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 Post subject: Sprague TO-4 restoration
PostPosted: Apr Wed 08, 2020 11:14 pm 
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Location: Mesa, AZ 85206
HI Folks

I am now rebuilding a TO-4 I have in my collection, and pretty clearly someone has been in there before with changing things about...in such a way that I cannot tell if they were variations off the schematic, or just poor component selection/placement. The soldering of the parts in question do not seem to be up to sprague standards, so I am thinking a bit of a hack job.

I have posted a photo of the underside of the push button section, along with a keyed partial schematic and full schematic.

The issue at the moment centers around R7, R21....R7 is correctly placed (per manual) and is marked 12K....it appears to me that R21 is either gone or replaced with what looks like to be a 330K, and there is a 680Ω resistor added after R12. I have marked these two resistors as "unknown". I have marked one of the connections "A" in blue both on the schematic and the picture which appears to maybe been moved from it's original place off of R7. "B" is marked in blue on both to show that C10 begins and ends where it should.

I have the higher res copy of the manual off of BAMA, but it isn't clear enough for me to make out the detail.

My ask is if someone has an original manual, maybe a picture of "view from left rear" would give me enough info. Any stock pictures or info about variations would be helpful!

Thanks!

Steve

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Last edited by drworry on Apr Thu 09, 2020 2:43 am, edited 1 time in total.

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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 2:07 am 
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Joined: Dec Wed 16, 2015 10:31 pm
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Location: Toronto ON
Here's a shot of my (as yet) unmolested TO-4.
Let me know if you need further detail.

Joe


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 2:51 am 
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Thanks! My mistake...I said 330K resistor, it is 3.3MΩ...sorry!

I wil look at the pics closely...

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 3:02 am 
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So, it looking like you have a 620Ω after the 12KΩ, and I have a 560Ω after the 12KΩ, so I can "assume" that these were added for R7 being 2% 12K...both of our R7s are 10% (silver band..)..so that makes sense.

It seems that ours then substituted a 3.3MΩ for a 1MΩ in R21 from my copy of the manual...

Thanks!!

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 3:46 am 
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Location: Toronto ON
I agree. The component listing does mention that R7 may have another in series to make up the correct value.
The 3.3M plus a few extra filter caps found in my unit may be mods to later revisions than the one shown in the manual.

Joe


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 8:14 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
Things like this can drive you nuts. The TO-4 was in production from about 1949 until 1959. Apparently they were made in small lots as demand warranted, and it seems like no two of them are exactly alike. No doubt when you bought one new, they gave you a manual that matched your instrument, but after the manuals get separated from the instruments you have to figure things out for yourself.

What I would suggest, before embarking on a major restoration is to replace just the things necessary to power the unit on safely. That would include the two electrolytic filter capacitors and the line-to-chassis caps. Then power it on and see what you've got. There may be other parts in need of replacement and it may not work perfectly, but no harm will be done and it should work well enough to reveal any major problems. If no serious issues are found, you probably don't want to un-modify it too much or it might stop working. Sometimes modifications were made in production or in the field to correct mistakes and you can make matters worse if you remove the corrections as part of a restoration effort. On the other hand, if the unit is wonky, then you have to dig in and see why, and removing parts that don't belong in there may be part of what you have to do to fix it.

In any case, it's always best to tackle just one thing at a time. There are some threads in the archive here from people who shotgunned all the resistors and capacitors in TO-4s and TO-5s in one blast, and then spent weeks or months trying to get the units to work right again. Some probably never did.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 8:54 pm 
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I agree, Chris. I think the one thing I would add is that "unmodding" may be different than replacing a component that is clearly out of tolerance to the value in the unit (as opposed to putting it back to the schematic)

For example, the 3.3MΩ resistor that he and I have in place of the 1MΩ on the schematic....If the 3.3MΩ has crept up to let's say 3.6MΩ, I don't think any of us would replace it if it were a 10% resistor. If it were instead 4MΩ, I might replace it with a 3.3MΩ so I could see if the unit worked with the mod..of course, maybe they picked a high 3.3 out of a box because they didn't have a 4.....

Just a side note....test equipment is quite different from the restoration of jukebox amplifiers, let's say. As you know, there were many "fingers" usually in these amps over the years, with many cost saving attempts to keep them working. Juke box amps changed every year or so through the gold and silver years, so factory mods were far less to an older amp. Usually, we restore these amps back to the schematic, and then troubleshoot if things sound poorly. It might be re-creating the wheel if the factory recommended a mod, but usually you are just putting it right again.

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 09, 2020 10:39 pm 
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Location: Long Island NY
It's hard to tell from the pictures but Radiotech's unit and yours appear to have the 3.3 meg resistor in the same place. I would conclude from that, that 3.3 megohm is the right value and the schematic may be from an older or newer version of the TO-4. Or it could be a misprint which is rare but has been known to happen.

Your 560-ohm resistor is a 620-ohm on Radiotech's unit. Could this be an old repair where somebody used the 560 because it was the closest thing they had? Possibly, since it looks like somebody cut the leads and tack-soldered it in. Your wire "A" does not appear to be in the same place in the other unit, but a lot of the wires are routed differently between the two and the colors don't match. (This is what was meant when I said no two of these units are ever exactly the same!) It does not necessarily indicate a problem since one of the units could have a slightly different switch assembly, or there's a parallel terminal on the switch which was wired differently in one unit. Test equipment was a niche business for companies like Sprague, so instruments like this were often hand-assembled by techs at the factory from parts stockpiles when orders came in--or when they didn't have anything else to do. So what color wires they used and how they were routed in each unit depended on who was at the bench that day and what mood they were in.

