9090db tuneup quick question

[wd409]

Hey everyone.

In the coming days?/weeks?, depends when it gets here.. im going to tune up a 9090db that is clipping once it reaches 50 watts.. (cap problem?)

anyway, while im in there I will address the known issues with power/driver/dolby/protection boards... and will specifically be having a look at the phono section and headphone section since he uses those regularly..

...and those ceramic resistors that deal with FM, make sure those arent desoldered...


anything else you professionals recommend I look at? ive done the fixing the boards bit before so im familiar with the different revisions and what not, I will have to figure out which one this is when it gets here

btw... curiosity sake, im gonna change them all, but, which caps specifically would make it clip @ 50 watts? he said the lights dim too when it does that. would that be the large filters or just the power supply..

 

[lbcgav]

No professional here, but I did want to make sure you replace the fuse resistors in the driver board.

 

[wd409]

Aye I will change those.
those are the blue ones arent they?

and i think theres some gray ones im supposed to replace too.. i havnt checked the manual since last time so just going on memory til it actually gets here.

 

[LBPete]

Start by checking the DC Offset and Bias Current then move on to the fuseable resistors. The clipping problem is not likely to be a cap.

- Pete

 

[wd409]

Start by checking the DC Offset and Bias Current then move on to the fuseable resistors. The clipping problem is not likely to be a cap.

- Pete

Fascinating, so you're thinking the resistors might have drifted, but not enough to go into protection?
so that is either over or undervolting the drive?

if all goes to current plan he will bring it out sunday. so what i will do is

check offset/bias
note cap values on driver/power/dolby/protection
note caps on phono/headphone
...should I mess with the caps in the tone section or no? and if so, are these bipolar?
order caps/resistors
deoxit
check/adjust bias/offset
???
profit!
(cap replacement is still on agenda because he has specifically asked me to do recap the problem spots)

oh yes, and those pesky ceramics that like to desolder. why do they do that? is this bad design, or by design. what should I know about that that I dont? regarding the 2 ceramics involved with fm power supply. these have been desoldered on all of them ive worked on, so far. (current count is 2)

 

[LBPete]

By all means, do the tone board. It has a lot of small caps that are likely to be out of spec. There are bipolar caps sprinkled throughout, you need to pay attention to that when you make your cap list.

When you say it's clipping at 50 watts, how do you know it's clipping at 50 watts.

I'm not sure what ceramic resistors you are referencing. The two 5 watt ceramic resistors on the tuner power supply can have their solder joints fail. So be sure to check them.

- Pete

 

[wd409]

as he described it, and on a side note he provided more info last night about it....


initial description: when the meters show 50w output, the treble sounds ok but the bass sounds terrible. there is a lot of distortion in it and the lights dim.



new information: the load is 4ohm (Canton CT-800) and he read somewhere that 9090's dont like 4ohm.



I deem the new information as irrelevant. as that would still not be pushing the amp, ~100w into 4 is not pushing a 9090 imo....

the resistors im talking about, as i remember it... there is a small power board to the left of the tuner board, it doesnt have a whole lot on it, infact these 2 resistors might be the only thing on it, i dont recall offhand.

ive been putting everything (limited information tho) that i know into one pool to "think" on, based on this it seems to me the coupling caps are fine as there would be hum if they werent, wouldnt there? but what would dc offset have to do with the lights dimming when it clips... this is throwing me because i woulda thought that unrelated.

 

[Ths364]

Lights dimming means excessive current draw. I'd be looking at the main filter caps.

 

[don philipe]

That small power board to the left of the tuner board,are you referring to the 2619 board? If so,would that cause the FM and AM to go out?I'm not in my shop now but I believe there's two resisters a couple of caps and a transistor with heatsink on that board.I read somewhere that it gets really hot and tends to fry the components .

 

[wd409]

That small power board to the left of the tuner board,are you referring to the 2619 board? If so,would that cause the FM and AM to go out?I'm not in my shop now but I believe there's two resisters a couple of caps and a transistor with heatsink on that board.I read somewhere that it gets really hot and tends to fry the components .

yeah, that'd do it. with the machine off put your finger to them and see if they rock back and forth. if they do then they are definitely loose. and even if they dont they still might be. tricky little things, those ceramics.