开云体育

Hello All,
Need help on HP-8903A, it took more than 2 years fighting,

1.it came with an ERR=18
after checked, -15V rail had bad regulator A11U2 and shorted filter cap C02, an easy fix.

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2 after few days playing, system suddenly did not respond to keyboard.
found A8 CPU board , the most CPU pins were black and rusted,?
easily crashed when touch, by soldering? IC? directly to another CPU socket with fly wires. then seat back to original socket,

it run perfectly again.

3. another few days running testing with GPIB program, thank you very much for great work from Mr. Pete Millett.

Software did control the measurement and actions,?
however the voltage reading something wrong, (not taking notes clearly for this issue)
through the block diagram, the RMS voltage measurement related with A4 board,

follow troubleshooting sheet A4 1-3, confirmed no DC checked on A4TP1,? A4U5 was suspicious.
Replaced with a new LM301AH, it run good again.

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4.few days later, the THD reading getting high just after power On, after 20mins warm up, the THD reading back to normal.
after long chasing/measurement on board, and took one lead out to measure film caps.
finally found out C45 3uf/100V reading is good, but leaking.
New film cap was replaced, it read quick again.

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5.One more issue recognized, when OSC. output lower then 0.5865V, the voltage reading and THD was not stable, up/down slowly.

it took long time verify this issue,?
Since not every time encountered, but says > 70% happened.

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I can't locate the issue either from Osc. board A6 nor Atten. Board A5,?
Until I got second unit in very reasonable low price,?
luckily, A6 Atten. board was confirmed defective board, after several swapping tests.

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6. issue is my A6 board is "3 relay" version, can't find matched schematics/troubleshooting yet.
the A6 board marked "08903-60105 / A-2242-10", Would someone could instruct where to have it.

A6 board from second unit is "2 relay" version matched the service manual.

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almost last mile to go, any hints would be super appreciated!

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Chen-shen

Hello All,

seems found the root cause of the issue?

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1. I re-checked the FET/IC caps, can't fins trace of issue?

2. the OSC. output unstable is not 100% happened.

3. the behavior is depends to operation time (temperature?)

the volatge/THD is jumping

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Presumed the issue should come from contacts?

try to raise Atten board with hand-made risers,

set to low output < 0.58V

slightly click the relay, only k2 is sensitive to touch.

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de-solder the replay and check the resistance with 5VDC apply to coil,

Pin 3/5=150Ohm, Pin6/8=0.4Ohm, yes it is a bad reply.

since not easy to get one,?

I remove the cover and try to clean the contacts by very fine sand paper with alcohol.

it works again, not sure how long it would last.

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Chen-shen