Fisher CA-800 Rebuild Thread!

[Bassblaster]

Looking at the mounts, I'd probably also go for the widest viewing angle possible on the meter LEDs

55 deg is my best choice. but i little trick a found out is to take sandpaper to the LED's. that will help to disperse the light

 

[Bassblaster]

Messin with the broken board, i went to move it and it just snapped off.

Oh well, no biggie! its only 1 trace. easy fix.

 

[Bassblaster]

Neato!

 

[Bassblaster]

Looks like the op-amps voltage is set thought this resistor network...but which ones...R41 R42 look like the ones. wonder what value il have to change that too to drop the voltage to +/-12 to 15V

 

[llwhtt]

D13 and D14 Zener diodes set the voltages.

Craig

 

[Bassblaster]

D13 and D14 Zener diodes set the voltages.

Craig

Ahhh! its starting to come together now. those are both 20V zeners. replacing them with lower voltage zeners will drop my op-amp voltages. Or is there more to it?

 

[llwhtt]

Depending on how hot R37-40 are running now you might have to bump them up a bit. What voltage are you planning on?

Craig

 

[Bassblaster]

Depending on how hot R37-40 are running now you might have to bump them up a bit. What voltage are you planning on?

Craig

12-15V. those 4 resistors are all 2W and i dont know how warm they get. i can always turn it on and just let it run for a bit.
EDIT: on for about a minute and i cant touch those 4 resistors. guess ill up those to 3W

 

[Goldie99]

R37-40 are all listed as 2W each, so I'd expect them to probably be ok - for 20V zeners I calculate about 580mW on each; for 15V zeners ca. 760mW.

I wouldn't go lower than you need to on the voltage, and you might need to remove R50 / R51, for the IC-03 supply.

EDIT - the calculated mW values for the resistors are based on the manual rail voltage of 55.8V, a 63V rail voltage adds another 40 - 50%.

 

[Bassblaster]

Look like what i need are these and these.
im still debating if i want to re-transistorize the main PCB. i dont see a need to but at the same time it might help the god awful noise floor and maybe lower distortion.

 

[Bassblaster]

i swear the CA-800 is a cheaper version of the CA-880. filter caps are 6800uF compared to 8200uF. 1 comment board instead of 3. emitter resistors are 5W instead of 7W. and this bummed me out and thats the 5 "peak level" lights level cant be adjusted. im sure if i sat and actually compared the 800 to the 880 i would find more. but thats not the point of this thread.

if i plan on replacing both the STK packs with all discrete components, it might be a good idea if @rcs16 chipped in and gave some suggestions on what transistors could/should be swapped out on the main PCB.

 

[rcs16]

I do not know why you want to change the bjts on the main pcb?
Do you hear noise/hiss out of the amp/speakers when the volume control is at minimum?
The specs on the 2sa1240 look pretty good and it is a nice matched pair. There is no offset control on this amp.
I am not sure why you are changing those 20V zeners, the NJR opamps and the NE5532 are fine with this voltage.
I did found another lower noise opamp, slightly better than the NE5532A, a NJR2114, something to try. If you take the opamps out, use some IC sockets so that you can test and compare.

 

[Bassblaster]

I do not know why you want to change the bjts on the main pcb?
Do you hear noise/hiss out of the amp/speakers when the volume control is at minimum?

You could say that. its quite audible.

I am not sure why you are changing those 20V zeners, the NJR opamps and the NE5532 are fine with this voltage.
I did found another lower noise opamp, slightly better than the NE5532A, a NJR2114, something to try. If you take the opamps out, use some IC sockets so that you can test and compare.

im changing the Zeners because the NE5532 i was looking at didn't like +/-20V. most op-amps are best with +/-15V. i tested and one of the factory 20V are a little out of spec as its got 22V on its rail. but i would rather not have op-amps running at their absolute max voltage. The NJM2114D looks like a good contender.

And im well aware there is no offset control, left channel is at 80mV while the right is sitting at a nice 200mV

 

[redk9258]

Hey Bassblaster, What is that scope? Does it work very well?

