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HP 8656B

 

I bought one of these off ebay and the 'only' thing so far is that the frequency makes large jumps when it gets close to, or at the end of a band. For example, going from 123Mhz to 124Mhz yields a frequency of ~224Mhz. The front panel shows 124 though. Then if I start hitting the up arrow when its off by a 100Mhz it will jump back down where it is supposed to be. Power on and rom1 self tests are okay. Amplitudes are right on. FM and AM modulation fine too. Also does the same when sending new frequencies through HPIB.

Any ideas? The frequency tends to be a little low, ~200Khz but I do not want to start making adjustments and maybe further complicate things.

-howdyrichard
 

I have only worked on a few of these, but there should be similarities to the 8640B¡¯s in the band switching. Before you adjust anything grab the manual and check the frequency tuning and band switch circuits for correct voltages. The trouble shooting in the manual should be pretty good. I suspect the band edge tuning may not be correct, either due to an adjustment or possible frequency DAC tune issues (failed low order bits in the tune DAC.). Check the manual troubleshooting, walk through the adjustment procedure without doing any adjustments, and look for any anomalies.
If you cannot find a failed component, then try the adjust procedure.
Don Bitters
 

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You mentioned that the frequency was in error by 200 kHz; that is a big error.

I suggest the first thing to check is the frequency of the internal reference. Hook a counter to the reference out socket on the back of the generator.

If it is not exactly 10 MHz, see if you can adjust it- see manual. If not, start your repair work there.

I have known the 8657 (similar to 8656 but with a frequency doubler to extend upper frequency) to have problems with the reference being outside adjustment tolerance.
This can lead to synthesisers failing to lock at some frequencies and generally strange behaviour.

These are useful generators, reasonable phase noise for a low price.

Hope this helps.

Regards,

Alwyn


_____________________________________________________

Alwyn Seeds, Director
SynOptika Ltd.,
114 Beaufort Street,
London,
SW3 6BU,
England.


SynOptika Ltd., Registered in England and Wales: No. 04606737
Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom.
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After spending days on my 8656B, I find that the correct bands are being selected, amplitudes of all the mixing signals are good (some are difficult to measure with o'scope probes but I am able to determine the loss with the SA/Tracking generator). However, when checking bias on the High Frequency VCO, I found that when I "scratched" any of the active components, the generator locked right back dead on frequency! (Scratched=moving the DVM probe on the base of the oscillator, for example) So, if I look at the base, as in my example, with the SA, I see the wrong frequency on the base until I scratch it. I can also scratch either varicap diode, and the amplifier's base that is AC coupled to the oscillator on same same PCB. Because so many active junctions here its hard to decide what to try first. It does act like it is locking on noise, for example, the frequencies it does lock on seem to be in 100, 200 and 400 Mhz differences. When I punch in 500 Mhz, it will go to 900 Mhz (if it does not lock on to 500 right away), then if I up arrow to 600 it goes down to 800, 700 = 700, and 800 = 600 (goes backwards).

Any ideas would be appreciated.

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