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HP 651B Oscillator Help
HP 651B Oscillator Help
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Old 4th December 2009, 11:24 AM   #1
bsgd is offline bsgd  Brazil
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Default HP 651B Oscillator Help

Hello all!

I just got a HP 651B test oscillator (sold as is) and Im having a hard time figuring out how to fix it, as my analog electronics skills are pretty much zero
So, could anyone help me out? I'll post a few pics of the instrument as soon as I can.

I'll try to give as much information as I can below. First the problems:

1- The output Amplitude is too low: I get a maximum of 2.2Vrms while the manual states it should be 3.16Vrms.

2- The analog meter indication is far off. With the HP set to maximum amplitude output is only 2.2Vrms but meter gives me more than 3Vrms. I did try to calibrate it by turning A1R23 but its range wasnt enough to calibrate the meter. I went thorugh the Troubleshooting tree described in the service manual and end up finding that PIN 16 had a voltage of 4.4V while the manual says it should be 2.7V. Voltage at pin 14 was close to 5.8Vrms (6.5Vrms) and at A1Q9 collector close to 0.75Vrms (0.85Vrms). Manual suggests to check the monitor rectifier circuit, but I could not find a problem.

3- The HP will not oscillate at the lowest range (10Hz). I noticed that the output at A2TP2 is -4mv in this range, and the service manual says it should be -370mV.

Now some information regarding the equipment:

- Although my chassis states HP 651B, the boards inside my unit are from the 652A oscillator, so Im using the service manual for the 652A. My unit however, does not have the A3 (expanded monitor circuit). Maybe HP used 652 boards on late 651 units?

- I have checked the PS voltages and they are fine at +29.91V and -24.63V.

- I replaced 3 electrolytic capacitors that were off spec (more than 22%) but that didnt make any noticeable changes. I'll soon post pictures.

- The service manual can be downloaded from HERE.

I believe that is all the information I have. I appreciate any help as Im really willing to bring this back to life to help me out in my DIY projects!

Thank you!
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Old 4th December 2009, 11:44 AM   #2
bsgd is offline bsgd  Brazil
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- Unit before capacitor replacement:HERE.

- Unit after capacitor replacement: HERE.

I also have the service manual for the 651B oscillator, if anyone needs it.
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Old 4th December 2009, 05:52 PM   #3
Elvee is online now Elvee  Belgium
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You should replace all the electrolytic capacitors, including the small ones in the non-overheated areas (not the tantalums however, they should be OK). In my experience, they do not age gracefully (neither do I). If you tested them with a simple capacitance meter, they might look good, yet the series resistance could be too high.
You should also check the carbon composition resistors; they tend to go off tolerance after such a long time.
And if it includes a LDR optocoupler (probable), it could also have gone bad.
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Old 4th December 2009, 06:14 PM   #4
Conrad Hoffman is offline Conrad Hoffman  United States
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Agreed. Though I'm generally against wholesale capacitor replacement, I've fixed enough equipment of that vintage to know many of those caps will be bad. The worst ones will be the high capacity low voltage types. If you see anything like 500uF @ 6V, they didn't age well at all. If it uses LDRs, hopefully they're ok, as replacement parts can be hard to find. Ditto thermisters, especially vacuum ones. I usually mark the setting of pots, then squirt them (open style only) with DeOxit and rotate a few times. Then reset back to the mark and continue troubleshooting. The small signal transistors can be checked with the DVM on diode-check, just to weed out any shorts or opens. Check solder joints. Even HP can get a bad joint on a PCB every now and then. Look for cracked traces, especially near the joints. Look for "technician blight"- the evidence of prior repairs not done correctly, and making the unit not conform to the schematic. Check that chassis grounds are good and not oxidized. Check that any floating internal shields are not shorted to ground. Good luck!

I may be barking up the wrong tree, but at least I'm barking!
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Old 7th December 2009, 11:44 AM   #5
bsgd is offline bsgd  Brazil
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Hi all!

Thanks for all the replies. I finally found the solution to problem 3. I dont know how but during my capacitance testing I guess I skipped C7. This is a 2000uF 1VDC (yes, 1 volt) capacitor and it was completely open. After replacing it the HP started to work at the lower range again.

Problems 1 and 2 are still here, but I guess I'll just replace all electrolytcs and see what happens. Im not a big fan of random capacitor replacing too, but I guess here, as you suggested, this might be the way to go. I only have a capacitance meter and there is really no way to tell if these caps are still OK only by checking their capacitance...
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