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8116A Code 42

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MarkL:
On BNC tees, Amphenol makes good tees for a reasonable price.  I always like Pomona components, but their tees are about double and I'm not sure worth it.  I would buy new from Mouser or Digikey.

Looking at the end points of C528/C530 in series, you can see it is between the base of Q504 and the collector of Q506.  A quick determination of their effect can be done by imagining a small positive voltage change on the base of Q504.  This would move Q504 closer to turn-off, and reduce the emitter current.  The emitter would move positive, being pulled up by the resistor network R530/R532.

This would in-turn cause the base of Q506 to go more positive, which would then pull down the collector of Q506.  The negative change here would be coupled back to the base of Q504 through C528/C530 which we imagined was going positive, therefore limiting extent of the positive change.

In short, what all this is doing is reducing the slew rate of this stage of the amplifier until the feedback can take affect and keep the waveform true.

On the positive driver side, C529 is performing the exact same function, and contributes to the slew rate control along with the C528/C530.  C529 is adjustable in order to tweak the overall slew rate of this stage.

Note that if C529 has insufficient range, C525 and C530 can be changed as per Table 9-1 with the procedure described on page 9-7.  C525, C551, and C552 also limit the high frequency response of this stage by the shunting the bases of Q503 and Q504.

A 200V rating on C528/C530 is not necessary, plus you have them in series which approximately doubles the voltage rating (ignoring imbalances).  You could substitute 50V, but I would be surprised if the existing 1pF caps were bad and needed replacing.

Mick B:
WOW! Mark, I can see it so clearly now a great explanation of that circuit. I have my doubts also about anything being wrong with c528 & c530, but I wonder about c529 being off, basing this on scope results while making adjustments on c529 and Bias adjustment R535 for adjustment on Pg.9-6. Maybe I’m overthinking this. Awhile back I changed all the Power transistors. In the process Q501 and Q502 are different Q501 is a TO18 pkg. and Q502 is a TO92 pkg. Q501 is a direct replacement Q502 suppose to be, and the specs are remarkably close, but not exact Part numbers Q501 is a 2N2369A and Q502 is a 2N5771 /with a heatsink attached. Perhaps the problem is here.
I thought it might be fun to play around with c529,c528 & c529 to see what difference it makes to the overshoot and slew rate.   
Before I do that I’m going to get on the HP5005A.

MarkL:
The power amplifier and other analog sections look like they've had many tweaks applied to get it as perfect as possible as HP engineers went through the design process.  And sometimes they still couldn't depend on consistent enough specifications for components (probably the semiconductors), as evident by Table 9-1.

I wouldn't be surprised if some of the semiconductors were standard parts, but "selected".  The HP Parts List and HP Cross-Ref has this info if you want to check on the ones you replaced with non-HP (generic) part numbers.

Mick B:
Hi Mark, I have a file that was downloaded as HP Master Xref.pdf Pg.1. Transistor-Diode Cross reference - H.P. Part Numbers to JEDEC Numbers
and it list Q502 as the same number as the manual. I don't remember how I came up with 2N5771 as a replacement just in TO92 pkg. I spent weeks tracking down original part numbers for this and found all the transistors except Q502. I don't know if this means anything in today's part market. Do you have a Xref. besides the ones I have that list the
1853-0405 as anything besides a 2N4209? I will upload the ones I have, for those that might not have them. CORRECTION! only one file the other mentioned above is too large.

Then I found this today.

https://www.microchipdirect.com/product/2N4209
availability: Order now, estimated to ship on 06-Jan-2025
1.  $16.91 per unit

MarkL:
That appears to be correct that for Q502, 1853-0405 is a 2N4209.  And all the xrefs I have show that 1853-0405 is not a selected part; just a run-of-the-mill 2N4209.

I also have a copy of the xref you posted, and it says 1853-0405 is a "TO-17" case.  I don't know what a TO-17 is, but everything else including the datasheet says it's a TO-18 (metal can).  Probably a misprint.

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