Ok Now that it is working again.... at 1 volt output into analyzer, no loading. Adjust for full scale etc and then put into thd mode on -80db and adjust oscillator (2K Ohm) for lowest distortion. what do you get?
THx-RNMarsh
THx-RNMarsh
The input to the the op amp after the filters has one Mega ohm resistor to ground. The input bias current through the 1meg ohm resistor and high gain of the amp could pin the output to a rail if that switch screws up. So it's probably a bad switch.
Don't know why HP did it this way. A smaller resistor and bigger coupling cap would work just as well.
Don't know why HP did it this way. A smaller resistor and bigger coupling cap would work just as well.
That's a Sallen Key filter. Lowering the impedance will make it less sensitive but will also cause the upstream stages to be more stresses and need bigger caps. That resistor is part of the filter.
My guess is still a short somewhere, possibly to a supply.
My guess is still a short somewhere, possibly to a supply.
I was referring to the post amplifier not the filter.
But he did say TP5 which is before the post amp so the point invalid.
Forget the filters for now. A lot of the used units have bad filter switches (open-- non-contact or intermittant) from years of wear. Leave them Out and jumper from j200 to TP5 and do the following, below -->
I"m not there yet.
Had been working through section 3-48, 3-49
the Oscillator section, and see what the readings
on the meter are.
passed until:
Failed 3-49 f. meter pegged to the right, beyond +2 (blue scale [bottom]
Passed 3-49 g., h, i, j, k. So when meter adjusted to each Oscillator Level & Input Range, it steps accurately 10 dB increments on the meter w/o any other changes.
So now I'm can get to your latest post.
Got it.
Pulled connector and jumpered it.
Will adjust and post results. I haven't gotten lowest null or anything yet.
Pulled connector and jumpered it.
Will adjust and post results. I haven't gotten lowest null or anything yet.
Pulling J200 and jumping to TP5 on the A2 board for those following along.
After setting it up, 1kHz, 1.volt, into Distortion Analyzer
of HP339a, @-80dB, 3V input range (no lights), and
function set to Distortion...
HP 339a meter has oscillation between -5.5dB to -6 dB.
Then adjusting oscillator level A1 R30 it moves slighly
to narrow oscillation between -5.8 dB to -6.1 dB.
Full CW -5dB
Full CCW 0
Just before full CCW -7dB
about middle range -6dB
When I plug into Monitor output, the meter gets pegged to right
>0 dB.
After setting it up, 1kHz, 1.volt, into Distortion Analyzer
of HP339a, @-80dB, 3V input range (no lights), and
function set to Distortion...
HP 339a meter has oscillation between -5.5dB to -6 dB.
Then adjusting oscillator level A1 R30 it moves slighly
to narrow oscillation between -5.8 dB to -6.1 dB.
Full CW -5dB
Full CCW 0
Just before full CCW -7dB
about middle range -6dB
When I plug into Monitor output, the meter gets pegged to right
>0 dB.
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You are on the wrong range. One volt out and be on the one volt (not 3 volt) input range. Then adjust osc output level for 1v full scale with function switch on the Input level. Then change function switch to Ref Level and adjust 'Relative Level' for full scale. Now change function switch to Distortion.
Next, adjust the internal osc level pot (A1R30) for lowest distortion. You might have to be at a higher Distortion range at first and lower it as you get a better null.
Let us know what happens.
THx-RNMarsh
Next, adjust the internal osc level pot (A1R30) for lowest distortion. You might have to be at a higher Distortion range at first and lower it as you get a better null.
Let us know what happens.
THx-RNMarsh
Richard,
I tried that, matching OSC LEVEL output with INPUT RANGE
but noise is so bad and meter too unstable to measure it.
Bouncing around as described in my 'She's So Fine' post # 409.
When I plugged in the QA400 to plot it the harmonics
are in the noise and QA400 can't measure it properly.
But then maybe I shouldn't have set the DISTORTION RANGE
to -80dB so I'll use a smaller range and maybe that will work.
Currently I'm trying to set the meter however with different
FUNCTION selections the meter doesn't what I expect.
I don't know yet if it's me or the meter or the set up.
1kHz, 1V at "full scale" on the meter, that is the
needle is on the Zero (top scale) and One (second scale).
These scales are together just above the platinum strip
on the HP339a meter. In this case they are in sync.
Yes, I confirmed the OSC LEVEL, INPUT LEVEL,
and REL LEVEL have the same meter indication.
Zero & One at the right side of the meter.
I tried that, matching OSC LEVEL output with INPUT RANGE
but noise is so bad and meter too unstable to measure it.
Bouncing around as described in my 'She's So Fine' post # 409.
When I plugged in the QA400 to plot it the harmonics
are in the noise and QA400 can't measure it properly.
But then maybe I shouldn't have set the DISTORTION RANGE
to -80dB so I'll use a smaller range and maybe that will work.
Currently I'm trying to set the meter however with different
FUNCTION selections the meter doesn't what I expect.
I don't know yet if it's me or the meter or the set up.
1kHz, 1V at "full scale" on the meter, that is the
needle is on the Zero (top scale) and One (second scale).
These scales are together just above the platinum strip
on the HP339a meter. In this case they are in sync.
