Author Topic: HP 34401A hacks and upgrades  (Read 17072 times)

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Online Ecogeek

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Re: HP 34401A hacks and upgrades
« Reply #50 on: May 30, 2024, 12:01:44 pm »
Yes I did similar, soldering some copper wings on the LRs to at least attempt to lower the peak temperatures and speed up the distribution of heat by way of a greater surface area.  My mains voltage is typically 240V but ranges from brief periods of 230V to 250V during the day with all our solar PV systems lifting the mains voltage. Most houses where I live have solar PVs. The other thing I did to paint the internal aluminium shield matt black, which goes from a very good IR reflector to a good absorber and radiator to speed up the thermal transfers to hopefully reach some equilibrium quicker.
The Linear Regulars are dumping a lot of heat, the designers of the 34401 made sure the DMM was under regulation down to 200V or below (for 230V mains) or so. 
The thermal environment of the 34401 has both external ambient and internal heating to contend with.
The ideal world would stabilise both the mains voltage to just above the LRs regulation point around say 210VAC and then the external ambient temperature.
I did plot the temperature of the internal shield, it takes hours to stabilise, a log curve, you need at least two hours before it flattens off then with both the mains voltage variations (variable LR heat dissipation) and ambient temperature fluctuations.
I did a quick check with all the covers off and used my soldering heat gun set to 100C i.e. a small point source of hot air. While watching the external Voltage plot from the DVM, applied the heat source briefly to the various components.  The good news most of the instrument makes only small differences, but the three Vref  0.01uF X7R capacitors are extremely sensitive and cause significant changes to the measurement voltage.
Replacing only three 0.01uF capacitors from X7R  to COG around the Vref circuit would eliminate one source of TC instability affecting the warm-up period in particular. One could ask why such unstable TC components were used in the Vref circuit in the first place apart from the cost 10+ years ago.
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Offline iMo

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Re: HP 34401A hacks and upgrades
« Reply #51 on: May 31, 2024, 04:49:26 am »
So, the potential changes around the Vref..
The 78/7915 are the placeholders for a low drop regulators (18.3->15V), there are just the 3V2 dropper zeners there now (and the same situation at the ADC powering too).
« Last Edit: May 31, 2024, 04:52:04 am by iMo »

Online Andreas

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Re: HP 34401A hacks and upgrades
« Reply #52 on: May 31, 2024, 05:50:46 am »

caution. 7915 (and usually LDOs) need more than 100 nF as output capacitor.

And do not forget the reverse bias diodes (anti parallel to output) to prevent latch up during start up.
(especially when putting a large capacitor across the heater).

with best regards



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