Hi!
I managed to get hold of great looking pioneer sa-7800 but in not working condition. The output transistors as well as Q24 were blown on one side. On the other side R35 and R59 were shorted and someone replaced R33 and R34 (560ohm each) with 9Mohm ones so I changed all of those. I fired up the amp with, as usual, a 40W light bulb in between. the little flash and then a very light glow as should be, after a couple of seconds the relay clicked. The trouble began when I tried to set the bias, 53mV by adjusting VR5 and 6 and then up to 66mV by turning VR7 and 8. As soon as I started turning VR5 the bulb started glowing way brighter and by the time I got to VR6 (on the other channel) the relay opened up until I got both pots back down. At the same time the vacuum power meter dimmed to almost nothing and a slight noise appeared around the radiator. Even at the bias set at 66mV I have only a few mV of DC before the relay. With the Bias at 0V the amp sounds great and even with one channel up to 66mV (left or right no difference) there is no obvious problem with it.The fault seems to occur when both channels are set at 66mV. All the chemical capacitors are new.
I couldn't find any faulty components and am stuck, if any one has some idea as to what could be wrong I would love to hear (read) it.
I managed to get hold of great looking pioneer sa-7800 but in not working condition. The output transistors as well as Q24 were blown on one side. On the other side R35 and R59 were shorted and someone replaced R33 and R34 (560ohm each) with 9Mohm ones so I changed all of those. I fired up the amp with, as usual, a 40W light bulb in between. the little flash and then a very light glow as should be, after a couple of seconds the relay clicked. The trouble began when I tried to set the bias, 53mV by adjusting VR5 and 6 and then up to 66mV by turning VR7 and 8. As soon as I started turning VR5 the bulb started glowing way brighter and by the time I got to VR6 (on the other channel) the relay opened up until I got both pots back down. At the same time the vacuum power meter dimmed to almost nothing and a slight noise appeared around the radiator. Even at the bias set at 66mV I have only a few mV of DC before the relay. With the Bias at 0V the amp sounds great and even with one channel up to 66mV (left or right no difference) there is no obvious problem with it.The fault seems to occur when both channels are set at 66mV. All the chemical capacitors are new.
I couldn't find any faulty components and am stuck, if any one has some idea as to what could be wrong I would love to hear (read) it.
40W is far too low for a bulb tester when you are working with consumer amplifiers in the area 60-100W in class AB 60-100W will be a wise choice Expect more when the amplifier is higher power
You need to recap and also you need to verify that secondary power supply feeding the first stages of the amplifier provides proper and clean voltage .
At this age there is more things to be done while few of them will not be cured with deoxit Relay is to be replaced Phono stage transistors can be also noisy ...
Kind regards
Sakis
You need to recap and also you need to verify that secondary power supply feeding the first stages of the amplifier provides proper and clean voltage .
At this age there is more things to be done while few of them will not be cured with deoxit Relay is to be replaced Phono stage transistors can be also noisy ...
Kind regards
Sakis
The SA-7800 has been working great until now but I started getting a hum almost right after turning on the amp today and it's getting louder and louder so I opened it up and started checking things, this is what I got when connecting to the left and right outputs and the last picture is of the power rails, both have the same problem. This is measured with the probe on X1 and at 10mV/div on the scope, 10 seconds after turning on the amp so it gets worse.
So far I've checked all the resistors in the power supply and the capacitor had been replaced before. Any ideas on what could cause this ?
So far I've checked all the resistors in the power supply and the capacitor had been replaced before. Any ideas on what could cause this ?
Seems like you nailed it Sakis, 1 tested ok but the second has 314ohm between gate and drain one way and 700ohm the other way, also in diode mode I have 1.74v between gate and drain and gate and source. I've read a lot of bad stuff about these, is there anything to replace it with or should I just go with 2 new ones ?
Okay so when I disconnect the collectors of both power transistors on the left channel the relay closes and the right channel works perfectly. I took some measurements and I found that the voltages on the bases of the drivers are very unstable (it should be +/-1.2V but on one I get a variation between 0 and -1.2v and on the other between +1.3 and +5.1v. The voltages on the collectors of q9 and q7 are equally unstable (between -2 and -2.8v where it should be -2.5v) as well as the collector of q5 (between +3.9v and +7.8v where it should be +3.8v) and q11, 13, 15 and 17. The right channel has all the voltages as stated in the manual and they are stable.
At first I thought q5 was bad but I replaced it by an equivalent I happened to have and nothing changed...
At first I thought q5 was bad but I replaced it by an equivalent I happened to have and nothing changed...