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8116A Code 42

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Mick B:

--- Quote from: MarkL on June 21, 2024, 02:36:33 am ---I've been out traveling for the last week.  Will review your latest readings and post some suggestions for next steps in the next day or two...  sorry for the delay.

--- End quote ---

Hope your journeys went well, Glad your back. I have since removed U33 also, measuring U32 with U33 missing
U32 OUTPUT PINS
  2.  -5.3 V
  5.  -5.3 V
  3.  -5.3 V
  6.  -039.53 V
14.  -5.3 V
11.  -5.3 V
15.  -0 V
  9.  -0.833 V

U32 INPUT PINS
12.  -5.3 V
  4.  -2.08 V
  7.  -010.00 V
10.  - 010.67 V
13.  -1.81 V

U34B PINS
  6. (I)  -038 .31 V
  7. (I)  -5.07 V
  3. (O) -0.828 V
 

MarkL:
Ok, some interesting observations...

With U32 out of the way, inputs U33.6 and U34.6 should both be -5.3V (-5.2V nominal), but are still close to 0V.  There's nothing else connected to these nodes, except the pull-down resistors (R62) which is what should be pulling them to -5.3V.  As an example, both U33.5 and U34.5 are now at -5.3V, which is what should be happening.

One additional verification you could perform is to hold a 470 ohm resistor to each of the pins U33.6 and U34.6 to see if they go to -5.3V.  Maybe the pulldown resistors are bad, but it's unlikely since you did this test before with U32 in-circuit.  If they stay near 0V with the resistor test, I think we can conclude both U33 and U34 are bad, at least on that gate.  It's odd that it's the same gate on both.

U33.11 is at -4.5V, but should also be at -5.3V with nothing else on that node.

And more... U33.4 and U33.5 are both logic low, yet the output for that gate on U33.2 is high.  This is wrong.  Same for U34.4 and U34.5.

With U37 out, U33.9 should be closer to an ECL logic level, but is not.  Again implicating U33.

U37.14 should be an ECL logic level, but is somewhere in-between.  This node is not involved with U33 or U34, but it is wire-OR'd with other outputs U35.3, U35.15, U32.9 (out), and inputs U203.10, U32.13 (out), U38.7.  Might need further investigation.


Ok, now read your next post about removing U33...

So now U32.5 is where it should be at -5.3V with U33 gone.  U33.6 was holding that node high, which it shouldn't be.  That input must be shorted high internally (probably base to collector short on the input transistor).

U33.11 is also good now at -5.3V (was -4.5V).  Maybe caused by some reverse emitter to base leakage caused by the overly high voltage on U33.6, but notably not present on U33.5,  But doesn't matter, clearly U33 needs replacing.

I would suspect removing U34 would do the same for U32.6, which would say U34 is dead too.

Obviously no need to do the 470 ohm resistor test described above, since removing U33 proved the pull-down resistor is working.

Mick B:
Hello there Mark, that is the conclusion I came to also with U34.6, I bought extra chips just in case I will get to removing it. and post the results tonight. :-+

Mick B:
OK, U34 removed, U32.6 & U34.6 are -5.3 
when plugging in new chips should I do U32 last? (also some of these are Ebay chips of unknown quality)
What do you think Mark?

MarkL:
I don't think the order is critical and I haven't seen anything to make me think the new chips are in danger, but I would definitely do it one at a time and check for nonsensical logic levels as you go.

Did you install sockets?

I think it would be mildly interesting to plug in the old U32 first and see if you get the expected outputs on it now.  As per the previous observations, one of the main problems with the nodes near 0V was either with U33+U34 or U32, and we've concluded now it was U33+U34.  So, U32 might be ok and it would be interesting to know.

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