SWR 550x Repair Nightmare

[bnwbass]

Hello all,

Well maybe someone can chime in here based on repair experience with SWR 550x head unit because I've invested way to much time on this thing. Without making this a huge post detailing every step I took in an attempt to repair this unit I'll just bullet point my actions:

• Unit came in dead and found internal axial lead 10A fuse blown. Swr put these in series with the 8AT main fuse.
• Performed though visual and check all bias resitors,diodes,Bridge rectifier,etc and found nothing abnormal.
• Pulled out all drivers 2sc3246 sanken devices and all were open
• Replace all 2sc3246 sanken parts.

• On power-up blows 8TA main fuse
• Isolated the power amp board by pulling J1 on the preamp board.
• Unit turns on and shows front panel leds, fan runs, with the power board diconnected.

The next morning as I plug in J1 to the pre PCB a huge arc occurs and burns the J1 molex connector. This must have been the potential stored up from C12,13,32,33. As a result all of the new 2sc3246 sanken device are gone (open) BCE. Needless to say this really pissed me off. Any good circuit design dissipates stored voltages after a short time preventing this from happening. This unit sat for over 12 hrs and still these caps are charged...what gives. Has anyone been through this with 550x? Is this thing ready for the pile..or should I spend more time. Open to suggestions?

Thx
Bw

[gbono]

Did you check the driver transistors - Q7/8? You need to build a light bulb limiter or get a Variac to use for power up/debug. Check all the transistors on the board.

[MarkusBass]

With +/-100V rails you have to be very careful (light bulb limiter is a must on first turn on). If you disconnect J1, there is no load for the capacitors in the power supply - SWR didn't foreseen that someone can do it :-).
I was fixing SWR Basic lately. It was seriously damaged - there was open fire inside, burnt PC boards, most transistors shorted. I turned the amp on even without a light bulb limiter - I had only 33 Ohms resistors in series with the power supply rails.
There is nothing special about this amp. You have to check all transistors, diodes and resistors. Usually, at least driver transistors fail (including resistors in the neighbourhood) and the bias circuit. You have to desolder the temperature sensing transistor to check it. Then you can also check the bias trimmer.

Mark

[g1]

It may not make much difference but keep in mind that the internal fuse is a backup in case someone replaces the mains fuse with a bigger one (or tinfoil).
Sometimes it will tip you off as far as how well the amp has been treated.
In this case it may have saved the amp from a lot more damage, especially if the culprit was tinfoil or a 20 or 30A auto fuse.