Hi,
Is there somebody who repaired/worked or have experience with HP 8656B signal generator? I try to troubleshoot a malfunction, when I change the output frequencies, I get variable output signal. The variations are significant around 33 Mhz, the output is more the doubled compared with 10 Mhz, then again around 60 Mhz and then drops again. The stepped (relays) attenuators works, also the variable attenuators- I checked the ratios from 0dbm down to -127dbm. Some advice will be of great help, please!
Cheers,
Daniel
If you haven’t already, check out the service manual and go through it. I have an 8657A, have not had to go inside it yet but if memory is right the oscillator also does some switching action of some sort (like the attenuators) to get the correct frequency. My guess is one section is having an issue. Try checking at several frequencies as well. The manual will provide a performance check to narrow it down.
Radios + Tubes + Scopes + Cars= Nothing better!
@radtekman Thank you for taking time to answer! The thing is the humpbacks I got on 30 Mhz, 60, etc. are coming from DC blocker I use for output but now the output amplitude varies with frequency. As my scope works up to 100 Mhz, I used VTVM to check the VHF output amplifier (which is used also to generate mixing signal for LF output) I found the Q2 on A6 board doesn’t work properly and apparently was changed with some Motorola part which is seems unidentifiable. The amplifying of Q2 drops with frequency by the point is beyond the output of previous stage, so either is defective (but it check good on transistor checker) either it has a much lower FT that amplifier requires. The ALC stage for the VHF works fine. I’ll change the transistor anyway but the service manual gives only the HP codes:
A6 Q3, MFR code: 28480, MFR Part Number: 1854-0720
On specs is writen: “Transistor NPN Si PD-500mW FT-4Ghz” Do you think I should replace it with some other transistor that meets those scarce specs?
@mikiduta I am sure there is a difference in what was originally there and what got put in. I just had this issue repairing a Tek type D high gain plugin. There was a major imbalance in the front end, found 1 GE 12AU7 and one random (different build) 12AU7. Found an identical build GE tube (not specifically a matched pair but much closer) and was able to calibrate it fairly close to normal. As far as a replacement goes, I am sure there is some kind of current production part that can take place of the original. Only issue may be that the form factor is different. I would stick with the known requirements and go from there, if you have to adapt it do so. Also, I would look for any other transistors in the unit that are the same part number that you could try testing for any missing specs to help select a new one. Good luck!
Radios + Tubes + Scopes + Cars= Nothing better!