Reparing B&K ST-140 Mosfet Power Amp

I do not know why I need BK ST-140 in my collection. I used to have BK multi channel Mosfet amp and I loved it. I kind like the way BK was built. BK's I had did not show much of cost cutting measures. It was sk  mosfet  amp.
First impression as I opened BK ST-140 was bit disappointment. It was more of empty box then full built amp. I thought about just using enclosure to house my home made amps. At the end I decide repair it and see if I like the way it was designed.

When I get back from golf on Sunday, I will find out what is fault in this amp.

Update: 4/30/2013

By looking at the amp board I found 4amp fuse on + side of rails on both amp board was newer than - side rails. Some one replaced fuse on +side of rails. This can be sign  of  shorted both side output mosfet 2sk135 or fault of both rail caps. It is highly unlikely to have both side rail caps fail at the same time or blow both output fets same time. Both side all output fets were checked good (hew!!!).

After disconnecting power supply from amp boards and unscrew power board caps and apply 15v to charge then recheck the voltage in few minutes to see if it is leaking voltage. Both big caps were holding 15V. So power caps are OK and output fets are OK. I am lucky on this amp.

Because of both + rail fuse are blown, I decide to re-cap both amp board first. Then I will be trouble shooting later. When you remove parts from old board, it is better to cut off all the leads of parts before de-solder them. Cutting will protect frail board from heat. It is better to put least amount of heat possible to PCB( printed circuit board).

I removed all electrolytic caps and found one cold joint. 8 electrolytic caps are 4x 100uf 100v for rails and 4x 47uf 16v (one for input one for feed back per board). These can be upgrade to better audio grade caps if I want to keep the amp. At this point I do not feel like I am keeping this amp. I will be replacing all the caps with Nichicon or Elna.

All 8 electrolytic caps were replaced and reapplied thermal grease and reseated all TO-3 fets correctly. But still kept power supply plugs to the amp board  disconnected. Hooked up the amp with Variac and start with low voltage and measure rail voltages and shut off the amp and remeasure rail voltages in few minutes to see if it is holding ( see if leaking). Once it is OK then drain power caps by shorting its terminals with light bulbs ( 100w 120v) to prevent shock. Then plug power supply plugs into amp boards and turn the Variac to 20 V AC and turn on the amp. Be watchful to see if any smokes sounds or smells.  If none , it's time to measure DC off set. Uncontrollable DC offset is a sign of malfunction on amps. Do Not increase Variac Voltage ( I used use 15 watt bulb serial to AC plug of amp. This works fine except transformerless (Carver) amps). 

DC Off Sets:

Hook up the volt meter across speaker terminals.

We want to get close to zero volt reading here. If it is not, adjust first trip pot ( one without resistor across) while watching meter. If voltage dose not get close to zero, we need to inspect based on voltage reading on the meter. Which ever side voltage is stuck on, you need to inspect same side of circuitry to find shorted faulty parts or opened opposite side parts.

Once DC off set is confirmed, turn off the amp and feel the outputs and see if it created too much heat. It is warm to touch at AC 20v , too much current is flowing through output fets.

Bias Adjustments:

Bias adjustment of B&K ST140 is tricky because of there is no emitter resistors on the amp. By looking at the data sheet of J50, I decide to set gate voltage to 0.8v. My unit had gate voltage measured to 1.8v and unit was really hot to touch in few minutes. I think some one try to make this amp in class A mode by setting up the bias higher. This was the reason both + rail fuses are blown on this amp.  Mosfet amps are typically set at little higher on bias current measure than bipolar transistor amps. Because of mosfets are less prone to thermal runner away.  I adjusted second trim pot with resistor across to get gate voltage of SJ50 to 0.8V.

Saw tooth and square wave function was good on scope and 200w bulb connected to speaker terminal was glowing bright on full swing of both wave forms.

I will be hooking up the amp tonight to do some listening.