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Old 4th April 2012, 06:35 PM   #11
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Last night I preformed a main board swap. With the main board from the 653 in the 654, the power supply is still being pulled down. OK so the power supply is sick. I substituded a bench power supply and the main osic produces a nice sine wave, something I have never seen in this unit. Its time to troubleshoot the power supply. I also noticed that on some bands the osic will not run reliability and has hash on it. so that is a seperate problem probally related to the range switch and the elaborate trimmer caps/adjustments. So more than one problem! And on it goe's.
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Old 4th April 2012, 08:48 PM   #12
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Could be.
Did you get your service manual yet?
I'm wondering if its powersuply is the same as mine.
The reason I wonder is cause, I thought the same thing. But it turned out the power supply had a current limiter in it. The main board was drawing to much and the powersupply was turning itself down.
I chased my tail for weeks.
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Old 4th April 2012, 09:01 PM   #13
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Yes I have the manual. Same sympton eh? That is a clue. Main board pulling down the power supply despite that fact that I have swapped it out. Hmmmmmmm.
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Old 4th April 2012, 09:57 PM   #14
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Not saying it is. It could be your supply is bad. Might not be the same type.
Mine was kinda bypolar 30v+25- ..... I think with a monitor cir on the same board.
Just the + is regulated.
Just spitballing.
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Old 4th April 2012, 10:06 PM   #15
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Yes, the supply is Bi-polar and not balanced your recollection of the voltages is real close. I wonder what happens if I disconnect just one of the voltages? At this point both are less than they should be. Could be that monitor circut.
Spit anytime you want.
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Old 4th April 2012, 10:21 PM   #16
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If your board is the same as mine you can disconect the mon cir pretty easy. Just lift one side of afew caps and resistors.
The mon cir on my board isn't the current reg part of the cir.
It dose somthing else I forget what.
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Old 4th April 2012, 10:22 PM   #17
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More research over at the Yahoo groups proved intresting.

"Hi Gang,

I have just fixed my HP654A audio oscillator,
after having received the manual from Dave (thanks).

The fault was fairly straight forward,
the oscillator was running,
but the balanced amplifier was faulty.
It was a blown zener in the output stages.
There are 13 transistors in this balanced amplifier,
consisting of 5 pairs of transitors, and 3 feedback loops.
With all of this, it has a gain of 2!
BUT has a flat bandwidth of 10hz to 10mhz.
The output stages have several zeners to set up the DC conditions,
(six zeners and 4 diodes in total).

I was wondering where I would get a replacemnet for the zener,
when I began to read the changes in the back of the manual.
There was a change, which invovled, removing both zeners
and replacing them with resistors.
I did this and the amplifier worked fine.
Obviously, there had been problems with these zeners before.

So,
when you get a manual,
read the change notices,
they can be helpful."
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Old 4th April 2012, 11:05 PM   #18
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Oh, cool. That could be good.
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Old 5th April 2012, 04:27 PM   #19
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So I now have swapped the power supply board and the problem persists. That is the + and - supply voltages are going into current limit and being pulled down a lot. Still if I disconnect the supplies from the main board the are fine. So to re-cap the 654 has both the main board and the power supply board from the 653. The donor unit did not have this problem. Of course the power supply main electrolydics are not on the board, but on the chassis. They could be poop I suppose.
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Old 5th April 2012, 04:55 PM   #20
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did you check the zeeners?
if your boards have the same values i, have part #'s from newark that fit the boards, and are simalir values i used.
for everything sept the resistors.
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