Specifically for the amplifiers that use 4of NEC small transistors is not just a fix for me is the best of the best solution .I could recon only that .
But for bigger amplifiers i cannot tell depends on the type of outputs
I've seen "repairs" that would make you cry. It amazes me what some places get away with.
Don't feel bad, Depaj, I just realized that the Pioneer SA-7800 doesn't even use TO-3s.
I have one sitting out in my shop that needs repaired. Not when I will get to it though. However, I don't think that I would replace four ouput transistors with two on that amp. It is a non-switching amp, and tweeking the design could be trouble.
Finding outputs for this amplifier will also be trouble if gone ....
I agree with that ...got my self a collection of pictures from that
I have a collection of similar factory made constructions like you described
That can also make you cryI had a Pioneer SX-3900, which as a very similar, but more powerful amp section, come through a couple of years ago with one channel out. I wish I would have taken notes as to what I did to fix it.
I know for one thing, I shitcanned the plug in leads, and soldered the wires directly to the transistor leads. I also added heatsinks to the drivers.
If memory serves me, I lucked out and had the correct outputs and drivers in my parts inventory. OP may have to study some datasheets to find a TO-247 or similar that will work and drill the heatsink to fit.
I know for one thing, I shitcanned the plug in leads, and soldered the wires directly to the transistor leads. I also added heatsinks to the drivers.
If memory serves me, I lucked out and had the correct outputs and drivers in my parts inventory. OP may have to study some datasheets to find a TO-247 or similar that will work and drill the heatsink to fit.
Thanks for the tip, for the outputs I salvaged some 2sa1095/2sc2565 which are a pair pioneer actually indicates as the best replacement and used on some of the latest sa-7800 so I'm good on the outputs. It has only 2 per channel so no need to change anything. I was looking for those drivers though and mouser is out of stock for the 2sc5171 so that's great
Talking about bad repairs : I bought this Marantz 1120 from a shop that said it had been restored to new, this is what I found, power amp board was not original and was just a mix of different transistors, no pairs just random transistors close to original specs, tracks were gone on other boards, to-3 outputs were two different pairs from 4 different brands, anyway I'm trying to get it back to original, still looking for the power amp board...
I have no idea, I went back to ask them when I saw this and they couldn't tell (or didn't want to, they seemed surprised that I even opened it). Anyway, got the power supply back to original and the preamp is working great now that I removed all the junk so I just need to find that board and than get the original parts (I already have the original outputs : 4 sj2519).
But let's not get on a side track, I will order the drivers and let you know how it goes with the SA-7800
But let's not get on a side track, I will order the drivers and let you know how it goes with the SA-7800
Bad news : when I first changed the drivers I took some that I had taken out of an other pioneer amp and as I said the sa-7800 worked fine, I decided though to replace them with the drivers you guys suggested (2sa1930/2sc5171) because the ones I put in had been in an amp with output problems so I didn't trust them too much. Now I'm exactly back to square one, got the exact same hum coming through the headphones and the exact same signal on both outputs :
and power supply rails :
I could just go and test the 2sk34 again but I feel there's no point if I don't know why it keeps going in circles.
and power supply rails :
I could just go and test the 2sk34 again but I feel there's no point if I don't know why it keeps going in circles.
just put IRFP240 , 9240 into sx 780 with minor changes. works perfectly
I am thinking of upgrading my sx-3700 as well
That Marantz is going to require alot of work. you could just builb a Honey Badger amp ,detailed in another thread, and put it in the case.
It looks like a ground may be bad.
check the offset, the input to the amp , the VAS stage voltages etc.
it could be a bad connection or bad transistor in the input stage
I am thinking of upgrading my sx-3700 as well
That Marantz is going to require alot of work. you could just builb a Honey Badger amp ,detailed in another thread, and put it in the case.
It looks like a ground may be bad.
check the offset, the input to the amp , the VAS stage voltages etc.
it could be a bad connection or bad transistor in the input stage
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Thanks for the tip but I don't think the drivers are the problem (not any more at least). When I just take random reading with the oscilloscope I got the first image everywhere on the positive side and the second on the negative side except for everything that comes from the power supply. Bias and offset are good, I changed the dual transistors with matched ksa992 (for the 2sa979) and ksa1845 (for the 2sc2291) which got offset down by 10mv, and did all the solder joints. I will check the voltages tomorrow. What is strange is that this is exactly the same symptoms as in the beginning
Yes it definitely is. I would really love to get it back to original, but I might keep this idea as a last resort.
Yes it definitely is. I would really love to get it back to original, but I might keep this idea as a last resort.
I checked all the voltages in the main amp stage and they are all good. A little correction the first reading on the oscilloscope is on one channel and the second on the other channel not negative and positive side of the same channel. I had the good idea at some point to disconnect the power to the equalizer stage and the problem was gone. As the signal is about 55hz I first checked all the ground connections but they're all good and the stage itself seems to work fine and the voltages are good on the transistors.
Well, I tested the amp without the dim bulb and... hum is completely gone, all voltages are perfect and the amp sounds amazing. I Actually have another SA-7800 here that works perfectly and I turned it on with the dim bulb and guess what, got exactly the same hum and same reading on the oscilloscope. I got a 100W bulb that only lights very slightly with the amp idling, strange this has so much impact.
Anyway here are some pics of the finished amp, love it !!!
Anyway here are some pics of the finished amp, love it !!!
