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Post by DHockey on May 19, 2022 21:24:33 GMT 12
I’m working on an HP 3310A Function generator that appears to have a fault in the output amplifier. There is a permanent positive DC offset on the output of around 8-10 volts when the DC offset control is set to 0. When I switch on the instrument’s DC offset control in the positive position I end up with over 18 volts of positive DC offset instead of a maximum of 10. In the negative offset position I can only get it down to 0.8V. I have confirmed the DC offset control is working correctly by checking it at the centre terminal of R2 (DC offset level section of schematic) It is switched to ground in the 0 position and I can vary it between -10V and +10V in the respective switch positions. I have checked the voltages on all the transistors (referenced to ground) in the output amplifier and aside from the supply voltages, some of them are way off compared to the values specified in the schematic:
Q1 B +9.7V C +25.1V E +9.2V
Q2 B +8.7V C -25.3V E 9.35V
Q13 B +12.3V C +25.9V E +12.2V
Q14 B +10.4VC -25.2V E +11.1V
Q10 B +19.7V C +13.1V E +20.3V
Q11 B -19.7V C +10.6V E -20.4V
Q9 B +21.2V C +20.3V E +22.0V
Q12 B -21.1V C -20.4V E -21.9V
Q7 B +1.4V C +12.6V E -21.9V
Q8 B 0.0V C +24.7V E -0.7V
I have checked all the transistors for shorts (in circuit) and they are all OK. All resistors values I have checked have been spot-on. The biasing diodes also test OK and the supply voltages to the amplifier are within spec. Aside from this offset, the instrument works well. I am not very experienced with troubleshooting solid state gear, so would very much appreciate any help!
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Post by Radiotech on May 19, 2022 22:54:55 GMT 12
Remove R32 and then look at the voltage at TP8. There should be zero volts.
You may have a DC offset coming from the previous stage which will affect the DC levels in the output stage and make it
impossible to zero out using the DC offset control.
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Post by DHockey on May 20, 2022 7:53:42 GMT 12
Thanks, with R32 removed I still get +1.2V at TP8 Also remains the same if I remove R31.
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Post by Peter Walsham on May 20, 2022 8:48:21 GMT 12
Hi Daniel
On your table of voltages, you show the emitter of Q7 as having a voltage of -21.9 Volts, and the emitter of Q8 as having a voltage of -0.7 Volts. Now, the circuit diagram shows these two emitters as connected together, so they should be at the same potential. Is there a dry joint on the emitter of transistor Q7, or a break in the PCB track?
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Post by DHockey on May 20, 2022 19:04:58 GMT 12
Hi Peter
Have double checked and that one was a mistake on my part. +0.6V volts confirmed on emitter of Q7
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Post by Peter Walsham on May 20, 2022 22:05:54 GMT 12
The low voltage on the collector of transistor Q7 might suggest that Q7 might be leaky. It looks like a common NPN small signal transistor may be suitable (like a BC337 or BC557 may do). It is best to test transistors out of circuit for leakage. If transistors Q7 & Q8 have the same type number, then try swapping them over & comparing the offset voltage before & after the swap.
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Post by DHockey on May 24, 2022 7:11:34 GMT 12
I swapped Q7 and Q8 over, no difference in offset voltage unfortunately. The PCB shows signs of previous component removal so I think someone may have been here before trying to solve this (or they caused it!)
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Post by DHockey on May 30, 2022 11:50:56 GMT 12
Pleased to report that I have now located the cause of the DC offset, it appears that I didn’t check all the resistors. I was reading about how DC coupled amplifiers like this needs to be perfectly balanced to ensure there is no (minimal) DC offset at the output, the current flowing through the positive and negative halves of the amp needs to be equal, if its not you end up with a DC offset at the output. This led me to check the values of R43 and R53 connected to the emitters of Q9 and Q12, these are 1% precision resistors. R43 measured 359.9 ohms just outside of tolerance and R53 measured 959 (!) ohms instead of 475 ohms. I replaced both these resistors by making up the values by connecting a 330 and 27 ohm in series for R43 and a 470 and 5 ohm in series for R53. I had to go through a number of resistors with my meter to get the values correct, 2% tolerance types were the best I had access to. The new resistors were installed and the offset disappeared! I was then able to apply the correct + 10V and – 10V offset with the DC offset control.
After this I finished off going through the calibration procedure which I had been doing when I discovered the fault and put the unit back together. It is a very nice instrument, pretty compact for something designed in 1969, but quite hard to work on compared to say, a piece of Tektronix gear from the same era. There is only one board that is easily removable, anything else and you have to do some serious disassembly.
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