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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Thu 09, 2004 10:25 am 
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Location: Burbank, CA and Thailand
Since no calibration procedure for these units seemed to be available, I did some "reverse engineering" and created one. I was very happy with the results on my own unit and hope this will be helpful to others.

I'd like to request that anyone who takes issue with something here, sees an error, knows "a better way", or has something to add write me directly, rather than put their comments in a reply. I'll edit the procedure with your comments, and give you credit. This is to avoid back and forth discussions that might make the steps hard to follow.

Thank you to Chris H, Dave (easyrider), and Leigh, whose ideas and comments helped jump start this project.

-------------------------------------------------

Note: Some models may have fixed or variable resistors in the R12 or R14 positions to expand the ends of the ranges. If your dial settings do not track properly, it may be helpful to add trimmers in these locations. See details below.

Caution: The TO-5 circuit uses high voltages for many of its tests. ALWAYS release all the black and red buttons before connecting or removing a capacitor. (This disables the test voltages and discharges the capacitor.)

1. Recap/repair/restore unit, check resistors where possible, verify normal operation. Be sure to install measured, precise replacements for C8 (.02 UF ) and C9 (2 UF).

2. Prepare the following 11 capacitors for calibration tests. As capacitor tolerances vary greatly, only measured, precise capacitors should be used. In some cases you’ll need to use parallel (or series) capacitors.

Range C1-2: lower 10 PF, center 200 PF, upper .005 UF
Range C3: lower 1000 PF, center .02 UF, upper .5 UF
Range C4: lower .1 UF, center 2 UF, upper 50 UF
Range C5: lower 50 UF, center 80 UF, upper 2000 UF.

If possible, use an oscilloscope or VTVM for calibration. This will be easier and more accurate than using the eye indicator. Connect your scope or meter to chassis ground and the + terminal of the TO-5. A balanced bridge condition is indicated by lowest level (deepest AC null). Remember that with some capacitors, the Power Factor control must be adjusted to get accurate readings.

3. Start by aligning Range C3. Check dial reading with the .02 UF cap, loosening the dial setscrew and re-positioning dial for exact reading, (Perform this step very carefully, it will affect everything else.) Then, check with 1000 PF cap. The reading should be very close. If the dial reads high with the 1000 PF cap, optional trimmer R14 can compensate. (If the dial reads low, this suggests a problem in the unit.)

Next, check the upper part of the range using the .005 UF cap. The reading should be very close. If the dial reads low, optional trimmer R12 can compensate. (If the dial reads high, this suggests a problem in the unit.)

Recheck dial indications with all three capacitors.

4. Check Range 4 using the .1, 2 UF, and 50 UF caps. The readings should be very close.

5. Check Range 5 using the 50 UF, 80 UF, and 2000 UF caps. The readings should be very close. If this range is off ‘across the board”, improvement is possible by replacing R15 (150K) with a 200K trimmer. (R15 only affects this range.) Set the trimmer to 150K, connect the 80 UF cap, set the dial for 80 UF, and then adjust the trimmer for balanced condition. Recheck with the 50 UF and 2000 UF caps, readings should be very close.

6. Align Range C1-2. This sensitive adjustment has been saved for last, so that the unit can be “left as is” afterwards. Connect the 200 PF cap, set the dial for 200, and adjust trimmer C7 for balanced condition. (C7 only affects this range.) Check with the 10 PF and 5000 PF caps, readings should be very close. If the dial reads low with the 10 PF cap, a 1-3 PF capacitor can be soldered to the to the - / + binding posts to trim this reading. A trimmer or “gimmick” capacitor could also be used.

At this point it would be good to recheck other ranges, though none of these steps should affect any previous ones. Then - done!

-------------------------------------------------

Note 1: R12 and R14 may have been added to the bridge pot during production. R12 would have been added to the clockwise end of the pot to expand range on the upper side of the dial, and R14 would have been added to the counter-clockwise end of the pot to expand range on the lower side. These resistors may be replaced with 100 ohm trimmers to correct range "spread" problems. To install the trimmers measure the existing resistors with an accurate ohm meter, then set the new trimmers to match the old fixed resistors. Install the R12 trimmer between the clockwise terminal of the pot and R10 (150 ohms, 2 watts) and install R14 between the counter-clockwise terminal of the pot and the wire to the switch board.

