HP 8656B repair attempt... ongoing

[Swainster]

Hi Tony,

Yep, you called it - the output was already as high as it can go. The 50MHz oscillator seems fairly simple, although based on a HP transistor for which I can't find a cross reference. I'll have a quick peek under the RF shield though - something might spring to mind.

[Tony_G]

Q50 is HP part 1854-0247.

Here is a thread on the HPAK group about it - https://groups.io/g/HP-Agilent-Keysight-equipment/topic/97228822#133021

TonyG

[Swainster]

If it crosses to 2n3866 that is great - I'm pretty sure I have a few of those from my search for an output transistor for my HP461A

[Swainster]

I checked the 1854-0247, and the DC voltages at the emitter and collector seemed a bit off compared to the schematic (+13V, -0.3V vs +11V, -0.7V). As I had some 2n3866As in hand, I swapped it out. The original transistor tested ok in one of those budget component testers, but maybe the issue is RF specific. Well, I powered it up and...

No change. 

So I decided to have a good think, and my conclusion is that there is nothing wrong with the reference output - the problem is with the user manual .

My reasoning is this: there has been a significant revision to the reference circuit in later models like mine. See the attached drawing where I have sketched it out (note that the drawing comes from the HP 8656A schematic, as it is much clearer, however the circuit design shown in the 8656B manual is identical except for different component designators). Basically, early HP 8656Bs (and all HP 8656As) run an emitter current of about 20mA, but the later 'Bs run an emitter current of about 11mA. As the gain of a common base amplifier is inversely proportional to the dynamic emitter resistance, which is in turn inversely proportional to the emitter current (i.e. gain is proportional to emitter current), the later design has roughly 6dB less gain than the earlier models. Therefore instead of +16-19dBm, the later model output should be more like +10-13dBm, which is closer to what I am seeing.

Well, ok, there is probably some dodgy math in there, but my overall conclusion remains that the 50MHz reference is later 'Bs should be expected to have a low level by design, and this is not reflected in the available service manual.

[Tony_G]

It's not unheard of for the manuals to be wrong. The one from Keysight that I'm looking at shows:



With the note in the revisions of:



Which seems to make it match what you're seeing (your bias voltages +11 & -0.7). The interesting difference is between your R112 of 1K and this schematic R25 of 348, I wonder if they also missed an update of what R26 was now supposed to be.

What does the level look like on your working one?

TonyG

[Swainster]

I was working from the same schematic as you - I just used the 'A version to illustrate the changes as it's a higher quality scan. The changes are also listed in the parts list but not noted in the schematic. I suspect that I might have got R25 and R26 mixed up, but it doesn't change my overall argument.

Actually, once everything is properly assembled (and the leads trimmed on the 2N3866A) , the 50MHz levels on both units are around +15.5dBm when peaked.

Now both units are performing pretty consistently. Although both are 2-3dBm down compared to my spectrum analyser, there is no longer any significant change in level as one goes from the heterodyned range to the normal range.

The SMC connectors that I ordered to make up some test cables have finally arrived. I'll probably use them to make up the missing external ref-in ref-out cable assemblies, though I'm not sure what to do about the BNC end. HP seemed to have used front mount connectors which are sized for rear mount cut outs I.e. much bigger than normal front mount cut outs. If I put a rear mount connector in then the nut will hide the label. I guess I'll have to open my poor abused wallet again...

[Tony_G]

I was working from the same schematic as you - I just used the 'A version to illustrate the changes as it's a higher quality scan. The changes are also listed in the parts list but not noted in the schematic. I suspect that I might have got R25 and R26 mixed up, but it doesn't change my overall argument.

I completely agree with you - They might have gotten that output value wrong in the manual right from the start.

Good to hear that they're working consistently even if a little down.

TonyG