At the right one is clearly broken, surely by a severe overload. Check transistor prior to replace them.
Take a reading at:
Amplifier with blown fuses.
for safety reasons while trying to repair it. Mainly the use of a lamp as current-limited power source.
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Yeah, the one on the right is split in two. The one on the left has a crack as well. Thanks for the advise about the transistor. Will try to replace those as well.
I'll try to pop one out one of these days and check the color code. Hope I can find suitable replacements for them (as well as the transistors).
I'll try to pop one out one of these days and check the color code. Hope I can find suitable replacements for them (as well as the transistors).
Those ceramic resistors usually are printed in a ceramic tile, and they break as a sudden thermal overload, or mechanical abuse. Prior to replace them, check transistors and bias of the stage. Again, use a limited current supply like those detailed in the mentioned thread, to prevent major damages to he amp and for safety for you.
You could probably replace them with what are called metal plate resistors. These resistos feature a serpentine track of thick resistive foil with leads welded on the ends, potted in a flattish ceramic case in a similar manner to the cheap ceramic tub wirewound resistors that are ubiquitous. Typical manufacturers are people like Micron, Fukushima Futaba, and Noble. I've found most of my plate resistors on E-prey.Use "plate resistors" as a search term, and you'll see some pop up.
The original power transistors appear to be RCA 40636/2N3055 that you see on right side. I have an Armstrong 626 receiver in storage and as I recall, it is also fitted with the RCA originals though SGS could well have been an alternate supplier to Armstrong. I believe these are hometaxial transistors, similar spec. to early 2N3055 product but unlike the epitaxial equivalent types such as Motorola's 2N3055 that several other manufacturers are still producing today.
What that means is we can't just replace the transistors with any old 2N3055 version that you find. Since hometaxials are relics of the 1960s-70s and rare in working condition, the bias arrangement which includes the plate resistors will need to be redesigned to suit epitaxial transistors. That's not simple if you expect the sound to remain good but it's also an opportunity to try much improved, multi emitter TO3 types like MJ21194.
There is similar problem with repairing NAD3020 models that originally used hometaxial 2N3055 transistors. I have repaired several of these which contained 2N3055 transistors made by the most unlikely manufacturers such as Toshiba and Matsushita but they were indeed hometaxial and were genuine product as marked. Perhaps you could also look at 3020 repair threads for help there.
What that means is we can't just replace the transistors with any old 2N3055 version that you find. Since hometaxials are relics of the 1960s-70s and rare in working condition, the bias arrangement which includes the plate resistors will need to be redesigned to suit epitaxial transistors. That's not simple if you expect the sound to remain good but it's also an opportunity to try much improved, multi emitter TO3 types like MJ21194.
There is similar problem with repairing NAD3020 models that originally used hometaxial 2N3055 transistors. I have repaired several of these which contained 2N3055 transistors made by the most unlikely manufacturers such as Toshiba and Matsushita but they were indeed hometaxial and were genuine product as marked. Perhaps you could also look at 3020 repair threads for help there.
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BTW, there is a schematic for 600 series power amplifiers, including that in the 626 receiver here. The site is a gold mine of other details about Armstrong products but does not discuss all variations that may have occurred over the years of production. It is maintained by Scots academic Jim Lesurf, who was also a former employee.
Thanks for this...
Read a datasheet indicating that 2N3773/2N4348/2N6259 are all hometaxial based. Should be suitable replacements right? Assuming of course their specs are in line with the originals (which I wrongly asumed to be the SGS 40636)
Ok, I misunderstood. Still new to all this. Just realized that you were talking about the production process, not the actual type of transistors. My bad.
As I understand it, my options are:
1. Source hometaxial 2N3055 transistors (unobtanium)
2. Substitute with "modern" epitaxial transistors but will need further work
Am I right?
I had to replace the original outputs (SJS 40636) as I was lazy and didn't replace the bias trimmers.
I adjusted the bias and poof.
Wonky trimmer took out the outputs.
Rooky move I know.
I then replaced the trimmers and outputs.
I used 'mospec' MJ15003.
No mods needed and it's been running fine for 5 years.
I adjusted the bias and poof.
Wonky trimmer took out the outputs.
Rooky move I know.
I then replaced the trimmers and outputs.
I used 'mospec' MJ15003.
No mods needed and it's been running fine for 5 years.
Attachments
Yes, sadly, (2) will likely be the only option for a DIY repairer. I have stripped hometaxial 2N3055s from wrecked NAD amplifiers because they were once easy to find in dumpsters and at recycle depots everywhere but this is no longer the case in my area and now I see a lot of gear that has already been tinkered with and wrong parts fitted.
For example, if you see Motorola marks on what is a shiny, new transistor, it's more than likely a fake since the Motorola trademark on semiconductors ended more than 20 years when On Semi was established. They no longer manufacture 2N3055 either. Recently, ST micro also ceased 2N3055 production.
