HP 8656A Signal Generator Repair

This is a HP Signal Generator, 100kHz to 990MHz, unfortunately not very popular with many in the trade, mainly due to design/quality issues. Most discussions around the web talks about issues on frequency stability, harmonics, and the notorious one is attenuator  failure.

This came to be from a flea market and as usual, I picked it up without realizing the history behind this model. The part which caught my attention is the  clicking you hear when you try to change the frequency or mostly output signal level. I was curious what was making this noise.

There is no repair attempt here at electronics level or component/board level. This is purely documenting the attenuator repair, which I could not find on web. So here it is for all those who wanna know what the heck is that which is making the noise when you change output signal level, what fails, and how to fix it.

Here is a video Explaining the Details –


Reference Picture –  with photos of actual components.

Screen Shot 2018-09-02 at 2.46.31 PM

Key points to remember –

1. Do not open the assembly with the unit in service position, do keep it upside down and open the A10/A11 assembly and tie it down to hold it up.

2. Keep a small magnet ready, to pick up the steel balls, in case they fall underneath the PCB.

3. The Cam Guide (Cover)  – A9MP4/A9MP5 screw needs to be tightened to specific torque. If you over tighten it, the cam will not move, if you leave the loose, the steel balls can come off and the switch contacts will move out too.

4. There are washers on both sides of the screw for the cam Guide (cover) – Part# in Diagram – A9MP4/A9MP5. Both washers (Spring washer and spacer) must be in place, else the cam will not operate/Move.

5. This is a very delicate assembly with lots of tiny parts. Careful disassembly and repair is needed, in a clean bench to locate part in case they fall off.

The solenoid pulling the cams in and out is controlled by a 24V Rail. in case you see the entire assembly dead – ie no movement at all – check the power rail and fuse. There is a SCR used as a crowbar in the circuit as overload protection.

Marking the section of the schematic here, highlighting key fellas. Remember, if the fuse is blown, do not replace until you are sure why it blew up in the first place. This is located on A10 Board.

Screen Shot 2018-09-02 at 3.05.10 PM


Thats all for this, all the best


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13 thoughts on “HP 8656A Signal Generator Repair

  1. Thanks for the video. I have one of these and I have two questions. 1–how did you get the cams with new grommets back into position? Is there a way to remove the solenoid assembly cover, or did you just slide them under somehow? 2–Where did you get replacement grommets? I’m finding them hard to source, and I don’t see how an o-ring will work. Thanks!


    1. Hey Brian, I didn’t remove the solenoid rather just pushed it in. I found the grommet in a local surplus store, If you have to O-Rings which are tight enough to hold on to the cam, you can insert two and hold the cam between them. Its a tricky circus anyways. If you have time/patience you could even make those grommets out of rubber sheets.


    2. HELP.. I cant find any grommets here or online suitable. can i get you to make me a set? also would love to know if i pull the top off of the solenoids? i pulled the 2 screws on the side, but it seems like they are attached to the arms. i dont want to get in deeper than i already am unless i can get out as well.


  2. Excellent, phenomenal presentation! One of the few useful links I found, we have one of these TODAY in our lab storage room that got liquidated after the tech left too ill to return. So I HOPE this is the problem, it was tagged too expensive to repair and 2 Diodes (strangely like in the fused circuit you show!!!) taped to the cover. As a HAM I always wanted at least an HP 8640B like the ARRL lab has…, this will do better!!! I think 2 O-Rings can work, maybe find some grommets or square section and try them.


  3. HELP.. I cant find any grommets here or online suitable. can i get you to make me a set? also would love to know if i pull the top off of the solenoids? i pulled the 2 screws on the side, but it seems like they are attached to the arms. i dont want to get in deeper than i already am unless i can get out as well. Sorry for double posting


    1. Joe, You can buy o-rings or Grommets from ebay, find the smallest size or assortment of different sizes so that you can match the best one. In case of O-ring, use them in pair to lock each cam down. That way you need not look for the grommets. I didn’t try removing the solenoid /cover as many parts are torqued to specific values and was worried I may mess it up. You can press the cam edge in through the slot. Be careful not to misplace the contacts (gold plated) or steel balls, All the best, remember this thing needs a lot of patience 🙂


  4. OHH awesome. Thanks so much. Ill buy some sets and see what works. In case anyone else is following along, Today is Tomorrow, and last night i found some small grommets and was trying to force the bugger on when SNAP. I managed to break the plastic fork (Cam-Switch) that these o-rings go on to.Arrrghh. Not all is lost. I spent rest of last night drawing a new one on solidworks and will print it out tomorrow. hope it all works out. Keeping fingers crossed.


    1. Oh so sorry to hear that and glad you were able to print it. Yes it is very sensitive and also remember to clean the contacts before final assembly. This unit was not one of HP’s best design. The cam does not take much load.


  5. Hi,
    I missing the 2 screws on the side top off of the solenoids assembly cover. Has any wan a picture or info about this screws?
    Thanks in advanced,



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