Lab-Volt 73-0P Variable Pas - Page 1
Forum section: electronics / repair

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 29, 2018, 08:09:33 pm
content: Any information about this unit? It seems like a very well built auto transformer but I can't seem to find any documents regarding this unit. AC side is controlled by a Staco 501C and works well. The DC has a problem with the voltage running almost double the stated voltage. I don't know what could be causing the elevated DC voltage. Any ideas?Sent from my XT1575 using Tapatalk

author: coromonadalix
author_rank: Super Contributor
timestamp: March 29, 2018, 08:22:33 pm
content: you have 4 stud punched rectifiers, normally in dc the formula is : vac x 1.414 for an full bridge rectifierhttps://www.electronics-tutorials.ws/diode/diode_6.htmlI would change the 4700 uf capacitor, may have degraded over the years, and i think the dc output is isolated from the main and the other 0-22 vac too ...

author: barry14
author_rank: Regular Contributor
timestamp: March 29, 2018, 09:01:20 pm
content: I think the voltage setting dial is arbitrarily labeled 0 to 10. For the AC output, this corresponds to 0 to 22 volts and for the DC output, this corresponds to 0 to 18 volts. That's why you think the dial is wrong because you are reading it directly as volts.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 29, 2018, 09:07:05 pm
content: Quote from: barry14 on March 29, 2018, 09:01:20 pmActually, I looked at the range settings at the DC output. The AC ranges are within 10% of the markings but DC is almost doubled. The 0 to 36VDC measures at 60VDC...Thanks for the input.Sent from my XT1575 using Tapatalk

author: Gyro
author_rank: Super Contributor
timestamp: March 29, 2018, 09:38:39 pm
content: Quote from: mr.fabe on March 29, 2018, 08:09:33 pmThese sort of units were typically used in school labs. They were intended to be fairly safe to use and also fairly bulletproof (thermal trip). The were designed to drive a lot of different loads, including lamps and motors - hence the variac and AC / DC output (unregulated).These days, I suspect that health and safety has taken more of a hand, also the need for closely regulated outputs for logic circuits etc, reducing the energy capabilities of modern units.The variable mains (not sure if that's direct or transformer isolated) and beefy isolated 0-22.5V AC outputs will be useful as-is for general fault finding. The unregulated DC output is of some use, but will need a post-regulator to use it for anything electronic and sensitive.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 29, 2018, 10:00:26 pm
content: Quote from: Gyro on March 29, 2018, 09:38:39 pmThanks for the information!I purchased this unit after having some bad luck with the low cost red auto transformer units on fleabay. I saw the parts used in this Lab-Volt and thought this was very well made for the cost with the Staco unit, linear PS, etc. I was surprised that there weren't too many users posting about this brand. Good to know about the DC stability in this thing.Sent from my XT1575 using Tapatalk

author: ArthurDent
author_rank: Super Contributor
timestamp: March 29, 2018, 10:08:04 pm
content: The D.C. side of the supply is not regulated and the transformer has two 23 VAC/6A windings that can be switched in series or in parallel to give either 23 VAC/12A or 46VAC/6A. After the rectifiers you have a choke (inductor) in series with the D.C. and a 4700Mf capacitor to smooth it out. The voltage with no external load will be about 1.4 times the A.C. or about 32VDC or 64VDV no load. There are resistors across the output terminals that probably get warm when the output is cranked up to max and probably lower the maximum output a little so I can see you getting 60VDC at max output.It looks like the two A.C. outlets are powered directly from the wiper on the variac but each goes through its own breaker behind the reset button. This just gives you two levels of protection on whatever you plug in to the A.C. outlets.Adjust the D.C. output to 12 volts and put a load like an auto headlamp across the terminals and see how much the output drops. With a heavy enough load the output will probably drop closer to what the rating says. I would agree that the 4700Mf should be checked or just replaced.

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 29, 2018, 10:20:03 pm
content: Quote from: ArthurDent on March 29, 2018, 10:08:04 pmThank you for the detailed analysis. I think the light bulb in my brain finally clicked on...Sent from my XT1575 using Tapatalk

author: mr.fabe
author_rank: Regular Contributor
timestamp: March 30, 2018, 05:03:18 am
content: Just pulled out the 4700uf capacitor and it was faulty as you suspected.Thanks again for your expertise!!