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 Post subject: Precision Model 68 Problem
PostPosted: Oct Sun 02, 2005 3:36 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1173
Location: Rome New York USA
Experts, Checked the tubes,repl bat., the meter will not return to zero, on Ohms scale goes to infinity. Does read AC/DC voltages, but not resistance. Is this a cap problem, or is the meter bad? If caps, what tolerance is allowed? Thanks<P>------------------<BR>


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Sun 02, 2005 4:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2098
Location: Waukegan, IL
Do you happen to have a manual?Precision used some odd voltages to power theur meters ohm range. I assume it is a vtvm? If so, try subing different tubes. If the bridge tube is bad, it will unbalance the circuit. If it is a vtvm, make sure the filter caps in the power supply are changed. I'd start with a 1.5 volt bat for the ohms range. If you can't get enough range with theohms adjust, try 3v. then 4.5 and some even used 6v. Make sure it is working in the volt ranges first.<P>I'd suspect filter caps in power supply if not changed, then tubes. You are getting meter deflection so meter isok.<P>------------------<BR>Tony


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Mon 03, 2005 3:58 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1173
Location: Rome New York USA
Hi Tony, Didn't come with a manual, and it is a VTVM. Will replace the caps, there are only three, and see how it works. Should I try to get the same mf caps, or can I vary somewhat as in radio repl. If still having problems will sub tubes, thou they do check good. It takes a 1.5v battery, will increase if not enough deflection, never thought of that. Thanks<P>------------------<BR>


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Tue 04, 2005 3:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2098
Location: Waukegan, IL
Filter capsinpower supply can be greater mfd if noneare available to be close. Get paper caps exact or very close. Be careful with the battery. Once it's working on the volt ranges, then play with the ohms. Stay with the 1.5 vbattery for now.too large a battery can blow your meter. Caps first, then tubes.<P>------------------<BR>Tony


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Tue 04, 2005 6:17 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1403
Location: Milford, PA
Hi Frank,<P> I'd look at BAMA for the manual. Use their mirror site if you have difficulty downloading from their FTP server.I didn't see the exact one , but since PACO is the same company, try a V-70 manual. It should be very similar, if not right on. Good Luck, <P> Bruce<P>------------------<BR>


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Wed 05, 2005 7:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1173
Location: Rome New York USA
Thanks Guys, tried bama site, page cannot be displayed, what am i doing wrong?<P>------------------<BR>


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Fri 07, 2005 3:40 am 
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Location: Milford, PA
Frank,<P> Try the mirror site that's got a link on the homepage [ EDEBRIS], this should work, and as I stated before, I believe that the Paco V-70 is very close to yours in design.<P>Good Luck,<P>Bruce<P>------------------<BR>


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 Post subject: Precision Model 68 Problem
PostPosted: Oct Fri 07, 2005 4:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2098
Location: Waukegan, IL
I think the 68 is 40's vintage. Could be wrong. What tubes are in the unit?<P>------------------<BR>Tony


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Mon 26, 2020 2:12 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
I have one of these in fair to partly cloudy condition - but still worth the effort to fix and use.

Does anyone have a manual and schematic for one of these? I opened it up and found a leaky vintage Ravovac battery (marked made in USA - and leakage remarkably was on the battery only and not anywhere else) and some old electrical taped connections meaning some well meaning but misguided fixer-upper was doinking around with it. Other than that, the inside looks about pristine and the exterior just needs a cleanup. Light rust in one spot on the case, but forgivable.

I want to get it up and going for my shop as the VTVM I do not currently have.

Much obliged if someone has this - please PM or respond if you can :-)


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Mon 26, 2020 2:26 pm 
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Location: Littleton, MA
It looks physically similar to the Paco V-70, so there's a chance it is also electrically similar. I converted the BAMA manual from DJVU format to PDF and attached it below. You might consider comparing your Model 68 to the V-70 schematic.

Attachment:
pacov70.pdf [1.31 MiB]
Downloaded 10 times

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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Mon 26, 2020 2:33 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
Some pix of the meter:

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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Mon 26, 2020 2:36 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
I does look similar - the 68 has two 6AL5s and a 12AU7 under the hood. I just want to look at a schematic to figure out what the previous owner(s) were trying to do with the rod of three resistors electrical tapped and undo any mods from it to the original. The only mod I would consider is a banana connector over the old Switchcraft type.

stevebyan wrote:
It looks physically similar to the Paco V-70, so there's a chance it is also electrically similar. I converted the BAMA manual from DJVU format to PDF and attached it below. You might consider comparing your Model 68 to the V-70 schematic.

