| Electronics > Repair |
| Old VTVM (Precise Model 909) |
| << < (5/7) > >> |
| blue_lateral:
Found a manual for the kit version: http://www.ko4bb.com/getsimple/index.php?id=download&file=06_Misc_Test_Equipment/Precise_Electronics/Precise_Model_909_VTVM_Meter_Construction_Manual.pdf |
| George Edmonds:
May I offer a word of warning. In truth this is a class 1 instrument without a protective ground, it also appears that the negative terminal is directly connected to the metal case work. Under no circumstances should this meter be used to measure the incoming mains supply or any voltage where the negative supply as above ground. G Edmonds |
| TimFox:
To my knowledge, all VTVMs of this vintage use transformer power supplies with the negative end grounded to the metal case. For DC measurements, there is a polarity switch for + or - full scale. AC measurements are also with respect to the case ground. Most have a two-prong mains cord, so the case itself does not go to PE. |
| TimFox:
--- Quote from: Analog Kid on November 12, 2024, 02:36:34 am --- --- Quote from: floobydust on November 11, 2024, 09:32:39 pm ---Sometimes in old gear like this I will add a couple 5W zeners in the B+ filter to regulate it. Hate it when VTVM needs moves around due to mains fluctuations. --- End quote --- Like this? (Attachment Link) If "a couple", where would the other one go? And the zener voltage should be the HV output voltage, right? Maybe I need a couple in series? --- End quote --- Note that in this power supply, there is a substantial resistance from the 6X5 cathodes to the input filter capacitor. In the limit RC >> 1/[ 2{pi}f ], the unloaded voltage across the capacitor goes to the average (mean absolute value) voltage of the AC waveform, rather than to the peak value found with a direct connection to the capacitor (neglecting diode drops). |
| floobydust:
Later models or more expensive VTVM's had a regulated B+. Precise 9071 is worth comparing to the 909. It uses 0A2 for 150V. EICO-250, Precision Apparatus EV-10-S, HP 410B etc. had the 0B2 for around 108VDC B+ I believe. It's just that I disliked the (Heathkit) VTVM's needle moving due to mains fluctuations which can look like a bad part/connection and it kind of threw me around, back in the day. Also mains voltage today is higher than decades ago so there is extra voltage. To clarify- a VTVM mod for regulated B+, I'll add a zener shunt reg. 1N53xx 5W like 1N5383B 150V/8mA or for higher voltages over 200V use a 1N5388B 200V plus 1N5368B 47V to make a 250V zener, this is the "two" I was talking about. Typical VTVM prices 1952: $25.98 (kit), $44.98 (wired) or $309 kit, $535 assembled in today's dollars. Average income 1952 on the higher end around $4,200/year for a professional, 1.5-2 month's wages. VTVM's were quite expensive in their day. |
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