Electronics > Repair
HP 8601A Sweeper Repair Help Requested
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Henry Finley:
I've acquired an HP8601A Generator/Sweeper in very good to exc condition at a favorable price. However it has a problem. Only one, hopefully. The particular problem is that I seem to be getting no output or response on the 110mc range. The 1-11mc range seems to be putting out. I just opened it up for a look-see and noticed 2 things off the bat. The posted pictures are of the bottom circuit board. One is the entire bottom and the other is a zoom-in of the area in question. One is the big capacitor is a 300mfd which has become unattached and the other smaller one (circled) has fluffy corroded looking trash at the negative end, and discoloration on the positive lead that is evidence of VERY hot overheating. The is also a black resistor circled in the full-sized photo that is marked 185 ohms, but measures 85 ohms, in circuit. The discrepency may be because it is in -circuit. Obviously the 2 capacitor swill need replacing. But I have no reason to expect it has anything to do with my initial complaint. Further, replacing obvious parts is not correct procedure, as something else was the causative factor. It may be a bad transistor, or something a mile away in a complete other section of the machine. I would appreciate any help the readers might have. I'm not familiar with this model. Thank you.
George Edmonds:
Hi

The two capacitors look like wet tantalum ones, the leaking electrolyte is highly corrosive and is very acidic,(possibly Sulphuric acid).

This must be cleaned away and neutralised as a first step.

G Edmonds
Henry Finley:
All I have on hand is a stock of new axial and radial lead ordinary 50V aluminum electrolytics. Will these be good enough, or do I need exact replacement with exact values and type? For instance the little one is 3.5mfd. I have plenty of 3.3. Is this a machine like most radios and tape recorders we all restore with close, but not exact value capacitors?
George Edmonds:
Hi Henry

The exact value of the capacitors is probably not critical, BUT, wet Tantalum capacitors have a very low internal impedance so normal aluminium electrolytics may not work correctly as replacements.

What are the schematic references for the faulty capacitors?

G Edmonds 
Henry Finley:
I did replace those 2 with the electrolytics I had on hand. Differing internal resistance notwithstanding, it was obvious I couldn't very well just leave the bad one in. But doing so did not help my original complaint. Why would the 1-11mc range be working but the 110mc range doesn't? I suppose my next step would be to see if al the voltage rails are in spec. I suppose what I need to do is find out if the 2 ranges use separate oscillator sections in the unit.
     I haven't been on this site much,  but am impressed with the quality of quick and knowledgeable answers. I CAN keep up with what I'm told, as my past successful projects have included some high end gear. I try to be worthy of the time and advice of my betters. Here is what I've got. The first is a scope photo of the low range which behaves as I would expect on-scope. The second is a photo of the 110mc range at any frequency setting of the generator. To achieve this photo I have to set the vertical input of the scope to a much more sensitive setting, and the horizontal to full sweep speed. Turning the frequency crank on the generator has no effect on this trace. the frequency. I intend to have a correctly operating piece of gear without languishing in ignorance for prolonged posts.
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