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Old 28th March 2012, 12:13 AM   #1
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Default Trouble with HP653A and 654A

I have a HP653A audio osic that I have been using for some time, but it's got problems. Reciently I bought a used 654A for a good price thinking I liked its condition and features better. Now the 653 has proper sine wave output, but the 654 puts out a bizzare chopped wave that looks more like a wide pulse than a sine wave. Here are pictures of the units and what the scope show's for each. Im going to hazzard a guess that the 654 has a problem with the osic. Suggestions for making one good out of two not so good? I am also looking for the service manual - might have to buy that on E-bay. Like $20
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Old 28th March 2012, 12:58 AM   #2
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Oh Oh --A little poiking around with a voltmeter in the power supply and there is not much that is the same between the two units. This could easily be the problem. Or not. Oh and on the 654 the meter is bouncing around alarmingly. DC on the AC sine wave?
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Last edited by firechief; 28th March 2012 at 01:00 AM.
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Old 28th March 2012, 12:58 PM   #3
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Have you checked out "deyjavont " s thread on the HP 654A .
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Old 28th March 2012, 01:37 PM   #4
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Quote:
Originally Posted by epicyclic View Post
Have you checked out "deyjavont " s thread on the HP 654A .
Ah thank you. There is some good information in that thread. Manual is on its way.
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Old 29th March 2012, 01:35 AM   #5
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Well - I know for sure that the two models are substantally different. Different part numbers on the boards, and different board layouts. That rules out swapping whole assemblies. Plus the power supply voltages are not even very close. Someone has been messing with the LDR assembly on the 654 and put it back together with black electrical tape. At least I can use the one from the 653 if need be, and thats a relief because I am confident that they are hard to come by.
Cant go much further without critical information.
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Old 29th March 2012, 07:05 PM   #6
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Old 2nd April 2012, 03:38 PM   #7
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So digging into this I find that the power supply comes up correctly with the main board disconnected. There are TO5 can style transistors in the drive leveling circut that have heat sinks on them and one is missing. (heatsinks that is). I pulled the transistor out and it is no longer a transistor, however the power supplies are still being dragged down. I'll bet that there are more damaged parts. Also if I look at the osicllator transistor with the scope it is not putting out anything resembling a sine wave. Not so suprising because the power supplies are being dragged down. More digging required.
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Old 4th April 2012, 12:07 AM   #8
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i had a similar problem on my 651B i had to change out allot of caps on the main board.....then i fried it again, powersupply this time.

Home
and
HP / Agilent Electronic Test Equipment Parts and Service Information

are great for this equipment
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Old 4th April 2012, 12:29 AM   #9
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Yes sakellogg- I will be trolling those links. Especally the transistor cross part reference. Still no success. This is going to take some time.
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Old 4th April 2012, 01:04 AM   #10
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Ive got a couple of pdf's of transistor xref and diode xref too if you need them.

it gives hp's part # and then the equivalent.

some are old equivalents but i just used alldatasheet.com to find repacements.
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