View Full Version : Found a Spectravideo SV-328 today!, without a wallwart :(

April 12th, 2014, 10:41 AM
Hi guys!

Been quite a while since I wrote anything on the forums, but today I found a Spectravideo SV-328 as the topic suggests. Sadly they didn't have a wallwart for it and the required 9VAC @ 1.5A and 16VAC @ 0.8A isn't something I have at home. So I thought it must be regulating that power inside and sure enough, 78-series linear regulators inside... I guess it's important to keep the Z80 warm and comfy.

Anyway, there's seemingly no easy way to access the rails to feed them with regulated power. So I'm a bit stuck in my attempts to test the SV-328. I could feed the +12 and -12 rails by hooking on alligator clips on the regulators but they are in bad spots and the required power doesn't allow for tiny hooks. So maybe I have unsolder the regulators to get some proper attachment points, but I'm not sure. The 12V rails are using a mains full wave bridge rectifier so I can't just feed it straight DC via the connector.


April 13th, 2014, 03:51 AM
Without looking at the pinout of the power connector or the schematics (i.e. this is off the top of my head) - if the external power requirements are a.c. then a couple of mains transformers of a suitable current rating should fit the bill. You will have to build them into a box along with a mains fuse. My guess would be that this would be easier than butchering the computer? 9V and 16V look to be 'standard' transformer voltage secondaries. You may be able to get 1 transformer with both of your secondary windings - or use two separate transformers (one for each secondary voltage). Torroidal transformers should be what to look for.

Just a thought.


April 13th, 2014, 03:21 PM
The 12V rails are using a mains full wave bridge rectifier so I can't just feed it straight DC via the connector.

Why not?

April 13th, 2014, 03:50 PM
Why not?Either +12 or -12, but not both...

April 13th, 2014, 06:35 PM
Either +12 or -12, but not both...

Why not? I would assume it is setup for a 24VAC input to the full wave rectifier where the ground reference is tied to the center tap of the 24VAC, not to the negative output of the full wave rectifier. In that case shouldn't feeding +12VDC and -12VDC into the inputs of the full wave rectifier work just as well?

Of course the input voltages would actually need to be higher to account for the voltage drops across the rectifier and regulators to result in the +12VDC and -12VDC outputs.

April 13th, 2014, 09:02 PM
Why "assume it is setup for a 24VAC input" when he says it's only 16VAC?

Sorry, I don't see any noninvasive way you can replace 12VAC externally with 12VDC and get both +12V and -12V.

Sure, if you hook two power supplies to the rectifier, but he did say via the connector, and I'm assuming that means without major surgery to the connector.

April 14th, 2014, 12:37 AM
Recently I found myself in the same position as the OP. I found this power supply:

DOSS AC9N16V (http://www.doss.com.au/ac9n16v-9v-16v-dual-output-ac-power/)

I had a hard time finding some to sell me one outside of Australia, until I contacted the company directly and they sold me one with minimal hussle.

I haven't had time yet to test it, but I see no reason why it shouldn't work.

April 14th, 2014, 05:52 AM
Why "assume it is setup for a 24VAC input" when he says it's only 16VAC?

Oh, sorry I missed that part of the original statement.

So what kind of circuit does it use to get both +12VDC and -12VDC from a 16VAC input?

April 14th, 2014, 09:21 AM

Doesn't use a FW bridge though.

April 14th, 2014, 10:33 AM
I have a video terminal keyboard device that has a similar issue, no AC adapter and I can't find any specs on what was originally used. The +5VDC regulator is fed directly from the input jack while the +12VDC and -12VDC regulators are fed through a FW bridge from the input jack. I was pretty sure it must have originally used a 24VAC CT input for the +/-12VDC supplies but I'll take another look. I figured with the device I have I should be able to feed it +5VDC and +/-12VDC (slightly higher) at the input as-is, or remove the FW bridge and regulators and install jumpers in their place and use an external regulated supply.

April 17th, 2014, 10:40 AM
Either +12 or -12, but not both...

Sure. But what do you need the -12V for (in the machine)?
I had a look at the schematics (http://www.samdal.com/SVIDOCS/STM-A_SVI328.pdf), and it looks like the machine is powered with +5V, -5V and +12V.
Ah, crap, I see it now: the -5V is generated from the -12V with a 79L05.

April 20th, 2014, 10:50 PM
You guys are probably right, just trying to find two propper wallwarts or transformers might be the best solution. I've seen the plug on some other system, TI-99/4A?, so maybe it isn't made completely out of unobtanium.

Anyway, thanks for talking me out of desoldering the regulators, might have been a disaster that might not have been worth it in the end. I'll keep you guys updated if I make progress on brining the Spectravideo into life again.

April 21st, 2014, 12:14 AM
Yes, the same plug was used on the TI-99/4A, the ColecoVision and the Spectravideo SVI-318/328, possibly also SVI-728. However the pinout and voltages vary a lot between those systems so while you could reuse the plug, you can't reuse the power supply between systems.

June 8th, 2014, 02:33 AM
Guess what I found?, the wall wart! It was laying around by the the junk and tools at that thrift shop.

And you know what!?! RED LEDs! TWO OF THEM!
Also, did I mention the Spectravideo need a bath really bad?

So, it responds to Caps Lock, now I just need to solder up a Video and Sound cable (I hope it's that easy) and see if I get a BASIC prompt on this puppy :)

Sorry, I'm just (rightfully) excited about this :P
I'll post an update when and if there's video.

June 8th, 2014, 10:31 AM
So I found the pin-outs in the SV-328 manual (http://msx.hansotten.com/uploads/msxdocs/svi328usermanual.pdf), the pin-out numbering is the proper DIN 5-pin pin-numbering, and it's not the plug as shown on the back of the Spectravideo. Something like that, very confusing, they always are for some reason.

And it actually works!, the keyboard is so dirty and the contacts are dirty as well, some cleaning is really required!
Sorry about the misplaced "s".

June 8th, 2014, 11:35 PM
Congrats!, now you only need to find "Frantic Freddy" ;-)

June 9th, 2014, 09:58 AM
Congrats!, now you only need to find "Frantic Freddy" ;-)

Hey thanks!
I'll have to try find Frantic Freddy, looks like a great game!

June 9th, 2014, 10:13 AM
So there was a disassembled Spectravideo in my bathtub.

The Spectravideo had a bath and is now drying :) It's previous user had mounted a reset-switch into the case and a DIN-5 plug that's hooked up to the back of the video/audio/RF-out connector. Thought it was for a tape deck at first.

It got missing screws and flux and solder form having been reworked at some point. So it's a real hobbyists computer :) I'm not old enough to have been around at the time when all you had was one of those 8-bit micros, and no Internet or friends. I really wonder what it was like being a lonesome Spectravideo user back then...

June 9th, 2014, 12:42 PM
I believe Ronex or someone else might have arranged some user groups. Spectravideo was reasonably common in Sweden, of course nowhere near the big brands like Commodore or Sinclair, but I'd wager it easily was among top 10 for home computers, on the verge on top 5 perhaps even.