Overhaul of an Armstrong 625 gone wrong! Please help me :)

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Hi, this is my first post on here, and I know this is an old post, but I am doing up my old Armstrong 626 amp. I have all this parts for the power amp which is the same board as the one on here, apart from the ITT44 signal in the diodes in the bias. Can anyone help me with some info. Could I replace the ITT44 with 1N4148? Thank you for any help.
All the best.
Tapehead.
 
I recently bought one of these amps, Armstrong 621 without the tuner, and it is working fine.
Some time in the past though, there must have been a problem as the right channel 40636 output transistors have been replaced with 2N3055. Reading this thread I saw that these transistors are out of spec for the amp so I bought some 2N3773 , as recommended, to replace them.
So I installed the 2N3773 in just the right channel, adjusted the idle voltage across the emitter resistor to about 4mV and now I get crackling in the right channel when the volume is turned up slightly. I checked the idle voltage again after the crackling started and found it had jumped to 120mV. I have checked the trimpots and they work smoothly so I don't think it is bad tracking. With the 2N3055 I was able to easily adjust the idle voltage to 10mV and it stayed stable and with.no crackling.
So I have put the 2N3055 back in and it is working properly again but I would really like to know what the problem is with the 2N3773 installed.
Thankyou for any help you may be able to give.
 
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I would really like to know what the problem is with the 2N3773 installed....
First, who was the source of your 2N3773 transistors? It seems you had no trouble with refitting the 2N3055s, so it does make them look questionable. If you bought Motorola branded (bat symbol) parts for instance or perhaps from a Chinese Ebay source, that may be the problem :eek:.
 
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Yes, I am thinking that may be the problem. I bought them on Aliexpress and they have Onsemi markings but they don't match the format in the datasheet and the printing seems to be coming off already. I suspect I have bought counterfeits. I will try getting some from a reputable seller.
 

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Just an update if anyone is interested.
I bought some more 2N3773, this time from RS Components. Onsemi again but this time they came in individual boxes and the markings match the format in the datasheet. Looks like these are genuine.
I fitted all four and the amp is working properly and sounding very good so I am very pleased.
Next step is to replace some of the capacitors. The output caps have been leaking and measure very high so they are going to be replaced.
 
You could also do a Vceo test of the "2n3055" you have now.
Put 100 Vdc c to e with the base open, read the current. If it is 10 ma or less they are probably fine in circuit. It was hard to hold off 80 v in 1965, the 5 digit RCA transistors were especially selected from the 2n3055 for higher Vceo. It is not that hard since 1980 to make transistors that don't leak much, the old homotaxial production lines have been demolished and plowed under.
The reason for 100 v, the print shows 60 vac rectified at 110 volt AC in, which would be 84 Vpp. The wall voltage is now 127 VAC in my town.
Read the safety thread for high voltage before doing this, you don't want the current to cross your heart or burn you through a ring or necklace.
As far as alibaba, there are some unique components that I could use, light Cds cell combinations, that I haven't bought because of stories like this.
 
I've just bought a 625, bought as non-working bit it does actually work OK, however one of the output capacitors is bulging so I will be carrying out a full re-cap.

I have read that stability on the output stage is marginal - has anyone got any detail on this? I am also considering replacing the output transistors with MJ15003, has anyone done it and had to adjust the compensation?

One thing I have noticed is that all four speaker returns go to the GND on the pre-amp power supply and not the main smoothing capacitor, which isn't ideal at all so I will probably change this when I am working on it.
 
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