TRS-80 Model 100 woes.. cannot get -5v to work

[Patrick.B (TTR)]

Model 100 with no -5v.. help? (long ...)
So I acquired (donated) a Model 100, I have never worked on these, I normally work on Model 3 or 4's but I am determined to figure out the issue.

First: the manual is weirdly structured and a single page of schematics for everything makes it very difficult to trace anything on this technical service manual, if there is a "Sams " or better publication to use, let me know.
Second: I tested the VB and VDD I get 5v on both, the CPU is active, the databus is busy and so is the ram, but I get no display at all and testing VBB provides a -0.5mV (so it is being drawn low somewhere.)
I checked the PNP at T21, and T22, and they tested good for PNP but that doesn't guarantee anything, and replaced most of the electrolytic Capacitors, except the non-polarized (I do not have any on order)

So VEE being basically 0V, I am searching for "points" on the board that could tell me where the "short" could be.

Questions: a) what value should pin 9 of the TA22 transformer should be? (vee circuit)
b) T21 and T22 Oscillate to make +5v and -5v, aside from the basic PNP testing is there a way to connect the scope and see this oscillation to ensure the T21/T22 are actually working properly?

much appreciated any help.

 

[daver2]

If you have +5V on VDD it is highly likely that everything is working fine up to the transformer.

The most likely possibilities are (in no particular order):

On open circuit transformer secondary winding between pins 8 (GND) and 9.

D15 open or short circuit.
C85 short circuit.
R92 open circuit.
D14 or C86 short circuit.

I would be inclined to remove diode D15 (this frees the transformer secondary) and then measure the ac voltage (with a multimeter) to see what the voltage is (relative to GND). You can also use your oscilloscope. The voltage shouldn't exceed 12V or so I would have thought.

You can also measure the resistance of the transformer secondary winding (between pins 8 and 9) with your multimeter and the power OFF.

Dave

 

[Patrick.B (TTR)]

well, your insight, got me to bite the bullet and pull components out and see what diode may be dead.. and well this was found (sad face)
I think my egg is cooked. so I decided to clear space below the transformer TN22A, and test diodes while I was at it and well...
This is what I am looking at, not sure what the lines are supposed to look like in this area.
I am wondering if it can survive this corrosion. (note the scratching was done by a previous owner obviously trying to scrape "something off the board" )

btw the diodes are all good and I have replacement caps... but I have doubts.

 

[Patrick.B (TTR)]

also, I tested the various p[oints on the schematics and seems like everything is still talking to each other like it should "electrically" anyways...
The transformer I could not find the pin out layout, but AC voltage on pin 4 is a 8v so seems to fit into what you mentioned earlier.
Lastly pin 8 & 9 give me 0.2 Ohms resistance
still testing the schematics of all possible pathways to see if there is any "cutoff by corrosion happening.

 

[Twospruces]

The M100 service manual contains artwork for the PCB. You can use that as a reference for what the copper is supposed to look like.
You probably know that already, just thought I would mention.
So far, I've managed to repair dead PSUs on the M100 a couple of times with careful inspection and repairs.

Have you looked closely at the area under the nicad battery? There are a few traces in there that relate to -5V.

In my experience, the electrolytic caps fail and cause corrosion, as does the Nicad. So, replacing those is job 1. But you do have to be very suspicious of board corrosion and impacts. It is a messy job to clean up and repair, but it is doable.

In some cases, the vias themselves corrode, which forces a tricky repair.

good luck. They are savable, just takes persistence.

 

[Patrick.B (TTR)]

Thanks for the heads-up the battery is already gone and no corrosion under it, I am being systematic about it, I have pads separated from the board, but with schematics and diagrams, I think I can "wire-wrap and bridge" the missing links pretty easily. but like you said, patience, identify what is bad fix it. one at a time.

 

[dosisvr]

Hey were you ever able to solve this? Im hitting the exact same problem. I ve cleaned it all up, replaced all the caps; sits about .5

 

[Patrick.B (TTR)]

yeah I found someone that was a pro at m100 repairs sorry but true. he stated the image you see above is deposit needed to be removed with a scratch pen and then he repaired the traces.

 

[dosisvr]

Okay that actually helps me, thank you! On my board there was similar corrosion where 2 different capacitors had leaked. Around c85 the same as your and also on the other side near c54. I will clean those spots up and repair the traces around them. It is most likely where the problem is... because that is where the obvious damage and mess is.