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HP8903A First impressions
HP8903A First impressions
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Old 19th June 2019, 07:59 AM   #11
tomchr is offline tomchr  Canada
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Replacing the supply caps is not likely to restore display brightness.

Step 1: Get the service manual.
Step 2: Measure the ripple voltage and supply voltage on the three supplies in the instrument. If they're within spec, don't "fix" anything.

Before you remove the filter mod, I'd look at what it does. It may be useful. If you decide to remove the filter, you'll find the schematics in the service manual useful in bringing the instrument back to its original state. The BNCs for the filter mod were clearly added. They have no HP labels and aren't even aligned vertically.

This instrument is NOT an OPT001 version. It clearly has the front panel connectors.

+1 for building a shorting plug. Or use a 50 Ω terminator when measuring the noise floor. If you perform a loopback test (1 kHz sine, 1 V RMS), you should see around 0.00x % THD+N (80 kHz BW). I seem to recall mine getting down to 0.002 % (best case, 1 kHz).

Tom
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Last edited by tomchr; 19th June 2019 at 08:02 AM.
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Old 19th June 2019, 08:29 AM   #12
as_audio is offline as_audio
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Quote:
Originally Posted by 27AUDIO View Post
I bought it in Germany for 370
This price is not cheap but fair enough I think.

Quote:
Originally Posted by tomchr View Post
If you perform a loopback test (1 kHz sine, 1 V RMS), you should see around 0.00x % THD+N (80 kHz BW). I seem to recall mine getting down to 0.002 % (best case, 1 kHz).
Tom
My instrument shows 0.0035% in this case.
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Old 19th June 2019, 09:06 AM   #13
27AUDIO is offline 27AUDIO
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Ok, thanks all, I will check the PSupply ripple!.

Any advice for a better display brightness?
Thanks
S.
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Old 19th June 2019, 11:20 AM   #14
as_audio is offline as_audio
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PS re post 1 etc the difference in "self noise" is not large here between open and shorted input : 14 and 10 μvolts.
Always observe with a scope connected to the "Monitor" output to be sure what is going on.
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Old 21st June 2019, 09:01 AM   #15
27AUDIO is offline 27AUDIO
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suggestions for better the display brightness ?


Thanks
S.
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Old 22nd June 2019, 01:57 AM   #16
PRR is offline PRR  United States
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Quote:
Originally Posted by tomchr View Post
Replacing the supply caps is not likely to restore display brightness.....
It "might".

If it is supposed to be a cap-input DC supply, and that cap is quite dead, the average current may be 2/3rd, or even less if LED voltage is a large part of design DC voltage.

And they will flicker.

I had a microwave suffer a lightning near-hit. After that the display was not as bright and if you waved your hand/head it "strobed" (flicker). My guess was that half the diodes went poof and it was eating half-wave instead of full-wave; but maybe the cap went poof. (This was a VFD so probably did not share a power supply with anything else in the box.)
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Old 22nd June 2019, 02:08 AM   #17
1audio is offline 1audio  United States
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HP8903A First impressions
Its quite unlikely since the audio channels would have severe hum as well.

The LED displays are old and early generation so not as bright as newer LED's. You may be able to change the current setting resistors. However the red filter in front may be quite dirty and scratched. Quickest and cheapest fix- turn down the room lights. . .

I recently upgraded the lighting in my shop and went too far. Even new monitors are washed out by the light BUT I can see everything inside electronics (and every bit of dirt and crud in the room).
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