Restoring Mark Levinson No. 331 Power Amp from Water Damage

Water damaged high voltage electronic equipments are very hard to restore. As soon as water gets into high voltage electric circuit, electrolysis happens. It is like  battery action happening in your equipment. Depends on polarity of electricity c osit of metals or metal oxides.  Loosing exposed printed circuit board trace is hard to repair because it requires extremely careful examination of any exposed current conducting parts or PCB traces. If the equipment is still worth the trouble, you can do what I am going to about this Mark Levinson No 331.

This is my first Mark Levinson amp that I worked on. For 30 years of amp restoration and repair and build, I have not worked on any Mark Levinson products. When I saw ML 331 on the Craigslist sellers truck, I knew I had to have it no matter what condition the amp was in. My heart was beating like a college  kid meeting the first date. I did not even bother to ask to open the cover of the amp to see what condition it was in.

It took me few hours to find out and disassemble the amp. It is best to take some pictures of the amp before fully disassemble for the in case your memory failed to serve you from remembering exact part locations and order of assembly.

Amp had extensive water damage inside as if it was sitting in the water for at least few days while electricity is flowing. I had more than few bad shaped amp for repair, this amp was new record low for its condition. I had white powders every where and few parts on the control board was missing  ( electrolysis corroded aluminum, zinc and copper if the part has live electric current flowing.  Most damage was done on control board because it has always on "supervisory power".

1) Disassemble Mark Levinson No331 down to last screws and bolts...

    Needed tools: SAE type (  type) Allen Hex Key sets and screw drivers and some hand tools.
     ply wood panel to cover the floors from getting marred by flipping amp around. 2x4 wood pieces.

    First open up the top case by loosening screws( 2 screws with black plastic washers) on the back.
    Remove small screws on the heat sink cover that covers output transistors on the both sides (4 pcs)
    Remove all screws on the top side of heat sinks on both channel to remove top rails that hold cover
    Remove screws on balanced input connectors ( mic connectors) to get ready for desoldering the part form voltage gain boards ( a board at the top with RCA plug).
    Remove voltage gain board. Be careful with 40 pin connector, do not force.
    Remove 2 top screws hat hold amp face plate ( thumb screws ) from inside.
    Flip the amp to upside down and remove bottom cover.
    Remove 2 hex screws  that hold bottom of the face plate to remove face plate.
    Remove 2 outer screws on backside on right sides of amp which hold heat sink/ current gain board
    Remove 3  Black screws on the right front side that hold heat sink
    Unscrew a copper screw  on current gain board that connects to speaker /buss connector to board
    Unscrew 2 screws on big filter caps that connect to current gain board while supporting heat sink/ current gain board from dropping ( Be careful its Heavy)
    Pull heat sink away to remove current gain/ heatsink module. ( do not force it!!- there is 40 pin connector that connects current gain/ heat sink to control board.
    Rest of disassembling is WYSWYG.
    If you are doing recapping  You need to remove buss bars and control board to replace all the caps. there are only few lytic caps on the amp boards ( current and voltage gain) but more than few on VSMB (control board).
   If you are doing recapping, you need to have heat sink compound ready to remove current gain board from  heat sink. It is better to mark the color of wires on the board before unscrew them. Also it is wise to mark the polarity of filter caps on the buss bar before unscrew them because recapping might take longer than you anticipate due to tight space limitations. When you remove parts from board to replace with new, it is better to cut off the leads to remove them then remove leads to prevent heat damage to cooper trace on PCB. Parts are replaceable but PCBs are not.  If it is IC's it is better to cut them out by clipping the pins before de-solder.

For replacing electrolytic capacitors:
   Main filter caps (four  47000uf 75V) can be found in Mouser and some might be back ordered. 1900uf 150v can be replaced with 2200uf 160v (Back ordered on Mouser). They should cost you total of 400 to 500 dollars for all the caps you need to re-cap ML331. Once you replace first is 1900uf 150v caps because these high voltage relatively small caps are known to fail first and will burn VSMB board and diodes. Replacing all the caps should increase resale value of your amp to at least 1000  to 1500 dollar more.


