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

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

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby Repairer » 28 Mar 2017, 21:50

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
Repairer
 
Posts: 4
Joined: 21 Jul 2015, 07:00
Has thanked: 0 time
Have thanks: 0 time

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby av500 » 29 Mar 2017, 09:15

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.
av500
 
Posts: 11
Joined: 27 Sep 2013, 19:02
Has thanked: 2 times
Have thanks: 18 times

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby cmdr » 11 Apr 2017, 22:40

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.
cmdr
 
Posts: 3
Joined: 11 Apr 2017, 22:17
Has thanked: 1 time
Have thanks: 0 time

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby cmdr » 17 Apr 2017, 16:28

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.
cmdr
 
Posts: 3
Joined: 11 Apr 2017, 22:17
Has thanked: 1 time
Have thanks: 0 time

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby av500 » 17 Apr 2017, 17:14

I have a working PSA 1 and an eprom reader so I will try to dump it next week
av500
 
Posts: 11
Joined: 27 Sep 2013, 19:02
Has thanked: 2 times
Have thanks: 18 times

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby cmdr » 04 May 2017, 08:14

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
cmdr
 
Posts: 3
Joined: 11 Apr 2017, 22:17
Has thanked: 1 time
Have thanks: 0 time

Re: Tech21 SansAmp PSA-1 PCB traces and schematics

Postby av500 » 04 May 2017, 12:51

slowly since coworker decided to give it to a friend so I am waiting for the EPROM reader come back... :(
av500
 
Posts: 11
Joined: 27 Sep 2013, 19:02
Has thanked: 2 times
Have thanks: 18 times

Previous

Return to Boutique Stompboxes dissected...

Who is online

Users browsing this forum: finom1, roseblood11, staslalune and 12 guests