If you can get NOS 2N6259’s, great. But if you get NTE “2N6259’s” what you really get is an NTE388, which is a re-badged MJ15024. There is no such thing as a modern 6259. The 15024 can replace it in all of these old amps, but do not mix. Entire parallel banks must be one type or the other, and preferably an entire channel the same. In any case I would feel safer with ON Semi 15024’s because NTEs can sometimes be questionable because they don’t always get them from the same source.
You may have more trouble finding the TO-66’s. There are two NTE types which can replace 2N5384, and Newark has them, but not the NTE38 forthe PNP. A common hack is to retrofit with TO-220’s. Do at your own risk. Last time I was at Skycraft (January this year) they actually had both 2N3585’s and 2N6241’s - a couple dozen of each at three bucks a pop. The vintage transistor section is tucked away in the southeast corner down on the bottom shelves, filed away in a bunch of little manila envelopes. Easy to overlook if you don’t know to go poking around there. No 2N6259’s this time, but I’ve gotten them there before.
From what I've read q9 (w/heatsink) is a bias transistor with a variable resistor. Driver board Channel 1 and channel 2 both measured the exact same.
This is how I was testing them, following instructions from the internet and cross referenced to make sure it was right.
DMM set to diode mode
Base to Emitter - good NPN 0.45v-0.9, good PNP "OL".
Base to Collector - good NPN 0.45v-0.9v, good PNP "OL".
Emitter to Base - good NPN "OL", good PNP 0.45v-0.9v.
Collector to Base - good NPN "OL", good PNP 0.45v-0.9v.
Collector - Emitter and Emitter - Collector should both read "OL" for NPN and PNP.
I also read that some of these small signals can read lower that 0.45 sometimes.
I'm fairly certain I've tested everything correctly. Here's a step by step of what I have done so far. If anything is incorrect please let me know.
First I removed all transistors from the rear heat sink, and actually only 2 of them did not pass the test. I'm going to replace all of the 120392 with MJ15024 anyways as it'll only cost around $110 to do it.
Next I removed all 0.5ohm resistors. Not all were bad some actually blew up, but about half of them or more left burnt marks on the board that need to be cleaned up good. The traces on the other side seems to be intact, thankfully. Some of the 0.5r were blown up, one broken in half. I will replace them ALL as it's another cheap thing to do.
Next I removed one pin and tested all 4.7ohm and 1ohm resistors on the output. All good.
Driver board, first initial transistor test for me is always C-E and E-C. If they show anything but OL I should consider it bad, but continued with the other tests as well.
Q3,4,5,6,7,8,9,11,12 did not pass the C-E, E-C or more test. When in doubt change it our right? or am I doing something wrong? I did not yet remove the base pin to do a further test.
It doesn't make it easier that somebody has already done lots of work to this. I see changed transistors, capacitors, resistors, power knob is foreign and soldered to ON. I don't believe the voltage was changing when switching from 40% to 100% but I'm not entirely sure if that's something I would of audibly noticed or not.
I have now opened up the control board the most I can but can't get the board completely off because of the mods done to the knobs. They won't come off and I don't want to break anything while trying. Looking at that Q7, 8 and 9 don't seem to be working right either.
All other transistors passed the test as they should have according to the dozens of videos and instructions.
I have the hfe and transistor socket tester on my DMM but I wasn't sure if it was even worth using for tests if they didn't pass those probe tests. I wouldn't really know much about the current gain and stuff like that either.
Thanks.
This is how I was testing them, following instructions from the internet and cross referenced to make sure it was right.
DMM set to diode mode
Base to Emitter - good NPN 0.45v-0.9, good PNP "OL".
Base to Collector - good NPN 0.45v-0.9v, good PNP "OL".
Emitter to Base - good NPN "OL", good PNP 0.45v-0.9v.
Collector to Base - good NPN "OL", good PNP 0.45v-0.9v.
Collector - Emitter and Emitter - Collector should both read "OL" for NPN and PNP.
I also read that some of these small signals can read lower that 0.45 sometimes.
I'm fairly certain I've tested everything correctly. Here's a step by step of what I have done so far. If anything is incorrect please let me know.
First I removed all transistors from the rear heat sink, and actually only 2 of them did not pass the test. I'm going to replace all of the 120392 with MJ15024 anyways as it'll only cost around $110 to do it.
Next I removed all 0.5ohm resistors. Not all were bad some actually blew up, but about half of them or more left burnt marks on the board that need to be cleaned up good. The traces on the other side seems to be intact, thankfully. Some of the 0.5r were blown up, one broken in half. I will replace them ALL as it's another cheap thing to do.
Next I removed one pin and tested all 4.7ohm and 1ohm resistors on the output. All good.
Driver board, first initial transistor test for me is always C-E and E-C. If they show anything but OL I should consider it bad, but continued with the other tests as well.
Q3,4,5,6,7,8,9,11,12 did not pass the C-E, E-C or more test. When in doubt change it our right? or am I doing something wrong? I did not yet remove the base pin to do a further test.
It doesn't make it easier that somebody has already done lots of work to this. I see changed transistors, capacitors, resistors, power knob is foreign and soldered to ON. I don't believe the voltage was changing when switching from 40% to 100% but I'm not entirely sure if that's something I would of audibly noticed or not.
