开云体育

ctrl + shift + ? for shortcuts
© 2025 开云体育
  1. YamahaDX
  2. Messages
Single Messages Topics Expanded Polls

Re: TX802 troubleshooting

 


Hi PeWe,

Op zo 5 feb. 2023 om 11:46 schreef PeWe <ha-pewe@...>:
Since my friend (tech) and me started to do inhouse repairs and service for all my gear many years ago, we never ever had to replace ALL capacitors in any machine, regardless of brand and type.
You have more experience here than me but this is also my experience. Only once have I had to do a recap of a power supply, and that was a JD990 that was failing to boot, and it had a clear 50Hz ripple on its 5V line.
?
Necessary service was usually battery change and soldering in battery holders, replacing a by roadies physically damaged display, tact switch replacements and PSU overhaul for REV-7.
Big fan of tact switch replacement. Not a fan of battery holders because the new battery should last 10-20 years and a holder can fail mechanically. Soldering it in is more reliable IMO.
?
Ribbon cables and sockets are often culprits ...
When cables and sockets aren?t available anymore, removing socket and solder the ribbon directly to the board was the simple solution.
Funny enough this is why I also think batteries should be soldered. :)

Because of how the TX802 is organized this cannot be a cable problem. The fault is happening on the main board and it does not involve data that goes through a cable.

The ROMs sit in non-precision sockets so that is suspicious. But if either of the CPUs was reading corrupt ROM instruction data it would be very likely that the CPU would crash. Still, replacing the ROMs is relatively cheap and non-destructive so I'm willing to try it.
?
So, PSU and ALL connections (soldering joints, sockets, plugs, ribbon cables) have to work 100% perfect before replacing anything else.
In an earlier email I wrote I suddenly saw a 1.5V ripple on the 5V line. It turned out that it was coming from a USB power supply sitting in the same power strip as the TX802. When I unplug that USB power supply the ripple goes away, and the bad behavior stays. I probably don't need to recap the PSU after all.

Finding the cause of failure is the most difficult and time consuming,- and I won?t replace anything before being sure what it really is.
Thank you, I agree very strongly with this. This is why I have been pushing back on blanket suggestions of recaps etc.

The common buyer and user of used vintage gear is often not able to identify cause of failure because of lack of tech skills and missing tools for diagnostics (and repair).
To save time it?s sometimes better buying a 2nd used machine and swap boards, then use the better one for work and the other for parts in future.
Yes it seems to be common for people to be enthusiastic and overestimate their chances of success. I know from experience what you are talking about.

I have sunk a lot of time into a broken DX7II that I had to give up on; I ended up buying another one and keeping the broken one for parts. I have experience with fault finding and repair on analog synthesizers. I know that my chances of success are much lower on a digital synth like the TX802 because of the proprietary IC's and because I don't have a logic analyzer. With an analog synth I can chase the control voltages using a single oscilloscope probe but in a digital synth the "control voltages" are byte streams and for that you need a logic analyzer.

I took on this challenge anyway because the TX802 service manual has a lot of good information, including a full schematic, and because I enjoy the fault finding.

My best guess at the moment is a partially broken SRAM on the secondary CPU. The SRAM probablye does not contain instructions so corrupt data would not immediately cause a crash. It's not expensive to replace and I have a heat gun so I will be able to neatly remove the old SRAM IC.

I see activity with my scope on all the SRAM pins that should have activity so I don't think it's a broken trace. Also, shorts on the address or data lines would corrupt data read from ROM too and then there would be a crash.
?
Cheers, Jacob
Join [email protected] to automatically receive all group messages.