MXR M175 Digital Delay nonworking

[Rob Strand]

I think if I can find where a delayed signal comes back from the computer stuff and gets mixed, I'll be onto something.  Or will know the digital crap is dead.  Enough for one evening of hair pulling tho!

If you can find where the audio come out of the D to A,  force the signal into the A to D to be zero  then measure or even listen to what comes out of the D to A.   You might need to get past the smoothing/reconstruction filters.  You might also be able to look at the data lines but if the value are signed you are going to see a lot of bobbling between 00's a FF's just because of noise.  Maybe look up the A to D datasheet to see what format the data is.

Those DRAMS ineed a refresh.    If you don't refresh it loses what is stored.  IIRC refresh needs to less than 10ms to 50ms.   In some delay schemes the fact it's always read/writing the memory is enough.   There are other schemes where the refresh is a separated "task'.   FWIW, I have had DRAMs fail in the past.

[GibsonGM]

If you can find where the audio come out of the D to A,  force the signal into the A to D to be zero  then measure or even listen to what comes out of the D to A.   

This is what I'm looking for, but don't really have a clue where it is.  I think what I'm hearing by audio probe is all clock at this point - which is also bleeding into the output.   Playing any guitar into it leads to some ring mod/R2D2 noises, patterns where 'the rhythm doesn't make sense', but no actual delayed signal.  Dry signal passes if mix is set dry.   

It appears that *something* ain't right - A isn't getting to D, and clock is coming thru.    I'll keep poking as I have time but boy - a schematic would really be helpful!   This one is made of unobtanium. 

[Govmnt_Lacky]

This one is made of unobtanium.

Not necessarily.

The TMS chips can be found (there are some on fleaBAY right now for $0.99/each.

I "may" be able to assist with some of the 74 chips. If you run into a situation, PM me and I'll see what I can do about those. No guarantees but, if I can dig some up.... they are all yours.

[GibsonGM]

This one is made of unobtanium.

Not necessarily.

The TMS chips can be found (there are some on fleaBAY right now for $0.99/each.

I "may" be able to assist with some of the 74 chips. If you run into a situation, PM me and I'll see what I can do about those. No guarantees but, if I can dig some up.... they are all yours.

I meant the schematic   Thanks tho, I may need a chip or 2 of course...but the first order of biz...how to tell if these things are doing their jobs??   The way I see it, some cap in the signal path could be dead, blocking audio from getting to where it's mixed or turned to digital or whatever is supposed to go on...but having no schematic, I don't know where to look!    What should be happening with the TMS chip, for example?

When we have a non-working transistor or opamp, we ask people for their voltages, and can surmise from the readings something about the device's operation.  I'm kind of lost with these things, lol.  They have power, I know that much......but maybe 'dirty power' from a dead cap could cause them to be erratic, and I can't tell at this point.    D@mn schematic.

[Scruffie]

Were it me, I'd give it a full recap as you pondered, it's not much of an outlay and would rule them being the cause out (they're 40ish years old anyway) and then I'd see if I can find another digital schematic from MXR which, assuming the same lead designer, might offer some clues to how the design worked.

[GibsonGM]

Were it me, I'd give it a full recap as you pondered, it's not much of an outlay and would rule them being the cause out (they're 40ish years old anyway) and then I'd see if I can find another digital schematic from MXR which, assuming the same lead designer, might offer some clues to how the design worked.

I found this site, not sure if they're reputable but thinking of ordering the (right) one for this unit.  Probably not going to get too far without one!

https://www.musicparts.com/products.asp?Company=MXR

[ElectricDruid]

Here's the datasheet for the dynamic RAMs:

https://www.datasheets360.com/pdf/-1140623823302668422

It's a 1-bit RAM, so they're probably all connected up the same way, 12 of them in parallel (a chip per bit). But you didn't say how many there are, so I'm guessing.

There's a pinout in the datasheet. If you've got a 'scope you should be able to see "stuff happening" (technical term) on all the pins except the power and pin 1 which isn't used. The address lines and all the read/write signals, row/column toggles etc should all be doing something exciting.

[Rob Strand]

I found this site, not sure if they're reputable but thinking of ordering the (right) one for this unit.  Probably not going to get too far without one!

If you want to dig around on the web there's also the MXR M-186 which seems to very similar to the M-175.

[PRR]

> 12 of them in parallel

I was corrected above. The SAR is 8-bit, so the memory array is probably 8-bit. (which explains the compander for almost 16 bit audible depth.)

MusicParts has been selling Xeroxes at least forever. The few comments I find are favorable:
https://www.hostboard.com/forums/f700/141478-altec-1220-mixer-post326556.html?s=6fec35524bfd3612147e5b038f472259#post326556

[ElectricDruid]

> 12 of them in parallel

I was corrected above. The SAR is 8-bit, so the memory array is probably 8-bit. (which explains the compander for almost 16 bit audible depth.)

Oh, crikey! I missed that, sorry. The compander wouldn't have been a bad idea with 12-bit, but at 8-bit ADC, it's going to be "essential" rather than "good practice"!
Incidentally, there were some clever non-linear DACs in this era that gave something like 12-bit dynamic range out of 8-bit data using "u-law":

https://electricdruid.net/wp-content/uploads/2018/06/AM6070-uLaw-DAC.pdf

Used in some of the early drum machines, Linn Drum and so on. The compander here is basically doing this same job using analog tech.