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Author Topic: 70's? Domino amp schematic  (Read 2889 times)

galaxiex

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70's? Domino amp schematic
« on: July 23, 2016, 10:14:53 AM »
Here's another "mini stack" amp that I found on ebay.
This one has the Domino logo on the speaker cab but no name of any kind to be found on the amp head.
The seller didn't say, but I assume it came from an estate sale, as that is where many of his items come from.
I suppose the 2 pieces could be from different places, but it came with an ancient looking speaker cord and everything looks untouched/original.

No schematic so I traced the board to create one.

The amp works ok, but has excess distortion, esp on the low notes.
I replaced all electro caps and many of the ceramics with plastic film caps. Same values.

Sounding better, but still too much distortion on the low notes. Very "Fuzz box" sounding.
I don't think it was meant to sound like this.
Backing down the volume to whisper quiet, it cleans up, but still distorts if you strum hard.

Putting a 1kH 200mV sine signal at the input (or playing guitar into it) and tracing the signal with a scope, all is fine until Q3.

At the collector of Q3 the bottom half of the waveform is severely clipped/flattened out, like it's hitting the rail.
Power supply is solid, very little sag, even at high volume.
Top of the waveform seems to have miles of headroom.

Any ideas?

Thanks  :)

Edit; Moved some things around in the power amp section to make the schem easier to read.




« Last Edit: July 23, 2016, 07:56:13 PM by galaxiex »
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J M Fahey

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Re: 70's? Domino amp schematic
« Reply #1 on: July 24, 2016, 01:48:36 AM »
Add DC voltage to ground in the schematic, specially +V rail and Q3 collector.
No signal.
« Last Edit: July 25, 2016, 12:24:08 AM by J M Fahey »

galaxiex

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Re: 70's? Domino amp schematic
« Reply #2 on: July 24, 2016, 09:37:33 AM »
Voltages added.

Sorry, I should have thought of that before.  :-[

Edit; I should note that the distortion happens mostly on the 2 low strings E and A.

It does occur on the higher strings if I strum (very) hard.
« Last Edit: July 24, 2016, 09:53:30 AM by galaxiex »
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galaxiex

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Re: 70's? Domino amp schematic
« Reply #3 on: July 25, 2016, 07:59:00 PM »
Well... this is weird....

Played this amp for about an hour despite the distortion, and it gradually got better,
as in, the distortion is almost gone.???

The only thing I can think is that I replaced all the electro caps with new of the same values, and possibly? some of those new ones were "old" and needed to reform?
I know it's also possible to get bad, brand new parts... it happens.
But I was quite meticulous and tested each new cap before soldering it in.

The negative half of the waveform still clips a bit, not as much as before, and if I back the volume down to around 7 or 8 It cleans up fairly well.
I kind of expect a certain amount of distortion at full volume anyway.

I'll keep playing it for awhile and see if it changes any more.
If it stays like it is now, I'll call it good and move on.  :)
« Last Edit: July 25, 2016, 08:02:39 PM by galaxiex »
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J M Fahey

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Re: 70's? Domino amp schematic
« Reply #4 on: September 05, 2016, 11:40:13 AM »
Quote
At the collector of Q3 the bottom half of the waveform is severely clipped/flattened out, like it's hitting the rail.
Power supply is solid, very little sag, even at high volume.
Top of the waveform seems to have miles of headroom.

Any ideas?
Since DC, no signal voltage there is fine, 1/2 +V , I guess Q9 is open (or some track/solder is oen) so it can not drive a 4 ohm load.
Try unloaded to see what happens.

galaxiex

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Re: 70's? Domino amp schematic
« Reply #5 on: September 07, 2016, 06:08:03 PM »
Quote
At the collector of Q3 the bottom half of the waveform is severely clipped/flattened out, like it's hitting the rail.
Power supply is solid, very little sag, even at high volume.
Top of the waveform seems to have miles of headroom.

Any ideas?
Since DC, no signal voltage there is fine, 1/2 +V , I guess Q9 is open (or some track/solder is oen) so it can not drive a 4 ohm load.
Try unloaded to see what happens.

Thanks JM,
I ended up replacing Q3, 8 and 9 with TIP series transistors.

Amp is working great!
Slight break up/distortion at full volume, but not unpleasant.  :)
Kind of a "crunch" transistor sound, very usable.  8|

Back volume off to "8" or so and it cleans up nice!
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

galaxiex

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Re: 70's? Domino amp schematic
« Reply #6 on: August 26, 2018, 04:24:30 PM »
Updated the schematic.

Reason for this update....

With amp volume at "0" and guitar volume turned up, strum guitar and you could hear quiet distorted sound thru speaker.

Spent some time trying to track this down.
Even if I lifted vol pot wiper, signal still got thru to the power amp.... somehow...

Kept on playing with the circuit and trying different things...

Discovered that if I lifted one end of C17 coupling cap to reverb Q4-Q5 drivers, problem went away.

Ah HA!!!
Signal at Q4-Q5 was modulating the power supply.

Added power supply filtering/decoupling.
R 36-37-38 and C 24-25-26

Makes a HUGE difference!

Amp plays waaaaay cleaner and louder!

I have heard (read) that the power supply is the most important part of an audio amp.
This lesson really brings that home.

Not something I will soon forget.

I was going to post asking for help with this, but finding out, learning on my own, really makes it real.  :dbtu:
« Last Edit: August 26, 2018, 04:47:42 PM by galaxiex »
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

 

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