Those MJE***** transistors were installed because some original output transistors were faulty and some were not original to the amplifier. Case colours were different - some green and some black.
If I remember correctly, I was advised to use MJE***** transistors because they were more robust than what were in there. As to the driver transistors, it is a good possibility I only replaced one of a pair because I did not realize there was a pair.
I do not have any TIP41C or TIP42C on hand. But I do have a half dozen or so each of BD139 and BD140. The TIP41C/42C are available at Digikey so can be ordered. Delivery is usually quite fast - 2 or 3 days.
There seem to be 3 versions of TIP41C and TIP42C transistors. Two of them have additional letter suffixes, G & -BP. Those two have max power figures of 2W plus a specified transistion frequency of 3MHz. The version without additional letter suffix (TIP41C & TIP42C) show max power to be 65W but no transition frequency is given,
Because they will be output transistors, even though no transistion frequency is stated, I assume the 65W version is the one to get?
Presumably this is what I should do once I have TIP41C / TIP42C on hand?
In the meantime, should I go ahead with installing BD139 / BD140 pairs as driver transistors? And, should I confine myself to the affected right channel only or perform the same work, at the same time on both channels?
The 2 watt figure will be when at ambient temperature of 25C with no heatsink (free air operation) and 65 watt is when cooled with a suitable heatsink. The other letters denote lead free (Pb free) and where made.
The important thing is that they must be the C version which is the highest voltage rating.
The modern MJE's are modern fast parts with much better specs than the TIP's BUT that doesn't always mean they are better replacements. Lots of old designs actually achieve a lot of their stability from the fact the devices used are slow and that is because that is all there was back then.
I would recommend only working on one channel at a time.
I ordered 10 each TP41C and TP42C with an ETA of 2 days. If not 2 days then, it will be after the weekend. I also ordered a few more BD139 and BD140.
Hopefully I will not burn through a lot of them trying to get this amp working again.
I will follow your advice of only working on one channel at a time. I hope the solder pads are up to withstanding the assault.
Hopefully I will not burn through a lot of them trying to get this amp working again.
I will follow your advice of only working on one channel at a time. I hope the solder pads are up to withstanding the assault.
My transistor order arrived last Monday and I procrastinated until yesterday. I only addressed the right channel in which no output transistors are in place, That is still the case.
Yesterday I removed the jumper wire from the bases of Q405 and Q411. Then I installed the new right channel driver transistors. A BD139 at Q405 and a BD140 at Q406. I did not install any of the TIP41C or TIP42C as output transistors yet.
FWIW, I fitted the old Q405 transistor, a 2SC1124, in my cheap component tester and it's display identified that transistor as a resistor. I checked it not that long ago and it registered as a BJT transistor so it was seemingly OK. Somewhere in the inervening time, it got damaged.
After installing the new driver transistors BD139 and BC149 (no output transistors installed) in the right channel and powered on the amp with DBT and variac. DBT remained dark at full power on variac but after a few seconds I noticed wisps of smoke. It took a few times of powering up the amp to see what was smoking which turned out to be R415, R413 and R414.
First thing I did was replace those resistors with fresh ones then unplugged the entire amplifier board to examine it under better light. I thought I may have gotten polarity confused so I pulled those and installed fresh ones.
With that done, I reconnected the power amp board , turned on the amp, again with DBT and variac. As before, DBT was dark with variac up full and once more, I saw smoke rising, It was originating again from the freshly installed 1K ohm at R415. I shut off power so I do not know whether R413 and R414 would have followed suit.
I cleaned up the trace side of that board with acetone to get rid of flux and any Sharpie ink marks I left. As far as I can tell, which is a questionable observation, there does not look like any solder anomolies left from my unsoldering and resoldering of components. The traces are not crystal clear to see, because or the masking I am not viewing it under ideal conditions and my eyesight is not what it used to be so I can be overlooking something.
What reading I had done on 2SC1124 (original Q405) and 2SA706 (original Q406) had me believe their pinouts were BCE same as BD139 and BD140. I began to question this so I pulled Q305, still the original 2SC1124 and still good, and placed it in my component tester which identifies what each transistor leg is. Turns out, their pinout is EBC not ECB. So eiither I got it wrong from bad info or I messed up. In either case I will pull Q405/406 again and install yet more fresh ones in their place and adjust their orientation accordingly.
That will be late tonight or tomorrow morning.
Lets make sure what is going on.
Replace or at least check the burned resistors are OK and also that the BD139/140 are OK and this time also remove Q403. Obviously check the BD139/140 are correctly fitted.
It is possible Q403 is faulty/leaky/breaking down. A BD140 (PNP) should work OK here if needed but again check pin outs.
