Author Topic: HP 8753C power supply questions  (Read 881 times)

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Offline TAMHAN

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HP 8753C power supply questions
« on: May 08, 2018, 07:49:07 am »
Hello Folks,
I would need some help. Like more than a year ago, I bought a HP 8753C. It arrived from the USA and worked well except for the dreaded ADC LIN error, but smelled really really odd. The smell is difficult to describe - imagine a musky perfume, as worn by some kind of sexy 50yr old female higher officer of a private militia. Alternatively, the sweet smell of a mouldy room in an old brick house - I am sorry, I can not describe it better.

Question 1
So, I merrily took the unit apart and found it covered, almost in its entity, but especially in the PSU, with a brownblack ash like substance. Not dust, but a very fine grained, ash like substance. And lots of it. Picture of it attached here.


At first, I assumed it to be the RIFA capacitors in the PSU. All of these just fell victim to my newly purchased desoldering station (bought just for this one repair), and - to a visual inspection without microscope - look fine.


Does anyone of you have an idea where this ash comes from?

Second question
Also, does it make sense to recap the PSU of the 8753C in its entity? I desoldered one of the three terminal capacitors from Sprague, and the G significantly drops off when going from 1KhZ to 10KhZ on my 4262A. But the capacitance seems the same.

Finally, are there any other things I should snipe while the badboy is open?
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Offline LateLesley

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Re: HP 8753C power supply questions
« Reply #1 on: May 08, 2018, 09:29:12 am »
Personally I have no idea where it would have come from, but I would suspect its from the environment it's been in, rather than from the device itself. Any chance it's come out of a fire damaged property? I've seen equipment come out of places after a fire, all coated in all sorts, and the company that dealt with it would take all manner of things apart and wash the boards and everything.

So that would be my guess, and would suggest just a thorough clean with a brush, vacuum, then wash with 99% isopropyl for any stubborn parts.
 

Online Cubdriver

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Re: HP 8753C power supply questions
« Reply #2 on: May 08, 2018, 11:58:30 am »
Tam -

The 'ash' looks to me to just be many years worth of dust accumulation, just like what used to collect inside old tube TVs.  It may have come from an environment where people smoked, or simply one with a more than normal amount of dust.  Given that it has a fan, and thus 'inhales' constantly when in operation, it will tend to draw in crud, which in my experience is inclined to deposit on things like that.  Take it outside and gently blow it out if possible, or gently brush the loose dust away while holding a vacuum nearby.  Be careful not to generate static by brushing too enthusiastically.

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 

Offline Bud

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Re: HP 8753C power supply questions
« Reply #3 on: May 08, 2018, 12:39:18 pm »
It was probably sitting for years in someone's dusty garage, proudly called "Lab" by the owner. There are such "Labs" photos in this forum. How the exterior of the device looked like? At the end of the day it is just dust, simply clean the device and it should be good to go. I have not heard of anyone recap-ing HP VNAs or other HP equipment for that matter.
 

Offline tombi

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Re: HP 8753C power supply questions
« Reply #4 on: May 08, 2018, 05:12:05 pm »
I once pulled apart a TV owned by a smoker. Yuck. Sticky brown stuff stuck to everything. I was just salvaging parts so didn't bother cleaning much of it but you had to wipe the stuff off (wouldn't blow off).

Could it be that?

Tom
 

Offline capt bullshot

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Re: HP 8753C power supply questions
« Reply #5 on: May 08, 2018, 05:25:01 pm »
To me, this looks like the usual dust that you find e.g. in an old TV set. I've experienced the smell you describe with various old electronics items that were stored for a long time in a domestic and maybe too humid environment, like the radio that lived in your aunties kitchen since 1966 or whatever ...

Regarding the electrolytic caps: From my experience it's quite normal for such caps to drop their Q/D at higher frequencies. As long as there isn't that typical "fishy" smell, they will be still OK.  As long as you don't find a too high ripple voltage, I wouldn't replace them.
Safety devices hinder evolution
 

Offline TAMHAN

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Re: HP 8753C power supply questions
« Reply #6 on: May 09, 2018, 10:21:50 am »
Hello Folks,
sorry for the slow reply. Mouser just shipped the parts today, so I have to w a i t ;(

I ordered, just for the fun and farting, a "cheap" replacement for one of the three leg caps. Before soldering it all together again, I will do a comparison...
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Offline TheSteve

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Re: HP 8753C power supply questions
« Reply #7 on: May 09, 2018, 10:36:57 am »
I replaced all of the electrolytic caps in mine. It's not expensive, I recommend you do the same.
VE7FM
 

Online hendorog

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Re: HP 8753C power supply questions
« Reply #8 on: May 09, 2018, 04:00:24 pm »
One of mine was possibly similar - lots of dust and a strange smell. Your imagination is much better than mine :)
Mine came from a humid country and I put the dust down to that combined with the environment.

I thought the smell might be due to a cap losing its lunch, I replaced one in the CRT unit which exploded. Have you opened up the CRT?
If you remove the CRT, be very careful removing the key/button cover. It is almost impossible to take that off without breaking a tab on it.
 

Offline TAMHAN

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Re: HP 8753C power supply questions
« Reply #9 on: May 10, 2018, 04:45:05 am »
I replaced all of the electrolytic caps in mine. It's not expensive, I recommend you do the same.

I might be stupid...but the only three leg cap with SOMEWHAT similar ESR and shit I found was a custom part from Cornell Dubliner at 17€ a pop and with 24wk leadtime.
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Offline TheSteve

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Re: HP 8753C power supply questions
« Reply #10 on: May 10, 2018, 04:57:18 am »
I replaced all of the electrolytic caps in mine. It's not expensive, I recommend you do the same.

I might be stupid...but the only three leg cap with SOMEWHAT similar ESR and shit I found was a custom part from Cornell Dubliner at 17€ a pop and with 24wk leadtime.

Why do you need a three legged cap? Two legs are connected together, as far as I know the third leg is simply for physical stability. Modern replacement caps are much smaller and are easy to fit.
VE7FM
 

Offline Bashstreet

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Re: HP 8753C power supply questions
« Reply #11 on: May 10, 2018, 04:58:22 am »
Yes it very likely is just old dust.

When dust accumulates for years it slowly seems to become that kind of dark almost ashy compound . It kind of brakes down and compacts.
I see this a lot in fan blades and components that otherwise are in relatively clean environments.
 

Offline Bashstreet

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Re: HP 8753C power supply questions
« Reply #12 on: May 10, 2018, 05:01:50 am »
I replaced all of the electrolytic caps in mine. It's not expensive, I recommend you do the same.

I might be stupid...but the only three leg cap with SOMEWHAT similar ESR and shit I found was a custom part from Cornell Dubliner at 17€ a pop and with 24wk leadtime.

Why do you need a three legged cap? Two legs are connected together, as far as I know the third leg is simply for physical stability. Modern replacement caps are much smaller and are easy to fit.

Or for ground connection.
 


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