hp 654a test oscillator - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st August 2008, 01:51 AM   #1
diyAudio Member
 
deyjavont's Avatar
 
Join Date: Aug 2008
Unhappy hp 654a test oscillator

Hi all, I just purchased this HP 654a test oscillator off evil-bay (local pickup though, so no shipping ) for $45. It has option H22( I am guessing it is a rare option because the impedance selector ranges are 75 unbal, 124 bal, 135 bal, 600 bal, and 900 bal) so no 50 ohm, but thats okay.

the main problem with it is that when I adjust the Amplitude control, nothing happens, but adjusting the attenuator does increase/decrease output.
One odd thing, and main reason for my post is that on my scope it shows that there is only lower half of a sine wave, which is def no good.
I have taken a bunch of pictures of the insides at perspective fault locations, if anyone wants to see, looks like a tad bit of moisture, but nothing too crazy.
So if anyone out there could help me diagnose this problem, thatd be awesome ( i know anatech is fond of these units). I know repair, but am still iffy with touching something im not supposed to and messing up the cal.
thanks!
regards,

chris
  Reply With Quote
Old 21st August 2008, 12:40 PM   #2
diyAudio Member
 
Pjotr's Avatar
 
Join Date: Nov 2002
Location: Netherlands
I own its forerunner, the 651. But that differs not much.

These are fine IF they work. If they don't, they are a nightmare. Be very careful handling the PCB boards. These are very high impedance structures and any pollution there, especially around the variable frequency cap will disable the thing.

To repair this thing you definitely need a service manual. It is probably around the amplitude stabilisation network or one of the buffer amps is broken.

  Reply With Quote
Old 21st August 2008, 02:28 PM   #3
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
I have service manuals for the 651B osc. if it is of any help??

My 651B has been my main Osc for years now. love it! very handy. I might be letting mine go soon. My HP339 Dist analyzer is due to be here today. and i haven't decided if i am keeping both or not. I have 4 Osc. now and having 5 just seems to be too much hahahaha


Zc
  Reply With Quote
Old 22nd August 2008, 03:53 AM   #4
diyAudio Member
 
deyjavont's Avatar
 
Join Date: Aug 2008
I took some pictures of the test oscillator. tell me what you think!

When I used to work at a communications shop we used alcohol to clean the boards (water damage) but I was reading the service manual for the 652A and it said DO NOT use alcohol, but water and mild detergent instead.
  Reply With Quote
Old 22nd August 2008, 04:12 AM   #5
diyAudio Member
 
deyjavont's Avatar
 
Join Date: Aug 2008
so here is the pictures, notice the 900 ohm and lack of 50 ohm..it says option H22

Click the image to open in full size.

Gold traces!

Click the image to open in full size.

Water damage??

Click the image to open in full size.

im not sure what that device is to the right of the blue electro cap
and a little more to the right there seems to be some corrosion on the little silver cap?

Click the image to open in full size.

those resistors got pretty hot!

so any suggestions as to what I should do next...I am not sure what kind of degtergent to use...
and I am still short on a service manual, and the one I have for the 652A doesnt really have that much in common with the 654A
  Reply With Quote
Old 22nd August 2008, 06:11 AM   #6
diyAudio Member
 
deyjavont's Avatar
 
Join Date: Aug 2008
here is the signal I am getting with the 654A...

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

anyone know?? thanks!!
  Reply With Quote
Old 25th August 2008, 12:46 AM   #7
diyAudio Member
 
deyjavont's Avatar
 
Join Date: Aug 2008
Has anyone any idea about these test oscillators? Anatech ?
I am not toooo familiar..
  Reply With Quote
Old 26th August 2008, 01:45 AM   #8
diyAudio Member
 
gain wire's Avatar
 
Join Date: Nov 2004
Location: NCR
LOL You keep bbq sauce on your workbench, now that's awesome!

I noticed, like you said, that "those resistors get pretty hot". Well personnally, that's where I'd start to look at the problem. it might be the transistor right next to the two red capacitors, to the right of "those resistors". Well my guess, without having the manual, nor knowing the circuitry of the 654, is that the output amplifier is damaged. It is a transistor amplifier, I can guarantee you that, and one half of it is gone, most possibly the NEGATIVE section, meaning the pnp transistor, because your oscilloscope shows the negative portion of the wave. you didn't have invert on your oscilloscope ON did you?

In any case, that is my first guess, although, there's another place on that circuit board that looks like it had heat damage: near Q26. I have no idea what that section is, but you could remove the transistor and check it. As a matter of fact, that's what I'd do with all the transistors that look damaged, or hot. I am assuming you can do this right?

Oh , and that "device near the blue electrolytic" well judging by the next picture where we can see the other end of it, it is most likely a Light-dependant resistor. You know, a photoresistor and a lamp encapsulated in a opaque or dark tube. I am guessing this is what keeps the amplitude constant depending on the frequency output of the oscillator section.

OK OK I'm done! I sound like a big know-it-all, which I absolutely am not. I just like to help people fix old stuff, because

OLD STUFF RULES[SIZE=3]
darn font better work
__________________
Trans-directional-servo-logamp non-zerocrossing autogain compressing thingamajig
  Reply With Quote
Old 27th August 2008, 01:46 PM   #9
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
Have you checked to see if both Power supply rails are working??? should have something like + & - 15V at a guess. are both working before and after the V-reg stage?? If yes, then do you have + &- at the output amp stage??

I would start looking for cold solder joints in the area's that got warm.



Zc
  Reply With Quote
Old 27th August 2008, 05:04 PM   #10
diyAudio Member
 
gain wire's Avatar
 
Join Date: Nov 2004
Location: NCR
Geewiz, I wish I had thought of that... It's like the common why doesn't it work? Oh, it's NOT PLUGGED IN problem.
__________________
Trans-directional-servo-logamp non-zerocrossing autogain compressing thingamajig
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
HP 654A oscillator dtmedin Swap Meet 0 27th February 2009 01:21 PM
Use of a Ribidum Oscillator as refrence oscillator in a DAC Mark A. Gulbrandsen Digital Line Level 18 2nd January 2009 09:57 PM
10 MHz oscillator possible to use ? Bernhard Digital Source 7 28th August 2007 09:07 PM
oscillator dsavitsk Tubes / Valves 9 15th November 2005 06:07 PM


New To Site? Need Help?

All times are GMT. The time now is 03:18 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2