HP 8903B voltage drop at higher frequencies  - Page 1
Forum section: electronics / repair

author: thinker1920
author_rank: Contributor
timestamp: October 30, 2015, 07:33:48 am
content: Recently bought a HP 8903b everything seems to check out but the power is dropping out at higher frequencies. When I set it to 1 KHz and 1 volt it reads 1000.7 Hz and 1.000 Volts which seems good to me but @ 100 KHz it reads 0.387 Volts, so I'm wondering if the signal generator transistor or power supply has something wrong with it that's causing it to lose power at higher frequencies? It begins to gradually lose power around 6 KHz. Thoughts?

author: macboy
author_rank: Super Contributor
timestamp: October 30, 2015, 01:16:50 pm
content: Have you verified the output voltage with another voltmeter? If not then you don't even know if the fault is in the oscillator/output section or the input/measurement section. When measuring the output, keep in mind the bandwidth of the meter you are using. Most handheld meters are not flat to 100 kHz. You will need to use a good quality bench meter or an oscilloscope at that frequency.

author: thinker1920
author_rank: Contributor
timestamp: October 30, 2015, 11:33:55 pm
content: Here are the readings,I used an HP 54200A Digitizing Oscilloscope and a Tenma 72-7735 digital MM. I tied the oscilloscope in to the HP 8903 with a BNC T and took reading with a digital MM.HP 8903b set to HP 8903 Readings Oscilloscope Readings MM Readings20 Hz 19.99Hz 0.997v 20Hz 2.75v 0.991v100Hz 99.98Hz 1.000v 100Hz 2.79v 0.993v500HZ 500.29Hz 1.000v 498.8-501.3Hz 2.79v 0.990v1KHz 1001.2Hz 1.000v 998-1004Hz 2.73v 0.980v5KHz 4994.3Hz 0.998v 4975-5013Hz 2.73v 0.779v10KHz 10002Hz 0.993v 9980-10020Hz 2.73v 0.519v20KHz 20007Hz 0.973v 19960-20040Hz 2.61v 0.253v50KHz 50018Hz 0.821v 49750-50250Hz 2.33v 0.014v100KHz 99985HZ 0.363v 99800-100200Hz 1.98v 0v

author: dacman
author_rank: Frequent Contributor
timestamp: October 31, 2015, 12:50:26 am
content: Check A3C2 & A3C3. These are 20 µF 50 V capacitors on the 2nd card in the card cage.

author: thinker1920
author_rank: Contributor
timestamp: October 31, 2015, 01:41:05 am
content: I think I found the caps but, does anyone have a circuit diagram from the HP 8903? I found the user manual and the service manual but, I haven't been able to find circuit diagrams.To test them I would have to remove them from the board correct? Should I just go ahead and change them? When I do work on older amplifiers I typically change out the electrolytic capacitors if they're over 20-25 years. If I do need to replace them is there any one capacitor that's better than another? or could I just use Nichicon Fine gold caps? And is there any other components in these that are known to go with with age?Thanks

author: dacman
author_rank: Frequent Contributor
timestamp: October 31, 2015, 02:27:07 am
content: There are two service manuals, and a calibration manual. They are available at the Keysight website, but you'll need to create an account.Caps with values of electrolytes can usually be tested in-circuit with a good LCR meter (if it uses a bridge circuit). If they are so bad that they are pulling down the circuit, then even a cheap ESR meter should work. I would not trust the capacitance function of a DMM for an in-circuit test whatsoever.I'd replace those with 22 µF mil spec tantalum capacitors (and they are not cheap). They are on the ± 15 Vdc input lines of the PCB.OEM:http://www.newark.com/vishay-sprague/te1305/aluminum-electrolytic-capacitor/dp/18F531What I'd use:http://www.digikey.com/product-detail/en/M39003%2F01-6234/1012-1075-MIL/319097235 Vdc cap should work on the 15 Vdc rails:http://www.digikey.com/product-detail/en/M39003%2F01-6111/1012-1068-MIL/3190965

author: thinker1920
author_rank: Contributor
timestamp: October 31, 2015, 03:24:03 am
content: You're not kidding those are pricey caps, I think I'm going to hold off on that upgrade for the time being and just go with the cheaper ones and see if it corrects the problem. I'll let you know what happens, Thanks for the Help!

