Curbside find: Fisher CP 7000

[robiles]

While out bike riding i found a Fisher CP 7000 power amp close to home, so made an effort and took it home with me, with the risk of it being thrashed/not working.

It ia really dirty on the outside, missing a knob, and one vu-meter seems to be stuck. But the front cleened up nicely with some water and dishwashing liquid.

Next step is to pull out the dim bulb tester and see if it is safe to hook it up, and check the fuses for continuity.

On the inside it looks original, except maybe the white coating (paint? thermal paste?) where the transistors are located, with no obvious signs of damage (burnt spots or similar). There are a few capacitors that are connected in series - i presume this configuration is used instead of a bipolar cap - but i might also be wrong. There is also goo on some caps, but to me it seems like gluemore than anything else.

As i have no advanced electronics skills (i know how to solder and use a dmm, but that is kind of it), i would apreciate some help, or any advice regarding this amp. Is there something else i could check before connecting it to some speakers?

Also did not find a manual online, so i have no idea where to check the voltage and such. But i might buy one from the bay if necessary. Being a power amp it is fairly simple on the inside, with a small board, some wires and a large capacitor.

Here are some photos of the unit, inside and out.

 

[robiles]

Small update: dim bulb test was a success - the typical lights up intense at first, than quickly settles to a dim light (iirc light it is slightly more intense than my pioneer sa 6500 II). Vu meter lights also light up which i take it as a good sign.

Any more suggestions before i hook this bad boy up?

 

[8xlaxx]

Looks like the amp has been tampered with. The power packs do not look original and have been replaced, hence all the thermal heatsink paste.

If that were my amp, I'd replace the power packs to a matching pair (or close to it), clean up the thermal paste, and re-check the caps and resistors to make sure nothing is leaking.

Good luck with the resto! Cool looking find.

 

[robiles]

Looks like the amp has been tampered with. The power packs do not look original and have been replaced, hence all the thermal heatsink paste.

If that were my amp, I'd replace the power packs to a matching pair (or close to it), clean up the thermal paste, and re-check the caps and resistors to make sure nothing is leaking.

Good luck with the resto! Cool looking find.

Thanks, that makes a lot of sense. From what i have found on a german forum, the two should be similar in specification, but a matching pair makes more sense (and does not trigger ocd). Is there a model that you recommend, or where to buy them from (pfeferrably Europe)? I know there are a lot of fakes on the bay. Should there not be any thermal paste between power pack and heatsink?

Should i remove the caps and resistors from the board (or at least 1 pin?) to test them? I have a small chinese parts tester (lcr tc1 i think) - is this ok for the job?

 

[8xlaxx]

From this pic it looks like one of yours is a Chinese knock-off and that stock were Sanyo power packs.

Interesting thread here:

https://www.diyaudio.com/community/threads/fisher-cp-7000-distortion.382341/

That big auction site has the Sanyo power packs for approx. $35/ea from China (maybe counterfeits ??), not sure about going that route and that's probably why someone swapped out for a cheaper version. You can lift a leg from each component or test in circuit with power down (OFF),...doesn't seem to be a ton of components in that unit to test, although I have not looked at a schematic. Yes, that heatsink paste-smear is a dead giveaway. Factory standards would be a lot neater, cleaner, and only painted under the power packs themselves.

 

[Tom B]

Sanyo was the producer of most of the Hybrid power amp modules. They made ones for generic use (STK types), and also made ones for companies like Yamaha, Sanyo, and Sony. The SS1001 Sanyo hybrid appears electrically and specification wise the same as the generic STK086 (both have the same pin outs, case style, suggested power supplies, and the same 70W RMS power out)
I would suggest trying the unit out on speaker first, before replacing any of the hybrids.
The previous work left a lot to be desired relating to the application of the thermal compound. Its fairly easy to remove the 2 screws each from the hybrids, the few screws holding the power amp/driver board in place, and clean up the excessive compound on the heat sink and IC's. Then reapply a very thin layer to the back of the IC's and reinstall. (I usually use an old credit card to spread a thin layer across these type of IC's)

This is a bit of a strange unit. Usually they would be designed with a dual supply (+42v, and -42V) but this unit has a single supply, (I assume 84V)
Also units like this, almost always had a protect circuit with a speaker protect relay. I don't see a relay, but do see some wiring that may have been modified by the speaker select switch. I wonder if previous owner had a bad relay as a result of the bad IC, and replaced the IC, and eliminated the protect relay, Jumping some wires together??

Looking again, the power supply cap may be a dual, supporting +42v and -42v.
And I found a schematic showing an integrated amp CA-7000. Similar design of power amp and power supply. This unit did not appear to have a protect circuit or relay, but relied upon fuses in line with the speakers, to protect the speakers. Cheap way, but not as effective as protect circuit and relay. Not sure what the splieced by the switch are. Maybe they replaced the switch and spliced the wires instead of soldering.

 

[robiles]

Thank you both for your replies, a lot of helpfull information.

I managed to take some time to hook it up to my main rig. Took a while as i have limited space, and a really difficult amp to work with (low height and recessed plugs), but it is an integrated with a pre out, so it's not all bad.

