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Old 18th January 2013, 08:11 AM   #21
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Lightbulb replacement transistors

MJE15034 and MJE15035 work fine as VAS transistors. (Q101 , Q105)
DIYHiFi.org • View topic - HELP with GAS Son of Ampzilla VAS transistors!
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Old 18th January 2013, 11:19 AM   #22
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Since there isnt much information about this nice amplifiers in the internet I decided to write here my experience resurecting one.
When I boght it, the amp had one channel distorting badly and the other with a lot of 50Hz hum. The first thing I did was to print the schematics and made a list of the electrolitic capacitors. I kept the same values everywhere except in the power supply, where I increased the values by 50%. Be carefull about the phisical size of the cans, because that area is crowded.
I also replaced the bias and DC trimmers and installed again the trimmers for the power meters in the front. Someone removed them at some time, and replaced them with resistors, that were now burned.
I also replaced the power cord and instaled a power switch in the back. I didnt want to make new holes, so I installed a new fuse inside, and mounted the switch in the hole for the old one.
Before removing anything take note how all the connections are made, number the wires and the plugs and only then start taking things out.
First I removed the transformer protection and the transformer. It is very big and heavy. It is much easyer to move arround withut it.
After that the PSU board (the one in the back with the big cans). Deox and clean all metal contacts and specially the one on the bottom that connects it to the metal enclosure. This will be the ground for the all thing, its very important that the contact is good.
To remove the PCB's on the side, unsolder the wires to the RCA's (that probably you want also to replace), remove the 4 output transistors by removing the plastic covers and gently pushing them out. Take note of the places.
The PCB's on mine where very dirty with dust, old flux and thermal paste. I cleaned everything, replaced the caps and trimmers and tested the transistors. One of the 4 small ones was off, replaced them all with the same type.
Abut the trimmers, choose multiturn precision ones, the normal one turn are not precise enough.
When reassembling take special care ensuring that the IC on the bottom PCB has thermal paste and is touching the heatsink.
If you put new RCA's, ensure that are not isolated. The ground for the input stage comes from the ground wire of the RCA. It should touch the metal enclosure. Even if you are not renewing them, take them out, deox and clean everything.
I removed the limiter transistors from the circuit (Q103 and Q104) and also the limiter diodes (D101 and D108), but its up to you to do the same.
To set the bias current and output DC it is much easyer and safer to first make some cable extencions like this:

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With several of these cables as extensions you can power the amp with the PCB's perfectly horizontal, and you have free access to the 0.22 ohm resistors.
For the next part disconnect the power meter wires from the speaker outputs.
To set up the amp you first have to shunt the inputs to the ground, connect a DC meter to the outputs and tune the first trimmer (near to the top of the case) until you measure 0V DC on the outputs, after this put the meter probes on the terminals of one of the big white resistors on the bottom part of the board and set the trimmer near to them until you measure about 25 or 30 mV. Check if the tension across the other resistors is about the same.
Leave it on for 30 min and then repeat.
After this, remove the extensions and assemble everything.
To set up the meters you need one dummy load. I built this one with a couple of 8 ohm 100W resistors and an old computer heatsink:

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To tune the meters you need first to reconnect them to the speaker outputs connect the dummy loads and a AC meter to the speaker outputs and a signal generator to the inputs.
If you dont have one you can download and record a CD with a 1kHz wave, or use some smartphone app.
With everything off, first use the screw in the front of each meter to set the meters to 0 (zero).
Start with the signal generator in the minimum (CD player, smartphone or PC) and slowly increase the volume untill you read 8,944V on the AC meter connected to the speaker output. That voltage over 8ohms equals to 10w, so tune the meters trimmer untill you read 10W and thats it!
If your dummy load is good enough, increase the volume untill you read 25,298V on the DC meter, by now the needles should be arround the 80w mark.

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Afterl all this work, happy listening
I hope this, togeather with the previous posts by everybody will help someone to keep one of this wonderfull amps in good condition.
The final result

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Ricardo
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Old 8th December 2013, 09:05 PM   #23
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Join Date: Dec 2013
Location: Aberdeen, NJ
I purchased a Son Of Ampzilla off eBay a few months back. I have been using it despite the fact that it has an intermittent hum in one channel, but now it is time to repair. I will be replacing all the caps as a few have tested bad.

My question is that I have seen in more than one place a recommendation to remove the 330 uF bypass capacitors. Anyone know what the advantage is to removing these as opposed to replacing them with new ones?

I also have found two schematic diagrams - one "clean" and one with a bunch of mods, including the removal of the caps mentioned above and replacement of some resistors with different valued ones. Anyone have any knowledge of this? My gut feeling is I am safer sticking with the original design.

Lastly one of the VU meters on mine is blown. Does anyone have a spec for a replacement?

Thanks in advance,

--Paul R.
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