author: thinker1920
author_rank: Contributor
timestamp: November 09, 2015, 02:04:38 am
content: Just put in the new capacitors, the voltage levels have slightly increased but, I also realized I'd made a stupid mistake when taking my reading. I had the 80 KHz filter on :p with it off I get 0.78 v on the HP8903 and 2.05v on the oscilloscope @100 KHz, 1 volt. And the distortion reading at 100 KHz is 0.0358 and at 1KHz is 0.0264 that's with no filters on. Does that sounds about right or is something off?

author: dacman
author_rank: Frequent Contributor
timestamp: November 09, 2015, 03:38:36 am
content: Voltmeter reading at 100 kHz is low and distortion at 1 kHz is high (100 kHz distortion is OK). What readings do you get with the output set to 6V?Go to section 5 of the calibration manual. Most of the adjustments are for the meter. There are several flatness adjustments for the attenuator.For adjustment 6, the output can be connected to the input and then it can be adjusted for the lowest distortion (-90 dB @ 1 kHz which is about 0.0032 %). The spec at 1 kHz is -80 dB (or 0.0100%).The only other common issue with these instruments that I know of is the Float Chassis switches sometimes need to be cleaned (but not very often). They would need to be lubricated after cleaning (or just lubricated).

author: thinker1920
author_rank: Contributor
timestamp: November 09, 2015, 05:34:00 am
content: With all the filters off and it set to 1KHz @ 6 volts the distortion reads 0.0059, with 100KHz @ 6 volts the distortion reads 0.0880

author: dacman
author_rank: Frequent Contributor
timestamp: November 10, 2015, 02:07:55 am
content: Quote from: thinker1920 on November 09, 2015, 05:34:00 amSlightly OOT at 100 kHz. Unit needs tweaked.

author: thinker1920
author_rank: Contributor
timestamp: November 10, 2015, 03:13:27 am
content: Do you think something needs to be replaced or does it just need to be re-calibrated? I tried starting at the beginning of the trouble shooting in the service manual and it instructs you to:1. On the Audio Analyzer, key in 41.0 SPCL to initialize the instrument. Set the INPUT and OUTPUT switchesto ground. Set IMPEDANCE to 600R. Key in AMPTD 3 V. Connect the HIGH OUTPUT to the HIGH INPUTthrough a tee adapter.2. Connect a high-impedance, ac coupled oscilloscope to the tee at the HIGH INPUT. Use the oscilloscope’svertical gain controls to adjust the display amplitude for 5 divisions peak-to-peak deflection of the sinusoidalsignal. (The period of the signal should be 1 ms.)3. Connect the oscilloscope to A2TP5 (SING END). Key in the Special functions listed in Table 8B-4. For eachsetting, increase the oscilloscope’s vertical gain by the amount indicated. The peak-to-peak amplitude, displayedon the oscilloscope, should be within the limits indicated. If faulty, see Service Sheet 1 and check the High InputAttenuator.Now I'm new to using an oscilloscope so I'm pretty sure I'm doing it wrong, I set it to auto scale and to a/c coupling and when I punch in V ampl measure channel I get 8.18 V so I tried V max and got 4.14v but according to the chart it's suppose to be between 4.9 and 5.1 amplitude limits (divisions peak to peak)So if I'm making a stupid mistake in my settings please correct me.

author: dacman
author_rank: Frequent Contributor
timestamp: November 10, 2015, 04:08:32 am
content: I'd do the calibration first. I would not know whether or not a repair was needed until after the calibration was attempted. Your unit isn't that bad, it's just out-of-tolerance.You need to take your oscilloscope out of auto mode. They want you to adjust for a 5 division peak to peak reference in step 2 using the oscilloscope's amplitude vernier (and V/DIV control), then connect to A2TP5 and check for the same 5 divisions in step 3. Later, when instructed to increase the oscilloscope's vertical gain by X10, if it were set to, for example, 1V/DIV, you would then set it to 100mV/DIV leaving the vernier alone.

author: thinker1920
author_rank: Contributor
timestamp: November 10, 2015, 07:40:32 am
content: When I attempt the DC offset adjustment in the calibration section the lowest it will go is -62.5 mVdc but the manual wants it to be 0 +/- 0.1 mVdc.

author: dacman
author_rank: Frequent Contributor
timestamp: November 11, 2015, 12:45:45 am
content: Check if A6TP4 is varying (referenced to A6TP1 or GND). If you have a BNC short, then place it on the OUTPUT LOW connector and see what happens.

