View Full Version : S-100 active termination circuit debug and repair

July 31st, 2007, 04:37 AM

I have a hand built S-100 chassis I am restoring and recently discovered that the motherboard contains active termination. However the circuit is not working. The S-100 bus motherboard still works but has a lot of noise on it and I think it would work better were the active termination circuitry still functional.

The homebrew active termination circuit is very much like the CompuPro/Godbout one mentioned on Herb's retrotechnology site.



The problem is there is no voltage coming out of the circuit and into the S-100 bus lines. When I measure the voltage at the LM4250 OpAmp at pin 6, I read 0 volts relative to ground. In other words, 100% of the voltage from the +8 volt rail is being dropped across the top section of the circuit.

I believe this means the top end of the circuit is not working and there is a problem with either R5, R6, Q1, Q3, or the LM 4250 OpAmp. Now, I do not think the resistors are bad since I measured those in circuit and they seem OK. After powering on the circuit, I measured V+ at pin 3 of the LM4250 and it reads 0 volts relative to ground which seems highly suspicious. I checked the output of the 7805 and it is providing +4.90 volts at its output so I think it is OK.

Moreso, I measured the voltage across R5 and it also measures 0 volts. (!?) Looking at the circuit again, I measured the voltage across pin 1 (emitter) and pin 2 (base) of the Q1 and it also is 0 volts which is suspicious. I would expect at least a 0.6-0.7 volt drop across the emitter-base PN junction in the circuit. When I power off the circuit, I tried to detect continuity across Q1 pin 1 and pin 2 but it reads as an open circuit. When powered on though, it acts like a short.

I measured the voltage across the pin 3 (collector) and pin 1 (emitter) of Q3 and it reads the entire +8 volt rail which is something like +9.5 volts or so.

My diagnosis is Q1 is bad and needs to be replaced. However, I am not 100% sure and would like to know if anyone has any thoughts or ideas on debugging this circuit.

Thanks in advance.

Andrew Lynch

PS, thanks to Herb for posting the information on his website for the active termination circuit. It has been a huge help and Herb is a pillar of the vintage computer community.

nige the hippy
July 31st, 2007, 05:32 AM
This is one of those "elegant circuits" where the op-amp power rails are used to drive the output stage & the quiescent current of the op amp through the 560R resistors is just enough to bias it on. The only problem with elegant circuits like this is that because everything is doing more than one job, they can be awkward to fault find. The business of 0v across r5 and 0v on the supply of the op amp also seems contradictory

(AAH! - unless there is a broken track on the pcb or dry joint on the IC leg)

You may well be right with the faulty Q1 hypothesis, but... transistors do tend to fail short rather than open so...

I'd be inclined to remove Q3 and Q4, that will mean that the op amp can be tested fairly independently.

1) While they're out, test Q3 & Q4
base-emitter and base-collector = 0.65V (ish) one way o/c the other. Collector-emitter o/c both ways (possibly 0.65V in the "wrong" direction if there is an internal protection diode) .
If you find one faulty, don't stop here, there could be more...

2) power up & look at the supply volts to the op amp, V+ should now be sitting 0.65V below the 8V rail, and V- should ve at 0.65V if it isn't I'd start to suspect either Q1 or Q2 (whichever one is nearest to the faulty reading)

3) look at the output volts - it should be at whatever the trimpot is set to (i.e. half supply) if not, replace the op amp.

E&OE... nig.

Dwight Elvey
July 31st, 2007, 07:59 AM
Two things can cause 0 volts across the resistor.
One is a short ( say Q1 emitter-base short ) the
other is no current flow.
Since you say that Q1 shows no short, I'd guess that it is
not short.
I'd more likely suspect that the OpAmp had an open
power lead.
Even so, there is more that could be wrong.
With the power off, measure from the output lead
of the OpAmp to ground. If it is short, that might
explain the bad OpAmp.
Check that none of the capacitors are shorted.
Then with all the boards on the buss removed,
Also remove the two output transistors.
The voltage should be the same as on the output as
the wipper of the pot. If not, the OpAmp is bad.
Make sure to test the transistors for shorts.

Terry Yager
July 31st, 2007, 10:01 PM
PS, thanks to Herb for posting the information on his website for the active termination circuit. It has been a huge help and Herb is a pillar of the vintage computer community.

Some of us even remember him from the days of 'Dr. S-100' in The Computer Journal (early '90s)...


August 1st, 2007, 03:30 AM
Nige, Dwight, Terry,
Thanks for all your suggestions and help. I apologize for not following up last night with all the suggestions as I was working on a hot project. However, I will do all the testing and report back on my finding soonest.

Hopefully, we can get this active terminator circuit working again soon.


Andrew Lynch

August 18th, 2007, 01:08 PM

Sorry for the long time no follow up. I followed your suggestions and was able to determine Q1, Q2, Q3, and Q4 all appear to be good measuring the appropriate diode junction voltage between collector/base and base/emitter. The more I measured in the circuit, the more it appears that U2 (the LM4250) was in fact the bad part.

I replaced that LM4250 and now the circuit works as intended. I am measuring 2.7v on the motherboard for unused pins which is just a tad higher than the 2.6v spec value. I can adjust R1 trimpot to lower the voltage a bit but will probably wait until the next time the motherboard is empty. It is working now and is good enough.

Thanks everyone for your suggestions and help. They were truly invaluable! Vintage-Computer.com forum saves the day again!


Andrew Lynch