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| Fool for the 8656A Sig Gen |
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| Tony_G:
Swainster's suggestion is a good idea - The attenuator control section draws from the +24V Unregulated & +5V Regulated rails (as shown on SS7) and there could be a problem with the ICs (like U6, U11, etc) that control the attenuator - Removing J3 might not solve the problem as the path to GND might be through the IC and not through the output of the IC and into the attenuator segment itself. You might want to consider removing U6 & U10 as that should break the path from the 5V rail to GND completely - the signal/5V goes into U10 and then into the inputs of U11 & U12 before flowing through to A9. Interested to see what you find. BTW It looks like both those ICs are still available from Mouser et al TonyG |
| jrharley:
Back to the HP here - As always, thanks for weighing in on the project guys! I've got the little Spectrum Analyzer squared away for later work now and I'm just back from "Hamcation" in Orlando FL. I won't lie, I was scanning for an old junker 8656A, but didn't see one. ;) A good day though, got some supplies and got some fresh air. Now it's time to get back to the HP. The J3 connector was actually unplugged when I first opened the unit swainster, and has been in and out of circuit a number of times during testing. You're spot on with it's importance. Having J3 in circuit, or not, does not seem to affect the 5V rail voltage discrepancy in this situation. Now though, if J3 is in circuit, I get some attenuator action. That wasn't happening before. I'm just getting back into the SS7 schematic Tony_G, printed it out for study. That could take a bit....... Thanks guys! JRH |
| Tony_G:
Great to hear that you had a good time and a bit of a break from the SigGen follies... It's a pity that the design of the 8656A has the attenuator control circuits on the A10 board otherwise you could just take it out of the circuit and see what happens... The ICs that directly control the attenuator are A10U7, U11 & U12 - These take a TTL control signal but will output the +24V to drive the attenuator. The ICs that handle the control latching are A10U6 & U10 - The CPU will send control signals to these and they'll then work out what they need to send to the driver ICs to toggle the appropriate attenuator segments. The A10U3 IC just enables the CPU to latch in the data before it is applied to the outputs of U6 & 10. I would tend to think the issue, if it is in one of these ICs, would be in U6, 10, or 3 but it is certainly possible that U7, 11 & 12 could be drawing too much on the output of U6 & 10 that could be dragging the 5V rail low but I would expect that to cause U6 or 10 to go tango uniform. * U7, 11 & 12 are SN75468N - Available currently for $1.04 at Digikey - https://www.digikey.com/en/products/detail/texas-instruments/SN75468N/380175 * U3 is an SN74LS123N - Available currently for $0.98 at Digikey - https://www.digikey.com/en/products/detail/texas-instruments/SN74LS123N/277282 * U6 & 10 are SN74LS273N - These seem to be popular/in-demand - Digikey & Mouser have 0 in stock (Mouser has 7K odd on order) and there are only 6 at Newark for $1.20ea - https://www.newark.com/texas-instruments/sn74ls273n/d-type-flip-flop-octal-dip-20/dp/60K6876 These are all probably available at Arrow, AvNet, etc though I didn't check as I have accounts with Digikey, Mouser & Newark. I happen to have a Hakko FR-301 so I'd probably just pull U6 & 10 and see what happens - If the rail is still low then I'd pull U3 and check again - If that didn't fix the problem then I'd look elsewhere. If the problem goes away by pulling U6 & 10 and they pass testing (breadboard it up and try applying power and see what happens) then it could still be because of U7, 11 & 12 so you could rinse & repeat that. Look forward to hearing what you find. TonyG |
| jrharley:
Thanks Tony_G! I've been studying the SS7 schematic and your thoughts have helped me see the organization clearer, so, thank you for that. :-+ I'll see what happens pulling U6 and U10, maybe U3. It makes sense to start with those ICs to me too, since removing connector J3 hasn't seemed to have had any effect on the symptoms during testing. I have a decent de-solder gun, so removing those chips shouldn't be too bad. (Gotta' clean that sucker up though!) In the meantime I noticed something while checking the passives associated with the chips. Capacitors C13 and C14 should be 1uF. They're both associated with U3. I lifted a leg to confirm, but they're both reading at 2.3uF with what seems like pretty bad ESR at around 4ohms. Since they're 10% parts, that doesn't look too good to me. I think I ought to replace those guys no matter what, make sense? It's interesting to me to finally find a component that doesn't test very good on this beast, since nearly everything tested so far has been acceptable. The resistors in front of each of the caps tested pretty much spot on @ 132.5K Ohms. After I pause to reflect on things for a couple days, I'll start pulling some ICs and report back. JRH |
| Tony_G:
They're the external timing caps for the IC - I'm not sure what would happen if you just left the legs lifted but you could certainly do that to see if they're the cause of the 5V Rail problem - You will probably have "undocumented" action from the attenuators but for the purposes of a short run to see if they impact the rail I don't think it would hurt. TonyG |
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