HP4274A power supply +/-12V issue - Page 1
Forum section: electronics / repair

author: tonycox01
author_rank: Contributor
timestamp: January 22, 2020, 09:41:00 pm
content: Hi,I have an HP 4274A LCR meter that is not calibrating itself properly and my first check showed the power supply is not behaving.After carefully adjusting the -5V reference (TP9) the +/- 5V supplies (TP17/TP10) are spot on correct. The +/-12V supplies as measured at TP1 and TP5 however are running at +/-16V respectively which is way out of spec.The schematic is attached below.Since both are out by a similar amount (except for sign) it seemed to me a common regulation issue may be affecting them both but I was wondering if anybody has any suggestions about where to start? I'm puzzled about what could be faulty here that would affect both -/+12V supplies.thanksTony

author: tonycox01
author_rank: Contributor
timestamp: January 22, 2020, 09:48:18 pm
content: Hmmm. I don't see the attachment for the schematic - trying again

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 22, 2020, 10:16:19 pm
content: There are two 16V Zeners hanging off the +/-12V rails, so the fact that both the +/-12V are reading +/-16V may just be a coincidence - in other words,the two supplies might be totally unregulated and the Zeners are clamping them.Do the normal checks for open/shorted transistors and diodes, and check that the inputs to the op-amps are about equal.There also seems to be a line coming down from the 22/44V supplies to the -12V supply, so check those voltages also.HP often designed "intertwined" supplies, so that if one voltage goes haywire, several other seemingly unrelated voltages will also be incorrect.

author: tonycox01
author_rank: Contributor
timestamp: January 23, 2020, 03:01:13 pm
content: Thanks - I had noticed the correspondence between the zeners and the voltage out on this rails. I hadn't connected those things (I'm not very experienced in this game yet :-). I'll check the transistors and diodes as you suggest.regardstony

author: tonycox01
author_rank: Contributor
timestamp: February 03, 2020, 09:58:04 pm
content: After replacing several leaking diodes and a couple of op amps the regulated +/-12V and 5V supplies are now spot on.One slightly troubling issue is that the unregulated supplies to the A21 DC Bias board (11R/12R/13R) are off by some margin. Manual says -22V/-44V/+44V but I am seeing -32V/-63V/+63V (this is on an unloaded, removed board BTW). It may be that the +/-42V and -15V regulators on A21 can cope with that excess supply but it seems a way off? The unregulated supplies are quite simple so I'm a bit puzzled as to why they are so far off.Any ideas please?Tony

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 03, 2020, 11:16:41 pm
content: You could take an AC voltmeter and measure the output of the two secondaries of the main transformer feeding the unregulated supplies. Multiply the RMSby 1.414, subtract off 1.4 VDC for the diode bridge and that should give you a rough idea of the nominal +/-44 supply values.Typically, when HP says "unregulated," the value can be way off from what they say. But your readings do seem a bit high.The A21 board regulators use 2N4920/3 transistors that are rated at 30 W. So, for instance if you have 63 V coming in and 42 going out, that means youcould draw 30/(63-42) or about 1.4 amps before you exceed the rating.Make the above readings with the board loading the supply - that's the real test. Unloaded, things may be higher than you think.

author: tonycox01
author_rank: Contributor
timestamp: February 04, 2020, 08:18:01 pm
content: Thanks wn1fju,I did as you suggested and measured the AC primary supply and it was 48V which gives about 66V at the DC supply - so pretty close to the figure I am measuring. This is unloaded though - so I think it is time to put it all back together and measure it loaded.thanksTony