Adcom 555

[djk]

"What most people don't realize is that there are already bypass caps mounted on the output boards"

I know on the later ones there are, but not all have them.

Adding bypass caps when there are none makes the bass sound tighter. Bypass caps get rid of the 'mud', and many people will perceive that as less bass. If you think you need more bass you need a tone control.

I find bypass values in the range of 22µF~100µF sound best. The GFA 585 came with 100µF.

These caps are also among the first to wear out, they seldom last more than 7~10 years.

[pchw]

Opened the cover and saw 2 47uF/100V caps (one for each channel) on the output board. Since this amp is close to 20 years old (the psu caps have a date of 1986), may be time due for a recap? ApexJr has the Hitachi 22,000uF/100V for $15ea, a little higher than the original 15,000uF, it shouldn't cause problem, right?

[anatech]

Hi Fred,
Good opportunity. Go for it!

-Chris

[changsecho]

hi all i have my adcom 555 up and running and i thank you all for your help.but i have another question i;m greatfull for your input.do you think i should replace all the transistors in the driver board if so what band do you suggest nte-on-fairchild ????.also if i put disc capacitors on the power caps would this give me better bass what size.if so what values would you suggest to use thank you

[anatech]

Hi changsecho,

do you think i should replace all the transistors in the driver board if so what band do you suggest nte-on-fairchild ????

You have just lost your right to own a good piece of equipment. I'll send you my address for delivery of your Adcom amplifier.

NTE? What is it really? Date code? Batch number? PRICE!!!!!!!!!

Fairchild? Please, tell me why you would install devices that the circuit was not designed for, or completely unknown parts? I am kinda shocked here. No good audio tech uses NTE or anything but original parts or suitable substitutes. Only TV techs and industrial maintenance guys use NTE because they do not understand how linear circuits work.

You can check the matching of the input pair and replace with a matched pair if you understand how to match those devices. If not, you had better leave well enough alone. Output transistors require matching. This isn't a "would be nice if" thing, this is a "must be done this way" thing. If you don't know how, same advice as above. Even applying heat sink grease seems to be an art (not really) because of the number of units I see with this improperly done. Hmmm, that means they were serviced and they failed again. There must be a connection here.

I normally don't do this, but I recently repaired a Citation 22 that came up from the 'States. Not working. It failed very quickly. So here is a picture of the channel as I removed it from the unit.

-Chris

Edit: Notice the obvious fake part, it had the right number.

[anatech]

This is how it looked going back in. These are new On Semi parts and they worked great.

-Chris