Repair of Tektronix TM501 Power Module

[xrunner]

I had received this along with a PG501 pulse generator. The checkout I did on it shows that the +/-33.5 VDC supply has an issue. Specifically the -33.5 supply is very low -

+33.5 = 37.1 VDC
-33.5 = 23.8 VDC

Another observation is that the + supply bleeds off very slowly after the power is turned off, but the -33.5 supply goes to zero almost instantly. I think it's obvious what is wrong, but for the record for any new players (or bored old players  ) I will go through the process of repair.

As you can see, an extender cable makes it easy to check the voltages (and work on the plug-ins).

[David Hess]

I think it's obvious what is wrong, but for the record for any new players (or bored old players  ) I will go through the process of repair.

Oh, so you do not need any suggestions. 

On my TM501, I replaced all three aluminum electrolytic capacitors with radial leaded parts bent at a right angle.  The hardest thing to do was to remove the original can style capacitors; I ended up lowering the temperature of my soldering iron while using the largest tip for maximum heat capacity so the printed circuit board would not be damaged and the copper would be be lifted.

I mostly use my TM501 with the cover removed in lieu of an extender to work on TM500 plug-ins.

[guido]

Somebody made some small PCBs to go from those style of caps to regular ones. The gerbers are on the forum somewhere. I use them all the time 
TM5xx and 7xxx scopes. If you use a regular cap instead you need to make sure you have everything connected. Tek uses the cap housing to cross other tracks.

I used a small sawblade to cut (most of) the leads of a cap to remove it.

[xrunner]

Oh, so you do not need any suggestions. 

Oh you can give any you wish to. 

I mostly use my TM501 with the cover removed in lieu of an extender to work on TM500 plug-ins.

Yep, but I like the flexible extender cable a lot. They are listed on Ebay as JAMMA extender cables, for troubleshooting video game boards, but they use the same connector. The only thing is you need to cut the trace between p1/p2 and p3/p4 on both sides, their boards require that configuration but it's a bad thing for these Tektronix chassis. 

On my TM501, I replaced all three aluminum electrolytic capacitors with radial leaded parts bent at a right angle.  The hardest thing to do was to remove the original can style capacitors; I ended up lowering the temperature of my soldering iron while using the largest tip for maximum heat capacity so the printed circuit board would not be damaged and the copper would be be lifted.

Somebody made some small PCBs to go from those style of caps to regular ones. The gerbers are on the forum somewhere. I use them all the time 
TM5xx and 7xxx scopes. If you use a regular cap instead you need to make sure you have everything connected. Tek uses the cap housing to cross other tracks.

I used a small sawblade to cut (most of) the leads of a cap to remove it.

Well, as shown in the scope capture, a capacitor has gone bad (surprise, surprise) - C22. Also, the resistors R12 & R22 show signs of getting very hot, in fact R12 is a compete open, which is why the positive supply does not bleed off. Both will be replaced.

As far as the capacitors go, yea replacement of all of them is in order. I might get radial leaded, or look into the gerbers that guido has shown. It looks like I can cut most of the leads with my nippers then pull each stub lead out after heating.

To be continued ...

[dave_k]

See here:

https://www.eevblog.com/forum/testgear/tektronix-76037613-repairs/msg804009/#msg804009

 

[xrunner]

On my TM501, I replaced all three aluminum electrolytic capacitors with radial leaded parts bent at a right angle.  The hardest thing to do was to remove the original can style capacitors; I ended up lowering the temperature of my soldering iron while using the largest tip for maximum heat capacity so the printed circuit board would not be damaged and the copper would be be lifted.

I think I'm just going to do this ^^^.

I found two 2200 uF caps @ 50V and a 6800 uF cap @ 35V in my new stock. The new parts  are so much smaller than the old ones at the same capacitance and voltage rating, it's no problem to put them in.

Also, the two 2k resistors need to be upgraded to a higher wattage. I calculate they dissipate ~0.6 W so the size they had in there (the replacements somebody put in) was way to small (I guess that's why they looked like overcooked hot dogs ), plus it was obvious they weren't the original ones. I ordered two 1W 2.2k resistors.

I'll work on the fun job of removing the old capacitors today.

[xrunner]

Caps removed.

I used the "helpful neighbor" method. I clipped two leads, then for the other three he held two soldering irons on two pins and I used another for the third, and pulled it out. 

