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Old 8th February 2015, 06:47 PM   #1
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Default Leader LFG-1300s DOA Fix Then Upgrade

Had one of these on the shelf for awhile now.
We all know the story line...went to use it and
not working.

My plan is to get it working first
then upgrade some parts, i.e., 741 opamp.

I'm learning too and this little unit is probably
not as demanding or as finicky as the HP339A
that has been giving me fits.

I originally picked it up because it does a variety of signal generating
and can perform sweep functions and provide carrier functions.

Haven't opened it up yet but guessing it won't be the PITA to
work on that the HP339a is.

I found the operators manual and the service manual online.
Kind of fuzzy hard to read but should be workable. I've read through
the dead thread about this unit and that should be a good reference to
start from. I would have posted there, but I want to start from dead
and then upgrade.
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Old 8th February 2015, 11:52 PM   #2
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I'll post some oscillator pics first
to show what the minimal output
looks like.

Now I've got to find them.
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Old 9th February 2015, 07:06 PM   #3
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Oh the un-Joy of using windows 8.1.

What ever you do, don't get anything with Microsoft 8.1.
It is the worst operating system I've ever used. Microsoft
trys to force you into some unknown MS world, most of the
old apps that I have don't work with it. You are forced to
use some klugy OS that wants to be some type phone on
your computer.

That aside, the bottom is what I've been able to upload.

Based on what I have found, I think there was a batch
of defective boards from Leader. These appear to be
defects on the output section where there are heat
and output problems. Unless this burned up so badly
that the traces and solder mask had to be rubbed out
for the board to function again.

Not a very good repair in my opinion. Once the carbon
is on a board it should, all of it needs to be removed other
wise it keeps getting worse over time.

Then, the first series of tests were in spec except the final
test on TP-3, which was -10.03 vs. -10.00 +- 1%.

Close enough not be the problem.

I then checked TP2, which was 44.2V P or 88.4 P-P.

Seems high but the doco I have doesn't show data for that
point.

Another problem seems to have the 220pf polystryene cap
right next to the hottest part of the main board. This is C207
which was underlined in my version of the service manual.

Also, since i've not used these things before, I guess they work
the following way:

A 1k sine wave is output, then if triangle is selected it places
a small triangle wave over the AC wave?

For some reason I was thinking the output would be one Large triangle wave
at a frequency of 1kHz.

Click the image to open in full size.


Click the image to open in full size.


And of course the scope pic at 44Vac peak I forgot to take.

In the process of cleaning up the board. The burned side of the board
is runny, looks like a layer of something is all over it running from back
to front.

I'm thinking I need to removed all these components then sand off all the
burned carbon, then cut a small portion of bread board without traces and
glue it to the old board....

Once dry, then reinstall new components or old components that still
work if I can't find replacements.
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Last edited by SyncTronX; 9th February 2015 at 07:17 PM.
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Old 9th February 2015, 07:26 PM   #4
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C207 is on that list non polystyrene (maybe a good polyproplene cap.
How would a Mica do in this position?

Tricky I think will be the encapsulated ICs

Q212 NPN 2SC1907
Q217 PNP 2SA571

Then the others which may be just fine:
Q211 PNP 2SA711
Q213 NPN 2SC97A

Q215 NPN 2SC1907
Q216 PNP 2SA711

On the solder side of the board, the Buss 1/10 fuse.

That is a start any way. Wondering what the root cause
that keeps frying the board.
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Old 10th February 2015, 07:10 PM   #5
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Got it working again.

First I noticed a coating of goo on the solder side of the board.
Cleaned it up with isopropyl alcohol on Qtips and poly wipes.
Also cleaned up the burned area on the top of the board.

Didn't have the replacement fuse so had to make do with what
I had. Soldered leads onto a regular fuse, placed it in heat shrink
to keep it from shorting out. Heat shrink is longer than the fuse
to prevent shorting. Soldered it in to see where we were with the
unit.

Plugged it in and turned it on to see where we were with the
unit. It powers up and I have output on the front panel.
I took some pics which I'll post when I can.

Output is fine and was adjusted to 20v P-P.
However, it clips the lower part of the waves
with the attenuator buttons are pushed.

It looks like it would if you were using diode clippers
on the bottom have of all waves, sine, triangle, square...
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Old 11th February 2015, 02:49 AM   #6
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Here are the pics of the "non-output" before fixed.
No attenuation. The only thing I think was going out
was a low level signal on a carrier, though I didn't
have carrier selected.

This is the sine wave problem:

Click the image to open in full size.



Triangle wave problem:

Click the image to open in full size.


Square wave problem:

Click the image to open in full size.
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Old 11th February 2015, 03:00 AM   #7
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This is the temporary fix I did
to the board so that I could see
if there were any other problems.

I want to see if the thing will even
work.

This is a 1/10A fust that was removed
I didn't have a mini one to reinstall but
I did have some AGC 1/10A fuses.

Here's the board:

Click the image to open in full size.


After that just checked the output per the manual:

Click the image to open in full size.

Then I checked the various attenuation levels.

Here is the output with 10dB attenuation selected:

Click the image to open in full size.

Nuts. It looks like a diode is clipping off the bottom half of
the signal, not good. More to check.

Here is the out with 40dB attenuation selected:

Click the image to open in full size.

Same problem with the 20dB attenuation switch also.

Will do some further checking on schematic and some switch
cleaning when I get the chance.
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Old 11th February 2015, 11:26 AM   #8
skidave is offline skidave  United States
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Sync,

Here are links to the manuals. Make sure you read the operator's manual and make sure you have AM modulation off and the Symmetry is set correctly before you chase your tail.

Operator: http://eeshop.unl.edu/pdf/LFG1300S.pdf

Service: http://exodus.poly.edu/~kurt/manuals...%20Service.pdf

Dave
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Old 11th February 2015, 05:20 PM   #9
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Ski,

Thanks for the heads up. Was going through the service manual.
from the part of the Operators Manual I can see now what I need
to set to.

The output control pot on the main board was set to max output,
about 23 Vac. I had backed that down to 20 v P-P.
Now I see that it should be 7 V rms.

Then, I bet (as I'm a betting man) the attenuation settings
will fall in line.

Hopefully no tail chasing. I never done that.

Cheers,
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Old 12th February 2015, 03:36 AM   #10
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I'm working my way through the calibration procedure,
and I'm unsure of calibrating the Functions.

section3.5 (page3-3)
I've gone through it but not much really happens.
On the scope, I just adjusted a nice round corner on the sine wave,
between flat and pointy. None of the pots did much even on the
analyzer--barely a movement.
Triangular wave, Square wave, offset no problems.

Section 3.6 (page 3-5) AM Modulation is a different story,
as I've never done AM modulation or carriers before.

Uncertain after connect the audio generator to MOD IN connector...

"...and set the frequency to 1kHz, output level for 1Vp-p."
I set this on the oscillator output, 1kHz and output for 1V.
This was way too high for the 1V p-p on the scope, so I
put the scope on the oscillator and set it for 1Vp-p, then
reconnected as per the calibration procedure.

Neither way I tried, could I adjust the the scope or
adjust VR207 to look like that of Figure 3-1 (page 3-5).
I was able to to center a waveform and was able to set
A and B heights, but the problem was there was a flat
spot in the middle of the band and my sine wave pos
and neg were 180 degrees apart. That is the peaks
and valleys weren't aligned as shown in Figure 3-1.

If anyone who's performed this before can give me
some pointers I would appreciate it.

Cheers.



Then I back the oscillator voltage down so the scope shows
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