author: Simon Spiers
author_rank: Regular Contributor
timestamp: November 12, 2015, 08:54:45 pm
content: Make sure your termination is correct.I would sweep the output through a scope of adequate bandwidth and see if you can see any rollout.

author: thinker1920
author_rank: Contributor
timestamp: November 12, 2015, 10:13:42 pm
content: Checked A6TP4 refrenced to ground not shorted reading was 1,002 Hz @ 2.631 Vrms Shorted gave me the same reading (had it set to 3 volts)

author: dacman
author_rank: Frequent Contributor
timestamp: November 12, 2015, 10:59:40 pm
content: Quote from: thinker1920 on November 10, 2015, 07:40:32 amIf this is Adjustment 9, and in step 3 you are measuring -62.5 mVdc at the OUTPUT High jack, then this could be caused by the Float Chassis switch on the OUTPUT side. Place a BNC shorting cap on the OUTPUT Low jack. This is equivalent to setting the Float Chassis switch to Chassis. (I know the manual uses ground vice chassis, but the symbol on the front is the chassis symbol.) If the output can then be adjusted to 0 mVdc, then the Float Chassis switch needs cleaned (I'd use LPS-1 on the switch).http://www.amazon.com/s/ref=nb_sb_noss_2/181-8212496-9497506?url=search-alias%3Daps&field-keywords=BNC+Shorting+cap&rh=i%3Aaps%2Ck%3ABNC+Shorting+capI'd want the 3rd one in the list above (the Amphenol).Also, The 8903B came with four of the below adapters (although they were gray).http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=Pomona+3430&rh=i%3Aaps%2Ck%3APomona+3430

author: thinker1920
author_rank: Contributor
timestamp: November 13, 2015, 12:21:41 am
content: I shorted the Low output and it works like a charm, I took apart the front and sprayed deoxit in the float switch and flipped it back and forth a bunch and put it back together and it's still not working, again if I take a piece of wire and short the low it works great I get 0.985 volts at 100 KHz and was able to adjust my DC offset to 0.1 mV. Thoughts? I was pretty thorough with the cleaning.

author: dacman
author_rank: Frequent Contributor
timestamp: November 13, 2015, 12:44:22 am
content: I've seen LPS PreSolve work when DeoxIT would not.Try some contact cleaner or IPA. (You would need to lubricate the switch after using one of these, else it could be destroyed for lack of lubrication. I use LPS-1.)If you can get to the contacts, try an ink eraser.If you want to know what I would have tried at the start, I would have drenched it in LPS-1 (and if this did not work, I would have then tried the above).In the meantime, I recommend getting a BNC Shoring Cap, and just using it (a minor issue).

author: thinker1920
author_rank: Contributor
timestamp: November 13, 2015, 12:51:42 am
content: Thank you for seeing me through this!!! I really do appreciate it

author: dacman
author_rank: Frequent Contributor
timestamp: November 13, 2015, 01:19:58 am
content: The value you were getting, -62.5 mVdc, is pretty bad, almost as if the switch is open, and if it were adjusted down to that, then maybe the switch is open. Could you check the continuity of the switch connections to see if it is actually connected to the motherboard? Could you measure the resistance across the switch?

author: thinker1920
author_rank: Contributor
timestamp: November 13, 2015, 02:50:09 am
content: It was the float switch and at the same time it wasn'tI dissembled the actual switch and cleaned the internal contacts which were black, I used a Q tip with deoxit, I reassembled it and measured it with my DMM, and it checked out. I reassembled and turned it on and got the same readings :p So I thought maybe the wire that connected it to the mother board was bad so I dove into it again and disconnected the molex type connector and used my DMM and the wire was good. So I then took off the bottom of the HP8903b and thought I would follow the traces, and it was right where the connector was solder to the board, a trace that looked like it had more or less vaporized, I soldered in a small piece of wire where the trace use to be and voila! All my readings are within tolerance, DC offset is now 0 mVdc, set at 1KHz @ 1 volt reads 1000.9 Hz @ 1.009 volts with .0024 distortion (with 80 KHz filter), set to 100KHz @ 1 volt reads 99,891 Hz @ 1.004 volts with .0214 distortion (no filter). Are those acceptable numbers?

author: dacman
author_rank: Frequent Contributor
timestamp: November 13, 2015, 03:53:19 am
content: All those numbers look fine. -92 dB residual at 1 kHz is very good.