Toshiba HX-20 PSG + SCC sound balance. Is it somehow fixable with some soldering like in Philips 82XX 's ?

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By sd_snatcher

Prophet (2573)

sd_snatcher's picture

13-02-2017, 23:25

If you reverse engineer and draw the sound circuit schematics, it will probably be easy to fix it. It's not much different from what you're trying to do, except that you actually draw the circuit.

It doesn't even have to be a professional circuit schematic. You can do it the homebrew style, like Dhampird did in this thread and this thread.

By friguron

Resident (44)

friguron's picture

14-02-2017, 08:46

Well, the computer appearing in that Flickr photoalbum I linked before is not mine. So, those pics are good, but not as good for being able to trace and give values to the pin-49 circuit.

As soon as I can phisically reach my Toshiba (maybe today), I'll try to draw that circuit.
Thanks.

By friguron

Resident (44)

friguron's picture

15-02-2017, 18:34

Hi, after some PCB line careful tracking I think I've come up with the schematics drawn on top of the main PCB of my toshiba HX-20. Here is the ultra high resolution picture:

http://bit.ly/2lhParZ

There you can see how I've traced PSG channels A, B and C, pin 49 from both slots, and some random R+C subcircuits apparently in charge of the whole mix thing.

To make SCC sound louder, could the solution might be lowering both 5K6 resistor values? (No idea...)
Electronic experts: now it's up to you :)

If someone needs additional values or has general doubts about the circuit, please ask, I have my Toshiba HX-20 completely disassembled nearby.

Greetings.

By friguron

Resident (44)

friguron's picture

16-02-2017, 17:35

I have no idea if I've correctly transcribed my Toshiba-HX-20 physical circuit onto this logical circuit I'm showing you below.
Please, go to http://www.falstad.com/circuit/ and import this .txt file:

$ 3 0.0000049999999999999996 10.20027730826997 50 5 43
R 56 160 64 208 0 2 160 5 0 0 0.5
R 64 64 64 96 0 2 200 5 0 0 0.5
w 64 64 144 64 0
209 144 64 184 64 0 0.000009999999999999999 -0.3402509822857658 16
209 64 160 184 160 0 0.000009999999999999999 -0.13180891422638075 16
R 56 232 64 280 0 1 40 5 0 0 0.5
R 56 304 64 352 0 1 40 5 0 0 0.5
R 104 344 104 392 0 1 40 5 0 0 0.5
w 248 64 248 160 0
r 64 232 216 232 0 1000
r 64 304 216 304 0 1000
r 104 344 216 344 0 1000
w 216 344 216 304 0
w 216 304 216 232 0
209 216 232 288 232 0 0.000009999999999999999 0.4720598965236835 16
w 248 160 288 232 0
w 288 232 384 232 2
r 184 64 248 64 0 1000
r 184 160 248 160 0 5600
o 16 64 0 2083 10 0.00009765625 0 -1 0

SCC are the two upper inputs.
PSG are the 3 at the bottom.

With the original 5k6 resistors for the SCC mixer subpart, SCC output seems small. Lowering their value, SCC amplitude seems to become louder at the output part, but I don't know "how much" louder the SCC part needs to be, or what could the expected result must be... I'm very tempted to solder 1 potentiometer in parallel with each of the the real PCB 5k6 resistors, and fine tune SCC output amplitude by hand/ear...

OTOH, I'm not sure the SCC/PSG subparts of these 2 subcircuits work the same way... The common C for the PSG inputs is differently placed compared with the separate C's for each SCC channel (not to say their position, PSG one placed AFTER its Resistors, SCC ones placed "before" them...)

Whatever, any electronics expert might shed some light onto this...

Thanks...

By Daemos

Paragon (1647)

Daemos's picture

16-02-2017, 18:43

Passive mixdown of the signals. Very bad approach. No active components used to amplify and mix down the signals properly. You get 1. Horrible crossover distortion 2. Weak sound.

Look up the nms8250 sound problems in the forums. There is a solution in there concerning a mixer to build and solve your problems. Contact user lupie. He may even have the board for sale

By friguron

Resident (44)

friguron's picture

16-02-2017, 18:55

There's an Operational Amplifier following fuchsia path (I haven't drawn all the schema, it's a slow process.
I don't even know if my components translation is 100% ok onto this online-circuit-simulator, I'm no expert in electronics).

What I've translated is just the beginning of the path from SCC + PSG lines onto the main amplifier circuits to see if just some tweaking of these initial R values might enhance SCC loudness...

One thing is sure, I've NEVER had sound distortion with my Toshiba HX-20, just quiet SCC. PSG is crystal clear no matter what, SCC is crystal clear no matter what (extremely quiet though).

I also have a NMS8250 with still not 100% completed R + C amplitude fixes... I'm not that sure 8250 fixes can translate easily to this completely different PCB.
I wanted to fix this HX-20 board "just soldering R's and C's". If due to amplifier circuit design this is not possible, then I will resing before trying more advanced stuff.

Thanks.

By Wild_Penguin

Champion (399)

Wild_Penguin's picture

16-02-2017, 20:55

This is interesting, considering the HX-10 - HX-20's little brother - uses an OpAMP circuit.

However I can confirm the opamp circuit can solve mixing issues at least on the nms8250/55. You can build it on a protoboard, too, if you can solder.

