Hello everyone.
Long time lurker / never poster.
Waned to see if anyone may have some ideas on an issue that I am having with an 8116A? function generator.
The output of the unit slowly started to become distorted over the last year in every waveform form mode. The distortion would come and go. Intermittent so to speak. One day it would work fine, and the next it would be unusable. It looks like it has finally broken, still comes good every now and then, but it seems to be in a consistent failure mode now.
I've followed the troubleshooting steps in the service manual starting in 10.4. The results are as follows:
The output at test point 4 is not distorted below 999 mV. I also checked the pins on U401, pin 18 and pin 19 and all looks well according to the manual. Once I increase the amplitude above 1.00Vp-p, TP 4 starts to become distorted for both the triangle and square waveforms. Amplitude - vernier control voltage is within spec.
I removed U401 to reseat it just to make sure there wasn't a bad connection there. Checked the current at pin16 and measured 2.366 mA. 2.5 mA is what it is spec'd for.
The current mirror on the output of U401 (Q401A and Q401B) is what concerns me the most. The measurements are as follows:
With the unit set @ 4.00V? amplitude output: (I can't find the datasheet on this matched pair, so I don't know if I have the naming convention correct below) The tab on the can is pointing to the right <-- as modeled by MAT01, or MAT03 matched PNP.
? C(2)? ? ? ?|? ? B(2)? ? ?|? ? ?E(2)
3.1090 V? ?3.9568 V? ? 4.7701 V?
? C(1)? ? ? ?|? ? B(1)? ? ?|? ? ?E(1)
3.8617 V? ?3.9564 V? ? 4.7746 V?
@ 999mV
? C(2)? ? ? ?|? ? B(2)? ? ?|? ? ?E(2)
3.1081 V? ?3.9626 V? ? 4.7722 V
? C(1)? ? ? ?|? ? B(1)? ? ?|? ? ?E(1)
3.8625 V? ?3.9626 V? ? 4.7766 V
The offset-vernier control-voltage is slightly out of spec for the negative side. Although I don't think this is my issue, however it might be related.
WIth offset set to -5.00 V, I measured 3.3122 Vdc. This is 62 mV out of spec.
WIth offset set to -7.00 V, I measured 2.0840 Vdc. This is 34 mV out of spec.
I diode checked all the output transistors from Q501 to Q513 and didn't see any usual values. No shorts or opens. Just some caps charging.
I also checked the A2 board for offset control and the DACs and didn't find anything out of sorts there either.
When switching from 999mV to 1.00V, it seems that the reed switches K2 and K3 are the only ones that change. If any of the circuits were pulling down the signal at TP4, I would suspect the issue to be on K3. However, the main output from the front panel is still distorted regardless of the reed switch positions.
The unit has no negative offset control. Only positive offset control. I can add the full positive DC offset, and the signal will rise above ground and the waveform will not be distorted. Looks nice and clean above ground reference.
I do have another good working 8116A that I could use to compare measurements from, but i'd rather not probe around in it unless I really have too. I want to try and keep this unit as I like the 8116A. Don't want to turn it into a parts machine unless I have too.
So I guess my question is, should I try to replace Q401, or is it possible that the issue could be else where. Was wondering if someone here had experience with these units, and if there are any common failure points with this error.
Sorry that I made this so long.
Thanks for the help,
David
