Hi
This amp cannot start up. When power switch is pressed, the red light comes on, then the red light goes off and the blue light comes on and this keeps repeating continuosly at each second. Sound comes on during the blue light and cuts off during the red light. One thing I realise is if I remove the NE555 ic, then the blue light comes on and stays. I am unable to get the schematic, but the attached schematic looks quite similar. I guess the NE555 provides a delay start and protect. Any advice.
Thanks
This amp cannot start up. When power switch is pressed, the red light comes on, then the red light goes off and the blue light comes on and this keeps repeating continuosly at each second. Sound comes on during the blue light and cuts off during the red light. One thing I realise is if I remove the NE555 ic, then the blue light comes on and stays. I am unable to get the schematic, but the attached schematic looks quite similar. I guess the NE555 provides a delay start and protect. Any advice.
Thanks
If I understand correctly, you remove the 555 and the power amp remains active with no evident smoking parts--- right?. If so, this sounds ideal for active troubleshooting of the problem.
There are several schematics in your attachment that appear to be very similar. I'm gong to use the first schematic unless you advise me otherwise.
I surmise, the amp was cycling into fault, bias disable, delay and repeat? With power off, I suggest looking for shorted transistors in circuit.
If nothing is found, I'd begin powered troubleshooting. Use your ohmmeter to ensure R21 is set for maximum resistance; this will adjust the Vbe multiplier/bias current for minimum current which is safest option. Apply power and, assuming no smoke, measure DC voltages to hunt for the fault:
Power supply rails (+/- 51V)
Amp output (eg. at L1)
Base Q16
Base Q22
Emitters Q22 and Q21
Emitters Q23 and Q24
Base Q24
Base Q18
All voltages at pins of Q2.
Please measure voltages re ground and report negative voltages as negative numbers.
With any luck, this data will point us toward the problem.
Thanks,
Steve
There are several schematics in your attachment that appear to be very similar. I'm gong to use the first schematic unless you advise me otherwise.
I surmise, the amp was cycling into fault, bias disable, delay and repeat? With power off, I suggest looking for shorted transistors in circuit.
If nothing is found, I'd begin powered troubleshooting. Use your ohmmeter to ensure R21 is set for maximum resistance; this will adjust the Vbe multiplier/bias current for minimum current which is safest option. Apply power and, assuming no smoke, measure DC voltages to hunt for the fault:
Power supply rails (+/- 51V)
Amp output (eg. at L1)
Base Q16
Base Q22
Emitters Q22 and Q21
Emitters Q23 and Q24
Base Q24
Base Q18
All voltages at pins of Q2.
Please measure voltages re ground and report negative voltages as negative numbers.
With any luck, this data will point us toward the problem.
Thanks,
Steve
Hi
Except for the NE555, this circuit looks abit different, the transistor markings are not the same and there are more transistors in the circuit than the one in the schematic. The DC power rails read +/- approx 43volts. After removing the NE555, the amp powered up and the voltage at L1 is approx 16mV. I have in fact double checked all the transistors, diodes, resistors and caps in the circuit and they are all fine. When a friend brought this amp to me, he said when turning on the power, there was some strong burning smell, he immediately turned off the amp. When I got the amp, I opened up and realised that C27 (0.1uf) was charred and broken into pieces. I decideded to check the power transistors, and while force opening the heatsink, 3 power transistors pins broke. The rest of the power transistors and drive transistors were OK. I replaced the broken transistors, resoldered the board. I took out NE555, soldered a socket and mounted in a new IC. After reassembling, The amp turned on as normal and was running for approx 30mins. Then the sound started turning on and off for a while and then went into protect. I am puzzled as all the components are OK but what is causing the problem.
Thanks
Except for the NE555, this circuit looks abit different, the transistor markings are not the same and there are more transistors in the circuit than the one in the schematic. The DC power rails read +/- approx 43volts. After removing the NE555, the amp powered up and the voltage at L1 is approx 16mV. I have in fact double checked all the transistors, diodes, resistors and caps in the circuit and they are all fine. When a friend brought this amp to me, he said when turning on the power, there was some strong burning smell, he immediately turned off the amp. When I got the amp, I opened up and realised that C27 (0.1uf) was charred and broken into pieces. I decideded to check the power transistors, and while force opening the heatsink, 3 power transistors pins broke. The rest of the power transistors and drive transistors were OK. I replaced the broken transistors, resoldered the board. I took out NE555, soldered a socket and mounted in a new IC. After reassembling, The amp turned on as normal and was running for approx 30mins. Then the sound started turning on and off for a while and then went into protect. I am puzzled as all the components are OK but what is causing the problem.
Thanks
I'm mostly in the dark without a schematic.
Try to coax the amp into the misbehaving state so there's a problem to observe. Perhaps the fault detection circuit itself is the problem. Measure DC at the amp output; if it's still good, that raises suspicion of fault circuitry.
Try to coax the amp into the misbehaving state so there's a problem to observe. Perhaps the fault detection circuit itself is the problem. Measure DC at the amp output; if it's still good, that raises suspicion of fault circuitry.
In general, it´s a BAD idea removing protection devices (equivalent to using an oversized fuse, etc.)
Here you were lucky it is not a real "protection" circuit, it leaves amp powered all the time (fine until a real fault develops), it only mutes it. 🙄
Talk about a fake sense of security.
Must we guess?
Tell us which ones are actually there, how many, and if possible a picture showing them (or the board at least).
The schematic is for a GK "350" model, typically twice actual RMS power so 175W, fine for 4 power transistors.
