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Dougallh · 15th Oct 2019, 9:38 pm

I have a power amp fault on my 1976 Goodmans 150 receiver. This uses monoblock 110 + 110 watts rms power amps, one of which failed. One of the MJ802 (100v 30A) output transistors blew, C-E short. This put +43V dc on the output.

On replacement, I cannot get any quiescent current to flow; the last 4 transistors: BFT37/MJ802, BFT34/MJ4502 are OFF. The previous drivers BC449, BC450 are ON - just, the voltage across the bias voltage transistor feeding the drivers is 1.1V (should be at least 1.2V) - No amount of adjustment will increase the bias current.

The voltage across the BFT34/37 bases - emitters is just 0.2V (fed from collectors of VT808,810)

The top & bottom constant current source voltages all measure correctly.

I have replaced all transistors - NO JOY
I have removed obvious suspects C809, 824 etc - NO JOY
I have disabled protection ccts, VT815,816, diodes WD01,02,03,04 - NO JOY
I have disabled VT801, overcurrent protection to bias current driver VT802 - NO JOY
All resistors are correct & connected (inc the 0.33 ohm emitter resistors)
I cannot see any shorts or burnt out tracks
The +43V & -43V supplies are correct, no noise evident using a scope

I am missing something ?

I have uploaded a schematic

Thanks

15th Oct 2019, 9:38 pm	#1
Dougallh Triode Join Date: Apr 2012 Location: Manchester, UK. Posts: 29	Goodmans Model 150 Amp Fault I have a power amp fault on my 1976 Goodmans 150 receiver. This uses monoblock 110 + 110 watts rms power amps, one of which failed. One of the MJ802 (100v 30A) output transistors blew, C-E short. This put +43V dc on the output. On replacement, I cannot get any quiescent current to flow; the last 4 transistors: BFT37/MJ802, BFT34/MJ4502 are OFF. The previous drivers BC449, BC450 are ON - just, the voltage across the bias voltage transistor feeding the drivers is 1.1V (should be at least 1.2V) - No amount of adjustment will increase the bias current. The voltage across the BFT34/37 bases - emitters is just 0.2V (fed from collectors of VT808,810) The top & bottom constant current source voltages all measure correctly. I have replaced all transistors - NO JOY I have removed obvious suspects C809, 824 etc - NO JOY I have disabled protection ccts, VT815,816, diodes WD01,02,03,04 - NO JOY I have disabled VT801, overcurrent protection to bias current driver VT802 - NO JOY All resistors are correct & connected (inc the 0.33 ohm emitter resistors) I cannot see any shorts or burnt out tracks The +43V & -43V supplies are correct, no noise evident using a scope I am missing something ? I have uploaded a schematic Thanks Attached Thumbnails Last edited by Dougallh; 15th Oct 2019 at 9:59 pm.