Hi Grant,

Op za 4 feb. 2023 om 14:54 schreef grantpbt <grantbt@...>:
>

Two things to avoid: dismissing input from Daniel Forro and making assumptions. This is first and foremost electronic troubleshooting.

I am not dismissing Daniel's input, I am reacting to his suggestions. I really appreciate that Daniel refers to actual problems that others have had with the TX802, such as the output muting transistors.

Check all the Power Supply voltages. I have seen an increasing number of Yamaha PSU failures which are happening because a LOT of the Yamaha designs use a Switch Mode PS. I have been able to repair some of the PSUs, but not all. There are many threads online discussing this or that synth or effects unit needing PSU repairs and various successes and failures.

Logical analysis clearly shows that the problem is in the digital domain. Everything from the EGM chip on down functions correctly. So the only power line that matters here is the 5V digital supply. I have checked that, with an oscilloscope, and it looks fine.

Take a look at the block diagram of the TX802 on page 4/5 of the service manual. Everything from the EGM on down is working correctly. That also implies that the "master oscillator" works correctly: if it didn't, you would never get clean sine waves (at the correct pitch!) out of the outputs.

If you look on the left of the block diagram you see that the "master MPU" runs on its own clock crystal. It is responsible for receiving MIDI. If that clock was not running, or not running at the right rate, incoming MIDI data would be garbled. MIDI works fine so I know that clock crystal is fine.

I am not pro-cap replacement, but as far as the PSU is concerned this is a common problem with them.

I agree. But this just doesn't look like a PSU problem. Remember that the main CPU works fine. The synth responds to MIDI, I can use all the buttons in the interface, the display works, there is no garbled text in it. It does not crash.

The BIN files are everywhere.

I wish that were true. I have spent a lot of time looking already. The only source I can find for the BIN files is
http://www.musictechnologiesgroup.com/mtg_eproms.htm, where you have to pay $10 a pop.

Do you know where to find them? The ROMs are called XB223, XB224 and XB536.

I agree that a byte for byte comparison is the right thing to do but I only have those checksums. And unfortunately, that page does not have the checksums for TX802 v1.2, which is what I have.

Reseating the EPROM was a good move. Sockets get dirty. Bad connections cause a myriad of problems. Look at the schematics and Service Manual.

Thanks, I have spent a lot of time looking at the service manual before I even posted here. That is how I know about the EGM and OPS2 chips.

Most of the Yamaha FM products have a huge similarity and some of the Service Manuals are very helpful, others less so.

Thanks, I agree.

The DX7II is a close relative of the TX802. I have spent a lot of time reading its service manual too. In spite of them having the same sound generator ICs, the architectures of the two are different. In the DX7II the primary CPU drives the EGM/OPS2 and the secondary CPU handles keyboard inputs etc. In the TX802, the secondary CPU drives the EGM/OPS. This makes the flow of data quite different. You can clearly see this if you compare the block diagrams in the respective service manuals.

The DX7/DX9 service manual has some high level information about the EGS and OPS sound generator IC's. The IC's in the TX802/DX7II are different however. For example, the old EGS has an 8 bit input address bus. The new EGM has only a 5 bit address bus. We have to guess what the inputs to the EGM exactly are.

The weird thing is that the EGM is getting input data that is mostly correct (amplitude envelope rates are correct, amplitude L4 is correct, pitch EG works correctly) and only incorrect in some places: the output level and L1/L2/L3 of the operator amplitude envelopes.

I see no evidence on my scope that they are broken, but I think I will try replacing the RAM chip of the secondary CPU and the 74HC138 address decoder that enables the EGM and OPS2 IC's. If I can get hold of the bin files I will also replace the ROMs because it's easy to do.

Jacob