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Old 7th February 2007, 11:05 PM   #1
NH TomG is offline NH TomG  United States
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Default HK Citation 17S trouble- HELP!

Maybe someone here has a clue, I certainly don't. I have a Citation 17S preamp on the bench that sends DC to the output of one channel whenever the subsonic filter is switched in. It swings erratically from + to - with a magnitude of 2.5 VDC or so, gradually decaying to lower voltages. There is a small measureable pulse at the the output of the filter board when the subsonic filter is switched in, on the order of a few MV DC. I suspect that the output stage may be oscillating, but since my scope is dead, I cannot view the output waveform. I am using a DVM to check for DC. Here's a link to a schematic, any ideas are welcome: http://manuals.harman.com/hk/Service...n%20s%20sm.pdf
So far, I have changed all the electrolytics on the filter board, as well as the ceramic caps related to the subsonic filter, and the op amp also.
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Old 9th February 2007, 08:03 AM   #2
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Was the OP amp replaced with the same chip or a faster one?
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Old 10th February 2007, 01:24 PM   #3
NH TomG is offline NH TomG  United States
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The replacement was an NTE, allegedly a direct cross.
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Old 10th February 2007, 02:12 PM   #4
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Was it the same problem before the opamp change? You need to get your o'scope fixed dude. This problem would be so much easier to narrow down.
Something else you might check is the switch itself it may have dirty or broken contacts or cold solder joints.
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Old 10th February 2007, 11:41 PM   #5
NH TomG is offline NH TomG  United States
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The problem was there before the opamp change, changing it made no difference. I removed the switch, took it apart, and burnished everything plus cleaned with Deoxit, the switch is now spotless, and the condition is the same. I believe I may have more than one problem- minor glitch on the filter board does something that results in the output board for that channel oscillating/swinging.
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Old 11th February 2007, 02:14 AM   #6
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Does the unit function without the subsonic switch engaged?
Which channel does it effect?Does the Mono switch or the Hi cut filter switch effect the problem?
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Old 11th February 2007, 04:07 AM   #7
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Do you have your +/- 15vdc on pins 4 + 7 of the opamps? I believe there is a typo error on your schematic on IC1 it states +15 on both pins 4 & 7...NOT pin 7 positive and pin 4 negative...Similar error with IC2.
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Old 11th February 2007, 03:53 PM   #8
NH TomG is offline NH TomG  United States
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Quote:
Originally posted by XEAGLEKEEPER
Does the unit function without the subsonic switch engaged?
Which channel does it effect?Does the Mono switch or the Hi cut filter switch effect the problem?

Quote:
Originally posted by XEAGLEKEEPER
Do you have your +/- 15vdc on pins 4 + 7 of the opamps? I believe there is a typo error on your schematic on IC1 it states +15 on both pins 4 & 7...NOT pin 7 positive and pin 4 negative...Similar error with IC2.
The unit does function with the subsonic filter disengaged; problem is in the RH channel. The mono switch has no effect, problem remains on the RH channel. With the Hi cut engaged, the problem changes- a single spike of around 70 to 90 mV DC appears at the RH output, quickly decays, does not swing or oscillate. I have -15.27 VDC at pin 4, and +15.46 VDC at pin 7. When I got this piece, it was a DOA/parts unit. It needed new regulators plus Q3 and Q4 on the power supply board for it to operate. Shortly after I got it operational, the subsonic filter problem appeared.

If the hi filter is engaged after the subsonic, the voltage swing is not halted by engaging the hi cut.
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Old 11th February 2007, 04:46 PM   #9
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Have you verified the resistances of R20,21,22, 23 & 24.?

Dave
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Old 11th February 2007, 06:10 PM   #10
NH TomG is offline NH TomG  United States
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Quote:
Originally posted by XEAGLEKEEPER
Have you verified the resistances of R20,21,22, 23 & 24.?

Dave

Tested them in circuit, and they measure identically in both channels; did not remove them from the board to check out of circuit.
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