electro-music.com   Dedicated to experimental electro-acoustic
and electronic music 		 
    Front Page  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 

 Forum index » DIY Hardware and Software
Roland CR-78 always running, no reset
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [2 Posts]		 View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
tupinamba



Joined: May 14, 2010
Posts: 24
Location: France
PostPosted: Thu May 26, 2022 5:47 pm    Post subject: Roland CR-78 always running, no reset Reply with quote  Mark this post and the followings unread
Hi guys!

I'm repairing a damaged CR-78 (those damn batteries...).

I'm nearly back to normal after a massive trace rebuild but I have a weird behaviour:
The unit always makes sound. If I hit START/STOP after powering on the CR-78, I have a louder sound depending on the "accent" setting (if at its lowest, no sound difference, if higher, some sounds are higher).
This means that the start/stop switch has at least some effect.

I've studied the Service manual : https://www.polynominal.com/roland-cr78/roland-cr78-service-manual.pdf

It seems that T1 (Pin 39 of the CPU) has to be at low level until the START/STOP switch is pressed. My T1 is always high...

The Service guide reads:
Quote:
1. POWER ON
Resetting of the START/STOP flip flop IC109A inhibits a rhythm from running by holding T1 of uPD8048 at low level until the START/STOP switch is first tapped.
When power is on, since both pins 12 ad 13 of IC111A are grounded momentarily, its output (pin 11) level swings to high resetting the RS flip flop IC109A which in turn develops high output at pin 2, setting T1 level to low (through Q5-Q7, IC117A and Q11). Pins 12 and 13 of IC111 will go positive as C103 charges, but IC109A output is kept high until the START/STOP switch is depressed.


My IC109 pin 2 functions accordingly to what is written above. It is high, and when START/STOP is pushed, it turns to low.

But is has no effect on T1 which always stays high.

I've replaced IC111 (4001), then Q5 to Q7, IC117 (4558) and finally Q11.

The issue remains...

I think there's a reset issue but I cannot find the cause. I think IC111 pin 4 is supposed to produce a "Master reset pulse" (a brief "high" state) when you first press start, but I cannot read anything on the oscilloscope.

This must be something obvious that I don't see...

All help welcome! Smile

Cheers
Back to top
View user's profile Send private message
tupinamba



Joined: May 14, 2010
Posts: 24
Location: France
PostPosted: Fri May 27, 2022 5:23 am    Post subject: Reply with quote  Mark this post and the followings unread
SOLVED!
There was one last damaged trace. IC117 (the comparator) did not receive the HIGH state signal from the flip flop (IC109) so its output always stayed high...
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [2 Posts]		 View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use