By sd_snatcher

Prophet (2573)

sd_snatcher's picture

16-02-2017, 21:36

You missed one slot. Here's the complete circuit:

$ 3 0.000015625 0.12840254166877416 58 5 43
R 160 136 88 136 0 2 330 1.8 0 0 0.5
R 160 40 112 40 0 2 55 1.8 0 0 0.5
w 160 40 240 40 0
209 240 40 280 40 0 0.000009999999999999999 0.4798392965305558 16
209 160 136 280 136 0 0.000009999999999999999 -0.057073590605474456 16
R 160 208 72 208 0 2 110 1.8 0 0 0.5
R 160 280 112 280 0 2 220 1.8 0 0 0.5
R 200 320 144 320 0 2 440 1.8 0 0 0.5
w 344 40 344 136 0
r 160 208 312 208 0 1000
r 160 280 312 280 0 1000
r 200 320 312 320 0 1000
w 312 320 312 280 0
w 312 280 312 208 0
209 312 208 384 208 0 0.000009999999999999999 -0.48169199892132636 16
w 344 136 384 208 0
w 384 208 480 208 2
r 280 40 344 40 0 1000
r 280 136 344 136 0 5600
r 344 136 408 136 0 5600
209 480 136 408 136 0 0.000009999999999999999 -0.07288262127837175 16
R 480 136 536 136 0 2 165 1.8 0 0 0.5
211 480 208 576 208 0 1 8000 1
o 16 64 0 2083 10 0.00009765625 0 -1 0

By RetroTechie

Paragon (1414)

RetroTechie's picture

17-02-2017, 12:50

Just some thoughts:

a) Start with replacing all electrolytic capacitors in this circuit. If they've gone bad (likely given their age), less signal will pass through if any. It doesn't hurt to take a bit larger value. That may improve low-frequency response, possibly at the cost of a bigger 'plop' when powering up the machine (nothing too bad Wink ). For example replace with 22 uF/10V specimens. And observe polarity! Replace one by one so you can remember how each one was oriented. And/or make a photo first where polarity of each of these is visible & check afterwards with the new situation.

b) No need to replace the entire circuit with some add-in board immediately! It's okay to experiment a little with values of series resistors in a circuit like this, as long as you don't go overboard. A good approach would be to increase the values of resistors coming from PSG a little, and decrease value of resistor(s) coming from cartridge slot at the same time. That would keep overall impedance seen by the circuit following it (transistor amplifier / opamp? RF modulator etc) roughly the same & thus not affect frequency response too much.

May I suggest: replace each of the resistors coming from PSG with 1.8kOhm (vs. 1k). And replace resistor coming from cartridge slot sound input with 3.3kOhm (vs. 5.6k). Do this for one cartridge slot only - the one where you'd usually plug in your SCC. This should make a very noticeable difference in PSG / SCC balance.

Then try your SCC & other audio-producing cartridges plugged in individually, together, in different slots, using different software to test, etc. Once you have a good picture how the new situation behaves as compared to before, decide what to do from there.

By friguron

Resident (44)

friguron's picture

17-02-2017, 17:37

RetroTechie wrote:

Just some thoughts:

a) Start with replacing all electrolytic capacitors in this circuit. If they've gone bad (likely given their age), less signal will pass through if any. It doesn't hurt to take a bit larger value. That may improve low-frequency response, possibly at the cost of a bigger 'plop' when powering up the machine (nothing too bad Wink ). For example replace with 22 uF/10V specimens. And observe polarity! Replace one by one so you can remember how each one was oriented. And/or make a photo first where polarity of each of these is visible & check afterwards with the new situation.

Capacitors look really ok. PSG + SCC misbalance has been this way since the very first day I've had this computer, so in case everything else fails, maybe I'll mess with original Capacitors.
I took around 20 macro pics of every possible angle before disassembling my Toshiba PCB.

RetroTechie wrote:

b) No need to replace the entire circuit with some add-in board immediately! It's okay to experiment a little with values of series resistors in a circuit like this, as long as you don't go overboard. A good approach would be to increase the values of resistors coming from PSG a little, and decrease value of resistor(s) coming from cartridge slot at the same time. That would keep overall impedance seen by the circuit following it (transistor amplifier / opamp? RF modulator etc) roughly the same & thus not affect frequency response too much.

That approach is the one that seems more feasible. I think everything is just a bad Toshiba desing of the pre op-amp phase (I think is the fuchsia circuit I drew on my uploaded pic, some posts ago).

RetroTechie wrote:

May I suggest: replace each of the resistors coming from PSG with 1.8kOhm (vs. 1k). And replace resistor coming from cartridge slot sound input with 3.3kOhm (vs. 5.6k). Do this for one cartridge slot only - the one where you'd usually plug in your SCC. This should make a very noticeable difference in PSG / SCC balance.

Then try your SCC & other audio-producing cartridges plugged in individually, together, in different slots, using different software to test, etc. Once you have a good picture how the new situation behaves as compared to before, decide what to do from there.

I'll start with just the slot-2 SCC resistor (lowering it to 3k3 or so), then depending of the outcome, I'll rise all 3 PSG r's to 1,8 or so, again depeding of the outcome, I'll go on telling here the result of the fix...

Thanks for your answer!

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