It's a Darlington - they are two transistors connected and usually have base to emitter resistance.
Hi dayking79
The cascode transitors Q667 Q669 I refered to, are they ok too ??? They referenced to the output, if the output swang to a rail because of a short that blew some output devices, these will go along and take maybe Q664 and Q614 along too.
The cascode transitors Q667 Q669 I refered to, are they ok too ??? They referenced to the output, if the output swang to a rail because of a short that blew some output devices, these will go along and take maybe Q664 and Q614 along too.
Hi,
yes, i have checked Q667, Q669 - they are the ones connected to the smaller heatsink. I have now checked all transistors on the main pcb and all seems ok but still now power at all.
There are 2 transistors on a small seperate pcb (x-1271D05 on service manual but in my amp the board has different name - x-1271B05C), the transistors are Q901 and Q902, do you think they would have blown? They are quite difficult to get to so i thought i would ask before i de-solder them.
If there is no power at all, then do you think it is likely to be on a pcb that is supplied by the main power input?
yes, i have checked Q667, Q669 - they are the ones connected to the smaller heatsink. I have now checked all transistors on the main pcb and all seems ok but still now power at all.
There are 2 transistors on a small seperate pcb (x-1271D05 on service manual but in my amp the board has different name - x-1271B05C), the transistors are Q901 and Q902, do you think they would have blown? They are quite difficult to get to so i thought i would ask before i de-solder them.
If there is no power at all, then do you think it is likely to be on a pcb that is supplied by the main power input?
Q901/Q902 - part of the stabilized power supply for operational amplifiers AD711. They may have be difficult to obtain, but there are a lot of other transistors to fit in their place. No problem at all. They should have some (±)35V on collector and (±)15.5V on emitter if everything is OK.
Ok something must have gone in protection circuit. Have you followed quattor suggestion about the other fuses. Its ok to check Q901 and Q901 altough I only see these to power the opamps for balanced operation but it could cause problems if one were down, and cause opamp to inject large dc into input of the amp triggering the protection. No need to take them out just check if they shorted or else that their is +15 -15 volts on their emmiters referenced to ground while you at it check for 35v at theit collectors. Also check that on neither r509 or r510 there be a large dc value. If you remove the two 10 amp fuses can you hear the relay cliking on ??
Of course. There should be one or two power resistors, thermistor and thermostat all connected in series. Furthermore, they should be bypassed by relay RY901. Depending on the version, there may be a dedicated soft-start circuit, driving that RY901 (small separate PCB with two small transistors on it - T11 and T12).
The main problem is that in order to get to the 2 smaller pcb's, i have to take the 2 big pcb's(L&R) out (disconnect). Surely i will damage something if i were to power the unit while both pcb's are disconnected?
I checked the 2 transistors (Q901/902) while in circuit and although there doesn't seem to be a short, there is a short beep on my DMM when i connect base to one of the other legs but i think this is just because it is in circuit.
The other thing is that because my skills are very limited electronics, i do not feel confident in measuring voltage when powered up. Unless you were to talk me through it.
I am very appreciative of all your help and i just need to get this amp working for myself as sending it somewhere will be very difficult and pricey (40kg's) with no original packaging.
I will re-read what you have all put and try to do as much as i can and then get back to you.
I checked the 2 transistors (Q901/902) while in circuit and although there doesn't seem to be a short, there is a short beep on my DMM when i connect base to one of the other legs but i think this is just because it is in circuit.
The other thing is that because my skills are very limited electronics, i do not feel confident in measuring voltage when powered up. Unless you were to talk me through it.
I am very appreciative of all your help and i just need to get this amp working for myself as sending it somewhere will be very difficult and pricey (40kg's) with no original packaging.
I will re-read what you have all put and try to do as much as i can and then get back to you.
There are 2 small seperate pcb's. One with a plastic 'CAUTION' panel over it (which i have removed) - this is supplied directly by mains power from kettle lead. It has a relay, 3 seperate white blocks taped together which i believe are the resistors and thermostat, a thermistor, a tiny diode and a blue circular capacitor which i have read is the spark killer.
I will try to send some pic's.
I will try to send some pic's.
This one is of the other small pcb which is located again at the bottom of the amp but on the opposite side.
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This is one of the main pcb's that had the blown transistors on it which have now been replaced.
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This is the other channel pcb mounted on it's heatsink. Notice on these last 2 photo's the 2 fuses on each pcb - these are the only fuses in my amp unlike what the service manual states.
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So what do you rekon guy's?
Like Quattor mentioned about the power resistor's, do you think it could be these? (see pic's)
Thanks.
Like Quattor mentioned about the power resistor's, do you think it could be these? (see pic's)
Thanks.
Lets take this step by step, i hope you comfortable measuring high voltage, just be careful not to physically touch the contacts or short them out. We need to know if their voltage going to the amps and only way to be sure is to measure voltage accross say like those big capacitors. Then we need to test for voltage output from that small pcb, 2nd foto you posted. There is no way around this. We need to know if there is power going to the various circuits.
Dont worry about damage whithout the amp pcbs, this wont happen, and its actually better so that nothing happens to the amps again till we figure out what else is wrong.
Thanks homemodder,
so in the present state of this amp (last photo), i can power the unit up and then measure voltage on the big caps? Then on the small pcb?
I will do this tomorrow and let you know the outcome.
Thanks for sticking with me.
so in the present state of this amp (last photo), i can power the unit up and then measure voltage on the big caps? Then on the small pcb?
I will do this tomorrow and let you know the outcome.
Thanks for sticking with me.
Ok, i have done some voltage tests and there is no power going to the transformers. It must be something that has gone on the first small pcb as there is definitely power getting to it but not coming from it. Is there something that won't let the relay click on?
What do you reckon? Change all components on this pcb? What normally will prevent the relay from kicking in? There are only a few components on this pcb, a diode, a thermistor, a thermostat, two resistors, a capacitor (spark killer) and then the relay.
I am so looking forward to getting my amp back! I can see the light at the end of the tunnel!
I know i shouldn't count my chicken's before they hatch but i am without an amp and can't wait to get some music back in my life.
Please help someone - homemodder?
I am so looking forward to getting my amp back! I can see the light at the end of the tunnel!
I know i shouldn't count my chicken's before they hatch but i am without an amp and can't wait to get some music back in my life.
Please help someone - homemodder?
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