8002 not working after recap.

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Grr Posted: Wed, Apr 11 2018 7:01 AM

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Hello, long time lurker here.

I seem to have fumbled while replacing the capacitors on a 8002. It did work before the operation, but since I had no knowledge on this units history, I decided to do the change. And indeed most of the caps seemed to be originals, it was about time to do this.

Anyway, after the operation the turntable does not power up properly. :(

• the dot between RPM digits does _not_ light up
• display flashes briefly 3_ __ (first digit 3, others nothing) when plugged in
• if I press play, display goes to 3_ __ but nothing else happens
• << < and > >> buttons do not work
• when pressing play again it goes 3_ _1, 3_ _2 etc for the repeat, and pressing stop shuts display off
• pressing 33 displays 3_ 33, and 45 goes to 4_ 00, also in this mode + and - buttons step the visible digits as supposed to.
• +5 V line is OK
• +15 V and -15 V are not there, there should be more than enough available after rectifying, roughly +-26V at C27 and C29...

Any suggestions? To me it looks like the digit 2 select strobe might be mute, and further cause this issue through uC circuitry. I'll get my oscilloscope from work and check this later. But any idea why the strobe would not be there? I did doublecheck the NN-NN wiring for cuts and shortcuts already.

 - G

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Grr replied on Wed, Apr 11 2018 4:40 PM

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OK, checked the uC digit select pins with scope. 16, 14 and 13 are OK, with about 76/24% up/down cycle. Pin 15 is stuck high at 5 V.

[edit] Then I did a quick and dirty test and soldered the NN wire into pin 13 of the uC. Now the turntable works but without the 2nd digit on the display... I also tried removing IC4, but pin 15 is still stuck high. What do you think, have I somehow busted the uC while recapping or could there be another explanation for this behaviour?

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Grr replied on Fri, Apr 13 2018 10:29 PM

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beogram_OK.jpg

Well I guess it is (almost) confirmed that I destroyed the microcontroller pin 15 while doing the recapping. :/ Pin is firmly stuck at +5V instead of 75/25 duty cycle as supposed to. A good reminder to everybody, be careful with the NN-NN wire while doing service on the system.

Since three out of four the display digit strobe/multiplexer pins were OK, I decided to go for a (little bit dirty) workaround solution instead of sourcing a functional uC: I stole strobe/mux signal from pins 13, 14 and 16 (R27, R29 and R30), fed it to TTL 7412 with 3-input NAND, and the result to R28 which I had lifted away from the circuit board.

Hey presto! The turntable is working again! And the workaround is reversable if I should find a fully functional uC later.

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Peter replied on Fri, Apr 13 2018 10:31 PM

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Clever work around - glad to hear is working again!