Note 2: It is advisable to add back to back protection diodes (1N4007 or similar) across the meter for overload protection.

Note 3: When using the TO-5, readings in the center of the dial will be the most accurate.

Some notes about the circuitry:

C1-2 range reference is C6 (180 PF) with C7 trimmer.
C1 range accuracy is +/-2% and +/- 1 PF
C2 range accuracy is 3%

C3 range reference is C8 (.02 UF)
C3 range accuracy is 5% from .001 to .005 and 3% from .005 to .1
With C3 range selected, R11 is inserted between the upper side of the range control and the power transformer. In all other ranges R11 is bypassed.

C4 range reference is C9 (2 UF)
C4 range accuracy is 5% from .1 to .5 and 3% from .5 to 10 UF

C5 range reference is C9 (2 UF), extended by shunt resistor R15 (150K).

R12 is an optional selected resistor on the upper side of the bridge dial, 0-100 ohms. Increasing the resistance of R12 will expand range at the upper side of the dial. (With R12 or R14 at 100 ohms, about 1/4” of “dial diameter” is added.)

R14 is an optional selected resistor on the lower side of the bridge dial. Increasing the resistance of R14 will expand range at the lower side of the dial.


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Last edited by David Kulka on May Sat 13, 2006 5:31 am, edited 1 time in total.

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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Thu 09, 2004 4:51 pm 
Silent Key
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Nice job, David. Thanks for your efforts.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Thu 09, 2004 9:41 pm 
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I agree, a most excellent job. Would any of the info apply to the TO-6 - or could we talk you into it for that one?<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Thu 09, 2004 11:11 pm 
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I agree also, and raise the same question about the TO-4.<P>Greg<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 2:19 am 
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Guys, thanks. Test gear restoration can be time consuming and I've found that sometimes, the piece you wind up with is not really worth the time and effort.<P>But the Sprague capacitor analyzers are really nice instruments. They are invaluable for the work we do, and I don't know of any modern product that does the job as well. <P>As for the TO-4 and TO-6, they can't be <I>that</I> different, and if someone would care to email schematics I'd be glad to look them over and offer some feedback. But it might be hard to give definitive answers without having a unit on the bench.<P>Of course, if someone would like to "contribute" a TO-4 or TO-6 to the cause, I'll get on it right away! <IMG SRC="http://antiqueradios.com/forums/cool.gif"><P>------------------


