| Electronics > Repair |
| HP 8601A Sweeper Repair Help Requested |
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| 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:
--- Quote from: drjprv2 on May 18, 2024, 08:45:29 pm ---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. --- End quote --- 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:
--- Quote from: drjprv2 on May 19, 2024, 02:53:16 pm ---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. --- End quote --- 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 |
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