Fisher ca-2030 help needed

dreamth

Member
2010-11-18 9:12 am
I just collected a Fisher ca-2030 and i just wanted to ask if anyone knows what would the missing components be on the preamp-tone pcb inside the red circle.I suspect they're part of a more refined preamp- tone section of a different Fisher model, but i can't figure out which one would it be as mine is found in at least one other preamp, the cc3000 model but only the mm preamp looks the same in both models.
 

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The big electrolytic capacitors and the SIP op amps indicate something like a big bass boost section. Look up NJM4558L if you're not familiar with SIP op amps. Datasheet on datasheetcatalog.com
Could be additional line level or mm input sections, but you've already got 6 buttons to select for inputs don't you?
If you want to populate them the NJM4562L comes in SIP and doesn't hiss as badly as the 4558. Although at gain 2-5 4562 the hiss is not as audible as at 50X for mm. NJM4562 not to be confused with Nat Semi LM4562, a much more modern part. If using higher slew rate 4562 be sure to bypass feedback resistor with 33 pf disk caps and provide close .1 uf disk power supply bypass caps. Higher slew rate parts can oscillate in positions designed for tame 4558.
 
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Opamps? These guys seem to be 5-pin SIPs and labeled Qxx, which would rather indicate double transistors conjoined at the emitter. Heaven knows why one would need four of them, possibly an MM prepre?

Actually I don't see a single IC opamp in this entire unit, seems to be all discrete, the STK hybrids excepted.

NB, NJM4562 is not unity gain stable, it's an older decompensated part.
 

dreamth

Member
2010-11-18 9:12 am
There aren't 4 op-amp footprints, there are 4 double matched transistors which tells me it's rather a mc pre-preamp section... The mm section is identical with the one in CC-3000 model .It might be also a variation of the mc preamp section in cc-3000 there. I will open the can completely and try to draw the circuit on paper. 4562 doesn't qualify in this circuit as it's supplied with +-22v.They have special op-amps that can take max +-25v.I think i'm gonna scrap the tone eq op-amps and place the tone network in the power amp's feedback.
 
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dreamth

Member
2010-11-18 9:12 am
Actually I don't see a single IC opamp in this entire unit, seems to be all discrete, the STK hybrids excepted.
The STK hybrids are only final Darlington transistors...STK0040
Actually there are two HA1457 op-amps that can be supplied at max +-25v.I won't bother change them with more modern high voltage op-amps , i will scrap them completely and move the eq network in the amp's feedback.Some mods might be needed, but i have a clear plan as i found the front end of this amp to be very similar to my all time's favorite amp: the Kenwood KA-1000.
I actually made an interesting discovery .Some of the best Kenwood designs are pretty similar with the best Fisher amps , like ba-6000. Finding this Fisher amp gave me a lot of food for thought. The only vintage amps i saw having an impedance adapter between the front end cacode and the VAS are Kenwood and Fisher at this moment.if anyone knows some other designs from that era having that impedance adapter please let me know.
 

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Well you have better eyes than I, being local to the board.
Good luck on finding dual transistors. I went looking, found some diodesinc in smt packages, which is totally useless in lots less than 10000. My hands are small but I am not an elf.
Sakis had a lot of contempt for amps with darlington output. Had to service a lot of them where they blew up. Thermal runaway? Drivers on a separate heat sink is a better design. His market maybe was mostly beach bars on islands, not the kind of gig where 1/4 phone plugs tripped over on a dark stage short the outputs. His shop was in Piraeus GR.
 

dreamth

Member
2010-11-18 9:12 am
Indeed , they are used to blow up...but their sound was quite good.I don't know if mines are ok.I'm still checking each component and just started replacing capacitors before any fire-up.It's a lot of work to do with this.I don't have those transistors either.Just contemplating what i can do with this amp to make it sound as good as as possible.