I am stumped. This amp is challenging me. I have recapped and all new outputs and drivers. Power supply was recapped also. I adjusted offset and idle no problems. I then load tested both channels separately. 8 and 4 0hms. Everything was perfect distortion was lower then specs and output was higher. I ran it for a good hour. I then decided to try to bridge it and thats when all the smoke came. I never even got to load it. I simply switched the switch to bridge and turned it on and smoke from a resistor and a power supply transistor popped. Main fuse blew. Any help would be appreciated.
No it was off. I am baffled. Taking the boards out now and checking things. Ugh..... All my components were purchased through Mouser and Digikey so they shouldn't be counterfeits.
So you switch to bridged, did the protection stay red or click the relays -(which I assume you changed) and turn off?
There was no load?
Do you know which components provided the magic smoke yet?
There was no load?
Do you know which components provided the magic smoke yet?
R252 is connected to Jp104, which is the soft clipping and Q401connects to TS301 and TS302 which are the thermal relays, which suggests that when you switched the bridge mode, there could be a short to earth on the input board involving only the right channel (R252).
Check the thermal relays for continuity and shorts
Can you bring the amp up on a dim bulb and variac?
Check the thermal relays for continuity and shorts
Can you bring the amp up on a dim bulb and variac?
Amp boards are ok it seems like issues with the power supply. 2 other fried transistors on power supply board I have -18 and +18 on the supply wires for the amp boards. I replaced all 3 transistors and now when I switch amp on the relays engage immediately then when switched off there is a delay with them releasing.
There should be a delay when switching on, the protection LED should remain on for a few seconds and then the relays click on. On power off the relays should disengage immediately.
Check the voltages on the protection IC and all the small caps for the protection, one of them determines the timing of the protection circuit.
also check R413
Check the voltages on the protection IC and all the small caps for the protection, one of them determines the timing of the protection circuit.
also check R413
Wow this thing is a mess. Resistor 413 all caps and transistors now are ok. I think it is the protection ic. I have a parts unit I can use that ic. On these circut boards the solder pads are terrible.
Yes the solder pads can be tricky, even with fancy Hakko desolder gear.
These guys had replacement protection IC's- https://www.ceitron.com/- which were genuine.
How did the voltages on the IC match up?
These guys had replacement protection IC's- https://www.ceitron.com/- which were genuine.
How did the voltages on the IC match up?
So I have officially given up on this amplifier. I measured the IC voltages and had 34v 64v on most of the pins. Then lost my -18 volts. This was a new 7918 regulator too. So what I did was remove the board and all components. I need repair the board solder pads and some of the tracks. My parts amplifier is an early serial number with the bridge rectifier in a different place so I don't thing it's compatible. I love these amps and wanted to restore this one.