HP 651B Test Oscillator Teardown and repair - Page 1
Forum section: electronics / testgear

author: Tony_G
author_rank: Super Contributor
timestamp: March 06, 2016, 12:59:54 am
content: Hi All,I've been trying to follow the performance checks for a 355C Step Attenuator (You can see my posthere) but I was running into either a problem with duplicating the test signal or a typo they really committed to.In any case I happened to find a cheap HP 651B on EBay so I decided to splurge and buy it (on the plus side, these things have about depreciated as much as they possibly can so after I use it with the 355C I can sell it and get most of my money back).The 651B arrived on Friday so I decided to follow the creed and open it up before turning it on. You can click on the images for the full high def versions (I think, first time I've hosted anything at Imgur).The unit is classically well made HP gear. It felt very solid when I unwrapped it and, to be honest, it is pretty cool looking - Reminds me of the 60's-70's Sci-Fi shows I watched as a kid growing up.I've seen a ton of the leather/plastic strap handles and of course the eponymous bail but I've never seen a piece of kit come with these spring loaded handles on each side. Very cool.Taking the top cover off you can see that there is a lot of open space on the top side around the capacitors and switches. The setup is very sensitive to stray capacitance so tuning it required a set of TV tube tuning wands. I can't believe that they still make them but I bought a set new of Amazon.Zooming in a bit and you can see the well machined gears for the main frequency dial - none of that plastic gear rubbish here.This complex arrangement is the main range switch which selects the various resistors and capacitors to configure the specific range. This is where you actually do the tuning to bring the dial range back into alignment. If feeds into a JFET that is used as an impedance converter (see section 4-9 of the manual). If this JFET goes Tango Uniform then you lose the lower 4 ranges (as I discovered).Because the system is so sensitive in the way the oscillator operates (read the "Theory of Operation" in the manual, interesting stuff - You should also read theHP Journal articleon it) they've put a complete shield around the output attenuator.Flipping the unit over, you can see it has these cool feet - They're a period design because I now have a HP 415E & HP 435B and they both use an almost identical model of attaching the foot to the case.The bottom of the unit contains the two main assemblies. The one on the left is A1 and it handles the power supply needs and the right is the oscillator and power amplifier circuits. It is very sensitive and you need to have the shields on for certain operations otherwise the results of the tuning will be incorrect. in fact one of the steps has you wrap a resistor around A2TP2 and connect a test lead to it, snake it through the case and out the top to the DMM. You then put the shield and bottom cover on before continuing with the calibration.Removing the shield lets us see the full board. As expected it was all through hole. What wasn't expected was the TO-92 packages that were sprinkled around. I was expecting TO-18/TO-5 packages. At first I thought someone had already repaired the unit. However on further research it appears that there were multiple versions of the assemblies and at some point they were updated to use the TO-92 packages. If you look to the lower right of the big blue capacitor you'll see A2Q1, the JFET I mentioned earlier. When I turned this unit on it was working on the top two ranges but not on the bottom ones. I suspect that the people who sold it simply turned it on with the range selector on a high setting, twiddled the knobs, saw the meter move and called it "good". I had to remove and replace that JFET to get it to work.When I went to replace that JFET there was a lot of random information around but I eventually settled on a NTE326 (which is a replacement for the 2SJ103 which is a replacement for the 2N5461 which was a replacement for U112). It went in and the lower ranges came to life. The peasants rejoiced!I spent the last day just going through the various tests and bringing it pretty much into spec (the lowest two ranges on the output attenuator don't land in spec with my modified test approach).The troubleshooting guide in the manual, even though themanual available onlineisn't accurate for my version of the 651B was invaluable in bringing this back to life. Step 15 on page 5-11 even says "if the instrument malfunctions only on the lower four ranges, A2Q1 is probably bad." Sounds like what someone would say to you in real life.This is a great vintage little unit but I'm not sure I'll keep it once I've worked through the 355C/D test issue mainly because I don't think it does anything more than my DG1032Z can do (except drive a lower level signal). If I was into ham radio though I'd probably keep this for its ability to provide a modulation signal, maybe, I'm not a radio guy so what do I know.Hope you enjoyed the images.

author: med6753
author_rank: Super Contributor
timestamp: March 06, 2016, 02:52:37 am
content: Nice teardown.Comes from the era where the "Big 3" each had their forte. HP for oscillators/counters, TEK for scopes, and Fluke for DMM's.