Hello all,

This one has me stumped. And there are a couple unknowns, but wanted to get some opinions anyways. This is being tested on the bench.

This is another -35 mpu in my stash (just have 1-2 more to fix then I'll be done! ) with its LED stuck on. These stash of MPUs I bought a long time ago from Herb Silvers for $5 a pc. He had about 50 mpus at his booth at a show (think it was pin-a-gogo but don't remember), so I cherry picked the ones with the least acid damage, and looked the best and got about 10 of these things. All the rest I've fixed so far. Anyways...

This mpu I believe was always LED stuck on, but I don't see much if any acid damage on it. However, there is this weird nicotinish-brown(tm) staining on it. I can't clean it off. When replacing the sockets etc, I used flux just in case (I don't all the time), with very low heat, but still pulled some pads in the U15 and U16 area. I have an order in with GPE for some replacement chips, but the poor guy is overloaded (Ed if you read this, just take your time. I'm in no hurry). So the unknowns are, I pulled a U15 and a U16 from another board that I am pretty sure at least reset correctly. Here is what I've done so far:

When LED is stuck on, if I short pins 39 and 40 on U9, nothing changes.
Replaced, and double checked continuity on sockets at locations U2, U6, U7, U8, U9, U10, U11, U15, and U16.
All chips in those sockets test good in another working board.
NVRAM installed. R11 removed, and CR5 and CR7 replaced with zero ohm jumpers.
R113 does look heat stressed, but still measures 2k. I will change it out anyways, but haven't yet.
Pulled U15 and U16 from a working board (I am 99% sure. I damaged a leg salvaging the chip, but it was just the vcc pin, so I connected that and soldered it in a lovely way to the cap right below the socket).
Re-jumpered the board for 2732s.

When following the FO560-3 technical manual, here are my measurements:

U9 Pin 3 s/b 2.4v -- Measuring 100mv
U9 Pin 36 s/b 2.6v -- Measuring 4.98v
U16 pin 4 s/b 2.2v -- Measuring 100mv
U16 pin 5 s/b 5v -- Measuring 4.98v
U16 pin 10 s/b 2.2v -- Measuring 5v

When pulling C14 and C15 and checking them, they measure correctly.

This is where I'm stuck, because U16 is the first suggested replacement, and once again I'm 99% sure that chip is good. If U15 is good, the 2nd suggested replacement is U15, and once again I'm 99% sure that chip is good.

So, with everything I've done so far my questions are:

Am I on the right track with U15 / U16? Or could it be something else in the reset section? Those voltages are way off though.
It very well could be a bad U16 or U15, and my memory could be wrong. But I'll have to wait to receive the parts order to verify.

Pics are attached. Can anyone see something I'm not seeing?

-Pat

Front of Board (resized).jpg

20220112_212239 (resized).jpg

Back of Board (resized).jpg

Thanks, but I have tested all the chips, and they all work in another board. And all sockets are good. So does Leon's test cover U15 and U16? Or am I missing why you want me to test the chips and sockets?

-Pat

So, let me see if I am understanding this right.

This would have something to do with pin 4 on U16 being no voltage, as that is the A1. The CPU pin 3 connects to U15 pin 8 through a 20ohm resistor. So either U16 or maybe R9 could be open not providing voltage?

Thanks guys!

The pads/traces on the top half of the board seem very weak. And that is where most of the brown crap is. I did the same things I did with the bottom half of the board while desoldering/soldering (was all the way down to 338C, should have gone lower I guess), and was pulling up pads at almost every location without a trace. The jumper was because I broke the pin of the chip pulling it out of a donor board.

I couldn't figure out from the schematics if U16 or U15 was were the circuit started, or was it U9. I've ordered both U16 and U15s, just waiting for them.

I will test R4 and R10 also.

-Pat

(My chip order is now in process, so hopefully arriving soon. I did order some additional replacements from Jameco to see if they would arrive sooner. I hope they are the correct replacements:

9602-- IC,DUAL RETRIG,MULTIVIB,16-DIP RESETTABLE MONOSTABLE VIBRATO Dual Retrigger
7437-- IC,7437,DIP-14, QUAD 2-INPUT POSITIVE NAND BUFFER Quad 2-Input Positive NAN)

Oh, something else:

I was so concerned about that brown staining, I really took extra precautions soldering and desoldering this board (as I explained a little bit about above). 338C, using flux, 650F Hakko FR300, etc. And, when desoldering things, I would check continuity with the open socket holes (both sides of board, from desolder location to component if present). I would add the socket, then checked continuity once again. Finally, when inserting the known good chip, I would check continuity a 3rd time. There were no variances in any of the 3 tests, so I'm extremely confident the soldering and sockets are making good connections. Besides the broken chip, and U15 and U16 towards the top.

Yes, I had the same anal methodology with U15 and U16 especially. With those chips in there, from chip leg to next component on the board, not just to the solder pad on the top or bottom. Just to ensure it's making connection going everywhere.

-Pat

Quench and Barakandl good eyes!

I received my new 9602 and 7437 and same issue. HOWEVER, I did notice a little solder blob on pin 15 of U16 which is shorting right next to the trace that goes underneath. (My 2nd issue now with flux on a new socket. At first I thought it was a good idea because it draws the solder to the top of the board once you solder from the bottom, however sometimes it draws too much, and shorts a trace if it is uncovered.)

Which also, shorts pin 15 to pin 4 (gee lookie there), and also to pin 14 of U15.

Going to try to fix this without removing the socket and post results. But I think this is it.

-Pat

Pin 7 was also not making contact with the board on U16. Once fixing that, all voltages are correct on U15 and U16, and I get a flicker. But that's it. Continuing forward.

However apparently I either missed checking U16 like I did with the others, or missed this whole board. I know all the chips work in a different board, so it's socket time.

-Pat

Checked every socket. Continuity on the back of the board, continuity from socket to pad, and continuity to next component on board. All good. A couple weird anomalies but got those fixed.

Board is now, part of the time, booting up with 6 flashes on bench. If I leave board powered off for a small amount of time, it will boot up fine. If I try it again right after it's booted up, just the flicker again.

(Edit: When it boots up with 6 flashes, the 5v is rock steady. When I just get the flicker, the 5v stays on for about 1/2 second, goes off, then comes back on. Weird)

I may need some help with this one, because I don't understand which components or which section (assuming valid power detection circuit?) would be the culprit.

-Pat

I believe I fixed this one, for the most part.

Sometimes when powering up the board on the bench, I get a locked on LED OR a flicker. When turning it off and on again, it works fine. No matter how long you wait to reset it. When I do get the locked on LED or the flicker, the 5v is acting kind of wacky. It goes from 4.9 to 5.08 randomly. If it boots, it is 5.08 rock solid. I may try to replace the other capacitors in the reset circuit, just in case one or more are a little leaky. (C3, C5, C13 and C80)

The culprit you may ask? It was VR1. It was measuring 7.75v, put in a new one, it measures 8.12, and now it works.

Also, for those of you keeping score with the U15/U16 debacle, it was U15.

-Pat

Are you referring to this? So, just take the eproms out, and rewrite them another 5 times or so, without erasing?

Also, is this good practice when burning eproms?

-Pat

Cool. I use the GQ-4x. I'll start setting the speed slower, and will rewrite those 2 sets.

-Pat