R and T are normally connected. (through insert jack, as J21 goes to J32) However, dirty contacts in the insert could make noise, so bridging R and T would be a good test, like we normally jumper fx loop send to return.
The reason I suggested cutting the traces to the DSP was in case noise would still bleed into the signal, if the DSP is at fault.
Like Phatt said, send and return is the signal going into and then coming out of the DSP effects board, just like amps have send and return jacks that we can connect stomp-boxes to. By placing the link in there you will no longer be affected by problems in the DSP board.
If you make the modification and still have problems, then its not likely to be the DSP board.
Next time you open your amp up could you please get some better photos of the board ? even better if you can remove the sticker on the large IC so it can be identified. It might not help to fix it but it would help understand what is happening in there.
Last post by phatt - November 28, 2022, 08:45:30 AM
Victor,Regards the Loop; Signal splits at pin 7 of IC4B. (see TP7)
Dry goes through R73 & R77 across to input of IC4A. Meantime signal also goes through R84 to input of DSP (the Send). The DSP FX is then sent back via Return line through R101 where it is mixed with the dry signal.
The CD or aux input is also mixed at this point.
Note Q24 is the mute circuit,, you will find label D is linked right at the input socket. So when you remove the guitar lead it kills all signal.
As you seem to imply that you don't really need the DSP then it's a fair bet some of the channel selections may work without the DSP. Which is why I mentioned joining R to T. It may at least bypass some of the DSP control over the analog parts of the circuit. Of course I'm just guessing as there is not enough info on the Schematics to work out how the DSP interconnects. Be aware that a lot of those pin header connectors are very cheap and often don't like being plugged in and out many times before they no longer connect. A gentle push around the wires and plugs can often find a faulty connector and amp comes to life again. they are not really suited for guitar amps which tend to get a lot of abuse due to speaker vibration and road handling. Phil.
A picture of the DSP board would be good, even better would be a part number if there is one on the DSP board. There are some DSP boards on Ebay, also some other places on the net, but a part number or a picture would be great to identify which one is needed.
Last post by phatt - November 27, 2022, 08:12:52 PM
Hum,,My thought, What about Bridging R and T? Find them at Test point 8 after IC4A also at J21
The DSP seems somehow interconnected with the channel switching. Hard to tell? Though it seems to me that Modes A,B & C are direct to circuit, while Solo OD, B2 & A-B modes are activated via the DSP chip. Maybe Post some pictures might give us some better plans. Phil
Can you read schematics ok and translate to the board traces? If so, cut the traces where red x's are shown. Then connect the 2 traces shown with the blue link. Reconnect any boards you disconnected. Amp should mostly work.
I'm not a real technician, nor an engineer. But I build pedal effect, so I have some experience, but no much in big amp like that. Anyway, I'd say it's impossible to repair because the issue, I think, is in the DSP board. Change a cap, a potentiometer... is ok, but if the problem is in the microprocessor (if I can call it like that) I just can change the board, but I can't find it. Actually, the fact I have to wait some minutes to warm it up to make it work could be a soldering or bad contact problem. But I'm not sure.
I don't say I won't try to fix it anymore, but I want to wait to have some clue about this problem. I read somewhere about another guy with this amp with the same problem here: I turn it on, I hear loud pops even with volume at zero (this is the reason I have to disconnect the speakers). When I disconnected the DSP board, taking away the ribbon wire between the tuner and the DSP board I "solved" the problem, but it was stuck in the second channel, with volume at zero (but I can hear it, because in this amp, the volume, is never mute!), and of course no effects and no tuner worked (maybe the loop effects, too). (Actually, I don't know why taking away that wire changes all this, because the DSP board is soldered directly on the main PCB, I just separated it form the tuner interface. Maybe the wire makes some jumper on the DSP board itself?)
It was enough for me if I can get even only the clean channel, no matter about the distortion channels, the effects and the tuner. But to make this I should know how to do it.
G'day Victor Van Dort I am very confused with your post, why do you think it is impossible to repair ? The schematic looks like a fairly easy one to read and follow, and parts are no problem to obtain that I can see. The DSP board would be more difficult for someone with my limited experience with digital audio boards but it might be nothing wrong with that part.
It might be so simple that it is a solder joint that was bad to start with and has corroded a bit with age, simply re-melting the solder would fix that problem. It could be dirt in a connection or plug somewhere or any simple problem that any technician would find in no time at all.
I feel that most amps can be fixed as long as the parts are available, and many parts that are not made anymore can be substituted with better modern parts.
There could be a young electronic engineer in the making among your friends or family, they might love to have a go at fixing it for you when they get a few years older and more experienced.
Do you have any electrical / electronic experience ?