Tech21 SansAmp PSA-1 PCB traces and schematics  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
Repairer
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Post by Repairer »

Hi,

Still have not fixed the amp. Turned out the new replacement transformer was shorted, same as the original transformer as a result of whatever the fault is in the circuitry. This happens very very quickly, perhaps almost instantaneously.

Problem has to be a short to ground that occurs with no inputs or outputs attached to the sansamp. Therefore suspects are op amps as previously suggested by forum or the QTC 2730 chip to which I am leaning because of its proximity to the power supply and the fact it forms part of the midi circuitry, which had been used immediately prior to the problem first appearing.

Gearing up to buy new transformers, will take a few weeks to get here. Hesitant to put into amp until I find the fault. Catch 22 situation.

Can anyone tell me what the QTC 2730 8 pin ic does/is? I think/guesstimate I know but would like a second, uninfluenced by me, opinion.

Repairer

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Post by av500 »

before you burn another transformer, check the board for a short to GND on any of the three power rails:

1) measure the resistance to GND on the three regulators, 5V, 15V, -15V on both input and output pins, see the internet for the data sheets (7805, 7815, 7915)

2) check the capacitors in the power supply section for being shorted, best is to desolder them to check them outside of the circuit.

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Post by cmdr »

Nice to see people are still interested in the PSA-1 today!
jsaily wrote:The 27C64 chip is a 64k (8k x 8-bit) EPROM which is not too hard to dump and clone since it cannot be write protected like modern microcontroller internal memories. It's truly old school device (I'd imagine early 90's) with a 68HC processor ;-)
Has anybody attempted to dump the EPROM? I have, however for some bytes I get a different result every time. This might be the reason why my SansAmp no longer works, with the EPROM contents decayed over the decades.

If anybody could dump the EPROM of a working SansAmp that would be great! To get you hooked: there is a secret message from the author in the beginning :D .

I have also begun to trace the "boring" digital part. So far it looks like a standard "Expanded Multiplexed Mode" circuit described in the 68HC11 docs. This basically means that the whole program is loaded from the EPROM. The internal flash memory in the 68HC11 is probably only used for user presets.

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Post by cmdr »

Repairer wrote: Can anyone tell me what the QTC 2730 8 pin ic does/is? I think/guesstimate I know but would like a second, uninfluenced by me, opinion.
It's an optocoupler (or opto-isolator) used for isolation of the MIDI input. See https://en.wikipedia.org/wiki/MIDI#Elec ... ifications for details.

IMHO it's rather unlikely that an overvoltage or short on the MIDI input would have influence on the power supply, because they are, in fact, isolated from each other. Worst thing that can happen is the LED inside the optocoupler burning out. That would just cause the MIDI input to stop working.

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Post by av500 »

I have a working PSA 1 and an eprom reader so I will try to dump it next week

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Post by cmdr »

Hey, how is the dumping going? I'm really looking forward to it!

I wrote a little disassembler which lets you analyse the contents of the EPROM. I was hoping to fix the broken bytes by the help of that, but it seems there are just too many of them. :(

You can find the source code here: https://github.com/cmdrf/dis68hc11

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Post by av500 »

slowly since coworker decided to give it to a friend so I am waiting for the EPROM reader come back... :(

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Post by av500 »

cmdr wrote:Hey, how is the dumping going? I'm really looking forward to it!

I wrote a little disassembler which lets you analyse the contents of the EPROM. I was hoping to fix the broken bytes by the help of that, but it seems there are just too many of them. :(

You can find the source code here: https://github.com/cmdrf/dis68hc11
I have the EPROM content now, but I won't post it publicly, so PM me please

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Dirk_Hendrik
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Post by Dirk_Hendrik »

Why not?
Sorry. Plain out of planes.

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av500
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Post by av500 »

Dirk_Hendrik wrote:Why not?
well, I don't have any idea what the legal implications are. so I err on the side of caution...

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Post by Dirk_Hendrik »

In which case,
Should I do the posting for you? I could not care less for the legal implications regarding an obsolete product.
Sorry. Plain out of planes.

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Dirk_Hendrik
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Post by Dirk_Hendrik »

Look what I happened to find during my endless internet endeavours!!
PSA_1_27C64.pdf
(8 KiB) Downloaded 143 times
and since the attachment function does notr allow the .bin extension.... rename the file above from .pdf to .bin :secret:
Sorry. Plain out of planes.

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bajaman
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Post by bajaman »

says it is corrupt even when renamed
be kind to all animals - especially human beings

av500
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Post by av500 »

bajaman wrote:says it is corrupt even when renamed
yeah looking at the actual binary data, it looks fishy. guess something went wrong reading it out. back to the drawing board...

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Post by av500 »

darn, the EPROM reader does not work, it always reads the upper and lower 4 bits the same :( and of course it's 20ys old with no support...

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Post by howmuch »

av500 wrote:darn, the EPROM reader does not work, it always reads the upper and lower 4 bits the same :( and of course it's 20ys old with no support...
I had an old parallel port Dataman that was doing something similar.
I opened it up and found some trimmers for adjusting voltages at test points.
After a bit of "calibration", it started reading consistently.
Your programmer is probably in the same situation and needs calibrating. It's cheaper than buying a new programmer (e.g. GQ-4X or TL866).

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Post by av500 »

turns out it was not the EPROM device but the PC, after unfreezing another XP machine with a working parallel port from cold storage it worked...

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Post by Dirk_Hendrik »

And what happened!!!?? I immediately found the correct image online!!
PSA_1_27C64_fixed.pdf
(8 KiB) Downloaded 250 times
Sorry. Plain out of planes.

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Repairer
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Post by Repairer »

Hi,
Update on troubleshooting psa-1. Since original forum posting, the device has been put on the shelf. Not had equipment to find fault.
It still remains on the to do list. Will one day solve and post findings.
Cheers

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