Speed Controller Testing

--updated 06-10-16--

The goal was to finally sit down and determine the issue in the 40+ year old failing saga with the Voice of the Mummy Board Game record players.  Most of the problems were from the old wet cell batteries.  The batteries quickly depleted shortly after being manufactured.   These batteries were left in the players after play and sent corrosion material and gases throughout the case. Anything metal was oxidized and destroyed including the motors.  Then the speed controllers were the next issue.  These controllers worked (very rare), and had the "too slow then too fast" voice speed with the slightest movement from the control knob.  This was very annoying as I remember this clearly as a little tike in the 70's.

I was able to find out that the quality of the resistance material wasn't carefully controlled to begin with as the material used degraded over time.  This created a high resistance value that gave the game these common quirks.  Plus the way the speed controllers were designed didn't help. The wipe contact attached to the knob rocked back and forth changing the resistance also.  Plus the speed "notches" were very few to begin with.  There are 31 separate resistance areas inside the speed controller where the voltage/current could have been controlled.  This would have been enough gaps, but the resistance material failed over time and caused the all of the problems.   

I used a test situation with a single battery as well as a dual battery setup.  The optimal resistance for correct voice speed control, for the single battery, is about 3 ohms and 11 ohms for the dual setup.  (+/- 30% depending on the battery strength).  The problem with the dual battery setup is that in order to get the speed of the turntable down with the extra current/voltage, more resistance had to be applied with turning the knob CCW.  At that point the turntable would not spin because the motor would not be getting enough power to spin.  I was able to test this a number of times to make sure it wasn't a random issue.  This is, most likely, why the record players only required a single "D" battery.  The resistance power window was better and the motor responded with enough power.   Plus, I remember in those days, these batteries were expensive and they didn't last long at all.  We can all remember those flashlights that had to be tossed out because of the extreme battery corrosion.

Sorting the Good from the Bad

		The heart of the speed controller is a small printed wafer board with copper contact pads. Then a resistance material "carbon goop" is applied in colored grey.  At the knob end is the copper sweep bent in an arc.  When these are installed together and crimped into the player case, this tension keeps the sweep and the contacts connected.
		The grey carbon goop material connects each gap between each copper contact pad. The resistance adds as the knob sweeps CCW around the contacts.
		Here is the Stack! All 60 of them ready to be sorted for the single battery test.    After sorting, only six of them passed with a usable range of resistance for the single battery setup.
		These would have at least 5-6 or more contacts tabs measuring below 3 ohms.  That would give better speed control.  Most had one tab below 3 ohms and then went past on the second tab.  These 5 marked with a red dot were common with an average total at about 30 ohms.    Wow! I found the Holy Grail! One piece had at least 10 contacts tabs measuring below 3 ohms! I marked it with a blue dot. It read at max of 6.44 ohms!

Measuring for the Single Battery

		I tested one of the 5 marked red. Starting from the small outer hole, and moving around the contact pads CCW, read the total resistance.  This one marked red had a max reading of 42.09 Ohms.
		First notch. 0.73 ohms
		Second notch. 1.16
		Third at 1.94
		The Fourth at 2.41 ohms.  The large jumps in resistance are what is causing the "Too Slow then Too Fast" problems with fine tuning the voice speed based on the strength of the battery used.
		The fifth was at 3.87 ohms for this piece.  This is out of range.  This just doesn't have enough tabs measuring below 3 ohms to be able to fine tune the voice speed.   BTW it was determined that 1.90 ohms and 3.20 ohms is a usable range for the motors.  Depending on the strength of the battery.

Measuring the "Holy Grail"

		So,  lets check out the "Holy Grail" marked blue.  It was one out of 60!  I'm amazed that I found it!
		Total 6.44 ohms!  Nice.
		First tab. 0.54 Ohms.    Wow! A really good start.
		Second notch. 0.89 Ohms
		Third 1.40
		1.75
		1.88
		1.97
		2.27
		2.40
		The ninth tab at 2.42 Ohms!   Awsome!   Notice the smaller increments in the resistance.  This will result in smaller speed changes in the voice so the pitch sounds closer to the original recordings and you can simply understand it.   They all should have been built in this manner.  I'll install this in a player later on!

