Blowing fuses on 1980's Fisher CA-880

[AdamWeissmuller]

Hi
I have a Fisher Studio Standard Home Amplifier. I am blowing 4 amp/250 watt fuses only in the left channel. I got the amp two years ago and it worked fine until last week. All of a sudden it started blowing fuses. I replaced the left fuse 4 times. It makes a loud buzzing noise, then the fuse blows. I am not very electronically inclined, but that noise probably is not a good sign.
It is an old amp, very heavy and great quality. I thought before I threw it away and bought another one, I would try to see how badly it is broken or even if it is broken.
I would appreciate any advice, I miss my stereo.
Thank you, AW

 

[duke37]

The fuse is more likely to be 4A 250V.

Do not replace a fuse without finding the fault, they do not fail for the fun of it.

I assume this is a transistorised amplifier. Have you got a circuit diagram?

You can solder a resistor across one of the blown fuses to put some power into the amp without further damage, then look at the voltages in various areas. The most likely fault would be a leaky capacitor in the power supply or a faulty output transistor.

Blown electrolytic capacitors may have a domed top.
Shorted transistors can be measured with a meter.

 

[AdamWeissmuller]

Fisher Home Amp CA-880 Blowing speaker fuses

Hello Everyone,
First off, I am a complete moron when it comes to electronics. I am asking for help because I love my old Fisher amp and before I go out and buy something that wont be as good, I want to make sure I cant fix it first...
I have blown five 4 amp 250 watt speaker fuses. Each time when I switch out the fuse, power it back on it makes a horrible buzzing noise and the fuse burns. Only on the left side. The right side is fine.
Can this truly be the end of my good ol' Fisher? Please give me some advice for trouble shooting or repair...If she is too broken, I'll give her a proper burial at the recycling center.
Thank you, whoever you are, for your advice and if I need a new one can anyone recommend a comparable amp?

 

[duke37]

I thought I had made a contribution here before.

Do not ever replace a fuse without finding the fault, they do not fail for the fun of it.

Do you have a circuit diagram (schematic) ?

Unplug the speaker and put a resistor of several ohms (say 100) across one of your blown fuses. Put this in place and measure the voltage across the speaker output with a 10 ohm load. The voltage should be zero. If it is not then it could either be the output capacitor if there is one or a blown transistor if it is directly coupled.

Have you been driving too hard?

 

[KrisBlueNZ]

As Duke said, you need to measure the voltage on the left channel speaker output. I would do it slightly differently: replace the fuse (again), disconnect the speaker, power it up, and measure the DC voltage across the speaker connections.

If it's a direct coupled output stage, most likely one or more (and possibly all) of the semiconductors in the output stage have lost their magic smoke. Amplifiers that this happens to are often considered not worth repairing, but you can try if you want to. You will probably need to replace a number of components.

As Duke said, we need to see a schematic diagram, to get an idea of what type of technology it uses. If it's old, as you say, the semiconductors in the output stage may be old, familiar devices that are still available. It depends how old "old" is

It would also be helpful to see a few photos of the unit, especially of the output stage (the components that are screwed to the heatsink, and the ones on the circuit board in the same area).

 

[docb]

The fuse is more likely to be 4A 250V.
. The most likely fault would be a leaky capacitor in the power supply or a faulty output transistor.

.

Bad cap in power supply would probably affect both channels, not one,

 

[(*steve*)]

You have the significant benefit that it s a stereo amplifier with a fault affecting a single channel. You can do comparative measurements between both sides. Significant differences are likely due to the fault.

I would first be looking at the output transistors to see if they were damaged or even shorted.

 

[duke37]

Adam,

Which of your threads do you wish me to comment on?

 

[(*steve*)]

duke37, you better comment on this one because I just closed the other one.

edit: I decided to combine them...

edit2: Adam, notice how people waste their time giving you advice in one thread that you had already received in the other?

 

[Travis kamogelo]

Hi guys my amplifier also blows fuses, below is the circuit,please help

 

[(*steve*)]

Ok Travis, all we need now are some good photos.

See here

 

[duke37]

And make,model number and schematic.

 

[Wayne Phillips]

The chap did say he was not that electronically minded..a bit of cation here is needed to not get zapped by the 60vdc or more!!

These old fisher amps do have large flat packed power transistors say 16amp at about 200w.. it will have a speaker relay circuit to aid connection and de-connrction of the speakers that will trigger if there is high volts is at the output end.


Using your test meter and fitting say 4amp fues back in, next do not power it but place the black lead of your meter to chassis metal case and set it to dc volts of 200vdc range.

Using the red lead place it on the filter caps to see if there is any volts still there?

You want to remove the fuses from the blown side before swtiching it back on or the lighting flash will make you jump!!

For those of us that carry out repairs work know how to test amplifiers and yes, there are some amps that will drive us to spending lots of time getting it to work or giving it a dam good talking too

Next a quick way to see if them large devices are goosed is to place your meter to beep mode and place them meter leads on the legs and if there is a shorted part or parts it will be down to A let a tech fix it or weight the cost up!