Yes, I confirmed the OSC LEVEL, INPUT LEVEL,
and REL LEVEL have the same meter indication.
Zero & One at the right side of the meter.
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Clarification: Most correct meter indication is One on
second scale which is the Volt scale on HP339a.
second scale which is the Volt scale on HP339a.
Update: with -80 Distortion range, the lowest
meter reading is about -12.5 +- 0.2 for -92.5 dB
This reading is with J200 removed and Jumper
J200 on A2 Board pin1 to A2 TP5 Jumpered.
This is after performing checks in post # 411.
meter reading is about -12.5 +- 0.2 for -92.5 dB
This reading is with J200 removed and Jumper
J200 on A2 Board pin1 to A2 TP5 Jumpered.
This is after performing checks in post # 411.
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That's excelent isn't is! Tiny bit of fine tuning and you got it back to specs..?
Yes, thats what it seems to me, also. Adjust the osc level on the pcb to get a null... below -92.5dB if you can. Can be very touchy if you didnt replace that control with a ten-turn. Then see what it reads. Final adjust is on the analyzer portion..... null is on the vertical plug-in pcb at the rear.
THx-RNMarsh
THx-RNMarsh
tschrama, I didn't think that was so good,
was thinking it should be below -120 dB?
Oh, that's just for the ocsillator.
A lot of work into this.
But, it is in spec now so that is good.
Thanks for letting me know.
Trying to recall what the spec is...
will have to look.
However, I have to remember what Richard and
others pointed out...HP339a can't resolve THD better
than about 100 dB from its own analyzer.
Wondering how to get around -120 dB or better.
Maybe I fat fingered the quad opamp when trying
to measure them....or bent a pin on the A1U2
quad opamp. I'll have to get back to it again
and check.
I'll have to get the 1st removable board, the A4 board
fixed, reconfig to stock and put back in before making
more adjustments for null debth.
I'll check the meter again with A2 J200 back in, so I can
see if their are any differences. Pics at 11.
One other thing, the hard part of using the HP339a with
the QA400 using 1V, 1kHz, is that QA400 starts clipping.
Demian, showed me how to make a simple low distortion
divider network that I can use right at the input to the
QA400. The following resistors are used:
POS +--10k----+----10k----1K -NEG
Can use either + pos terminal for 10k or 20k resistance
and the NEG terminal.
More to come.
Cheers,
was thinking it should be below -120 dB?
Oh, that's just for the ocsillator.
A lot of work into this.
But, it is in spec now so that is good.
Thanks for letting me know.
Trying to recall what the spec is...
will have to look.
However, I have to remember what Richard and
others pointed out...HP339a can't resolve THD better
than about 100 dB from its own analyzer.
Wondering how to get around -120 dB or better.
Maybe I fat fingered the quad opamp when trying
to measure them....or bent a pin on the A1U2
quad opamp. I'll have to get back to it again
and check.
I'll have to get the 1st removable board, the A4 board
fixed, reconfig to stock and put back in before making
more adjustments for null debth.
I'll check the meter again with A2 J200 back in, so I can
see if their are any differences. Pics at 11.
One other thing, the hard part of using the HP339a with
the QA400 using 1V, 1kHz, is that QA400 starts clipping.
Demian, showed me how to make a simple low distortion
divider network that I can use right at the input to the
QA400. The following resistors are used:
POS +--10k----+----10k----1K -NEG
Can use either + pos terminal for 10k or 20k resistance
and the NEG terminal.
More to come.
Cheers,
You shouldn't get any clipping from the 339A monitor port on the QA400.
I did all the mods on the 339A with nothing but the QA400.
That was it's intro to DIYAudio as it wasn't quite ready for release to the public.
I was on the beta team.
I did all the mods on the 339A with nothing but the QA400.
That was it's intro to DIYAudio as it wasn't quite ready for release to the public.
I was on the beta team.
IF the meter reading is at full scale, and the monitor output is !VRMS -- any error between the two might cause a problem....
David,
I don't know as it was always too noisy. Before I do anything else,
that is the next thing that I was going to do, check out the monitor
port with everything configured the same.
Then I will reconnect A2 J200 connector and check again.
That will give me some good trouble shooting information.
At 1Vrms the QA400 shows clipping, the led blinks like crazy.
I don't know as it was always too noisy. Before I do anything else,
that is the next thing that I was going to do, check out the monitor
port with everything configured the same.
Then I will reconnect A2 J200 connector and check again.
That will give me some good trouble shooting information.
At 1Vrms the QA400 shows clipping, the led blinks like crazy.
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RichEEM,
With the HP339a meter reading -12 dB
When I plug into the meter, Monitor
the meter goes beyond full scale.
Then the input range front panel LED
lights up and wants me to change the
Input Range switch to the 3V selection.
When that happens and I select the
3V input range, the meter then resumes
normal operation and I can read from it.
Also, the noise floor completely drops out.
I've got timely plots if you guys are still around.
With the HP339a meter reading -12 dB
When I plug into the meter, Monitor
the meter goes beyond full scale.
Then the input range front panel LED
lights up and wants me to change the
Input Range switch to the 3V selection.
When that happens and I select the
3V input range, the meter then resumes
normal operation and I can read from it.
Also, the noise floor completely drops out.
I've got timely plots if you guys are still around.
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