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 5:14 am 
Silent Key
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I'm going to try this on my TO-6A. Will report the results, probably in about a week.<P>David, I'll email a TO-6 schematic in a minute. The TO-6A schematic will have to wait until tomorrow.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 8:21 am 
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Interestingly, I just finished rebuilding my TO-6 a few weeks ago, and ended up using a very similar procedure to calibrate it. So what you've documented does work on a TO-6 also. <P>In the end of the "Multitude of Sprague..." topic, Leigh provides a procedure for calibrating the electrolytic leakage test also (and many thanks to him for that as well). The only calibration area that's left is the insulation resistance test. <P>Just to get that one started, I'll throw out the general process I used to calibrate mine last month. It was a "reverse engineered" process also, and I'm sure there are better ways to do it, so please speak up if you see any issues.<P>Make sure the 5mA meter shunt is correctly set before doing this (see other TO- post). Start on the higher voltage range ("L"), and adjust the meter to "Set" using the "F" button. Connect a VERY large resistor, preferably a value that is near the middle of the meter scales (I used one from inside an old high voltage probe), to the test terminals. Adjust pot R35 to establish the correct meter reading for the resistor you're using. <P>Switch to "K" range and readjust the meter to "Set" while holding in "F." Potential showstopper: if your R18 is only a fixed resistor, you'll have to change it to a pot or combination fixed and pot to calibrate the "K" range. My R18 was a quasi-TO-6A setup, with a 5.6k fixed resistor and a 2k pot in series. Another Sprague ad-lib. Vary the resistance of R18 to establish the correct meter reading for your resistor. Double check the "L" range, which shouldn't have been affected, and you're done.<BR><P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 7:05 pm 
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Leigh, I got the TO-6 schematic, thanks. At first glance the capacitance measurement section looks very close to the TO-5, perhaps identical. I wish I had a legible image of the dial, so I could see how the dial markings compare to the TO-5. Any easy way to send that?<P>I know that the TO-6 has an added low voltage range for the leakage test. I'll compare the two schematics in the next few days. But from Erik's comments, it sounds like the procedure is at least close for the TO-6, and perhaps can be used as is.<P>Erik, thanks for the insulation resistance info. I've noticed that in that mode, my meter is jumpy and sort of erratic, although it seems to measure ok. It acts as if the needle is rubbing, but I don't think it is. I'd expect it to fall back smoothly under test, kind of like "curve" of a discharging capacitor.<P>Also, I'd still like to know why these units use nearly 150 volts for the high range (C-5). Why is this much voltage necessary? Leigh, any thoughts on this?<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 8:25 pm 
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David,<P>Pictures sent.<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 8:26 pm 
Silent Key
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Leigh wrote:
<font>quote:</font><HR>Originally posted by DavidK:<BR><B>...I wish I had a legible image of the dial, so I could see how the dial markings compare to the TO-5.<BR>...Also, I'd still like to know why these units use nearly 150 volts for the high range (C-5). Why is this much voltage necessary? Leigh, any thoughts on this?</B><HR>
<P>Hi David,<P>I'll try to get a shot of the TO-6A dial tomorrow. (The batteries in my digital camera died <IMG SRC="http://antiqueradios.com/forums/frown.gif"> ) I don't have a TO-6.<P>Regarding the high voltage: Probably because the impedance is so low that a high voltage is needed to produce a measurable error voltage.<P>------------------<BR>73 de Leigh W3NLB | | Leigh@AtwaterKent.Info<BR><A HREF="http://www.AtwaterKent.info" TARGET=_blank>http://www.AtwaterKent.info</A> | | <A HREF="http://www.Synchrophase.info" TARGET=_blank>http://www.Synchrophase.info</A>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Fri 10, 2004 8:48 pm 
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Leigh, I got the pictures from Mort (m82), so no need to bother with that. Thanks Mort.<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 11, 2004 12:01 am 
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DavidK,<P>I was going to E-mail this to you, but decided to post it in case it affects anybody else. I had a similar problem with my TO-6 acting strangely on insulation resistance tests. I'm assuming you've replaced all those carbon resistors and the grid cap in the d.c. amplifier circuit of the TO-5. I actually don't even have a schematic for a TO-5, or I'd give the specific references. The switches really need DeOxIt too, but I'm assuming you've already done that also.<P>Anyway, even after I'd replaced all the insulation resistance test circuit parts in mine, I still had strange needle behavior. It took my Chris H-calibrated Heath TT-1A to find the problem: grid leakage on V4, which is a 6C4 in the TO-6. My 539C, TV-2/C, Jackson 658, 648, Eico 667, and Sencore MU140 all found the grid perfectly acceptable, by the way, so I at first doubted there was even an issue. Five NOS 6C4's later, I finally found one the TT-1A liked. Erratic meter behavior -- gone for good after that. Short story made long, but try a few other 6C4's (if that's what's in TO-5's), unless you have a TT-1A to screen them first. Please don't flame me for saying this, but nothing else can snuff out grid leakage like one of those.<P>By the way, my experience with the TO-6 I restored last month, which cosmetically was perfect inside and out, was that basically every passive in the thing had to be replaced. I don't know what everybody else has seen with theirs.<P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 11, 2004 1:10 am 
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Erik, excellent info, thanks. The TO-5 does use a 6C4. Discouraging that you had to try 6 of them, but maybe I'll round up a few and give it a shot. I guess it makes sense in a way, that's an incredibly sensitive measurement, and I suppose any little bit of leakage could muck things up.<P>While I replaced every capacitor in my TO-5 (except for 1 or 2 micas) the resistors seemed to measure "right on" so I left them alone. I suppose that some of them could have oddball leakage or noise issues that wouldn't show up on a quick resistance check (I've seen that before). But they looked fine to me, and I would be concerned that by replacing them, even with "more accurate" resistors, I might introduce new problems.<P>I certainly agree about cleaning the switches.<P>Which tube has a grid cap?<P>If someone has theirs open, I'd still like to know what the plate voltage on the 1629 is. I've ordered a new one and suspect this will solve my dim eye tube problem, but would like to be sure that my B+ is really up to scratch.<P>------------------


