Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

[shelby5041]

hi

ill try to explain best i can

i have this old fisher studio standard home stereo system that has CA-39 Amplifier

it has worked fine ever since i bought it back in early 1980s

however, i have had it my garage as a garage sound system for about 15years...last time i used was about 5 years ago..today i decided to change out the speakers with some better speakers so i connected the speakers. left side works fine but for some reason the right side speaker was not working and was making a loud humming sound..then stopped....found out a fuse on amplifier board blew..so i replaced it and same thing happened..fuse blew...i tried this with orig speakers and the new set...and same result.

left channel works fine..right channel no work

also during hooking up the speaker wires today i did touch R channel + and - speaker wires by accident 2 times..and they sparked pretty good..not sure if this is relevant

anyways..i did notice on the board above the large stk chip one of the small caps is leaking bad.....and its a fresh leak..very liquidy...if u look at the 2nd pic you can see the leak all over the cap legs...

here are some pics

not sure if the capacitor was the root cause issue or the capacitor was damaged due to another reason...the Large IC is STK 4191 II

hoping someone can guide me to repair..

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

If the fuse is blowing it is very likely the stk is bad, I can't see where the cap is connected in the circuit but I doubt it caused the problem.

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

thanks..i saw a couple youtube video on the ca-39 amp exact same issue but diff causes...

could the leaking cap at the cap legs...cause a short issue? both legs are in a puddle of fluid.

i dont want to replace the stk chip unless 100% sure since its a lot of work and costs a bit of money and time...

the leaking cap can not be good..just not sure if it was prexistng or happened today when i powered amplifier on for first time in 5 yrs...or maybe it was a bad speaker that casued it...this all happened today it when i tried to change to a diff set of speakers......1 of the repalcement speakers does not work..but im not sure if it ever worked..maybe it was no good and fried somerhing on my amplifier?

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Do you have a schematic? I don't, so you would need to provide a high quality picture of both sides of the ciruit board so I can follow the trace. Does the cap go to ground or is it just a audio coupling cap?
Did you try the amp with the original speakers before switching them?
If you look at the datasheet for the ic, is that cap on the sample application diagram?

[budm]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Look at the spec sheet of the STK-4191 II to get the pin out of the output transistors inside the IC then the resistance of the output Transistors to see if any one of them shorted out or not.
BTW, funny to see Fisher made by SANYO called Vintage stereo.

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

correction....the capacitor may not be leaking it appears it could be old glue between the cap legs.not sure though ...im thinking maybe the old glue is being conductive? shorting out the cap legs??,..becasue its not hard like all the other glue areas its soft and melty which im assuming casued by short/heat when powered on?? why else would 40yr old glue be melted in that area?

hi rj.no unfortunately i did not test the orig speakers prior to changing to my new speakers....so i do not know if this was a preexisting condition...or this happened from touching the speakers wires together or from a bad new speaker...etc..note: the right channel (bad channel) is the channel that i touched the speaker wires together by accident 2 times and sparked...

.i watched a video on a diff ca-39 repair and the person noted that the 2 fuses from amplifier should have zero volts on them..if not zero then there is a problem..according to the video 48V is what should be going to the STK chip..on a few pins.....

i tested both fuses..the right channel(which was the left blown fuse) has 48 volts going to it with amplifier on and no speakers connected..so maybe the stk chip like you said is bad? since the 48v should not be coming from the stk to the fuse?

the left channel fuse has 0 volts going to it..

i hope this helps a bit..i plan on fixing this

it is vintage lol..its over 25yrs old...its about 40yrs old..only reason why i want to fix it is i bought this when i was a kid with my own money and i have kept it all these years becasue it sounds so much better than todays crap made electronics...

thanks

it is vintage lol..its over 25yrs old...its about 40yrs old..only reason why i want to fix it is i bought this when i was a kid with my own money and i have kept it all these years becasue it sounds so much better than todays crap made electronics...

thanks

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

here is pic of bottom ob board..cant take pic of top unless i remove stk chip which dont want to do at this time until i make sure thats the issues

the cap is 47uf 50V

one thing i noticed is that i assume the board in separated into 2 channel..

..the 50V cap on the left side has 48V going to + leg and i have continuity from that leg to the left fuse mount (which is the blown fuse bad channel
the 50V cap on the right side has 0V going to the + leg and have continuity from that leg to the right fuse(which is the good fuse good channel)

and sorry for all the info but im trying to provide as much as i can..hoping someone can pick something out that is the root casue...like rj said it could be the stk chip is bad..maybe from the speaker being bad or me touching speaker wires together or from the 40yr old glue at the cap legs that could have casued a short damaging the stk chip...