Attachment:
pacov70.pdf


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Tue 27, 2020 2:37 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
Looks like they are not very similar. The Pacotronics 68 VTVM manual quest is turning up nothing - I was hoping someone had it.


stevebyan wrote:
It looks physically similar to the Paco V-70, so there's a chance it is also electrically similar. I converted the BAMA manual from DJVU format to PDF and attached it below. You might consider comparing your Model 68 to the V-70 schematic.

Attachment:
pacov70.pdf


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Tue 27, 2020 5:45 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
little research reveals that this could have been a Paco kit someone bought. I am not sure if it is assembled correctly and it works strangely when plugged in and I try to use it. I did replace the filter cap. I cannot be sure it is capable of working correctly. It is able to be zero adjusted but that's where the orderly behavior ends. The Ohms adjust does not work and the needle moves backwards from a 1/4 position on the scale. Whatever it is doing is sensitive, but the deflection reading is not accurate to discern a measurement and then accurately showing it doubled.

Dead in the water without some sort of schematic and a procedure to calibrate it :-(


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Tue 27, 2020 6:47 pm 
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Joined: Jun Sat 15, 2019 7:43 pm
Posts: 1002
Try this and see if it matches.
http://bama.edebris.com/manuals/paco/98/

DM


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Tue 27, 2020 7:28 pm 
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Joined: May Wed 18, 2011 2:40 am
Posts: 5065
Location: Littleton, MA
Here's a PDF of the BAMA DJVU file.

Attachment:
paco98.pdf [698.01 KiB]
Downloaded 10 times

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Steve Byan https://www.byan-roper.org/steve/steve-at-play/


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Wed 28, 2020 12:52 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
I am going to try to fix this, but I do not have the setup and calibration process so I cannot be sure it is working. There appears to be a line adjustment (zero) that has to be done and an ohms zero set. I am not sure on the 68 where the zero set is and the ohms as using similar model instructions seems to not work. But none of the other Paco VTVMs had the same knob arrangement I have exactly.


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Thu 29, 2020 8:45 pm 
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Joined: May Wed 20, 2020 1:33 am
Posts: 229
Location: Rockford, IL
The meter is WORKING - and seems pretty accurate (and surprisingly linear). Basically, I used the Paco V70 model schematic as a guide and the Heathkit M-11 (and 18) schematic. I was surprised to find out that there is really 1 design to the basic VTVM. There are very minor differences but the elements of the classic VTVM design (of which the roots are said to go back to 1916). This meter may have even been a kit but the build is very good except for a couple instances I fixed. It is fully able to be disassembled down to the rotary switch plates for a total rebuild (my type of equipment).

I did convert it from the switchcraft microphone type to the 1/4" plug as my signal tracer is setup for this type of probe input.

The precision resistors for the most part are right on the printed values. But the other resistors for tubes were way off - majorly. I replaced the 10uF filter and the lone 0.47uF that was hiding where the previous owner could not get to it and then replaced the critical resistors leaving a sole 1K that was good enough to leave as is.

The other disconnect I had was that it was not properly used or calibrated by me - Now that I understand it it does work and is unbelievably sensitive. There are a could of VTVM info sites out there that helped me understand the circuit.

If someone has a Precision Model 68 (pacotronics, or paco company) rest assured that if the transformer, switches, and meter are working the effort to get it going is very minor.

This is the meter cleaned up:

Attachment:
unnamed-10.jpg
unnamed-10.jpg [ 460.19 KiB | Viewed 151 times ]


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Thu 29, 2020 11:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3631
Location: Seattle WA US
The PACO V70, like many other VTVMs, uses a 1 meg isolation resistor near the probe tip for DC voltage measurements. A switch was provided in the probe to shunt this resistor for AC and Ohms measurements.
Your V-68 may, too. IFso, DC voltage calibration would be off, without the resistor.

There have been extensive previous discussions about use of a quarter inch phone plug/jack for the probe connector - and the plug's insulation suitability for the upper end of the voltage range.
-Chuck K7MCG


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 Post subject: Re: Precision Model 68 Problem
PostPosted: Oct Fri 30, 2020 12:20 am 
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Posts: 5065
Location: Littleton, MA
mrrstrat wrote:
This is the meter cleaned up

Very nice!

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