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 11, 2004 2:41 am 
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Hey Dave, <P>I epect to EVENTUALLY try calibrating my TO-4, but in the meantime there's a manual and schematic here: <A HREF="http://bama.edebris.com/manuals/sprague/to4" TARGET=_blank>http://bama.edebris.com/manuals/sprague/to4</A> <P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 11, 2004 6:05 am 
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DavidK et al,<P>You may just have better quality resistors in your TO-5 than my TO-6 had. I had lots of carbon comp. ones, and every single one of those made an exit. I tend to follow the Alan Douglas capacitor testing theory on carbon comp resistors: if they made a sound when they hit the bottom of the trash can, they were bad. <P>Sounds like there are a few differences in the tubes in TO-5's versus TO-6's. TO-6's have a 6E5 instead of a (cheaper and easier to get) 1629. My 6E5 was dim too, and had to be replaced. BTW, the 6BL7 in the power supply of mine had grid leakage too. I'm not convinced that in the role it played that even mattered, but it's something you might want to check. <P>The grid cap that was bad was the 0.005uf on the grid of V4, C16 in a TO-6. <P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 11, 2004 8:23 am 
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Oh! When I first read the term "grid cap" I thought you meant a clip that connected to the top of a tube, and searched the TO-6 schematic for one, in vain of course. <IMG SRC="http://antiqueradios.com/forums/rolleyes.gif"> <BR><P>------------------<BR>


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 Post subject: A Calibration Procedure for the Sprague TO-5
PostPosted: Sep Sat 18, 2004 6:20 am 
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I took some time to compare the TO-4, TO-5, and TO-6 circuits. Electrically the capacitance measurement circuits are nearly the same, and it looks to me as if the TO-5 procedure should work on the others.<P>However, the parts designations are different, which might be confusing. These are the differences that I see:<P>1. The C1-2 reference capacitor,C-6 (180 PF) on the TO-5 is C-3 on the TO-4 and C-8 (changed to 190 PF) on the TO-6. <P>2. The C3 reference capacitor (.02) is C-8 on the TO-5, C-4 on the TO-4, and C-9 on the TO-6.<P>3. The C4 and C5 reference capacitor (2 UF) is C-9 on the TO-5, C-5 on the TO-4, and C-10 on the TO-6.<P>4. The 150k shunt resistor that extends the hi UF range is R-15 on the TO-5, R-6 on the TO-4, and R-13 on the TO-6.<P>Following up on some other things we discussed…<P>I finally got a NOS 1929 for my TO-5, which took care of my dim eye problem, and seems to indicate that 95 volts on the plate is about right.<P>As for my jittery leakage readings, it looks like I had two separate problems. Replacing the 6C4 seems to have cleared up the “drunkenly wandering needle” problem. The meter still acts “sticky” in one spot but when adjusting the cal pot the meter voltages look fine on the scope, so I think the meter movement is hanging up in one spot. Not something I'd look forward to working on, and for now, not a major problem.<P><BR>------------------


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PostPosted: Mar Sat 07, 2009 8:18 pm 
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5-year bump in case someone has tried to cal the Insulation Resistance.

Erik has the 50K Insulation Resistance (R35) adjustment backwards. If you follow intuition and adjust for correct reading with test resistor in place, you will end up farther off than before. This is because R35 affects the full-scale reading more than the mid-scale reading. Here is the correct procedure:

1. Establish Set Level.
2. Apply test resistor.
3. Note how far off, and in which direction.
4. Adjust R35 to read the same amount off, in the opposite direction.
5. Repeat steps 1-4, because Set Level and R35 interact.

One more note: The entire cal procedure (as published on BAMA and amended here) works for the TO-6A same as the TO-4, TO-5 and TO-6.


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