So I guess your "500" model is actually 250W RMS and probably 6 power transistors.
Only guessing until confirmation, of course.
C27 is Zobel capacitor, that means your amp was strongly oscillating. Bad for its health.
Does the cabinet have any tweeters?
A shorted tweeter will leave you with a (crossover) capacitor straight across speaker out, no amp likes that and many violently complain (by oscillating > overheating > burning in that order)
😱
Sledgehammers and crowbars are not recommened in Electronics repairs 😉
Serious, first you must remove all transistor screws, then they are attached to heatsink only by grease stickiness.
Then the amp is basically working.
It is either oscillating (scope the output) or overbiased , measure voltage drop from top emitters (Q21-22) to bottom ones Q23-24 or whatever you have in your amp.
GK provides two handy test points for that: J1 and J2.
Please post whatever you find.
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Hi
I am sorry, there are lesser transistors on the board than compared to the schematic and the numbering is not as in the schematic. There are only 15 (T0-92) signal transistors on the whole of the power board. I have not taken any reading as I am not sure of the transistors concern as the numbering is different. As I said the voltage at L1 is approx 16 to 18mV. The bias pot is set at minimum (maximum resistance). I have removed all the 15 transistors out of board and tested them. I guess it is not due to some shorted component but due to some problem with the protection circuit itself or oscillation as suggested by you. Any idea what I should check for apart from the transistor readings.
Thanks
I am sorry, there are lesser transistors on the board than compared to the schematic and the numbering is not as in the schematic. There are only 15 (T0-92) signal transistors on the whole of the power board. I have not taken any reading as I am not sure of the transistors concern as the numbering is different. As I said the voltage at L1 is approx 16 to 18mV. The bias pot is set at minimum (maximum resistance). I have removed all the 15 transistors out of board and tested them. I guess it is not due to some shorted component but due to some problem with the protection circuit itself or oscillation as suggested by you. Any idea what I should check for apart from the transistor readings.
Thanks
Attachments
I'm sorry you don't have a schematic to work from--- makes the effort far more tedious. But the low output offset is very reassuring. Do you have a scope to look for oscillation? If the 0.1uF Zobel cap is good and there's no warmth in the Zobel resistor, perhaps oscillation isn't the culprit. Then a problem within the Fault circuit is a good guess.
With the new 555 in place, is the cycling like it was in your first post? Try to confirm that activity from the Fault signal triggers the shutdown sequence. Remove the 555 again if necessary to get a stable failed state. With luck, the Fault signal will be showing activity. Try to trace Fault line from the known location near the 555 back to the originating components. The circuit will probably resemble what's used in other versions.
Keep us posted. Good luck!
With the new 555 in place, is the cycling like it was in your first post? Try to confirm that activity from the Fault signal triggers the shutdown sequence. Remove the 555 again if necessary to get a stable failed state. With luck, the Fault signal will be showing activity. Try to trace Fault line from the known location near the 555 back to the originating components. The circuit will probably resemble what's used in other versions.
Keep us posted. Good luck!
Hi
You would not believe what happened today. I removed the heatsink, removed all the screws on the transistors. Looking at the board especially underneath the transistors, there was solder paste all over the place, as I resoldered the whole board earlier. As I dont have a heat gun, I placed the board under the hot sun for about an hour. After cleaning the board, I apply a thick layer of heat compound on all the transistor. Without putting back the screws, I placed the heatsink and screwed the heatsink on to the chasis. When I powered on the amp, the red LED came on. the fan started spinning for a while and stopped and about 2 secs later the blue LED came on. I plugged in a CD player and it remained on for an hour. I turned the amp on and off several times and it never failed; not even once the red LED came on. Before this the fan never came on. I am going to put back all the transistor screws and try again tomorrow. Will inform.
Thanks
You would not believe what happened today. I removed the heatsink, removed all the screws on the transistors. Looking at the board especially underneath the transistors, there was solder paste all over the place, as I resoldered the whole board earlier. As I dont have a heat gun, I placed the board under the hot sun for about an hour. After cleaning the board, I apply a thick layer of heat compound on all the transistor. Without putting back the screws, I placed the heatsink and screwed the heatsink on to the chasis. When I powered on the amp, the red LED came on. the fan started spinning for a while and stopped and about 2 secs later the blue LED came on. I plugged in a CD player and it remained on for an hour. I turned the amp on and off several times and it never failed; not even once the red LED came on. Before this the fan never came on. I am going to put back all the transistor screws and try again tomorrow. Will inform.
Thanks
Hi
It is not over yet. I fitted back the screws on the transistor and powered the amp on. It turned on without any problem. After about 30mins, It started turning on and off again. The heatsink was fairly hot, but the fan was not turning. I used my finger to push the blades and then it was running. I feel it has something to do with the thermal protection circuit. There is a terminal breaker and a heat sensor (LM35DT) under the heatsink. About 3hrs later I turned on the amp, and it started without any problem. I turned off the amp about 20mins later as I had to attend to an appointment. Will keep you informed .
Thanks
It is not over yet. I fitted back the screws on the transistor and powered the amp on. It turned on without any problem. After about 30mins, It started turning on and off again. The heatsink was fairly hot, but the fan was not turning. I used my finger to push the blades and then it was running. I feel it has something to do with the thermal protection circuit. There is a terminal breaker and a heat sensor (LM35DT) under the heatsink. About 3hrs later I turned on the amp, and it started without any problem. I turned off the amp about 20mins later as I had to attend to an appointment. Will keep you informed .
Thanks