Righto, after all the discussion let's see who's right. When the new devices arrive I'll put them in in a day or so and see the results.
If the box starts working, I win.
ML
If the box starts working, I win.
ML
Seems to me that this circuit is a basic, Miller stabilised amp which would work with almost any output/driver pair. For example, the drivers could be BD139/BD140 and the output pair could be MJL21193/21194's or perhaps a lower power version (even TIP3055/2955). THey would need an additional bias stabiliser transistor and separate emitter resistors (as already mentioned) and I suspect it would work even with TIP142/147 Darlingtons, (also these would need a bias stabiliser and emitter resistors).
Few people asking for help here have a full kit of repair tools such as 'scopes , signal generators, dummy loads etc. If they already possessed this gear and could use it effectively on linear amplifiers, they likely would not need help to repair a simple audio amplifier.
I think increasing numbers of new members join primarily as "fixit" people with broken audio equipment. We have to assume that being a DIY repairer usually means an average person with access to only basic equipment and a small budget, not a professional repair technician with the spare cash, skills and tools to do professional work. OPs don't often tell us their situation so there will be a variety of replies, including those for intellectual curiosity and perhaps more interested in investigating the problem or to demonstrate the royal way to find it, than the time, cost or solution.
With such basic products and circuits, I find it more efficient to begin with or to eliminate the fault that is already known to exist with almost every example of the model that is reported. This is a popular, budget model with a well known problem, common to most products using these output devices. Proving an output device is ok or not may only require comparing bias currents because here it is a self-contained circuit and the OP has shown its control in this faulty channel now affects DC offset. Unless there is something wrong with one of the complementary output devices, you have to ask yourself "how could this be?"
Yes, a scope is helpful as a diagnostic tool if you first understand what the screen displays in contrast to what it would with a good device. However, it doesn't point to faults and isn't essential to an audio amplifier repair where most problems can be directly identified as DC anomalies like junction voltages drops, bias currents, output DC balance. These can just as easily lead us to the fault.
Sure, faulty semis should be confirmed before tearing parts out unnecessarily. As far as passives are concerned, there are very few in the compact output stage. 'Easy enough to disable the base input to prove the point. Note there is no DC offset adjustment. The input transistor match of Q203,4 that affect this, was probably fine within the batches of MPSA93 used.
I think increasing numbers of new members join primarily as "fixit" people with broken audio equipment. We have to assume that being a DIY repairer usually means an average person with access to only basic equipment and a small budget, not a professional repair technician with the spare cash, skills and tools to do professional work. OPs don't often tell us their situation so there will be a variety of replies, including those for intellectual curiosity and perhaps more interested in investigating the problem or to demonstrate the royal way to find it, than the time, cost or solution.
With such basic products and circuits, I find it more efficient to begin with or to eliminate the fault that is already known to exist with almost every example of the model that is reported. This is a popular, budget model with a well known problem, common to most products using these output devices. Proving an output device is ok or not may only require comparing bias currents because here it is a self-contained circuit and the OP has shown its control in this faulty channel now affects DC offset. Unless there is something wrong with one of the complementary output devices, you have to ask yourself "how could this be?"
Yes, a scope is helpful as a diagnostic tool if you first understand what the screen displays in contrast to what it would with a good device. However, it doesn't point to faults and isn't essential to an audio amplifier repair where most problems can be directly identified as DC anomalies like junction voltages drops, bias currents, output DC balance. These can just as easily lead us to the fault.
Sure, faulty semis should be confirmed before tearing parts out unnecessarily. As far as passives are concerned, there are very few in the compact output stage. 'Easy enough to disable the base input to prove the point. Note there is no DC offset adjustment. The input transistor match of Q203,4 that affect this, was probably fine within the batches of MPSA93 used.
Box already works. We can't "win", but we also can't "lose", only "'martinlangley" can.
I could also guess about DC offset and looking at YouTube I could guess it has nothing to do with sexy obsolete transistors just a not so sexy 33k resistor, and blindly replacing that 33k resistor would be just as silly.
https://youtu.be/-SSCcjW6EHc
I could also guess about DC offset and looking at YouTube I could guess it has nothing to do with sexy obsolete transistors just a not so sexy 33k resistor, and blindly replacing that 33k resistor would be just as silly.
https://youtu.be/-SSCcjW6EHc
Yes, a lot of videos like that posted about a500 by same company/person, all just show fault and repair in few letters below same still+music. Not real sexy videos, but does give impression they ain't afraid to charge for hours 🙂
On these amps, most of the time it is the SAP's that expire. SAP16's will work perfectly in this circuit. You may possibly find Q207/Q208 damaged, but they are cheap enough to just "shotgun replace".
Sometimes, Q206/Q210/Q211 snuff it, but this tends to only be when the output stage has failed catastrophically - ie when there's holes in the SAP's, or evidence of burning/smoke on other parts!
Sometimes, Q206/Q210/Q211 snuff it, but this tends to only be when the output stage has failed catastrophically - ie when there's holes in the SAP's, or evidence of burning/smoke on other parts!
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