Back in 1975 I built this TAT from an article in Model Railroader. I don’t remember if it was a TAT III or TAT IV. It was never used and now I want to give it a neighbor who is starting out in HO. Unfortunately, I don’t have operating instructions or a complete understanding of what each control does/interacts.
Can anyone help with a copy of the operating portion of the article?
Thanks, Jim
Jim.
Linn Wescott provided us with many innovations in the '60s and '70s. The throttle you’re displaying is one of those. At it’s time, it was a great device… Many of us could build it and employ it without having an EE degree.
Today, however, I suggest your friend will be well pleased with one of the many offerings from Model Rectifier Corporation (MRC). They usually advertise on the back cover of MR Magazine. At this point, for your friend, I think “plug and play” will be a whole lot better.
Chip
Chip, excellent advice which my friend is currently following. He has a simple. modern transformer in use. We both want to try the TAT (for me it’s 35 year later) and would appreciate finding the instructions.
Jim
I just received the MR disk library of all past issues. If you can narrow down the publication date for your throttle article, I might be able to get what you need.
RSVP
You may have built it in 1975, but when was the article? [(-D]
I looked in the magazine index and came up with an article called Improved Transitor Throttle - TAT-IV. It is listed as being in the March 1969 issue.
Oh, and it helps the search if the name is spelled correctly: Westcott
March 1969 is probably the article. I know it was built in 1975 because I put 75 and my initials on the front face. The thread title has been edited for spelling; thanks.
The control labels are somewhat self-explanatory. The adjustable pot on the side is unlabeled. I have no idea what it does. I’ve put a volt meter on the output and fiddled with things. The side control does not seem to do anything. Results may differ if I had a load on the output. I don’t have a layout. I also have some concern about the capacitors after 35 years. These TATs were designed for the old permanent magnet motors; I wonder if they even work with can motors.
Thanks for any help. Is it possible to copy the operating instruction portion of the article. If not I can look for a used magazine.
Jim
I can look through the Club’s magazine collection tonight (I’ll try to remember) and see what I can find.
Right now this issue is on ebay for $4.95, Buy It Now, free shipping.
I just did a Google search. Seems to be the lowest price.
Rich
I built the TAT IV. I think the pot on the side is for the starting voltage. Put a loco on the track, set the Pulse sw on, PW to 1/3 or so, crack the throttle, then adjust the pot on the side until the loco starts creeping.
EDIT:
Sorry. Turn the Pulse SW to OFF.
I have done the same with a couple trim pots on a Pacematic throttle by Thorne. The pots only set the minimum speed for the main throttle pot. This could vary from loco to loco.
I had one for standard speed and one for switching speed. The main throttle had a resistor added in parallel with the throttle pot via a switch that modified the pot curve.
Rich
I have the Model Railroader collection on DVD. I would just send you the article, but Kalmbach might frown on that. From your picture I can not tell for sure how many pots you have on the throttle, but it looks like maybe five(two on the face, two on the left side, and one on the right side). The circuit actually shows eight pots. Initially I thought you might have a TAT III, but the pulse width setting rules that out. I’m guessing you have some of the less critical pots either inside the throttle or have resistors substituted for them. For example, there is a load compensation circuit that the article downplays the significance of(it was more significant in the TAT III). I would not be surprised if you placed those pots inside or replaced them with resistors. There is also a brake rate pot that he has suggested values for resistors to use in it’s place if you don’t need it to be adjustable.
Here are the pots he has listed for the circuit along with the listed value of each pot:
I have only three pots, the two on the front face and one on the right side. The right side is unmarked. The terminal posts for the output are on the left.
I need to buy the magazine. I have absolutely no recollection of what I did when i built this.
The others may be internal, although the later TAT V allowed for DIP plugs with fixed resistors for each loco so after you tweaked all the settings you could measure the settings and build a fixed setting plug for each loco with optimum settings.
This should drive modern locos just fine, since it is a transistor output. It’s rheostat throttles that have issues with modern efficient motors - they rely on the current flow across the rheostat to drop the voltage, and when the motor draws a fraction of the current there isn;t enough voltage drop to slow it down. That’s why they used to have different models for N and HO scales, the N scale motors would run away on most HO packs. But transistor packs only come in one flavor, there is an absolute maximum current draw but otherwise the amount of current has little effect on the output.
–Randy
Thanks to everyone who contributed to this thread, with a special thanks to Chip and Greg.
Unfortunately, my throttle does not seem to be either a TAT III or IV. These are characterized by a multi position brake lever; my throttle has only two brake buttons. The MR articles show schematics only. I could never have built a complicated circuit like that without a parts layout.
There must have been an article sometime after 1969 and before 1975 that described building a momentum throttle - maybe based on or using parts of the TAT circuits. Heaven forbid, could it have been in RMC?
I need to spend some time with the several articles I now have to understand the workings of these throttles to see if I can decipher the functioning of mine. Or, better yet put an engine on the track and try it.
For now I have a mystery home-built throttle.
Jim
The timing is right that it could be one of the throttles from Peter Thorne’s Practical Electronic Projects for Model Railroaders by Kalmbach.
I found another searching the 75 year collection. Don Hansen, Transistor Throttle with a Future. It looks a lot like yours although without a meter and it doesn’t seem to have the momentum switch or the third potentiometer, but it does have the two pots on the front plus the two brake buttons for service and quick service. And mounts the power transistor on the heat sink on the bottom of the unit like that.
–Randy
Randy, can you tell me the issue for Throttle with a Future?
Jim
MR, June 1975. Somewhat similar. Just no meter.
One pot controls speed.
One pot labeled, Cycle Adj.
Fwd/Rev swittch.
Pulse switch.
Service switch
Q S switch.
No meter.
Power Switch.
The diagram shows the Power Supply stage, the Pulse Circuit, the Momentum circuit and the Output stage.
There are also diagrams of the perf board, and metal box photos, plus full schematic and parts list.
Rich
Great, Rich, that sounds more like what I have, including the layout. Now I need a copy of that article.
Jim
The magazine is on the way, thanks to all.
Jim
The mystery of this throttle is now solved (mostly). The circuit matches that in the June 1975 article “Transistor Throttle With A Future”. My throttle has two additions: a volt meter and an extra potentiometer. The pot is added to the momentum circuit at terminal 16. R7 is replaced with a 10k ohm resistor; the pot is 50k ohms.
The purpose of the additional pot is to vary the amount of momentum/braking.
There must have been an addendum/letter to the editor in a later issue describing the additional features, which were incorporated when I built mine. As yet, no locomotive has been run with it.
Again thanks to everyone for the help.
Jim