Holt Instrument Laboratories Model 611 Voltage Calibrator - Page 1
Forum section: electronics / repair

author: vsmith
author_rank: Regular Contributor
timestamp: January 06, 2024, 10:38:52 pm
content: Hi AllI have a Holt model 611 Voltage Calibrator, for which I have no information whatsoever. It is an AC calibrator with built in 400 Hz capability and an external signal input accepting 50-1000 Hz. It is a mid 60s instrument, judging form the components, manufacture style and nomenclature (cycles!), with a mainframe and plug in card cage type form.It powers up, but the output does not agree with the switch settings (3v is actually 2.28 v for example), so it appears to be repairable but without a manual or schematic, a pretty difficult undertaking!Can anyone help me with documentation? Does anyone on the forum have or ever worked with such a beast? What even are it's capabilities (I like the promise of 1000V output!)photo of the panel attached.Thanks!Vic

author: TheDefpom
author_rank: Frequent Contributor
timestamp: January 08, 2024, 11:23:20 pm
content: Generally speaking, if the output is low it could be due to bad caps causing AC ripple which then effects the output control... 1st thing is to check the power supplies and make sure the ripple is not bad on the DC rails.

author: vsmith
author_rank: Regular Contributor
timestamp: January 10, 2024, 07:56:32 pm
content: Thanks for your reply, that is good advice, and will be my next step. My difficulty is even finding the "rails" as the power supplies are not the usual "full wave bridge-3 terminal regulator" types used in later era equipment. Everything is discrete (no op-amps that I can see, yet) There are three large electrolytic caps, and definitely two rectifier (heat sinked) diodes in a full wave - center tapped transformer secondary type configuration, but I have not found the return path (the "ground" point) yet. I have checked those caps and they are close to their nominal values. I will have to do some more detailed sleuthing. Really could use a schematic!!

author: vsmith
author_rank: Regular Contributor
timestamp: January 11, 2024, 01:48:00 pm
content: Just an update.Tracing out the circuit (and I did find the "ground" return for the full wave rectifier) it is a common positive, regulated negative power supply. Center tapped transformer secondary, two rectifiers, a 1.5 Ohm series resistor, 2 parallel 1800 uF input filters, another 1.5 Ohm resistor, another 1800 uF cap (raw unregulated voltage aprox 30V) and then a 20V supply built on two zeners and a series of PNP (Germanium) transistors, with the final Pass element heat sinked. It appears as though the 20V is used for all the circuitry. All the filter caps measure "good" both with an ESR type meter and in circuit. The final regulated output is "quiet" (4mv ripple) and stable (as expected, some warm up drift). It is adjustable with a trimpot, and was noisy until I exercised it, leaving it set at 20.0V. I assume that further downstream regulation is used on each circuit card (there are 4) to produce a stable 400Hz sine wave for the output. More on that to come.

author: vsmith
author_rank: Regular Contributor
timestamp: January 11, 2024, 02:16:50 pm
content: some photos of the insides

author: vsmith
author_rank: Regular Contributor
timestamp: January 11, 2024, 02:18:15 pm
content: and more photos showing detail - in the photo of the card cage, the second card in has a nuvistor!

author: coromonadalix
author_rank: Super Contributor
timestamp: January 14, 2024, 02:56:01 am
content: if its a repair you are not in the good sectionsecondo your lytics seems bad just looking at themor some crust on them

author: Haenk
author_rank: Super Contributor
timestamp: January 14, 2024, 10:52:29 am
content: Ouch, those caps really look like they are returning from the undead soon.Before anything else, these certainly have to be replaced, they look beyond awful.

author: factory
author_rank: Super Contributor
timestamp: January 14, 2024, 12:52:50 pm
content: It's only the sleeving on those capacitors that looks bad, they are probably fine, with the exception of the smallest diameter ones, that go high ESRoropen for most brands.David

author: vsmith
author_rank: Regular Contributor
timestamp: January 14, 2024, 03:11:00 pm
content: Thanks for those "tips" about the caps. Yes, they look "beyond awful" but yes, it is mostly external. The speckled yellowish axial caps are Sprague "TE" types, they have a soft poly sleeving which seems to have reacted with the storage conditions (garage) over a decade or so. It can easily be scraped away and is a waxy sort of substance. The circuit boards are all conformal coated and these caps may have been coated with something as well, not sure. At any rate, I have actually checked each and every one (in circuit) using a handy capacitance/esr meter. I found only one that definitely needed replacement - it was a small (10 uf at 25v) one that had gone open.I have reposted this topic in the Repairs section, I agree that it probably belongs there, although the unit is a piece of calibration equipment. Thank you all for your comments. I really hope to luck out and find someone who has a schematic or even better a manual for this.In closing, at this point I have a working calibrator, but the output is not stable. I have traced out a lot of the signal flow from card to card, but don't really understand how the output level is regulated (I have found a "black box" on one card which is completely unlabeled but I suspect is a thermo-electric ac-dc converter for the control loop) but I really need some sort of "how is it supposed to work, as designed" resource!