Son of Ampzilla - Rebuild & Restoration

[Mayank]

As some of you know (and advised me on D&S) I recently rescued a pair of very sorry looking Son of Ampzilla units.

The previous owner had taken them in a trade several years (decade?) ago ... and never used them. They were is such a sorry condition, dusty, rusted screws and even cobwebs on the heatsinks, that I thought I HAD to give them a better home.

He had advertised these being in "good shape & lights up" - both these statements were unfortunately not true.



I have decided to try and rebuild these to their former glory. Here's the plan:

1. Start with the "dirtier" unit and perform initial preliminary tests.
2. Clean outside dust, grime and sticky residue and air blow the inside
3. Check fuses and power on with a Dim Bulb Tester
4. Check DC offset on speaker outputs

If these tests "pass", move on to more advanced work:

I'll disassemble and rebuild with new capacitors, replace the imput transistors, rebias the amplifier, replace VU meter lamps with LEDs, add an IEC power socket, replace the RCA input jacks and speaker binding posts. Maybe later I could try adding a power switch in front (yes, these units do not have a power switch) and replacing the VU meter circuity.

I will document my progress here for future reference.

Summer is going to be fun!

Rgds
Mayank

 

[Mayank]

Insides appear original with no butchery. The VU meter housing have long since burnt out with the heat from the bulbs. The wires still have the zip ties - perhaps a good sign







Lot of dust bunnies inside

 

[blhagstrom]

Hmmm, that reminds me. I have an Ampzilla tucked away for rebuild.
Pretty much in the same condition as this, maybe worse.

My plan is
1) start with the AC line and see if the transformer is OK.
2) work into the power supply and see if that holds up to produce DC.
3) depending on PS condition (repair if necessary), see if it passes a dim bulb.
4) loop back and rebuild everything.

 

[BilboBaggins]

There is plenty of room to work inside, so that is a good thing.

 

[MATT0404]

I'm looking forward to watching your progress. I have a restored and modded Ampzilla that sounds amazing. I hear the Sons sound great, too.

 

[CKDC]

You are a true optimist! Good luck!

 

[Mayank]

Following blhagstrom's plan, I started with the AC line.

Main external 7A fuse was missing
All 4 internal 8A fuses were found open
This amp has +/- 51V and +/- 45V supplies. With all rail fuses open, it appears this may not be an easy fix.

I'm starting with the Power Supply board. I understand there have been several versions of the amp/boards. Mine is the "newer" 420C.

Getting to the PSU board requires removing both the driver boards and the transformer and the heat sinks. Quick connects made the disassembly fairly easy with no de-soldering required at this stage. I've taken several pictures and made detailed notes while comparing against the schematic.

Issues found: The main KBH 2502 diode bridge has a dead short between two of its prongs; One of the 4 1N4003 diodes in the 51V supply is open.

Next - I'm going to remove the PSU board out of the chassis and test/replace the faulty components. Will also order replacements for the aging 10,000uF/50V and 2,200uF/10V capacitors.

Rgds
Mayank

 

[Mayank]

Hmmm, that reminds me. I have an Ampzilla tucked away for rebuild.
Pretty much in the same condition as this, maybe worse.

Why not get it out and do a rebuild together?

Rgds
Mayank

 

[Mayank]

Attaching the schematic if someone wants to follow along.

 

[Mayank]

You are a true optimist! Good luck!

Thank you!

Are you coming for the Mid Atlantic AK event? It will be great to meet.

 

[blhagstrom]

Why not get it out and do a rebuild together?

Rgds
Mayank

Time. Shortage. Got a pile of custom work in flight. People seem to want their stuff fixed.

I saw missing fuses in mine and figured it's going to be a total rebuild. Frame off as it were.
As rough as mine is, it's just as well. It will need a lot of polishing to make that turd shine.

That's why it's in the some day pile.

At full speed I manage to stay one month behind.

 

[CKDC]

Thank you!

Are you coming for the Mid Atlantic AK event? It will be great to meet.

That is too far way for me to commit. May tends to be a busy work month for me.

 

[willyrover]

Looks like a fun project. I rebuilt a Son last summer.

Watch out for incorrect silkscreen markings on the boards. My 420B power supply board has incorrect diode markings.

 

[MCM_Fan]

Looks like a fun project. I rebuilt a Son last summer.

Watch out for incorrect silkscreen markings on the boards. My 420B power supply board has incorrect diode markings.

Also watch the cap polarity markings. My 420 board had the polarity marked incorrectly for C002.

 

[FootFungus]

Which outputs do you have? The Toshiba, Sanken or Fairchild?

 

[Mayank]

OK ... now that I filed my taxes (11 hours earlier than last year), back to the SoA rebuild.

Looks like a fun project. I rebuilt a Son last summer.

Watch out for incorrect silkscreen markings on the boards. My 420B power supply board has incorrect diode markings.

Thanks Willy. The PSU PCB on mine is 420C. I read through your very informative thread on recapping and biasing your SoA. I hope to get that far with mine soon if everything check out

Also watch the cap polarity markings. My 420 board had the polarity marked incorrectly for C002.

Thanks for the heads up - I making notes on the orientation of all the capacitors prior to desoldering.

Rgds
Mayank

 

[Mayank]

All disassembled - except for the VU meter board which I'll look at once the rest check out.

 

[Mayank]

The main KBH 2502 diode bridge has a dead short between two of its prongs; I'm temporarily replacing it with KBH 2506 which I have from an Adcom GFA-555; one of the 4 1N4003 diodes in the 51V supply is open - I'm replacing it with a 1N4004 from my parts bin in its place to test the transformer - while I wait for Mouser to ship the correct replacements.

Which outputs do you have? The Toshiba, Sanken or Fairchild?

It has the GAS re-branded Sanken devices GAS 118C and GAS 119C. I've already found 2 of the 4 are short on the left channel. Probably will find similar situation on the right channels as well.

Rgds
Mayank

 

[Mayank]

Question for willyrover and MCM_Fan:

When you rebuilt your units, did you find they were factory "modified" as per handwritten edits on the above schematic?

My SoA appears to be the unmodified version.

Also, does anyone have a better schematic?

Thanks
Mayank

 

[Mayank]

Two of the 4 GAS transistors in the Right Channel had been replaced earlier with "equivalents"

Q201 ECG 280 no short
Q202 GAS 118C short
Q203 SK 3359 short
Q204 GAS 119C no short

I'll be replacing all by Onsemi or Toshibas

Rgds
Mayank