For this reason it's sometimes better to do only the one or two things you must so the unit can be operated safely, and then put it through its paces to see which functions work and which don't. It's not a jukebox amplifier where you have to worry about things like frequency response or distortion which may be good or bad depending on how fussy the listener is. A TO-4 either reads capacitances and resistances accurately, applies test voltages and measures leakage, or it does not. Commercial test equipment was a capital investment for some business or company, if it did not work and couldn't be fixed, it got trashed. So 99% of the time one can conclude that it must've worked the way you found it. Now if it's not working correctly you can dig in and start replacing parts and tracing wires out, starting with the ones that look like dubious hacks. If you make the performance better, that's great. But if performance begins to suffer, then you know maybe they put a different part in there because it was necessary and you need to put it back.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Fri 10, 2020 12:12 am 
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Great advice. Thanks!

Hopefully, the rest of this restoration will go smoothly...the last two TOs I did were pretty much dead on to the schematics..

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Fri 10, 2020 9:51 pm 
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Well, one more hiccup...

I have circled R28....The schematic calls for 10Ω; but this is clearly factory so is just a variation...

The head scratcher is the value...It measures 380Ω...It looks brown-green-brown, but then it would be way off from 150Ω..

If the first band were a faded orange, then it would be close....

As you know getting to this part of the unit is easiest when it is all apart...so I would like to get it sorted out before I put it back together.

Maybe someone can have a look at theirs....

Thanks!!

Steve
Attachment:
R28.jpg
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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Sat 11, 2020 12:43 am 
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Location: Seattle WA US
My TO-4 was near the top of the stack, so I opened it.
R-28 is marked 150 ohms (br gr br) 1 watt carbon.
The half watt carbon resistor in series with R7 is marked 820 ohms (grey r br)
R21 is 3.3Meg 1/2watt carbon.

It appears that some values may have been selected during final assembly. That's one approach to production.

The "precision" bridge capacitors look pretty ratty. One pair of wax-dipped aluminum jacketed 0.1mf paper caps with the wax breaking off, one pair of bumblebee black beauty ( .018 + .0018 ) with one end deeply buried in the switch assembly. I may just leave these if the bridge will balance at all, as I own a real Fluke bridge for when I need a good measurement.

-Chuck K7MCG


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Sat 11, 2020 2:04 am 
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Thanks, Chuck!

With luck, this thread may help others in their restorations documenting changes to the schematic.

BTW, this might be one of the largest percent drifts I have seen in a carbon resistor that wasn't completely shot....150Ω moving to 370Ω...

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Mon 13, 2020 8:03 am 
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Joined: Jan Wed 08, 2014 7:52 am
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Location: Arlington, Washington
I went through my T-04 about two years ago, and found the discrepant R-28 value too.
This 150 ohm resistor is connected across the binding posts when all of the push buttons are released. Value is not critical, in that it's function is to discharge the capacitor under test, connected to the binding posts, when testing is complete. Mine is marked 150 ohms, but reads 174 ohms.
One less thing to worry about.
Stan Carter


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 16, 2020 8:26 pm 
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Bummer..totally.

Finishing up the restoration, and should have checked all the pots at the beginning rather than the end.

The 3W 7.5K WW bridge dial pot is open on one side....

It looks as if there is a satisfactory replacement on Ebay...none at our regular places. Still in lockdown, so shouldn't go out to Apache Reclamation to sift through their boxes...

I will post in classified...maybe one of us has one...

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 16, 2020 9:52 pm 
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Posts: 3491
Location: Seattle WA US
Among my notes I find a page attributed to Donald Resor N6KAW, <www.hammondorganservice.com>
with useful notes on bridge pot replacement. In searching for the downloadable source of this page, It turns out to be ARF !!

See ARF thread titled "Sprague Tel-Ohmike can damage capacitors?" from February 2017.

https://antiqueradios.com/forums/viewto ... 8&t=316128

-Thanks, Don !

-Chuck K7MCG


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 16, 2020 10:26 pm 
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Thanks! I have seen this, but didn't note that.

The exact 7.5K is available on the 'bay, so I might spring the $20 for it.

I will check out the other one, though...

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Thu 23, 2020 11:58 pm 
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Got the replacement on Ebay..it was exact military specs and an exact replacment. The guy has over 200 of them...I might pick up a few just to have...he wants $20 shipping included. I can give the info if any needs one.

Finishing up, and after some tracing found cold solder joints and broken wire not visible but seen through tracing. All now looks well.

I posted this so people knew where to find the 7.5k potentiometer.

Barry, if you are looking at this, I had to replicate Mike's work to find the cold joints...do you want the voltages and circuit paths for the TO-4 to go along with the TO-5?

Steve


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 Post subject: Re: Sprague TO-4 restoration
PostPosted: Apr Fri 24, 2020 2:59 am 
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All done and calibrated.

Attached is the schematic, known resistor changes, and voltages for posterity.

Steve

Attachment:
TO4 voltages.jpg
TO4 voltages.jpg [ 218.64 KiB | Viewed 350 times ]


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