HP 8601A Sweeper Repair Help Requested

[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.

[drjprv2]

Henry,

I have repaired two of these HP8601 sweep generators within the past year. The two capacitors that you circled control the sweep time and, as you have found out, have nothing to do with the frequency control. The smaller capacitor is A9C1 and sets the time constant for the “FAST” sweep setting. The large, broken capacitor is A9C2 and is placed in parallel with A9C1 when the sweep mode is set to “SLOW”. Both capacitors are electrolytic, not tantalum, per the HP8601A service manual parts list. The slow sweep speed is extremely slow and was intended to drive one of the axis of a pen plotter through the sweep out connection.

Your second scope picture confirms that you do not have any output on the high frequency range.

The service manual is available free for download. Best source for a quality scan of the manual is the Boat Anchor Manual Archive (bama), bama.edebris.com. The bama archive also has a zip file of stitched together schematics which I found to be extremely helpful. The manual is written in the classic old school HP style and includes an excellent troubleshooting flowchart and test procedures for each of the modules. One of the problems with repairing one of these units is dealing with the SMC connectors. SMC adapters are not cheap.

I concur that you start first with evaluating the power supply. Both units I repaired had power supply issues. Both had missing -75V rails which was caused by leaky electrolytic capacitors (A10C11). One of the units had excessive ripple on the +20 V rail which was caused by a bad transistor A10Q5 (Current source, HP P/N 1854-0071). I replaced all of the electrolytic capacitors in the power supply but kept the original tantalum capacitors.

I would guess that you might have other issues besides the power supply. I suspect that there might be a problem with the A1 Discriminator or A2 Divider or some problem with the control signals to those two modules. Only one crystal oscillator and VTO is used for both ranges but the frequency divisor used in the A2 module is different for the two ranges. The A3 Loop Amplifier, A4 Crystal Oscillator and A5 VTO, A6 Amp, A7 Attenuator might be in working order since you have output on the lower frequency range.

Dave

[factory]

The two capacitors that you circled control the sweep time and, as you have found out, have nothing to do with the frequency control. The smaller capacitor is A9C1 and sets the time constant for the “FAST” sweep setting. The large, broken capacitor is A9C2 and is placed in parallel with A9C1 when the sweep mode is set to “SLOW”. Both capacitors are electrolytic, not tantalum, per the HP8601A service manual parts list.

For A9C1, A9C2, the manufacturers code is listed as 56289 (aka Sprague) and the manufacturer part numbers listed start with 109D, this is a wet tantalum type. Of course the larger one fitted to the OP's 8601A is a different brand, but still a wet tant. Mistakes can happen with the descriptions, but at least A9C2 is listed as TA (tantalum).

The 109D is still available from Vishay if you want to look it up, they are hideously expensive, due to the casing materials required for resisting the acid contents, the rubber seals are the weak point of these, hermetic variants with glass seals are much better and should almost never leak.
https://www.vishay.com/en/product/40023/

David

[drjprv2]

Yep, my bad. A9C1, C2 are indeed listed as 109D series caps. I should have checked the manual instead of trusting my memory plus the fact the A9C1, C2 are AL Electrolytic caps in one of my HP 8601A (the small blue and large black caps in the image). I don't know if they are original or not. Probably not but they work. I don't know what type the caps are for A9C1, C2 in the other HP 8601A that I have.

[George Edmonds]

Hi Henry

Perhaps I did not make it clear enough.

The leaking capacitors ARE wet Tantalums, I don’t need manufacturers information to know that as I have over sixty years of experience of designing and repairing electronics the colour and nature of the contamination tells me just what they are, however I thank Factory for his absolute confirmation.

The FIRST step is to ignore the lacking scan range and deal with the PCB contamination which is quite likely to be fairly strong Sulphuric acid.  When cleaning the PCB and its edge connector you MUST use eye protection and gloves at a minimum as the acid is normally of sufficient strength to cause you eye and skin damage. This contamination is both corrosive and highly conductive so the total removal and neutralisation of it is essential.   Then and only then start looking for the problem you detailed.

G Edmonds

[factory]

Yep, my bad. A9C1, C2 are indeed listed as 109D series caps. I should have checked the manual instead of trusting my memory plus the fact the A9C1, C2 are AL Electrolytic caps in one of my HP 8601A (the small blue and large black caps in the image). I don't know if they are original or not. Probably not but they work. I don't know what type the caps are for A9C1, C2 in the other HP 8601A that I have.

The rest of the parts are consistent with a unit made in the early 1970s, including the TI 4-071 transistor, none of those capacitors are original, except the Sprague 150D solid tant.

For reference the wet tantalum capacitors are usually smaller & heavier than regular aluminium electrolytics.

David