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

i was able to get a somewhat viewable imahe of top..in 2nd pic u can see the cap with the substance in between legs...

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

attached i pdf file for stk chip

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

That looks like the same 47µf that is on the datasheet, (from pin10 to 3.3k to ground) Based on the voltages the and the resistance, the output transistor in the ic is shorted.
TR9,11,24,22 are the outputs and can be checked for e~c shorts. check pin 10 to 11 and 13 to11, pin 10 to 9 and pin 13 to 14

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

thanks rj

i just did tests and found out i have short betwwen pins 13/14 output/v- which is right channel circuit so that supports my problem with right channel and im assunming STK chip is bad? if so could that have happened from touching the +/- speakers wires on right channel by accident? or bad speaker? or the capacitor legs old glue issue?

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Touching the speaker wires together would do it for sure, a shorted speaker (rare) would also do it. weak cap, unlikely, but for something this old I might just replace the electrolytics anyway.

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

thanks rj

well i did touch speaker wires 2 times when working on the right speaker install...i was in a tight space hard to work on...and also the speakers were untested they were given to my by a friend who wasnt sure they worked or not..and like i said the speaker that i was trying to connect does not work at all at this time..so maybe the speaker was bad inititally or maybe i fried speaker when i touched wires

whats odd to me is why would the glue be melted in between the cap legs? what would cause that? the shorted IC?
also i looked on ebay they have cheap china chips for $4 and they have more coslty versions for like 10-20$

does it really matter if i buy the cheap version? ..most everyhting comes from china anyways

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

I suspect that when you shorted the speaker wires the first time the ic may have shorted, and blew the speaker. If the speaker was connected when you replaced the fuse It may have blown the speaker. This amp has no speaker protection, so the speaker likely saw the full 48vdc. Speakers don't like DC.

[petehall347]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

might be hard finding a genuine stk pack these days .. you could maybe build your own . there is a board available that does as a replacement for some stk packs .

[eccerr0r]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

I used to have an old Sanyo^H^H^H^H^H^H Fisher integrated amp/tuner whose STK module broke... actually it was a pin that broke of the STK module. Bleh.

I'm surprised so many stereos don't come with short circuit protection, or even sufficient protection. I have another stereo, a Realistic(Radio Shack) branded one that has rudimentary short circuit protection...well, it failed to protect the outputs and fried it anyway.

Fortunately it's a discrete amplifier implementation and was simple to swap out just the defective transistor. Only 3 pins but I had to make sure nothing else was taken out with it, unlike the Fisher which replacing the STK chip with its 18 pins (ahh... rework station...) took care of everything.

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Thanks..ill just order the cheapest stk4191 II on ebay....its only about 10$ or less....as inhave said i bought this fisher 40yrs ago brand new and i plan on keeping it my whole life...its a very basic system but ..it sounds so freakin great....sounds way better than the junk they make today..and yes rj when i replaced the fuse i had the speaker still connected...the 1 speaker will go in trash...they are bookshelf size kenwood 100watt speakers 3pc set ......they sound decent though...once i fix the amp i will just use the center speaker as the right speaker...should be fine...its just for garage workshop,use..

[R_J]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Don't throw out the speaker it could still be fine. Check the speaker on something, If it went bad from the amp it would likely just be open, so there would be no sound from it. or check its resistance with a meter, it should read around 6~10Ω. If it checks ok I would use it. Also be aware of cheap fake stk's

[shelby5041]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

Ok thanks...i know speaker is not working since it wont work on left channel either..

It has 3 speakers within in it...i assume 2woofers and 1 tweeter

they are small like 4 in ...but pretty good sound model ks-401ht

Maybe ill mess with it ..it even has a small capacitor near connections inside the cabinet

I have not opened the speaker since it stopped working..maybe ill crack it open and d,some testing as u recommenededr

How the heck do i know if the stk is counterfeit..?i know ebay is known for lots of chinese fake counterfeit stuff like memory cards and laser modules and much much more

They all look the same on ebay

[budm]

Re: Vintage Fisher Studio Standard CA-39 amplifier R channel bad?

BTW, check the Voltage on the speaker terminal output after replacing the IC to make sure there is no DC Voltage, it should be only in mV range).
BTW, I used to work for FISHER authorized repair station, fixed a lot of original old tube 400c 500c, etc, then the solid state series, and then after SANYO bought out Fisher name. I still keep a lot of FISHER service manuals.