Adjusting the power limiters on technics SU 8080

[thes]

Hello everybody
I must say that at the moment i have 4 Technics SU 8080 but that i owned 8 in the last 3 years (4 are now in better hands i hope).
The point is that each one sounded different from the other! At one point i decided to keep only one so i chose the one free from scratches that has been cleaned inside from the previous owner (his tech?) only to find out more than a year later (when i purchased 3 more Su8080's) that this was the one that was sounding worse.
It always sounded worse to me but initially i thought that was only my impression.
In reality,compared to the other SU8080's i have now it sounds louder,more compressed if you like (like those wonderful cds they try to sell nowadays) so less detail and more headache.It runs very cool so i don't think it's the idling current that needs to be adjusted.
The other 3 Su 8080's i have sound a little better,better and much better,the best being the one that sounds "quiter" (less loud'?!?).
I've read in the service manual that to adjust the power limiters of the amp (which i think are causing this) takes more than a multimeter?
Can these power limiters be swapped from one unit to another? I tried,undid a screw but the pcb of the power limiter won't come out.
Please help

 

[chazix]

I've read in the service manual that to adjust the power limiters of the amp (which i think are causing this) takes more than a multimeter?

Hello,

I have no particular familiarity with SU-8080, but my reading of the manual is that the adjustments require either a distortion analyzer (which was translated to "strain gauge") or an oscilloscope. So, yes, more than a multimeter.

(I think there is an omission in the manual, by the way. I think the adjustments require the amplifier output to be loaded, probably with 4 ohms, but the manual doesn't call for this.)

I think it is unlikely that the power limiters would have an audible effect at normal listening levels if they were merely mis-adjusted. If they were defective, however, they certainly could.

I can't tell from the manual whether the power limiter boards are "connectorized". If there is no connector, then in order to swap boards, you would have to de-solder the 6 pins that connect the limiter board to the main amplifier board. Perhaps that means that this would not be a "do-it-yourself" project.

I hope this is of some help. Cheers,

chazix

 

[thes]

Thanks for your answers!
So these power limiter adjustments shouldn't be what's causing these differences in sound (one amp sounding louder than another)?
Maybe someone who knows these amplifiers better than me can explain what could be causing this?
Jeromach please help!

 

[jeromach]

I agree with inductor, lots of things can cause that.

The SU-8080 is a very nice amplifier, but it for sure will need TLC when this never before was done to it and in fact perhaps a full rebuild.

 

[jeromach]

In fact it is not so much different from the SU-8088 rebuild.

 

[slimecity]

I've had a couple of these amps to. They are great fun to work on and very logical with their layout.

Perhaps if the amp is running "cool" as you say and the sound seems to be lacking - maybe the bias isn't high enough? Just a thought.

 

[jeromach]

Yes, that is a very plausible cause. But also too high (or low) DC offset. Or bad contacts (switches, speaker relay). Or bad capacitors. Or bad 2SA798's. Or dirty pot meters. Or.... There's just a lot on these amps that would need attention.

 

[thes]

Ok,thanks for your replies and thanks to Jeromach!
It seems that the power limiters are not what's causing this amplifier to sound louder.
Also those can be adjusted only by using professional tools.
Low bias can make the amp sound louder? I thought that higher bias could cause that!
Also,Jeromach in another thread pointed out that all internal adjustments of these amps (dc,bias,etc) were not
always accurately made by the employees at the factory,causing them to sound different from each other...well i think that this could be the reason (i need to point out that the 3 8080's that are sounding better being less loud and compressed are all early models from 1977,so maybe they decided to change the internal adjustements at some point to impress people at the hifi stores?!)
I remeber something similar with marantz 1060's....someone stated that early ones (with grey big filter caps) sounded better than later ones (with black filter caps)..someone else pointed out that marantz 1200B's sound better than early marantz 1200's and so on...

Jeromach,could i ask you how does a technics se 9600 (and su 9600) sound compared to a su 8080?
I love that power amp but it's very pricey (900 usd and more) and i don't know if it would sound like a machine of that price..

 

[jeromach]

The SU and SE 9600's are beautiful machines. But if they sound so much differently... They have more adjustments to chance the sound though.

I have a complete set, near mint from the outside, but it needs the same TLC as all the amps from that era will need by now. This means that the buying price is not all there's to it, I am afraid a lot of people are not so much aware of that.

The bias adjustments of the power transistors will not make the overall sound louder or less loud. A wrong adjustment either causes distortion (bias too low) or lets the amp run too warm (bias too high).