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SOLVED: HP 6227B - slave channel voltage meter slammed right

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Tabloka:
Hello folks,

I don't know what to do next. What has happened so far.

One day the slave channel went dead. I checked the F2 fuse and found it blown. Okay maybe an easy repair. I changed the fuse against a new one and since then the slave channel is not adjustable and the meter of the slave channel slammed to the right to over 30 volts. The measured output (slave) is 47.5 volts at the front terminals.

As the manual says at 5-62 d. 'I isolated the crowbar from supply by disconnecting anode of CR4' to measure the reference and bias voltages. Step 2 to 6 is no problem and the numbers are quit good but step 1 I get odd numbers. For the master I read +24.7 volts and for the slave -16.3 volts. Both should be +27.5 volts.



I connected the common lead to the anode of VR2 and for TP12 I connected the positive lead to the plus of the C6 cap.

My next steps were according to table 5-4 (Series Regulator Troubleshooting, High Voltage Condition). For STEP 1 I shorted the driver Q7 from collector (-OUT terminal) to emitter (TP66) and the output voltage decreased as in response b. described. So far so good. I moved further to STEP 2 and shorted the predriver Q6 base (TP64) to emitter (TP65). The result was that the output voltage remained high. The manual says, 'Q7 open'. Unfortunately that doesn't get me anywhere. I checked all the TPs and here is the result:



Does anyone have a tip as to where the error could be?

Swake:
HP 6627B ?

I know of a HP 6627A, but that is 4 channel power supply.

Tabloka:
Sorry my fault. It‘s a HP 6227B

Swake:
'Q7 open' means it is damaged as an open circuit. With your DMM in diode mode, measure the junctions of Q7 to verify that this thing is still a transistor. You're best to unsolder it to have a good measurement.
If you don't measure the 2 junctions (roughly about 0.6 V each) then it is dead.

Swake:
The manual tells us that Q7 is a 1853-0052, and that translates to 2N3740.
Always check on the transistor itself if is matching this.

Search for 333488487983 on Ebay, that is from a German seller and to me at least seems legit from the pics and reputation.

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