ROLAND DAC-15D only dry signal

[Mabot17]

Hi,i have this 80's vintage guitar amp in the lab for repair.
The amp works pretty good,except the built-in effects,delay-chorus-flanger section.Led switch are working but cannot hear any effect.
I have original schematics and they are pretty similar to Boss DD-2 delay pedal,but i don't understand very well how the signal is processed through IC 7 main controller,DRAM ic9 ic10 ic11,and other sections.
I have an audio probe,poke with 1khz sine wave signal at input jack,i have also a mini oscilloscope,so if someone can lead me to the right way it will be so appreciated,thank you so much
Regards,
Mauro

[Mabot17]

Re: ROLAND DAC-15D only dry signal

Hello,i made some progress,just poking around DRAM IC's pins with sound probe i can hear a modulated effect sound coming from speakers so it seems that something is working.Now i had to track the signal path until it stops,but i need help in reading schematics,i hope that someone more skilled and expert than me can help me in narrowing down the problem.

PS: I thought I could see a waveform at the output of transistor Q9 but there appears to be nothing, just a voltage drop,sorry i'm a bit confused.
Regards,
Mauro

[budwich]

Re: ROLAND DAC-15D only dry signal

Only have limited experience with roland (dac50xd) and operation in general. Looking at the schematic, if you feed in your 1khz tone, you then can check to ensure what you are seeing at the output of the master volume (the 2.9dbm)... the value isn't so important at this time... just ensure that you are seeing a good signal. That signal is fed to the processing stage via op amp ic4a. Check at that op amp output to see what signal you are seeing there... ensure that it is similar to what you saw at the output previously mentioned / checked. You should be seeing a similar signal at the input of Q9... if I understand what you posted, you are seeing nothing there?

Further, check to ensure you are getting the proper "bias" voltages on Q9 (ie. +/-7v).

[stj]

Re: ROLAND DAC-15D only dry signal

got any pictures?
is the ram or other chips in sockets?

[budwich]

Re: ROLAND DAC-15D only dry signal

Check to ensure that IC5 has proper supply voltages ... this is a "chopper IC" thus after this point and after Q9, your 1khz input might look a "bit different" (not as well defined).

[Mabot17]

Re: ROLAND DAC-15D only dry signal

Hi and thanks for the replies, I checked with the oscilloscope and the 1khz signal arrives correctly up to the input of ic5, however I don't see a signal at the ends of Q9, however I wanted to point out that the signal without guitar effects can be heard perfectly from the 'speaker, both clean and distorted, the problem is that I don't hear the chorus delay and flanger effects

[Mabot17]

Re: ROLAND DAC-15D only dry signal

the Din signal is present at the input of IC7 at pin 31, and is also present at the output of the resistive DAC pin 14 which connects to the ends of C56. Now the signal should go to the input of Q12 which is part of the circuit EXPANDER, this thing where I don't hear the effects is really strange, the signal is there and when I adjust the delay time or chorus feedback potentiometer I see it modulating in frequency so it's there, I have to figure out where it stops, it's a nice puzzle

[budwich]

Re: ROLAND DAC-15D only dry signal

are you able to post a scope shot of what you are seeing at ic7 when you are saying "the signal is there"... what is the signal? My understanding is that it should be some form of "chopped analog" as I think that is what the two inputs appear to come from... ie. chopper chip... although i am not sure how the op amp before ic7 will handle that.... ie smoothing or otherwise.

In terms of the "only the chorus and flange effects" don't work... those are the only thing handled by this portion of the processing so this is the right area to look at (ie. the bottom half of the schematic).

[Mabot17]

Re: ROLAND DAC-15D only dry signal

Sure,this is what i see at pin 31 of ic7 Din,an analog signal before digital sampling i suppose

[budwich]

Re: ROLAND DAC-15D only dry signal

ok... thanks. I am starting to get beyond my limited knowledge / experience (my roland suffered from noise / leaky cap). Anyway, I don't know what it means.... but you could be right as it is certainly digital. I think the levels seem "high". How big is your input 1khz signal. Further, how are you generating the 1khz... do you have other frequencies available just to see if the characteristic at IC7 changes with the frequency change?

At IC6 (after q9), both inputs should have a similar look. What are you seeing there?

[budwich]

Re: ROLAND DAC-15D only dry signal

you should be seeing some sort of similar 1khz wave form at the output of the LPF section at q8.

another question about your input.... is it a nice clean 1khz sine wave? and further along the input signal path, does it continue to be nice and clean or are you seeing a totally "distorted" wave at some point.... maybe the input portion has some "weird" gain or such that the input can be handled in the further processing down stream.

[Mabot17]

Re: ROLAND DAC-15D only dry signal

It’s a 1khz 50mv sine wave coming from my laptop

[Mabot17]

Re: ROLAND DAC-15D only dry signal

you should be seeing some sort of similar 1khz wave form at the output of the LPF section at q8.

I will check on that as soon as I get my old 40Mhz analog oscilloscope back in the lab, I don't believe much in this Chinese toy I have in my hands now. I will keep updates next week, if you have any suggestions I will be grateful, in the meantime have a nice weekend,
Mauro

[Mabot17]

Re: ROLAND DAC-15D only dry signal

Ok i have an update,this is my input signal,1KHz at 700mv if i read correctly,and tje second image is what i see at master volume

[Mabot17]

Re: ROLAND DAC-15D only dry signal

And this is what i got at IC7 DIN

[Mabot17]

Re: ROLAND DAC-15D only dry signal

This is the signal at TP2,effects are off because with chorus or flanger ON the signal is modulating in frequency so i don't have a stable image.

[budwich]

Re: ROLAND DAC-15D only dry signal

I can't add much more other than "good stuff". Are you able to get a trace "point 3" which is just after Q9... it should be some sort of chopped signal similar to what you posted in #16. I am not sure why you aren't seeing a "normal" sine wave. It almost looks like you are getting some form of "rectified waveform" instead.

[Mabot17]

Re: ROLAND DAC-15D only dry signal

thanks Budwich for not leaving me alone, I feel a little lost haha, ok I'll take the measurements again next week in the laboratory and I'll let you know for TP3 signal, meanwhile a little curiosity, putting the audio probe on the pins of the 3 DRAMs next to IC7, I can hear the sound of the effects! wow I didn't think so, heck why can't they be heard on the output

[budwich]

Re: ROLAND DAC-15D only dry signal

sorry don't know much about the dram function or for that matter, the overall processing itself. When I was dealing with the noise problem, I did the same as you... put in a nice clean 1khz waveform and followed it along til I found where it went bad / noisy and then started check components in the area thereafter.

Another question for you, your post showing the schematic shows a better "picture" than previous posts which I think you took from other sites. Do you actually have the "service notes" for the DAC15. Mine shows some wave forms at some noted points....
in particular, tp1 and tp2... but I think you use them via a two channel scope and cause some triggering there in.

One more thing... I was referring to "reference point 3" (square box 3) and not TP3.... just to ensure we are talking the same thing. Further, the chorus should be on to check waveforms at 1-5.

[Mabot17]

Re: ROLAND DAC-15D only dry signal

Yes i have the original schematic of Roland Dac 15D,so now i understand what u mean when you say Box 3,they are waveform points,i will check on those in the lab on tuesday so i hope i can figure out something interesting,the thing that i would track is the effetcs signal path from the IC7 controller outupt to the output loudspeaker,but i’m not so confident in schematic readings,i try to do the best i can