Measuring One of the Rejected Wafers

		One example of the many complete rejects.  186.36 Ohms at max.   Way out of range.   Some were in the 2M Ohms Range!
		First Location.   Still in Range at 1.71 Ohms.
		Second Location. Out of Range already at 3.56 Ohms.  Way too big of a jump! Toss it!  This controller would played too fast and then stopped the motor with the slightest CCW movement of the knob.
		Third at 5.99 Ohms Ok, Ok.  I'm sure you get the point.

Player Testing with Single Battery

		So then, I moved on to testing each of the 6 passing pieces.  The next 2 probes were in line from the battery and I used the needle tips to dig into the contact tabs to test.  As expected, the voice speed could be "understood" until I moved to a contact tab that was near about 2.50 ohms.  Again, the voice would be too slow, then too fast on the next contact tab.
		Then I tested using the new replacement speed controller.  These parts are made very well.  I can rotate the knob and the resistance measured is very precise and linear.  They measure out consistently at .02 ohms per small "click" of turning the knob.  This gives you great voice speed control based on the strength of the battery used.

Player Testing with Dual Battery

		Next was dual battery  sorting and testing.     The lone wafer in the lower left didn't work for either battery setup.  It read too high within the first 5 contact tabs for the single battery setup, then the total didn't reach beyond 8 ohms.  Not enough to slow down the dual battery setup.
		The large group in the middle are wafers  that would either marginally work or work very well.  These would have at a minimum of 6 contacts or more that would measure below 12 ohms.  12 had contacts measuring that about 2/3 to 3/4 around.
		The group on the right were completely useless.  They have either 2-3 contacts measuring below 12 ohms or total corrosion and uhhhh, hmmm, splashed with some solder or just too high of a reading. 2-3 started out at 50 or more and one went as high as 240 ohms.

Summary is to say that the resistance material used, either degraded over time or they didn't have a good manufacturing process to yield a consistent part used in the single battery setup as it was issued from Milton Bradley in 1971.  Mechanically the design was great.  The wafer had plenty of room with 31 separate contact tabs for the needed resistance jumps for enough control to manage the voice speed.  The knob with the copper sweep was large enough with plenty of surface contact from the spring metal to do the job. The largest problem was the separate shaft that was secured through the case.  It would wobble loosing good contact causing the speed changes.

Most important would that the resistance in between each contact tab be carefully controlled.  There should not have been the large inconsistent jumps in resistance from one contact to the next, continuing around on the wafer board.  No more than, say,  .4 ohms in single battery configuration and .7 ohms for the dual battery configuration. The smaller the jump the better control of the voice pitch you would have.  A lot of these had huge jumps of a full ohm or more.  That would result in the common problem of attempting to change the voice speed to sound "normal",  it would sound too slow and then with a small knob adjustment, too fast.  Even rocking the knob around without turning it will cause this severe change in pitch.

Using this in a dual battery setup would still not would work. Turning the speed knob to slow down the voice speed with the added power from the second battery would just add more resistance.  This would reduce the voltage/current to the motor and it would not spin consistently.  You would have to help the motor by spinning the turntable.  There were still other issues with the knob contact that wasn't designed well.   The knob contact had a small problem with warping or excessive bending and occasionally the metal tab just popped off.  They really should have simply purchased an electronics grade linear potentiometer and saved themselves a lot of problems. 

In the end,  I installed the "Holy Grail" piece that had very good resistance measurements!  Yep, it works.  It has enough speed control throughout the entire knob rotation.  I tested using batteries at different depletion levels and the voice speed control worked fine.

Here is a small chunk taken from a website about resistive materials to shed light on how these units made it from the factory and then failed soon after.  Used without permission...

" Some slightly more recent history: Carbon Composition Potentiometers

...Carbon composition resistive elements are a combination of finely ground carbon granules, clay-like insulating filler, and a binder to hold everything together. This mixture in varying proportions produces widely differing resistance values. Values as low as a few hundred ohms to as high as several mega-ohms are possible. Unfortunately, some composite resistive mixtures can be quite susceptible to moisture with the result that values will shift when pots are exposed to changing humidity..."