 

[rcs16]

Where is that measurement been taken from?
Your scope pic looks to me to be more AC line ripple than noise. If it is ripple, changing bjts is not going to fix it.
Noise measurements are better taken with a audio analyzer, so that you can apply filters to differentiate noise vs ripple.
I would suggest to increase the resistance of R41,42 to say 100 ohms, as another option, if you want to reduce the opamp supplies a bit, it would also help in reducing ripple, so would beefing up some of those ecaps, like C36,37
You have to watch when you replace those slow opamps with higher GBP opamps as they can oscillate on you. Applying local decoupling, using 100n is always a good idea and recommended. If you read the new TI NE5532 datasheet, it shows the recommended way to decouple the devices.
I guess 200mV offset is starting to get higher than one would like, but it you do not get any speaker popping I guess that you are okay.
C19-22 could be increased a fair bit more as well. See what fits, could go up to 1000uF, but starts to get expensive

 

[Bassblaster]

Hey Bassblaster, What is that scope? Does it work very well?

it works for what i do with it. i know its not the best but it the DSO138.

Where is that measurement been taken from?
Your scope pic looks to me to be more AC line ripple than noise. If it is ripple, changing bjts is not going to fix it.

scoped right at the output across an 8ohm load.

Noise measurements are better taken with a audio analyzer, so that you can apply filters to differentiate noise vs ripple.

i dont own one

I would suggest to increase the resistance of R41,42 to say 100 ohms, as another option, if you want to reduce the opamp supplies a bit, it would also help in reducing ripple, so would beefing up some of those ecaps, like C36,37

What does upping R41,42 to 100 Ohms do to the circuit? factory ones look like 33Ohm fuseables.
C36 and 37 on the tone control board? factory 1uF what do you suggest i up those to?

I guess 200mV offset is starting to get higher than one would like, but it you do not get any speaker popping I guess that you are okay.

the right speaker does have a pop sound on power on. or switching the speaker switch on and off.

C19-22 could be increased a fair bit more as well. See what fits, could go up to 1000uF, but starts to get expensive

Guess ill up those to 220uF due to space limits.

EDIT: Main PCB C27 and C28 i already plan on upping those some. Nichicon PW 680uF 25V are the same size as the factory 470uF 25V.

 

[rcs16]

What does upping R41,42 to 100 Ohms do to the circuit?

It would reduce the supply voltages to the opa's and also reduce the supply noise/ripple a bit more.

the right speaker does have a pop sound on power on. or switching the speaker switch on and off.

So 200mV of o/p offset is noticeable and maybe unacceptable to you, so you could add a offset control if you wanted to. A simple way to copy how Sansui has done it in the BA-3000 power amp. Basically it is a 5K pot connected across two Si diodes (1N4148) acting as references (+/-0.6V), the wiper connected thu a 100K to the base of the ips bjt,q01,02. You want regulated voltages to supply the Si diodes with supply, so you have D01,D02 to do it for the + half, all you need is to duplicate it for the -ve half. Of course reverse the zener diode. The 1N4148 diodes need about 1mA through them.

 

[Bassblaster]

i could add that offset control, but dont your replacement modules have an offset control on them?

since IC03 op-amp is for the meters, if i change that out could it throw the accuracy of the needles off? there's no way to adjust them without completely redoing that part of the circuit. looks like the only op-amp thats going to get the swap is IC01. since im never going to use the front MIC jack or the phono input the factory op-amps can stay.

i figure ill up tone board C36 C37. to 680uF as well to match main PCB C27 C28. the factory op-amps get quite warm i found out. Hmmm, wonder if its because they're running at almost MAX voltage.

 

[rcs16]

i could add that offset control, but don't your replacement modules have an offset control on them?

no they have ops bias current adj not offset.
i would leave IC03 op-amp is for the meters, serves no purpose to fiddle with them.

looks like the only op-amp thats going to get the swap is IC01

sounds like a good plan.

the factory op-amps get quite warm i found out. Hmmm, wonder if its because they're running at almost MAX voltage.

it is okay for them to run warm, they have lasted this long, without issue

 

[cademan]

I own the 880 but have never owned the 800 but from what I have read here, the only difference between the two is the 800 uses the extra circuit boards for the external voltage taps for the matching tuner and cassette tape deck. They look identical from the front.

From what is known, the tuner and cassette deck are not 120 volt and do not have conventional wall plugs so the 800 supplies the 15-24 (or whatever) volts it puts on the connectors in the rear.

The 880 does not have this option but it still uses the same STK-0080 amplifier blocks. Even though the meters say 100, the STK reference book I have says these 0080 amp blocks are only rated at 80 watts.