2) Cleaning & Rinsing and Repeat again and again...
    Over passed years I found Simple Green spray and solution has been my choice of cleaning agent for my all restoration projects. It is mild and biodegradable as long as rinse thoroughly.
Warm water with diluted Simple Green solution was used to soak the board to loosen up deposits. At this point I do not worry about trimpots and relays which water can leak through. Because all need to be replaced anyway. I used Tarnex to remove some oxidation on copper screws and zinc stains.
I even used my dental water pic to clean unreachable areas.

Cleaning revealed extent of water damages more. Electrolysis happened most on VSMB board and lot of copper trace has been etched away. I used silver coated copper wire of 32GA as soldering buss wire to repair PCB traces. There were 4 missing parts on VSMB board. Tow are resistors that connect to main relay circuit and 2 are for energizing main relay circuit inputs near 5 volt regulator circuit.

After all the missing parts were replaced and visible all copper traces were repaired, amp was put together to test if it turns on. Amp failed to turn on I new it is much bigger problem than I hoped for.
I Tested and found 2 energizing circuits ( one for to turn on main relays (soft start and on) and other for small signal EA12-NJ relays ( all 7 , 3 per each board and 1 for control board) ). Pin 18 and Pin 17 of 18cv8 20 dip IC has key to the both circuits. If you supply 5 volt to those  2 pins of 18cv8, both circuit will energize.

18cv8 supplies 5 volts to all control circuits. Some 5 volt circuits are always on if amp is plugged in. All electrolysis by live 5 volt circuit try was fed by 18cv8 chip. This makes 18cv8 chip prime suspect of failure but most IC's should be OK because they do not feed any other circuits but 18cv8 input pins. All other IC on the VSMB board is replaceable without too much cost or hassle. I decide to replace all other logic chips  and put them on the sockets for future convenience. With all new chips but 18cv8, amp still fail to turn on. My most fear of failed 18cv8 has just realized. I need to either reverse engineer 18cv8 chip or get working copy of program logic files to burn new 18cv8 on programmer.

I decide to remove 18cv8 chip  from board without damaging it. It I destroy it then I can not get the replacement for sure. Mark Levinson refused my schematic request by keep reciting " send your amp to our repair centers"  I am sure repair center will cost me to almost buy used ML331 to repair.  I put 20 pin dip socket  in the place of 18cv8. I shorted pin 18 and pin 17 to manually turn the amp on. Right channel produced beautiful sound but left channel was dead as mouse. Accidentally I left it on over night. That was unscheduled overnight burn test for the amp and amp was fine next morning.
This tells me amp is not turning on because of control board not because fault in amplifier section.

Lack of schematic really makes things many times harder. I start to trace controller circuit manually. I made some progress. At the end I conclude that 18cv8 chip is probably faulty.  I purchased Topmax Universal eeprom programmer to handle 18cv8 chips. It is hard to find new EEPROM programmer that handles 18cv8 chips because all 18cv8 varieties are discontinued. Only some from China or old used ones from ebay are available on the market. Once I got my Topmax programmer, I tested pulled out 18cv8 chip. Chip read was OK with no error. I made copy of the chip and I received chip read from a guy in China. I tried both copies of chips on the board and amp still fail to turn on.  I decide to take some break from working on the ML 331 because I feel like I am burned out of long process.

My battery to restore this amp has been completely drained.  I packed up ML331 with custom programed chip to just turn on and off without feed backs from amp to just enjoy the amp for a while until my battery  to get charged up again for next battle.

Here is PLD file code I used on Wincupl ( free ware from Atmel):

Name: ATF16V8;
Device: ATF16V8;

Pin 1 = clk;
pin 2 = d0;

pin 11 = !oe;

pin 19=q0;
pin 18=q1;
pin 17=q2;

q0=!d0;
q1.d=!d0&!q1;  /* d  flop latch  while pin 11 is low */
q2.d=!d0&!q2;  /* d  flop latch  while pin 11 is low */

( There will be up date soon)