If it was my amplifier or to any of my costumers
yes i would replace the 4700 /50 with 10.000/63
yes i would replace the 4700 /50 with 10.000/63
That is certainly a good suggestion.
One always hopes the designers fine tuned the sound extensively because sound is all that matters.
However, reality is different and many cost cuts are made to also be able to sell the amp with profit. Having a bit of experience in amp design helps to find these cost cuts.
I tested the bias and set it within 10mV (spec is +- 50mV). The left channel was at 256mV. Amp still "pops" and now and it sounds awful. The main amp section current should be between 28-32mA and it is 23DCA and will not adjust. I don't know how 23A is making it past the 5amp fuse?! Or why there is no adjustment. Any ideas?
How are you measuring this massive 23A? A direct measurement with a budget DMM would destroy its fuse, the amplifier, power supply, PCB traces and probably the test leads and meter too. Are you sure of your procedure and any calculation?
I think you may be out of your depth here. Fault-finding an amplifier needs some basic skills and understanding of electronics test procedures. You don't need to be a pro. but you do need to learn more about electronics and amplifiers before digging in and trying to adjust things before determining whether the amplifier is damaged or not. Not everything can be tested with a DMM either. You often need instruments like an oscilloscope to see what is happening dynamically and with certainty.
Setting bias or offset are finishing touches that may be impossible to do if output stage transistors, electrolytic caps etc. are damaged or have simply died, as electrolytic caps certainly do after so many years. Caps are wet cells, rather like batteries, and have lifetime specifications of so many thousand hours. That lifetime varies with the cap. grade and price. If your amplifier was working 24/7 for just a few years, they would probably be due for replacement but most good quality domestic amplifiers will survive 20 years in typical use before the sound begins to deteriorate and hum appears with the audio peaks as power levels increase. Simply put, the sound quality deteriorates.
Ian-You are correct I am out of my league here and I would appreciate suggestions on how to become more educated. I am certain I frustrate a few people with my obviously uneducated questions but I enjoy restoring these old amps and want to learn more. Since I already own the amp, and it doesn't work properly AND I have sent it to a supposed EXPERT for repair with poor results I figure why not give it a shot. The 23 amps is really puzzling. How does it not blow the 5amp fuse? Maybe my reading is incorrect and I hope it is. I measured as the service manual dictates, but my DMM does not measure in mA so I have one on order that does. It has been suggested that resistors connected to the pot controlling the amperage may be bad. they are cheap and easy to replace. I also intend on testing all of the transistors and replacing any caps that have not been replaced in the last 43 years.
Here is a good start to understanding all things Audio from a DIY enthusiast's POV.
Elliott Sound Products - The Audio Pages (Main Index) The author is also a long-time member here.
There are many other sites with less or more formal coverage and some none-too reliable ones as well. Take your time and study the articles and design section rather than gloss over the difficult parts and daydream about the projects. If it is still hard going, the referenced Lenard Education site will give a more basic, but attractive and informative intro. Introduction: Greetings and introduction
Note that tutorials on "how to test XYZ" are all over the web and Google is your friend. I suspect though, that what you are measuring is more like 23 or 230mA rather than 23A.
Otherwise, as I have said, there would be serious damage and smoke as you tested.
Sansui's procedure for bias current setting is a crude one that places an ammeter in series with the + power rail. It measures the total current flowing to the amplifier rather than just the output stage but this not a problem if the other stages are all operating correctly and you are only considering that particular design.
Low currents like the specified 28-32mA, should not trouble any DMM but it seems to me that the scales are confused or perhaps you don't have the meter leads in the right sockets for current measurement. You can check the current approximately by fitting a 100R resistor across the fuse clips, in place of the fuse and then measuring the voltage across this resistor instead. By ohms law, the current will be V/R, so you would expect to measure around 2.8-3.2V for the required bias to be flowing. If 23A had been flowing instead, the resistor and amplifier would burn out in a flash.
Beware that the small transistors are not standard, easily replaced parts. Those weird numbers are house codes for selected gain or matched transistor sets and I would not replace those semis without very good reason. The same goes for the modern substitute types you will likely need to fit elsewhere. Unless you are absolutely sure of what you are doing and aware of how much that will affect the performance, you may just be degrading tyhe sound of what is an old but highly regarded Quasi-complementary design.
Elliott Sound Products - The Audio Pages (Main Index) The author is also a long-time member here.
There are many other sites with less or more formal coverage and some none-too reliable ones as well. Take your time and study the articles and design section rather than gloss over the difficult parts and daydream about the projects. If it is still hard going, the referenced Lenard Education site will give a more basic, but attractive and informative intro. Introduction: Greetings and introduction
Note that tutorials on "how to test XYZ" are all over the web and Google is your friend. I suspect though, that what you are measuring is more like 23 or 230mA rather than 23A.
Otherwise, as I have said, there would be serious damage and smoke as you tested.
Sansui's procedure for bias current setting is a crude one that places an ammeter in series with the + power rail. It measures the total current flowing to the amplifier rather than just the output stage but this not a problem if the other stages are all operating correctly and you are only considering that particular design.
Low currents like the specified 28-32mA, should not trouble any DMM but it seems to me that the scales are confused or perhaps you don't have the meter leads in the right sockets for current measurement. You can check the current approximately by fitting a 100R resistor across the fuse clips, in place of the fuse and then measuring the voltage across this resistor instead. By ohms law, the current will be V/R, so you would expect to measure around 2.8-3.2V for the required bias to be flowing. If 23A had been flowing instead, the resistor and amplifier would burn out in a flash.
Beware that the small transistors are not standard, easily replaced parts. Those weird numbers are house codes for selected gain or matched transistor sets and I would not replace those semis without very good reason. The same goes for the modern substitute types you will likely need to fit elsewhere. Unless you are absolutely sure of what you are doing and aware of how much that will affect the performance, you may just be degrading tyhe sound of what is an old but highly regarded Quasi-complementary design.
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