Hi Gerd. Yes R6 changes the output of the op amp by about 15 mv.. During further investigation I found the input to Q1 was sitting at -400 mv when it should be at 0 +/- 100uv. While disconnecting things to try and isolate where the 400 mv was coming from the resistors in series with the +15 on the p/s regulator board decided to smoke! The excess load was on the A7 power amplifier board, so something appears to have died on this board whilst I was testing, wether I caused it or it was just a coincidence I do not know. Using the transistor tester I first tested all the socketed transistors and found the n channel fet Q1 that I had previously replaced had failed, however this was not causing the overload condition on the +15 rail, nothing shows up with an ohmmeter test on this rail by the way. I then tested all the soldered in transistors and found a number that give incorrect readings, so I will have to dismantle the board again and remove these transistors to properly test them. Your suggestion to run extension cables is a good idea as continually removing the board to solder wires on to the points I want to measure is slowly damaging the socket the board plugs into, as well as making it difficult to do measurements. So when I have the board disassembled I will run some cables to allow me to operate the board out of the chassis. Ken. Date: Sun, 13 Jul 2014 21:42:23 +1000 From: Gerd <admin@controlelectronics.com.au> To: Discussion of precise voltage measurement <volt-nuts@febo.com> Subject: [volt-nuts] Fluke 5200A repair. Message-ID: <53C2709F.50406@controlelectronics.com.au> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Hello Ken, If I understand this correctly, R6 adjustment produces a few millivolts change at U1 output but no change at all at the power amp output. That seems to point to the input differential stage or the current to voltage converter stage following that. This will be difficult to find without removing the board from the machine. Given the amount of time you have spent so far it would be quicker to rig up enough extension leads to operate the board outside on the bench. I've done this many times when making or obtaining extender boards was impossible. Regards --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com

Ken wrote: >Yes R6 changes the output of the op amp by about 15 mv.. >During further investigation I found the input to Q1 was sitting at -400 mv >when it should be at 0 +/- 100uv. Suggest lifting one end of R3 to isolate U1/Q1 from the oscillator. >Using the transistor tester I first tested all the socketed transistors >I then tested all the soldered in transistors and found a number that give >incorrect readings, so I will have to dismantle the board again and remove >these transistors to properly test them. You need to apply more analytical thinking and less doing. A very valuable technique I often use to teach troubleshooting is to have someone troubleshoot a device that they can't put their hands on. The student has a schematic and an initial description of symptoms (and usually a few measurements and observations of mine) that I give him/her. The student asks me for measurements by phone or e-mail. I will measure anything the student want measured (I'll generally ask why they want it measured, to keep them focused, but I'll always measure it if they ask). This reduces troubleshooting to its essence -- thinking and making logical deductions -- and tends to prevent the bad habits we so often fall into, to tear into the equipment before we have fully digested the information we already collected. Anyone can duplicate this method without actually being separated from the equipment -- it just takes discipline. I strongly recommend against using any sort of transistor tester. The best transistor tester is the circuit you are troubleshooting. Think about the circuit, then measure DC voltages, then think about what those voltages mean. Think until you can explain why ALL of the voltages are as you measured, looking only at the schematic. When you can, you will generally have a few potential culprits. Sometimes your thinking will lead to other measurements you need to make. Make them, then continue thinking, looking only at the schematic, until you can explain why ALL of the voltages are as you measured. Don't remove any parts if you don't know why you're removing them. ("To check them on the transistor tester" does not count as knowing why.) I understand that you are limited by not having a card extender. But that is no excuse for doing ineffectual things just because it seems that's all you can do, like the drunk looking for his lost keys under a streetlight -- "Did you lose them here?" "No, I lost them over there, but it's dark over there and here I can see where I'm looking." Make an extender, or bring out pigtails from the important nodes until you have the measurements you need. You may have finally put your finger on the fundamental problem -- leakage current into the Q1 gate node, possibly through C24 or CR29 (and possibly due to excess DC offset in the oscillator amplifier -- see schematics p. 8-27/8-28 and 8-33). If so, lifting one end of R3 should give you a good clue. Best regards, Charles