Dammit. I faced the same situation many years ago/1992-93y/. Pioneer SA-7800 non-switching amplifier. Smoke, flames, fireworks... MIRACLE! Lesson I learned the hard way - THE TRIMPOTS! Yes, I made a lot of dumb mistakes back then. I was obviously very young. Well, I'm stile young - let's be clear, I just make fewer mistakes now
. Anyway. Transistors, capacitors, diodes, NPN, PNP, EBC, ECB, +, -, anode, cathode etc - I won't comment on that. Further more, it's no secret that service manuals often have mistakes in them. But, FUSING resistors /in case there are any/ - do yourself a favor and address this as a possible issue. So, the genie is out the bottle. All components testing is the precise and most effective method /#11-madis64/. This is more imperative than suggestive, given the circumstances...
. Anyway. Transistors, capacitors, diodes, NPN, PNP, EBC, ECB, +, -, anode, cathode etc - I won't comment on that. Further more, it's no secret that service manuals often have mistakes in them. But, FUSING resistors /in case there are any/ - do yourself a favor and address this as a possible issue. So, the genie is out the bottle. All components testing is the precise and most effective method /#11-madis64/. This is more imperative than suggestive, given the circumstances...
I did not start with the variac at full power. I began at "0" then then turned it up. I was watching the DBT. Earlier, the DBT would glow at 50/60 volts and get brighter as voltage increased, DBT was full on bright when variac was at full power. It was disconcerting to have the amplifier on with a strongly glowing DBT but no smoke issued from anywhere.
In this round, my focus again was the DBT. It did not begin to glow at 50/60 volts but stayed dark as I upped the voltage all the way to full. I notitced the smoke within 5 or 10 seconds after the variac was turned up to full power. DBT stayed dark.
Yes, that is how this episode began, trying to adjust the trim pots. The amplifier was working after I installed new transistors 3 years ago. I even managed to adjust the bias trim pots back then with no consequences.
What kicked this off was me recently replacing all the electrolytic capacitors. The amp had a mild turn off "thump" and I thought new electrolytics might cure that. I replaced the electrolytics in the power supply while installing those new transitors 3 years ago but all of the other capacitors in this amp were original so 50 years old. The new electrolytics were installed successfully and the amp was working.
The finishing touch was to be re-adjusting the bias and then....disaster.
EDIT: There are no fusing resistors in this amp that I am aware of.
When you had the original burn out, did you lift the legs out of all the immediate resistors in circuit and check each one
No, I did not. It didn't occur to me because the outward appearance of all the resistors looked fine. At least they did before those I replaced got scorchingly hot.
I was not measuring voltage, just observing where the pointer was at on the dial scale of the variac.
I have replaced all the burned resistors also the BD139/140. And I just now removed and checked Q403 which was a KSA1015. Q403 was damaged so there is now a fresh KSA1015 in place.
I have not proceeded further as in turning on the amp. At this time I am calling it a night. It's been a long day, I've been awake for 18 hours and do not wish to try falling asleep knowing the DBT glowed bright or more smoke was created.
EDIT: Both the new BD139 and BD140 are now oriented correctly. C legs and B legs are crossed and their C legs were insulated with yellow shrink tube.
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I continued on this evening from where I left off last night. DBT in place, dialed variac up easy to full power. DBT stayed dark and no smoke.
Next was installing the TIP41C and TIP42C outputs one NPN/PNP pair at a time as per @madis64. Neither pair affected the DBT which stayed dark and again, no smoke. So then I installed the full compliment - two NPN's and two PNP's. DBT stayed dark and nothing burned.
So that is some progress presumably.
The only thing I thought to check was offset which was very low. It began at 4 or 5 millivolts then drifted down to 0.1mv. The other channel, without a BC139/BC140 driver pair and still with the MSJ**** output transistors shows about 32mV.
EDIT: Is that ultra low offset a good thing or no. Any time in the past that I've measured offset there was always more than 0.1Mv.
Next was installing the TIP41C and TIP42C outputs one NPN/PNP pair at a time as per @madis64. Neither pair affected the DBT which stayed dark and again, no smoke. So then I installed the full compliment - two NPN's and two PNP's. DBT stayed dark and nothing burned.
So that is some progress presumably.
The only thing I thought to check was offset which was very low. It began at 4 or 5 millivolts then drifted down to 0.1mv. The other channel, without a BC139/BC140 driver pair and still with the MSJ**** output transistors shows about 32mV.
EDIT: Is that ultra low offset a good thing or no. Any time in the past that I've measured offset there was always more than 0.1Mv.
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That is progress
Offset should be as close to zero as possible but 100mv either way is acceptable for a design like this. The matching and balance of the input stage is the main decider. Offset will slowly drift and wander on most amps like this.
Offset on the channel without drivers fitted isn't a valid result but it does show the output transistors are not short or leaky.
So all looks more promising at this point.
Offset should be as close to zero as possible but 100mv either way is acceptable for a design like this. The matching and balance of the input stage is the main decider. Offset will slowly drift and wander on most amps like this.
Offset on the channel without drivers fitted isn't a valid result but it does show the output transistors are not short or leaky.
So all looks more promising at this point.