Author Topic: HP 8569B - How to engage (or fix) Resolution Bandwidth control  (Read 450 times)

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Offline pfm

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HP 8569B - How to engage (or fix) Resolution Bandwidth control
« on: December 23, 2017, 10:35:10 AM »
I have a HP 8569B in otherwise pretty good condition. If you are familiar with the unit you know that the Frequency Span/DIV and Resolution Bandwidth are two coaxial knobs. (Center knob in the pic). The outer knob is Freq Span/DIV and works good.  But the inner Resolution Bandwidth control does not have any effect on the actual resolution bandwidth, it always shows 10Khz on the screen. It does turn and 'click' fine all through the positions but no change whatsoever on the screen.
This happens regardless of the "optimum" coupling (push outer knob in), the bandwidth always reads 10Khz on the screen whether the knobs are coupled for 'optimum' or decoupled.

Any advise or help in troubleshooting and fixing this would be highly appreciated. Except for this the unit works great so I really want to get this function working properly.

If you think there might be a user error and I may not be operating these controls properly please advise on that as well.
Thanks!
« Last Edit: December 23, 2017, 10:36:51 AM by pfm »
 

Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #1 on: December 24, 2017, 11:38:04 AM »
What usually happens with these is the contacts fall off the switch rotor.
See below...

The switch contacts are held on by posts molded into the plastic. They are hot staked in place by heating the post and pressing it down to form a clamp for the contacts.
First if you opened up the analyzer be very observant and find the loose contact.
You will have to glue it back on in the proper antiabortion.

In one respect you are lucky. the resolution bandwidth switch rotor is the rear most one and is affixed to the inner shaft.

Get a copy of the manual; therein you will find disassembly instructions for those controls.

You will need some special tools...
Spline wrenches to remove the collets holding the plastic rotors in place.
You will need a good camera to take pictures of the switch assembly as you take it apart.
The reason for this has to do with the positioning of the collets, they don't explain that in the manual......

You will need allen wrenches (hex wrenches) to fit the knob set screws.

Take lots of pictures and don't overlook the position of the collets. As I said above THEY HAVE TO BE INDEXED PROPERLY otherwise the switch will not rotate through its complete range.


Be warned. The knob set screws are mild steel in a brass hub, pressing against a stainless steel shaft. They can become seized. Pennitrating oil can help...
In stubern cases you will have to purchse a left hand drill (actually two one small enough to fit in the hex head set screw, and one ether just at or just under the drill tap size for that set screw. (they are #4 set screws. ) ) I had the sweep time knob o the second 8569 that needed to have the set screw removed with a left hand drill. I had to chase the threads with a 4-40 tap... Don't use a starting or thread forming tap...

I'll be starting on the second 8569 front panel this January... There has been a number of delays in getting that project started.  I will repost links to my pictures from my Google photos account. in the HP-8569 thread.

This is not a project for the faint at heart, good mechanical skills are a must, along with attention to detail. I had to rebuild the Reference Level / Input Atten knob assembly three times before getting it right on the first 8569, so be patient. 

In the end it is worth it, that is a damned good analyzer.
Sue AF6LJ
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Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #2 on: December 24, 2017, 12:03:32 PM »
A few other useful notes...
I bought my manuals from here... http://artekmanuals.com/
They produce some really fine manuals, good scans, at a reasonable price.

If you haven't taken the analyzer apart yet here is a picture of the back of the front panel module.


This is not hard to remove, and all the RF plumbing stays put.
I did take a marker and draw index lines to make sure the connectors went in the right direction.
Once again make sure you find that small contact...

Inspect the other contacts while you are at it.
« Last Edit: December 24, 2017, 12:05:39 PM by AF6LJ »
Sue AF6LJ
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Offline pfm

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #3 on: December 24, 2017, 03:29:13 PM »
Thank You for your response!
I will open the analyzer and take a look inside, seems like a tricky job.
I wish I had taken pictures when I took out the Ref Level and Attn knob! I  have the manual but it does not show an assembly diagram that tells the sequence of the parts and so I couldn't figure out how the retainer ring and metal disc with the window and plastic washers go so had to leave those out. Luckily it functions fine though.
 

Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #4 on: December 24, 2017, 11:20:35 PM »
For this analyzer the manual comes in three volumes.
The HP archive has the service manual, and it does describe the assembly / disassembly of the controls.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline KrudyZ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #5 on: January 14, 2018, 09:24:59 AM »
Thank You for your response!
I will open the analyzer and take a look inside, seems like a tricky job.
I wish I had taken pictures when I took out the Ref Level and Attn knob! I  have the manual but it does not show an assembly diagram that tells the sequence of the parts and so I couldn't figure out how the retainer ring and metal disc with the window and plastic washers go so had to leave those out. Luckily it functions fine though.
I have the same spectrum analyzer and it also suffers from losing the wiper contacts.
I fixed some of the them after finding them inside the unit, but then was left short as some must have fallen out through the vent holes sometime over the last 50 years...
 

Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #6 on: January 14, 2018, 09:55:36 PM »
I have been on the lookout for those contacts. If anybody ever finds a source of those wiper contacts please post it here.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline pfm

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #7 on: January 15, 2018, 03:30:52 AM »
One would think considering that this problem is so common and widespread that by now someone would have come up with pre-built rotors and contacts to just swap them out with the broken ones.
 

Offline pfm

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #8 on: January 15, 2018, 03:36:29 AM »
Thank You for your response!
I will open the analyzer and take a look inside, seems like a tricky job.
I wish I had taken pictures when I took out the Ref Level and Attn knob! I  have the manual but it does not show an assembly diagram that tells the sequence of the parts and so I couldn't figure out how the retainer ring and metal disc with the window and plastic washers go so had to leave those out. Luckily it functions fine though.
I have the same spectrum analyzer and it also suffers from losing the wiper contacts.
I fixed some of the them after finding them inside the unit, but then was left short as some must have fallen out through the vent holes sometime over the last 50 years...

you might have looked at this thread but here it is anyway -
http://www.eevblog.com/forum/repair/hp-8569-spectrum-analyzer-repairs/100/
 

Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #9 on: January 15, 2018, 06:22:24 AM »
One quick note...
When I reassemble my front panel module, I will include the sequence for tear down of the reference level control group.
This one is hard because snap ring pliers are needed for the removal of the retaining ring holding the reference level knob baffle in place.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 

Offline pfm

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #10 on: January 19, 2018, 11:58:59 AM »
I found this in the Service manual parts list. Could this be the RBW rotor ? Has anyone tried sourcing it ? From Agilent Keysight or any surplus contractor ?
 

Offline AF6LJ

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Re: HP 8569B - How to engage (or fix) Resolution Bandwidth control
« Reply #11 on: January 20, 2018, 12:29:48 AM »
That is it and you can go to the Keysight website and see if they have any in stock.
There is a search page there you can plug in the part number.
Sue AF6LJ
Test Equipment Addict, And Proud Of It.
 


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