Major Havoc Build - no sound (but hums nicely)

Pots are genuine Atari 50 ohm pots, correctly wired and tested and range from about 2 ohms up / down to 54 ohms.......

It is a very strange problem and it must lie somewhere, I just have to find this little devil......
 
That's your problem... I think MH needs 10K ohm pots.

I agree with this 100%.

First off, I've never seen a dual-gang 20W wirewound rheostat. Nothing in the MH schematics or parts lists actually specifies the dual-pot value.

Second, the audio output channels from the main game board, both of them are output through an RC filter. The R value is 2.2kohm, same as on Star Wars. The volume pots are wired as a variable resistor (not as a voltage divider) in series with the 2.2k to ground and this forms the overall voltage divider for controlling the amplitude of the gameboard output audio signal. These two now-divided signals then go into the ARII power amps.

So yes, even if the rheostats you have on yout UT panel are wired correctly, they will just be 50ohm (at best) in series with the 2.2k resistor on the gameboard and that works out to 50/(2200+50) = 0.022 or 2.2% of the original signal going to the ARII so you probably won't hear anything even with the rheostats set to 50R.

I can check the pots on my Star Wars machine tomorrow after work to verify that they are indeed 10k. Even without checking, it makes far more sense that a 10k variable-resistor-wired pot would be used in series with 2.2k output resistor.

This is why I suggested previously to actually try the panel in the Star Wars. None of the other Atari color vector games are wired like this and that's why they use the 50R wirewound rheostat. MH and SW are wired the same and use the dual-gang pot (or for you, two separate 10k pots). Any pots rated 1/2W or more will work. Pots with an audio taper would likely work best but those would have to be wired with the taper in the proper direction (you might say that they are polarity sensitive). Obviously linear pots would work but fine control at the low-volume end would not be as good.

Bill B.
 
Last edited:
This is why I suggested previously to actually try the panel in the Star Wars. None of the other Atari color vector games are wired like this and that's why they use the 50R wirewound rheostat. MH and SW are wired the same and use the dual-gang pot (or for you, two separate 10k pots).
Bill B.


Not that I'm disagreeing with any of what you said, but if this is the case, how is it that a MH board (with conversion card) can plug into a Tempest cab and the audio works fine?
 
That's your problem... I think MH needs 10K ohm pots.

REP'D. :)

I agree with the confusion here as the conversion docs (XX to MH) don't discuss modifications to the utility panel.

Needless, MH dedicated TM states -> Atari part # 119006-103 dual volume control
>> where 103, if decoded as a value, is 10Kohm.

NOTE: same part used in SW (and PaperBoy and perhaps others)

NOTE: as Bill mentioned, other games have different sound configuration and use single 50R or 100R POTS (Atari part # 19-9032, as example); other raster games use other values, I believe.
Values depend if the POTs are in the AUDIO lines or in the SPEAKER lines wrt I/O of the ARII board. Therefore, any MH conversion cabinet is pre-wired correctly in the main harness to allow volume control. A MH dedicated harness, however, needs a correct utility panel and POT values.
 
Last edited:
I agree with this 100%.

First off, I've never seen a dual-gang 20W wirewound rheostat. Nothing in the MH schematics or parts lists actually specifies the dual-pot value.

Second, the audio output channels from the main game board, both of them are output through an RC filter. The R value is 2.2kohm, same as on Star Wars. The volume pots are wired as a variable resistor (not as a voltage divider) in series with the 2.2k to ground and this forms the overall voltage divider for controlling the amplitude of the gameboard output audio signal. These two now-divided signals then go into the ARII power amps.

So yes, even if the rheostats you have on yout UT panel are wired correctly, they will just be 50ohm (at best) in series with the 2.2k resistor on the gameboard and that works out to 50/(2200+50) = 0.022 or 2.2% of the original signal going to the ARII so you probably won't hear anything even with the rheostats set to 50R.

I can check the pots on my Star Wars machine tomorrow after work to verify that they are indeed 10k. Even without checking, it makes far more sense that a 10k variable-resistor-wired pot would be used in series with 2.2k output resistor.

This is why I suggested previously to actually try the panel in the Star Wars. None of the other Atari color vector games are wired like this and that's why they use the 50R wirewound rheostat. MH and SW are wired the same and use the dual-gang pot (or for you, two separate 10k pots). Any pots rated 1/2W or more will work. Pots with an audio taper would likely work best but those would have to be wired with the taper in the proper direction (you might say that they are polarity sensitive). Obviously linear pots would work but fine control at the low-volume end would not be as good.

Bill B.

