Today I went through some of our legacy boards at work looking for a Relex PID board. I found several how ever, After making repairs to a couple of them I notice a problem that wasn't coinciding with the schematic.. These units have simple zener shunt voltage references of +/- 6 vdc for the external device if needed. That seems to be fine. This PID uses a LM324, Buffer, Lead stage, Delay stage and a comparator with Gain. On the last stage that has the gain there are 2 zeners (10 Volts) back to back with optional rail selection for output. The schematic spec's this stage to operate 0..+/-10v out how ever, I could only get 0..+/-7 Volts. After a little investigation. It appears the 4 zener diodes in the board are all of the same values. The ones used in the supply are 6.8V and those used as the feed back limiter also 6.8V part numbers.
Now my question here is, was some one sleeping on the job or did they just didn't care about putting all the same part # zeners in places that didn't belong? After I found this, I check 10 units total and only 3 of them actually had the correct diodes in the feed back limit section of the last stage. And these were all OEM originals never repaired in those area's.
So what's your take on that? I just wonder how many before me had problems using those units not getting the full scale out :/