Trying to repair an HP10811

[jpb]

I am in the process of repairing (or attempting to repair) an HP10811-6111 I got on e-bay. It oscillates ok but with a very distorted waveform.

I have been using the HP repair manual but have found an inconsistency which unfortunately is in the very area I think my problem lies (AGC circuit).

I was wondering if anyone else has delved into these and knows the answer.

My question is, what is the voltage Vcc applied to the collector of transistor Q3 (the AGC input).

In Figure 8-7 attached it clearly states that it is 5.7V (the same as for Q1 and the rest of the oscillator circuit).

On my sample it is 11.8V!

But on the standard Voltage table 8.4 show that the collector of Q3 is at 11.8V so my value there is ok.

Unfortunately my Emitter of Q3 value (the agc feedback value) is 8V instead of 4.9V which is above the 5.7V supply of the oscillator circuit.

Any feedback from anyone with experience will be most welcome.

[wn1fju]

Looks like the actual schematic shows 12V on Q3C and 5.7V on Q3B.  Probably just a misprint on the figure you showed.

I've repaired a few of these, but my problems have always been in the circuitry involving the oven, not in the actual oscillator.  The reason I am posting a reply is to warn you to be VERY careful soldering/desoldering parts.  The board material is almost like cardboard and it is easy to burn right through it, taking out traces.  I nearly destroyed one of mine.

[jpb]

Looks like the actual schematic shows 12V on Q3C and 5.7V on Q3B.  Probably just a misprint on the figure you showed.

I've repaired a few of these, but my problems have always been in the circuitry involving the oven, not in the actual oscillator.  The reason I am posting a reply is to warn you to be VERY careful soldering/desoldering parts.  The board material is almost like cardboard and it is easy to burn right through it, taking out traces.  I nearly destroyed one of mine.

Thanks for the warning - I've not yet got to the stage of swapping out parts but will be very careful when I do so.

I'm trying to probe components in place and they seem ok (which is frustrating as I was hoping to find an obviously faulty cap that could be replaced).

My main issue is I only have a standard (500MHz) oscilloscope probe and I think the capacitance is affecting my readings. The AGC circuit seems to be stuck in an off state which leads to massive oscillator swings but I can't work out if it is the AGC or the tuning of the oscillator.

Anyway, thanks again for your post.