remember using a
method of 'Ring' testing a flyback using nothing more than a cap and a
scope. It was from a UK Practical Electronics article and it worked
very reliably to find shorted turns. I seem to remember it using the
horiz flyback pulse from the scope to inject a pulse and the rate of
decay was seen on the scope in the form or decaying oscillations, if
the txmr was shorted the decay was none linear and very short.

A great trick.

It works on the principle of shock excited oscillations which decay in
a nonlinear rapid fashion if there is a shorted turn damping the coil.
Interpreting what you see needs some experience and understanding of
how tuned circuits behave. As others have observed, you need a scope
with some sort of pulse output from the timebase. This can be either
the sawtooth itself, or a flyback or gating pulse. Many older analog
scopes will have such an output. This is not a technique for modern
digital practice.

The procedure is something like this:

Connect the vertical input of the scope through a suitable high
impedance low capacitance probe to one end of the inductor and ground
the other end. Connect a small (a few pf) cap between the pulse output
from the scope and the ungrounded end of the coil. You might need to
fiddle with the cap value to get a good trace. Set the timebase to
free run and adjust the sweep speed and vertical sensitivity to get a
nice damped oscillation. High Q coils will give a surprisingly long
oscillation. It's worth experimenting with a few different inductors
to get the feel of it.

Morris

 

[Bob Parker]

See: http://members.ozemail.com.au/~bobpar/fbt.htm for Bob's LOT
tester.

............. Phil

Thanks Phil!

Getting back to the flyback transformer tester.
That tester by Bob Parker looks good. All the info is there for Al
to build the thing from scratch if kits are no longer avaiable.

Oh the kits are definitely still available. However I'm often
seeing more than 200 downloads per week of the kit notes from my
website, so I wonder how many people build their own instead of buying
the kit?

Cheers,
Bob

 

[Al]

Thanks for everyone that responded, fortunately I got a lead from
another NG for Bobs Bar graph tester I think I will build, I was
thinking of miniaturizing it further by using the small dip socket LED
bar graphs.
Thanks Again.
Al
On Mon, 19 Apr 2004 00:41:49 +1000, Bob Parker <bobp@bluebottle.com>
wrote:

See: http://members.ozemail.com.au/~bobpar/fbt.htm for Bob's LOT
tester.

............. Phil

Thanks Phil!


Getting back to the flyback transformer tester.
That tester by Bob Parker looks good. All the info is there for Al
to build the thing from scratch if kits are no longer avaiable.

Oh the kits are definitely still available. However I'm often
seeing more than 200 downloads per week of the kit notes from my
website, so I wonder how many people build their own instead of buying
the kit?

Cheers,
Bob

 

[John Crighton]

On Sun, 18 Apr 2004 12:18:31 +1000, "Phil Allison"
<philallison@optusnet.com.au> wrote:

** Sorry John, but I have never owned a BWD 820.

I own a BWD 821 and a BWD 824a, plus two others that are rather basic
including the May 1966 RTV&H 2 MHz all valve CRO which I built while at
high school. Only the valve CRO has accessible time base (sweep) output.

Hello Phil,
thanks for the e-mail with the circuits, very much appreciated.
My return e-mail to you bounced. Never mind, I'll answer here..

You mentioning the RTV&H reminded me of the Feb 1966 vacuum
tube voltmeter project. I was a first year apprentice at the time and
the radio trade school for Adelaide was in a couple of transportable
wooden huts at the rear of Thebarton boys technical high school.

Anyway, the school used VTVMs on every bench so I thought I should
have one also. When I finished building the VTVM, I checked every
bleeding connection upteen times but it would not work properly.
One of the technicians at work helped me modify the unit to make
it measure properly, but that just didn't seem to me to be the right
thing to do as it was built exactly according to the magazine
instructions.
Years later, by chance, I saw the March 66 RTV&H, errata and notes,
with the missing links on the switch circuit diagram. We had modified
the unit correctly after all.

My wages were five pounds 10 shillings and sixpence for a few
pay packets then changed to $11.05. The meter movement
alone cost $10. An expensive project if you compare todays
first year apprentice wages of around a couple of hundred dollars.
Your CRO project would have been heaps more expensive than the
VTVM project. Cripes, you were keen for a high school student.

Regards,
John Crighton
Hornsby

 

[Phil Allison]

"John Crighton" <john_c@tpg.com.au>


(snip)

Your CRO project would have been heaps more expensive than the
VTVM project. Cripes, you were keen for a high school student.

** The parts for my CRO were bought out of money earned from a holiday job
and accumulated over a period of 12 months or more. I bought the Philips 3
inch CRT first ( from Radio Despatch Service) and the Mu-Metal shield and
rigged them up on so I had a green spot on the screen using the PSU voltages
available from a my 2 x 6BQ5 valve amp.

I think all up it cost maybe $85 in parts - my brother in law
fabricated the steel case from the drawings, he was a welder by trade. I
certainly got my money's worth out of it over the next 10 years both in
hobby and work usage.




............. Phil