Unfortunately, fake parts are a growing disease that causes damage, errors and uncertainty about what we are doing or trying to learn. Best to reject anything doubtful before you waste time proving something is ok that you probably can't. If we're going to waste money as well, better to spend that on genuine parts in the first place. Counterfeit Transistors
I suggest trying the original circuit in only 1 channel with standard 0.47R 5W cement resistors and known good, standard and genuine 2N3055 transistors that are perhaps old stock from ST micro or other trusted brands. MJ15003 is a belt and bracers approach and a little expensive but may just work out better too. Notice that the specified bias current is quite low for these types at 20 mA and this is because it is a quasi-complementary design where both power transistors are the same NPN type but may require less bias current for minimal distortion. As user/abuser says, check that pots are also in good condition before trusting their adjustment.
Bias current is measured as the voltage across either 0.47R resistor and calculated from Ohm's law; I = E/R. Use clip leads or meter clip probes to avoid shorts and burning your transistors if you slip as you gradually adjust the pots and watch the meter carefully as you go, in one channel followed by the other until each measures correctly and is stable.
Take care - If one were to rapidly increase the bias current to see whether the sound improves, it may just cause another meltdown. Also, consider that the amp. was designed for use in a 20C max. temp. climate and we both live in much warmer climates, so our risks of overheating are probably much higher than the designer may have considered.
Good luck
For example, if you see Motorola marks on what is a shiny, new transistor, it's more than likely a fake since the Motorola trademark on semiconductors ended more than 20 years when On Semi was established. They no longer manufacture 2N3055 either. Recently, ST micro also ceased 2N3055 production.
Unfortunately, fake parts are a growing disease that causes damage, errors and uncertainty about what we are doing or trying to learn. Best to reject anything doubtful before you waste time proving something is ok that you probably can't. If we're going to waste money as well, better to spend that on genuine parts in the first place. Counterfeit Transistors
I suggest trying the original circuit in only 1 channel with standard 0.47R 5W cement resistors and known good, standard and genuine 2N3055 transistors that are perhaps old stock from ST micro or other trusted brands. MJ15003 is a belt and bracers approach and a little expensive but may just work out better too. Notice that the specified bias current is quite low for these types at 20 mA and this is because it is a quasi-complementary design where both power transistors are the same NPN type but may require less bias current for minimal distortion. As user/abuser says, check that pots are also in good condition before trusting their adjustment.
Bias current is measured as the voltage across either 0.47R resistor and calculated from Ohm's law; I = E/R. Use clip leads or meter clip probes to avoid shorts and burning your transistors if you slip as you gradually adjust the pots and watch the meter carefully as you go, in one channel followed by the other until each measures correctly and is stable.
Take care - If one were to rapidly increase the bias current to see whether the sound improves, it may just cause another meltdown. Also, consider that the amp. was designed for use in a 20C max. temp. climate and we both live in much warmer climates, so our risks of overheating are probably much higher than the designer may have considered.
Good luck
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Thanks for the info. It was actually your thread on another forum that convinced me to give the repair a shot in spite of my limited experience.
Again, thank you for this. I would have gone ahead and just replaced the transistors with whatever was readily available if it weren't for you. You saved me a lot of time and money. Authentic transistors are hard to come by where I'm from. Speaking of which, where are you from? Given that these amps were designed to operate at 20 deg C, what can I do to ensure the amp can operate safely? Temp in PH ranges from 30 to 40 for most of the year.Yes, sadly, (2) will likely be the only option for a DIY repairer. I have stripped hometaxial 2N3055s from wrecked NAD amplifiers because they were once easy to find in dumpsters and at recycle depots everywhere but this is no longer the case in my area and now I see a lot of gear that has already been tinkered with and wrong parts fitted.
For example, if you see Motorola marks on what is a shiny, new transistor, it's more than likely a fake since the Motorola trademark on semiconductors ended more than 20 years when On Semi was established. They no longer manufacture 2N3055 either. Recently, ST micro also ceased 2N3055 production.
Unfortunately, fake parts are a growing disease that causes damage, errors and uncertainty about what we are doing or trying to learn. Best to reject anything doubtful before you waste time proving something is ok that you probably can't. If we're going to waste money as well, better to spend that on genuine parts in the first place. Counterfeit Transistors
I suggest trying the original circuit in only 1 channel with standard 0.47R 5W cement resistors and known good, standard and genuine 2N3055 transistors that are perhaps old stock from ST micro or other trusted brands. MJ15003 is a belt and bracers approach and a little expensive but may just work out better too. Notice that the specified bias current is quite low for these types at 20 mA and this is because it is a quasi-complementary design where both power transistors are the same NPN type but may require less bias current for minimal distortion. As user/abuser says, check that pots are also in good condition before trusting their adjustment.
Bias current is measured as the voltage across either 0.47R resistor and calculated from Ohm's law; I = E/R. Use clip leads or meter clip probes to avoid shorts and burning your transistors if you slip as you gradually adjust the pots and watch the meter carefully as you go, in one channel followed by the other until each measures correctly and is stable.
Take care - If one were to rapidly increase the bias current to see whether the sound improves, it may just cause another meltdown. Also, consider that the amp. was designed for use in a 20C max. temp. climate and we both live in much warmer climates, so our risks of overheating are probably much higher than the designer may have considered.
Good luck
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