Re: TM506

Harvey White

The NPN and PNP transistors for each slot are completely independent of anything.  They're nothing but transistors connected to socket pins.

1) if any transistor is mounted and any lead goes to ground/frame, bad things happen.

2) if you reverse the base and collector, or base and emitter, that's not going to help.  The worst case is that you get the BE junction connected where the EC junction ought to be, and you get a series diode, therefore, bulk voltage goes to the regulated output.

I'd suggest the following:

1) using one of the adaptors you bought, make a plugin to bring out the wiring for each slot.  You're interested in the power supply voltages, and the BEC junctions of the transistors.  Using a transistor tester (the cheap ones built with a PIC or MEGA ought to be fine), test each transistor. The advantage here is that the tester identifies which lead is which.

2) Check the power supply voltages.  The bulk DC supplies are fused.  The AC windings are not fused,

3) Make a supply tester, that at least breaks out the voltages to banana jacks or something like that.  You want to check each slot.  For now, the current available may not be a factor, but you want to make sure that all the voltages are where they should be.

4) that you have transistor failures suggests that at least one plugin has a supply problem, and that you may have put in the transistors wrongly.  An ohmmeter cannot really tell the difference between the BE and BC junctions, a beta test or transistor tester can.  If you have the C and E leads reversed, you generally get a transistor with very low beta, but it'll be a transistor.

5) note that you have discovered that using plugins as test loads is not the best idea.  A dedicated tester rig makes a lot of sense.

6) that tek-wiki article on the test module for the TM500 series frames makes a lot of sense.


On 7/22/2020 9:38 AM, Stephen wrote:
On Wed, Jul 22, 2020 at 02:34 AM, Michael W. Lynch wrote:

On Wed, Jul 22, 2020 at 07:42 AM, Stephen wrote:

No voltage at pin 1 on any of the slots. Schematic shows 24.8V RMS.
Are you checking this voltage between 1A and 1B of the back plane connector?
It appears to me that the 24.8VAC source at each slot is supplied by
independent winding in the Main XFMR, It seem unimaginable that every one of
these windings in the main XFMR would have shorted out. However, I am
certainly no expert on the TM506, I suppose it is possible? You can check
the individual windings at each slot by connecting an ohmeter between 1A and
1B and looking for continuity of about 1.6 ~1.8 Ohms and then between each
contact and the chassis for no continuity. I just did this test on my TM506,
so you should see something similar on yours.
Michael Lynch
Dardanelle, AR
Will do when I get back. At the store at the moment.
This is very strange. I even thought for a minute that I had wired everything backwards. I checked, and rechecked, and re-double checked against the schematic and the pinouts, and I don’t see any problems...
So unless my unit is, for some obscure reason, wired differently, all should be ok. But it’s not...

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