While sound comes out of both speakers, the right one sounds normal but the left one is quieter, and seems distorted. I will look into it later, or tomorrow, as the circumstances permit.

I thought about connecting the left speaker to the right B channel, to spare me the time of changing it all back (music will be heard while i am at work), but the speaker connectors are real $hit, and basicly unuseable with thicker wire.

I did not take a detailed look on the inside, but it appears that only the the one (or both) modules have been changed. One of the vu-meters was stuck but i managed to unstuck it by unscrewing the small screw on the front. Were turned off during my test so i do not know if they work.

Now the only question that remains is what should i do regarding the so called amp modules... Only source seems to be china, and at aprox 17€ a piece i could take a risk, but would like to use a more dependable source. And also have to source a knob that is missing, but after a clean it looks quite good.

edit: i might have to rethink what i do with this amp, as i have turnet it on again, and it made some weird sounds, even with the volume on the pre turned all the way down, so i immediately turned it off and it made a really loud "bang". hope it did not thrash my speakers - that would turn out an expesinve free power amp

 

[turnitup]

Be careful about hooking that thing up to any speakers that you value (in that condition). You might do significant damage to them. Just a word of caution...

 

[robiles]

Be careful about hooking that thing up to any speakers that you value (in that condition). You might do significant damage to them. Just a word of caution...

I will not be hooking it up to anything until i change the obvious suspects. I will also try to get my hands on some cheap disposeable speakers.
Hapily the heresy's made it out alive. I do not necesaary value them, but it is a shame to damage any half decent speaker.

 

[robiles]

I wil also seize the opportunity and ask a couple of questions that just came to me.
While hooking up a pre-amp to a power amp, which is the one that you turn on first, and are there any other things to consider?

Today i have noticed that, in my case, it does make a difference - but i think that my Braun integrated might have a slight problem - it "plays" static about 3 seconds after turning on, but with the Fisher connected it was not affected by the volume setting (was turned to 0), with the braun it was much quieter.

The Braun does not "bang" while turning on and off, but the Pioneer i had befire (SA 6500) did, and was louder than i would have liked. Can this be somehow countered, or it is something normal? This is one of the reasons i really like the Braun (it is also capable of driving 2 pairs of 4ohm speakers), and i have a hard time finding a replacement that has all the features i that i value

 

[Bradbrews]

Amp on last and off first is how I do it.

 

[rickl]

i'd carefully measure the dc offset too. post results. anything less that 10mV should be fine.

here is what I do:
turn down volume on pre amp
turn on pre
turn speaker selector to off on power amp
turn on power
turn on speaker selector
turn up volume
enjoy music
turn down volume on pre
turn off speakers power amp
turn off power amp
turn off pre

 

[Tom B]

I suggest that you check the 100 ohm resistors and make sure they are ok, and also examine the soldering work done on the IC that was changed, as well as the original. It is easy to damage the foil traces when the pins are solder to the board and then remounting to the power supply. Usually you can see the cracks in the foil trace, but you should also use an ohm meter to verify all connections. I wish I could help you out with a NOS STK, but I don't have any STK086's. I have lots of STK084, but they are only rated at 50W, not 70w like the STK086
Good luck

 

[robiles]

I suggest that you check the 100 ohm resistors and make sure they are ok, and also examine the soldering work done on the IC that was changed, as well as the original. It is easy to damage the foil traces when the pins are solder to the board and then remounting to the power supply. Usually you can see the cracks in the foil trace, but you should also use an ohm meter to verify all connections. I wish I could help you out with a NOS STK, but I don't have any STK086's. I have lots of STK084, but they are only rated at 50W, not 70w like the STK086
Good luck

Thanks for the help, i wil do so as soon as i find a source to order the parts. I have already taken it apart a few times, and next time i want to be do more than just check things out.

I found out that there is also a stk-086 g version used by some dual amplifiers that support 4 ohm speakers (apparently only difference), which would interest me - i live in Germany and 4 ohm speakers are abundant and with good value for money (a L710 can be easily had for 100€).

Do you by any chance have a service manual? I would like to check if the components match the schematic.

 

[robiles]

i'd carefully measure the dc offset too. post results. anything less that 10mV should be fine.

here is what I do:
turn down volume on pre amp
turn on pre
turn speaker selector to off on power amp
turn on power
turn on speaker selector
turn up volume
enjoy music
turn down volume on pre
turn off speakers power amp
turn off power amp
turn off pre

I will check the bias as soon as i get the dmm from the car (having some electrical problems that need checking when time permits). Could this be done without connecting the power amp to anything? As it sits idly at the moment, and hooking it up woult take more time than than i would like.

 

[jheu02]

One thought....if the pinout is the same, you might consider going with the 50W packs vs 70W packs. Volume difference will be inaudible, but if it gets you clean power...

 

[robiles]

So i just checked the dc offset and it is -5 for left channel and 10 for the right one. From what i understand, this should mean that the amplifier is not in a bad state.

I am currently in the process of thining the heard, and as soon as i move some things around, i will tri to hook it up again. But this time i will clean all the switches and such, an take the neccesary precautions.

I like the idea of sourcing lower wattage packs - especcialy if i know they are the real deal. Thank you all for the help.