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
Attachments
Ah thank you. There is some good information in that thread. Manual is on its way.
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.
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.
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.
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
Home
and
HP / Agilent Electronic Test Equipment Parts and Service Information
are great for this equipment
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.
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.
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.
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."
"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."
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.