74HC221, diode trick and waveform timing?

[Kevco]

I want to ask several questions regarding the 74HC221 ICs, and the four multivibrators within those two chips. I've only seen one authoritative waveform plot of only TP6 and TP7, and and trying to determine if there are problems with the timings I am generating.

The one plot (mentioned above) was at the end of Carl's Hammerhead documentation, showing TP6 & TP7. Both are the same width. Can they be differing pulse widths?

I am seeing a wide pulse on TP6, and a narrow pulse on TP7 (about 5X longer, and leading TP7 in time) - and they overlap in time. That is not what Carl shows in his documentation.

Are there any clear application notes, or descriptions on how these multivibrators can use the same RC pair on IC12b, via the two diodes? I have been out of the loop with electronics for many years, and want to manually figure what timing pulses I should be seeing. I would feel much more comfortable in diagnosing this, with my own independent confirmation of waveform timings.

It may be even more hidden bad solder connections, due to the unexpected higher temps required for RoHS soldering today - which was NOT what I planned for. I've seen way too many of those in the last couple days! I've flipped the PCB over so many times, I am beginning to break connections (I have the main pots for panel mounting, on wires), etc - what a hassle! See this thread for what I'm talking about.
http://www.geotech1.com/forums/showthread.php?t=17311

[Qiaozhi]

I want to ask several questions regarding the 74HC221 ICs, and the four multivibrators within those two chips. I've only seen one authoritative waveform plot of only TP6 and TP7, and and trying to determine if there are problems with the timings I am generating.

The one plot (mentioned above) was at the end of Carl's Hammerhead documentation, showing TP6 & TP7. Both are the same width. Can they be differing pulse widths?

I am seeing a wide pulse on TP6, and a narrow pulse on TP7 (about 5X longer, and leading TP7 in time) - and they overlap in time. That is not what Carl shows in his documentation.

Are there any clear application notes, or descriptions on how these multivibrators can use the same RC pair on IC12b, via the two diodes? I have been out of the loop with electronics for many years, and want to manually figure what timing pulses I should be seeing. I would feel much more comfortable in diagnosing this, with my own independent confirmation of waveform timings.

It may be even more hidden bad solder connections, due to the unexpected higher temps required for RoHS soldering today - which was NOT what I planned for. I've seen way too many of those in the last couple days! I've flipped the PCB over so many times, I am beginning to break connections (I have the main pots for panel mounting, on wires), etc - what a hassle! See this thread for what I'm talking about.
http://www.geotech1.com/forums/showthread.php?t=17311

The pulse widths at TP6 and TP7 need to be the same width for the differential integration to work properly. But first check that you are referencing these signals to ground (the +ve terminal of the battery) to prevent confusion.
Also, make sure you are using 74HC221s from the same manufacturer, otherwise the pulse widths can be substantially different.

[Kevco]

Thanks Qiaozhi!

At least I know they should be the same pulse width (as I had understood). And yes, I am referencing the scope against the ground (battery + in this case). And both 74HC221s are from the same manufacture, and the same lot number. Actually it is the CMOS 74C221, and NOT the TTL version.

After much head scratching over the sharing of the RC via two diodes, I finally see how that is generating the timing pulses. I did that before, but got confused with the positive ground, while IC datasheets reference their signals against pin 8 ground ( battery - ).

It's got to more bad RoHS solder connections (but they look good). I'm using Pb/Sn solder (60/40), but those RoHS component leads are not as good as the old-days soldering! Seems I've run into other postings of HH problems, which end up being bad solder joints. Too bad.

Kevco

[Qiaozhi]

Thanks Qiaozhi!

At least I know they should be the same pulse width (as I had understood). And yes, I am referencing the scope against the ground (battery + in this case). And both 74HC221s are from the same manufacture, and the same lot number. Actually it is the CMOS 74C221, and NOT the TTL version.

After much head scratching over the sharing of the RC via two diodes, I finally see how that is generating the timing pulses. I did that before, but got confused with the positive ground, while IC datasheets reference their signals against pin 8 ground ( battery - ).

It's got to more bad RoHS solder connections (but they look good). I'm using Pb/Sn solder (60/40), but those RoHS component leads are not as good as the old-days soldering! Seems I've run into other postings of HH problems, which end up being bad solder joints. Too bad.

Kevco

It might be worth double checking that you do not have the R42 and R45 trimmers set to one end of their travel. With certain combinations of trimmer settings it is possible to get some weird behavior. With a high TX pulse rate and R45 set for maximum pulse delay (for example) the secondary pulse can occur during the next charging cycle. Have a look at Figure 6 for the correct timings. If your trimmers are correctly set, then it may indeed be a soldering problem.