SRM-717 - two questions

[yabba235]

Hi!

I am renewing 25-years 717's and I observe strange things:

I noticed a lot of wet places - as if sweaty - I suspect it has something to do with high voltage, have you noticed such symptoms in your old Stax? It's probably old flux - but these are places where it shouldn't be - output sockets, bottom of heat sinks (between PCBs).
The second issue - how does the output protection system work - it draws voltages only from two outputs +L and +P? Because that's what I saw on PCB. What are the soldering points marked 81,82, 83 and Test: R,L,V for?
I know that Kevi n had a diagram of this system somewhere, but it is no longer available .

[kevin gilmore]

[yabba235]

Thank's !

[yabba235]

Kevin, one more questions:

under original schematic resistors r45 and r46 is connect between  base q19/20 and emitter q23/24. But in real pcb is connected to base q23/24.

I checked it a few times, no mistake - I do not know what to think about it...

Edited May 3, 2024 by yabba235

[kevin gilmore]

suggest you redraw that part of the schematic that you think is wrong.

there have been mistakes in stax schematics in the past.  especially the t2.

once it enters production and works...

[spritzer]

I've been told by more than one source that they put those "mistakes" in on purpose, same as they always labeled the balance pot "offset" and vice versa. 

[Craig Sawyers]

Similar thing happened with Tektronix back in the day. A big part of their business was supplying to the US DOD. But in an attempt to reduce cost the DOD placed an order with Hickock and Lavoie, saying that they wanted them to copy the Tektronix scopes. Tek got wind of it and started putting a random hole in the rear panel of the vertical plugin.

When this appeared in the Hickock copy, Tektronix sued the UK government in 1961. Took a while (18 years!), but Tek won $4.5m in damages from the US Government.

Described here https://vintagetek.org/clone-scopes/

So "mistakes" can show up a plagiarizer of the design. 

Edited May 7, 2024 by Craig Sawyers

[dsavitsk]

8 hours ago, Craig Sawyers said:

Similar thing happened with Tektronix back in the day. A big part of their business was supplying to the US DOD. But in an attempt to reduce cost the DOD placed an order with Hickock and Lavoie, saying that they wanted them to copy the Tektronix scopes. Tek got wind of it and started putting a random hole in the rear panel of the vertical plugin.

When this appeared in the Hickock copy, Tektronix sued the UK government in 1961. Took a while (18 years!), but Tek won $4.5m in damages from the US Government.

Described here https://vintagetek.org/clone-scopes/

So "mistakes" can show up a plagiarizer of the design. 

"Map traps"

https://en.m.wikipedia.org/wiki/Fictitious_entry

[waltzingbear]

so where does the emitter q23/24 go to instead?

 

 

[kevin gilmore]

someone needs to post very high resolution images of front and back, then will correct schematic if necessary

[hyperma]

I guess the real circuit bit different with the official circuit pic.

I got this from internet:

and back layer:

then align layers with through holes and apply "overlay" on top layer got this:

It's easily to redraw copper route and add on BJTs polar sign:

 

Finally got this:

Hope it does work in simulation, my Multisim collapsed need to be reinstalled.

 

[kevin gilmore]

makes sense.

[yabba235]

5 hours ago, hyperma said:

I guess the real circuit bit different with the official circuit pic.

I got this from internet:

and back layer:

then align layers with through holes and apply "overlay" on top layer got this:

It's easily to redraw copper route and add on BJTs polar sign:

 

Finally got this:

Hope it does work in simulation, my Multisim collapsed need to be reinstalled.

 

This is exactly what I wrote in the first post - the diagram is not the same as the connections on the board.

[kevin gilmore]

it could have originally been 2 different transistors for the darlington in which case the original diagram might have been correct.

since this is 30 years old, any original ideas are long gone.