I am glad that there are so many gentlemen that are willing to help, I never would have figured this out on my own as I was under the impression that I needed the 50 ohm pots and even though I tested them at the least, 10 times, I probably would have tested them again just to be sure I was getting the 50 ohms......... (textbook definition of insanity).....

And thank you for the detailed explanation with formula, that will come in handy sometime in the future!

Thanks again to all for your time and effort, I will report back once I get the chance to give this a try.

Jeff
 
I just got home from work and took a look at my Star Wars utility panel. I posted pics on Dropbox here...
https://www.dropbox.com/sh/ac7s8qkndvtvlaj/AABzuQJ3tCuCcLZ7wDte-JEsa?dl=0

It's a dualgang pot as expected. when disconnected, they measure 8 kohm each even though on the back the stamped part number includes -103. Well, they are 30+ years old and probably somewhat worn down.

@andrewb, VC already pretty well answered this question already "how can a MH board audio work okay through a Tempest (or Space Duel) conversion board?"
I'd like to add this. As mentioned earlier, if you unplug the utility panel in the dedicated MH, the full undivided audio signals (at pre-power-amp levels) goe straight to the ARII, so there are audio signals going all the way through to the ARII audio outputs. On a Tempest or Space Duel machine, the single 50R 20W rheostat is wired to divide the ARII audio output signal down to reduce the volume. This arrangement wastes audio amplifier power (on the ARII) and also burns up power in the rheostat windings but it works. To summarize, It's kind of nifty that you can use an MH board in any of the three game cabs and the audio still works. T and SD don't have a variable-resistor pot in series with the gameboard low-level audio outputs but they fix it at the ARII output side. The MH cab divides the low-level audio before the ARII input side.

Bill B.
 
Bill, I think that utility panel needs some cleaning!

I ended up using two single 10K pots on my build (couldn't find a quality dual gang one). I got these nice small ones on ebay. Worked fine.

http://www.ebay.com/itm/232099688337

Btw, for those of you using conversion boards in a dedicated, you should know that you likely need to add a varistor (Bill sells them) for the X-Linearity adjustment to work. It wasn't populated on my board. After adding the one from Bill it worked like a charm. YMMV.

-Jeff
 
Bill, I think that utility panel needs some cleaning!

I ended up using two single 10K pots on my build (couldn't find a quality dual gang one). I got these nice small ones on ebay. Worked fine.

http://www.ebay.com/itm/232099688337

Btw, for those of you using conversion MH boards in a dedicated, you should know that you likely need to add a varistor (Bill sells them) for the X-Linearity adjustment to work. It wasn't populated on my board. After adding the one from Bill it worked like a charm. YMMV.

-Jeff

Here's a few eBay pots that look a little bit more original and have 1/4" diameter shafts and are dual-ganged as well...
example1
example2

Here's a nice looking single one on eBay Link.

There appear to be many to choose from on eBay. Maybe AliExpress is another place to look for deals on pots.
example1
example2


Re: "panels needs cleaning!"
Yeah I know huh. That SW is one of the 4 machines that's been sitting in my garage waiting to be restored. Trust me, none of the finished games in the house look like that. It'll look practically new when I'm done with it, maybe within 2017, I hope.

Yes, the conversion-version MH boards were shipped from the factory with one VDR missing, maybe to save a little cost as they were intended to operate along with a T-to-MH or SD-to-MH converter that provides its own VDR. Since there was one on the converter board, the one on the mainboard wouldn't do anything at all because its output wasn't carried through the converter. So, for anyone still planning to use the converter, don't worry about it, but anyone using/changing a conversion-version MH board in/to a dedicated setup, yes, you'll have to add the missing VDR and yes, I sell the little BXVDR board for this purpose for $5 each. The accuracy of the BXVDR reproducing the exact same response of the original Atari VDR component is extremely good, practically perfect.

For MH, I also sell the BX287 module programmed to replace the BPROM at 6C for $20. It's 6H on Quantum (same part). I'm out of these atm but I'll assemble more asap.

Wish I could say that I had produced a LETA replacement but I figured why bother since Vectorlabs makes one. I was planning to produce a new QPE PCB for MH as well but the last time I brought it up, nobody responded so since apparently there was low interest and there are some other sources for them already then I didn't worry about it.



Bill B.
 
EXCELLENT NEWS!

The 10k pots did the trick!

I now have wonderful audio!

The only issue is the linear taper pots have to be swapped with the audio (log) taper pots as the audio is just barely audible and then with a very slight turn, loud-loud-loud.

Found them at Mouser, (sorry ;) ) and should have them by the end of the week.

Thanks to everyone again for your time and assistance, very very much appreciated.

Jeff

:)
 
Back
Top Bottom