Manual επισκευής/αναβάθμισης ενισχυτών vintage (English)
Vintage amplifier repair/restore tips by east electronics
1-All vintage amplifiers need recapping (most electrolytic inside a vintage amplifiers are likely to be gone. Apply not only to rectifier board but also the all device.
2-By the same logic resistors should be taken care off or at least take a look at to see for variations from the original value. (Replacing also from cheap carbon type to a metal film is also an upgrade)
3-Often replacing electrolytic with higher quality caps will make difference in the overall performance.
4-All changes to be made have to have a symmetric effect .IE adding 100mf bypass cap next to positive rail ,next to output transistor should followed by the same on the negative side .
5- Adding bypass caps of 0.1 mfd to rectifier board electrolytic makes them faster.
6- There is almost no limit of how much rail capacitance you may add .There is only waste of them .IE in a Japanese AKAI amp that have a max out of 35+35W and capacitance of 2x4700 mfd exists on the power supply unit …. You may upgrade up to 2x10.000 mfd but farther than that to my opinion is a waste.
7- Replacing outputs have to be done with caution .If outputs are obsolete you can go to the nearest pair that is on production today. Changes also have to be symmetric! You can not leave the existing PNP out put BJT and replace only the NPN. Also may be other channel of existing amplifier is working ok but replacement of all outputs also will guaranty equal performance between the two channels.
8-Outputs have to be matched for optimum performance .Especially if more than two exist on the circuit .Never buy transistors of different brand .Always take care of the brand and the supplier you buy from since a lot of fakes are in the market world wide.
9-All the above apply for any other transistor (BJT/ MOSFET) you will need to replace. All the above concept applies to the entire transistor needed to be replaced no matter what the stage of the amp exist in.
10-LTP transistors (first stage) should be chosen for low noise and high hfe and frequency response.
Ccs transistors (first-second stages) should also be equal to the task
Vas amplifier / driver transistors like the above plus they need to also have enough power to be able to work as a replacement.
11- A schematic of the amplifier is always a good thing to start .To be able to restore/repair one amp you may be able to make replacements and changes but up to a small margin .Schematic is always useful in case you have components burned beyond recognition. Original values like offset and idle of the amplifier should be kept with in factory settings more or less.
12- Anything you change respecting idle offset and/or others will have side effects on the performance of the amplifier. Power should not be your conquest always. Combination of quality, power, and stability will result to performance.
13- Extra care should be taken together with caution were transistors (output/drivers/ vbe multipliers are to be attached on any heat sink. Insulation required most of the times between transistors and heat sinks .Thermo transfer paste ( white ) should be replaced with fresh and you can apply as much as you like …the excessive amount can be cleaned later .
14-Trimmers existing inside the amp adjusting offset and bias might be dirty or broken (often a good reason to take you to immediate death even though most of other components are already replaced) Replacing them to a multi turn trimmer will provide you with more precise adjustments and this can also be very useful if you replaced with other outputs/drivers that might require more or less bias or adjustments.
15- To repair restore any amplifier you will need equipment like signal generator, scope, and dummy load as a basic equipment. Precise repairs, upgrading, and restoration cannot be done with out them.
16- Look very carefully for any cannibalization signs IE somebody in the past repaired this amp and used anything available in his workbench. There a schematic might be handy once again but also you may start to look and recognize if any of the components replaced is fake .This is mostly happening to the output transistors. Please notice that proper number of transistor doesn’t make a replacement .Brand and quality is also very important. You have to think of your amplifier as a horse car ……with 4 horses for example ….All horses have to be the same family the same age and the same build. If you run a horse car with two pony and two stallions ….then the pony will slow down the all horse car …..Even between the same brands of horses same age and same build, horse people in the past used to place male horses upfront and female in the back …. So male could not see or smell the female ….And goes on….
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