I have now opened up the control board the most I can but can't get the board completely off because of the mods done to the knobs. They won't come off and I don't want to break anything while trying. Looking at that Q7, 8 and 9 don't seem to be working right either.
All other transistors passed the test as they should have according to the dozens of videos and instructions.
I have the hfe and transistor socket tester on my DMM but I wasn't sure if it was even worth using for tests if they didn't pass those probe tests. I wouldn't really know much about the current gain and stuff like that either.
Thanks.
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Transistors have to be tested out of circuit to get reliable results. The exceptions are when a failure has resulted in a dead short (or near short) and then there is little doubt the device is faulty.
I would be very very surprised if all those transistors were defective.
Whether a transistor (when checked out of circuit) reads between C and E when reverse biased by the meter leads depends on the test voltage the meter applies to the device. When forward biased it should not read.
I would be very very surprised if all those transistors were defective.
Whether a transistor (when checked out of circuit) reads between C and E when reverse biased by the meter leads depends on the test voltage the meter applies to the device. When forward biased it should not read.
Thanks for the quick reply. Maybe I'm just overthinking it then because of the difference in result (in circuit). I'll work on getting the mj15024s and 0.5ohm R.
Also I've notice in the schematics C10 and C11 say 10uf 100v. On my unit it has 10uf 75v. They seem to be original and look the same as pictures of units I've seen online that have not been recaped. Why would they be lower in value or is that a mistake on the guy who wrote the schematics? I know if I just replace them with 100v I won't be looking for trouble, right?
C5 and C3 on both boards have been changed but they are mismatched. They will get replaced as well. C1 too.
Edit: I think you are right about the driver boards though as both channels were exactly the same. Fingers are crossed!
Also I've notice in the schematics C10 and C11 say 10uf 100v. On my unit it has 10uf 75v. They seem to be original and look the same as pictures of units I've seen online that have not been recaped. Why would they be lower in value or is that a mistake on the guy who wrote the schematics? I know if I just replace them with 100v I won't be looking for trouble, right?
C5 and C3 on both boards have been changed but they are mismatched. They will get replaced as well. C1 too.
Edit: I think you are right about the driver boards though as both channels were exactly the same. Fingers are crossed!
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C10 and C11 are across the supplies, and the manual shows those to be -/+80 volts. That means 100V caps are needed.
You would have to measure the DC rail value to know if there was a mistake anywhere but fwiw, manuals can be full of such errors.
Wire wounds are fine for the 0.5 ohms. Although they should be non inductive, the actual inductance of a couple of turns of resistance wire is next to nothing.
You would have to measure the DC rail value to know if there was a mistake anywhere but fwiw, manuals can be full of such errors.
Wire wounds are fine for the 0.5 ohms. Although they should be non inductive, the actual inductance of a couple of turns of resistance wire is next to nothing.
It wasnt actually a spark from a transistor on the rear like I initially thought. It was one of the thermal sensors. It shorted against the heatsink. To test these, resistance should rise with heat correct? I have one that rises, one that doesn't read anything. Do I even need them? I don't plan on using the fans.
I would concentrate on fixing the basic amp before looking to swap caps. The more work you do, the more chance of introducing further issues. Keep C5 and C3 as electrolytic.
Thermistors can work either way in response to heat i.e. resistance go up or down depending on whether it is a PTC or NTC type (pos or neg temperature coefficient).
I couldn't just spot them on the diagram anywhere.
Thermistors can work either way in response to heat i.e. resistance go up or down depending on whether it is a PTC or NTC type (pos or neg temperature coefficient).
I couldn't just spot them on the diagram anywhere.
Oh I'm not taking it to lightly. But not paying someone $100 an hour to swap resistors and power transistors, other things yes. Was gonna just order caps to save money in the future. After more reading I think the caps are fine beside the one coupling cap at c1 that has a slight buldge but no worries for now. The only cap that ever really needs changing every time is C3 which has already been changed. I will change the resistors and power transistors and nothing else Then get my tech to look at it before it gets fired up. Even the power transistors are probably ok. Who knows, the 2 that didn't pass probably were never working properly in the first place. They were original from 1974 and not secured very well at all. I know age doesn't matter but when they are loose from vibration or what not, and the thermal paste is disintegrated to nothing, that can't be good for it.
Channel 1 diagram. RT1 beside Q13. (Thermistor)
Channel 1 diagram. RT1 beside Q13. (Thermistor)
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I'll still get the mj15024 to play it safe. Besides using a scope to make sure everything is functioning properly, I can surely change out bad components if I can identify them as being bad. Not my first rodeo, but my first since I was a teenager. So my knowledge isn't very high anymore. Ive been focused more on speakers and speaker building since then, not amps. Still is my passion though. I'm here to learn, teach me.
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Yes i don't plan on sending power to it. I do plan on changing bad parts myself instead of paying someone to do it (as much as possible) I don't have a soldering station for nothing. Any and all help with this is appreciated. I am going to school like I've always wanted for electronic engineering this fall so this is a good place to start I guess.
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