[xrunner]

Checking out the PS with new caps. Don't have the bleeder resistors I ordered yet so using substitutes. All is well. 

[xrunner]

OK!

Got the parts replaced and all cleaned up. New caps and new 1 W resistors. Also replaced mica insulators under the transistors with silicone pads. All the voltages check out.

Ready to rock! 

[KD0CAC John]

I've got one of those around here , have to open up & check out .
Now to look into the small boards for the upgrade ?

[xrunner]

I've got one of those around here , have to open up & check out .
Now to look into the small boards for the upgrade ?

Go for it - if you want the little cap boards sure why not? I didn't use them but they make it look very professional.

[tautech]

OK!

Got the parts replaced and all cleaned up. New caps and new 1 W resistors. Also replaced mica insulators under the transistors with silicone pads. All the voltages check out.

Ready to rock! 

Why ? 
Were they busted ?

Soldering looks school boyish. 
Bloody rough in fact, have you no pride in your work ? 

[xrunner]

Why ? 
Were they busted ?

No, these transfer heat a little better.

Soldering looks school boyish. 
Bloody rough in fact, have you no pride in your work ? 

There's nothing where that solder is anyway.

Man dude it works at least I can manage to fix things I don't see any repair threads by you LOL ... 

[David Hess]

I finally got new batteries for my camera.

As can be seen, I cleaned the board after unsoldering the original can capacitors but did not bother removing the flux from soldering in the replacements.  The soldered loops of bare wire that I added are test points.  I had to replace the switch and while the original exact part number is available, it has a slightly different shaft configuration so I replaced the original clip which no longer fit with a nylon wire tie. 

If I had repaired this for someone else, I would have cleaned it up and tied down the aluminum electrolytic capacitors.

[xrunner]

I finally got new batteries for my camera.

 

As can be seen, I cleaned the board after unsoldering the original can capacitors but did not bother removing the flux from soldering in the replacements.  The soldered loops of bare wire that I added are test points.  I had to replace the switch and while the original exact part number is available, it has a slightly different shaft configuration so I replaced the original clip which no longer fit with a nylon wire tie. 

If I had repaired this for someone else, I would have cleaned it up and tied down the aluminum electrolytic capacitors.

Looks fine to me, but it might not pass muster for some others around here ... 

[WastelandTek]

is that the "JAMMA" cable used as an extender?

edit   never mind, I really should read whole threads before commenting, lol

I need to get me one of those  /edit

[xrunner]

is that the "JAMMA" cable used as an extender?

edit   never mind, I really should read whole threads before commenting, lol

I need to get me one of those  /edit

If you get one take notice that the two reds and two blacks on one end are bridged. I cut the traces on mine so bad things wouldn't happen if you put that end down where the pin numbering starts (1-2, 3-4 , ...)

[David Hess]

Looks fine to me, but it might not pass muster for some others around here ... 

There was another Tektronix discussion thread where I said I would post pictures of my TM501 repairs just to troll the more OCD of us.

[WastelandTek]

so xrunner, all the full 56 JAMMA extenders I can find are a full meter long, have you experienced any trouble with so much unshielded wire laying about?

[xrunner]

so xrunner, all the full 56 JAMMA extenders I can find are a full meter long, have you experienced any trouble with so much unshielded wire laying about?

Nope, not at all. Signals can be passed from the module connected to the extender back to the backplane but I don't look at those signals when trying to fix the particular device. All I care about is that the particular power required gets to it from the other end. So no, I've repaired several modules with the JAMMA extender and have had no issues. 

[alm]

The power transistors in the mainframe are also connected via the backplane connector, and could suffer from the extra impedance. I have gotten a PS503A to oscillate when I extended the emitter connection too much. That was not through a ribbon cable, however. I believed I wired one of the power lines through a DMM to measure current, and that made the PS503A oscillate and me scratch my head about what was wrong until I took the DMM out. The oscillation triggered the crowbar and subsequently blew the fuse. This made it somewhat tricky to debug.

[xrunner]

The power transistors in the mainframe are also connected via the backplane connector, and could suffer from the extra impedance. I have gotten a PS503A to oscillate when I extended the emitter connection too much. That was not through a ribbon cable, however. I believed I wired one of the power lines through a DMM to measure current, and that made the PS503A oscillate and me scratch my head about what was wrong until I took the DMM out. The oscillation triggered the crowbar and subsequently blew the fuse. This made it somewhat tricky to debug.

Well noted. All good info is always a good thing.