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8116A Code 42
Mick B:
Hello all, Quick catch-up. This was given to me because a screwdriver was dropped into it, it was working before this. It had multiple issues & killed the -24V rail. I replaced the Shaper output amp transistors Q505 ~ Q513 (except Q507) and a couple of emitter resistors. R559 & R560. The display came up now. and so did CODE-42 I started on the troubleshooter at chapter 10.4.1 as per the manual.
the first test, Input pins 18-19 I have a signal, but they are NOT 180° OUT of phase, but ARE IN PHASE. TP4 looks ok. At this point
I got sidetracked by Q502 being too hot to touch. (only this transistor) the others aren't even warm. Testing output transistors
Q501 ~ Q513 (except Q507) when testing PNPs, Q511 & Q513 in diode mode red lead on the emitter black in the collector to shows a cap charging and then open circuit. OK. When testing Q510 & Q512 red lead on the collector black on the emitter I see a cap charging up but never gets to open circuit.? i pulled 10 &12 they test fine caps charge up and goes to open circuit. I'M LOST.
MarkL:
(So for other readers, this would be an HP 8116A function/pulse generator...)
--- Quote from: Mick B on January 18, 2024, 09:05:42 pm ---Hello all, Quick catch-up. This was given to me because a screwdriver was dropped into it, it was working before this. It had multiple issues & killed the -24V rail. I replaced the Shaper output amp transistors Q505 ~ Q513 (except Q507) and a couple of emitter resistors. R559 & R560. The display came up now. and so did CODE-42 I started on the troubleshooter at chapter 10.4.1 as per the manual.
--- End quote ---
That's an awful lot of replacements. Did you verify they were all faulty after being removed?
--- Quote ---the first test, Input pins 18-19 I have a signal, but they are NOT in phase. TP4 looks ok.
--- End quote ---
So, when you say TP4 looks ok, you mean the test configuration is producing the waveform in figure 10.5-6? And when you perform the test at amplitude 9.99V and 16.0V, TP4 is following those voltages too? Do you have any screen shots or scope photos of what TP4 looks like? Does the unit produce any output?
You might have to come back and address the pin 18 & 19 issue, but if TP4 is operating normally (which is the input to the output amplifier), I'd recommend working on the output amplifier section first.
--- Quote ---At this point
I got sidetracked by Q502 being too hot to touch. (only this transistor) the others aren't even warm. Testing output transistors
Q501 ~ Q513 (except Q507) when testing PNPs, Q511 & Q513 in diode mode red lead on the emitter black in the collector to shows a cap charging and then open circuit. OK. When testing Q510 & Q512 red lead on the collector black on the emitter I see a cap charging up but never gets to open circuit.? i pulled 10 &12 they test fine caps charge up and goes to open circuit. I'M LOST.
--- End quote ---
When in diode test, your multimeter is applying a test voltage to the probes. On a good PNP transistor, it should display a voltage of 0.6V to 0.7V, which is one diode drop, when measuring red (+) to Emitter and black (-) to Base. You should also get a diode drop with the black (-) on the Base again, and red (+) to the Collector. Reverse polarity in both of these positions should show open. On NPN, the polarity for the four tests is reversed. Try it on a known good transistor out of circuit if this isn't clear.
Note that it's not always possible to do the diode test in circuit because other components can interfere, and often do. If there's a capacitor, as you seem to be seeing, it might take a while for the test voltage to settle to the final value. Or there may be a lower impedance path that will prevent the diode inside the transistor from turning on and your meter will show an incorrectly low voltage. Or the transistor could be open, but another component is conducting the test current and creating a "good" reading on the meter. It's just not 100% reliable. In one case however, if you get a reading of 0.0V that's usually a good sign it's shorted (but again not *always*).
So, unsoldering a transistor, testing it, and then putting it back is one way to do it. But that is time consuming, and risks damage to the board. A better way, if the board can be safely powered up, is to look at the various voltages around the transistor leads. If the transistor is in an active state, you should be able to measure the 0.6V to 0.7V diode drop across the B-E pins with your voltmeter (called "bias"). Although there's a lot else that can go wrong, looking for the bias voltage is usually a good test for a first pass.
If you don't see a bias voltage, a closer look at the schematic is needed to determine if the bias voltage should be there. A transistor might be normally off (for example, when used as a current limiter) and would not have a B-E bias voltage of 0.6V to 0.7V.
So, if Q502 and other components are not smoking hot or turning odd colors, I would power up the board for short intervals and check the voltages described on page 10.4-14. This section is checking voltages around some of the transistors that you replaced. Feel free to post your readings. There might be some clues where to look next.
You can also use your meter's diode test to check the various diodes in this circuit. Although it has the same caveats described above for in-circuit testing of transistors, if a diode is bad it's usually shorted and that will show up as 0V.
If you remove a component that you suspect was bad as a result of in-circuit testing, I'd recommend always testing it again out of circuit to confirm your diagnosis.
Mick B:
Hi Mark, Thanks for showing up. I put the Q510 & Q512 transistors back in They tested good out of circuit. Once again Q502 is to hot to touch. I checked TP4 as you can see at 999mV its just a blond one high and just gets nasty at 9.99V & 16V I corrected an error in my post above about pins 18 & 19 it now says pins 18 & 19 are in phase but supposed to be 180° out
I have not retested them again.
Mick B:
Here is the output @ 1K 1V this is sad. The output files are named output S and T and SQ = sine, tringle, square Mark what time zone are you in?
MarkL:
Ok, so as you've noted, when the unit is set on the 999mV range TP4 looks normal, and when set higher it starts to distort. This is because less attenuation is switched in between TP4 and the output amplifier input on the higher ranges (relays K501-K504). As you set the unit's output range higher, there's less isolation between TP4 and the output amp, and misbehaving voltages from the output amp input are making it back to TP4 (probably).
And looking at the unit's final output, clearly it's stuck above 0V when the waveform should be more or less symmetric around 0V. So something is wrongly dragging the output high. Q502 is one of the intermediate drivers trying to pull the output low, so it's not a surprise it's hot. It's probably fighting against something that's keeping the output high.
The next thing I would check is voltages around Q501 and associated circuitry. Q501 is likely the component Q502 is fighting, since Q501 is the high side of the Q501/Q502 pair.
The suggestion of looking at the voltages described on page 10.4-14 is still valid, although perhaps we only need steps 1 to 3 at the moment. I would further look at the voltages on EBC of both Q501 and Q502 with respect to ground. If you post your findings, it will probably provide some clues where to look next. If needed, we can also compare with a working 8116A I have here for voltages not listed or not apparent from the schematic.
(I'm on the east cost of the US, so EST.)
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