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Hewlett Packard HP461A Wide Band Amplifier

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Peter_O:
Not shure if this one should be here or in 'Beginners'.
It's grown up lab equipment, but so far my knowledge might be kind of beginners level.  :scared:

2023 this little guy needed to be rescued for 30 Euro.



The 461 provides a gain of 20 dB or 40 dB with a frequency response of +/- 1 dB from 1 kHz to 150 MHz and a max output of 0.5 V RMS into 50 Ohm.
https://www.keysight.com/us/en/assets/9018-02904/user-manuals/9018-02904.pdf

Obviously there have been some repairs in this one already.

 



What bothers me are these points:

1)
The PSU voltage is in spec, but the ripple is 8 mV RMS instead of < 1 mV spec.
Would you change the caps?

2)
The bode plot shows quite OK behavior below 10 MHz but goes wild above that.

 

Following the service manual I tried to adjust the inductors, but there was no change in the frequency response at all, so there might be broken coils.

3)
The transformer is running quite hot with 60 °C as well as some resistors. Is this normal for this kind of device or do I need to search for a fault?

 

Has anybody of you been an one of these points before?
Hints etc. appreciated!

Stray Electron:
  Nice save!  What's the date code on that unit?

   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.  But I would check those large silver electrolytic caps.

  I've never worked on one of those but IMO the transformer should not be getting hot. 

  Looking at the second thermal image it looks like that capacitor that's partially hiding under the black heat sink is getting very hot. I sort of doubt that it's bad but I think something is overloading it.  (Possibly that amplifier is oscillating and is dumping high frequency into the capacitor).

Peter_O:

--- Quote from: Stray Electron on January 01, 2024, 02:00:46 pm ---  Nice save!  What's the date code on that unit?

--- End quote ---

Thx for your ideas!  :-+
It's a 0946... .


--- Quote from: Stray Electron on January 01, 2024, 02:00:46 pm ---   Those small silver capacitors shown in picture #4 (and some in Picture #2) are wet slug tantalum caps and are VERY expensive and very high quality and rarely fail. I would not touch them unless you can prove that they're bad.  But I would check those large silver electrolytic caps.

--- End quote ---

Will check the PSU caps.


--- Quote from: Stray Electron on January 01, 2024, 02:00:46 pm ---
  I've never worked on one of those but IMO the transformer should not be getting hot. 

  Looking at the second thermal image it looks like that capacitor that's partially hiding under the black heat sink is getting very hot. I sort of doubt that it's bad but I think something is overloading it.  (Possibly that amplifier is oscillating and is dumping high frequency into the capacitor).

--- End quote ---

Will check the temperature of the cap and try to do some more measurements.

coppercone2:
mine had a cracked attenuator resistor that was driving me nuts. Its slightly out of spec now by 1dB but I can live with that.

Stray Electron:

--- Quote from: Peter_O on January 01, 2024, 02:33:06 pm ---
--- Quote from: Stray Electron on January 01, 2024, 02:00:46 pm ---  Nice save!  What's the date code on that unit?

--- End quote ---

Thx for your ideas!  :-+
It's a 0946... .


--- End quote ---

  Wahoo! Built in 1969.  Nominally the "46" week of the year but HP actually marked a date about 8 weeks (IIRC) AFTER the actual production date to allow time for shipping, etc.  So your's was probably being built at about the same time that Apollo 11 was landing on the moon!

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