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Fisher CA-860 one channel blown not fuse

 
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sharkbyt48



Joined: 10 Nov 2010
Posts: 6

PostPosted: Wed Nov 10, 2010 11:17 pm    Post subject: Fisher CA-860 one channel blown not fuse Reply with quote

had unit at work and was rigged with 3 speakers with a jumper on the left side of B. somone turned a off and naturally bang. took it apart found blown fuse replaced the fuse but now the speaker slector switch was smoking-ok not good. took it apart and checked the left side transistors and the posistive and negative were shorted. my question is, is this the right direction? and is the driver transistor ok?
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vtech



Joined: 08 May 2006
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Location: USA

PostPosted: Thu Nov 11, 2010 1:41 am    Post subject: Reply with quote

It is the right direction but more likely you have other components in the circuit blown. There is no way to tell without testing them. Used to see a lot of the CAxx series.
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sharkbyt48



Joined: 10 Nov 2010
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PostPosted: Thu Nov 11, 2010 3:22 am    Post subject: Reply with quote

vtech wrote:
It is the right direction but more likely you have other components in the circuit blown. There is no way to tell without testing them. Used to see a lot of the CAxx series.


Would you say it could be safe to replace the audio amp transistors and tturn it on and see if the driver is also shorted? or will this cause the gate to be over driven and blow the new transistors?
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sharkbyt48



Joined: 10 Nov 2010
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PostPosted: Thu Nov 11, 2010 3:37 am    Post subject: Reply with quote

sharkbyt48 wrote:
vtech wrote:
It is the right direction but more likely you have other components in the circuit blown. There is no way to tell without testing them. Used to see a lot of the CAxx series.


Would you say it could be safe to replace the audio amp transistors and tturn it on and see if the driver is also shorted? or will this cause the gate to be over driven and blow the new transistors?


no physical damage was found to any components. i did also find a 3 legged resistor with one side open but no heat damge to it nor any physical damge.
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vtech



Joined: 08 May 2006
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PostPosted: Fri Nov 12, 2010 2:53 am    Post subject: Reply with quote

When it comes to blown audio output circuit, there are no safe bets in anything. especially dealing w/ discrete components . If this used a STK module you could be pretty safe in assuming a good fix.
Unless you are willing to gamble with new outputs and if I were you, I would NOT do that. It can get expensive quick. The three legged white resistor is the emitter resistor and almost always goes when outputs short out. Why not at least test the associated components with a meter? Transistor(s),diodes, pots, etc.
Hopefully you still have an identical good channel/components that you can use as reference to compare to. Another way is to utilize an isolation transformer where you can bring up the AC slow as you monitor the dc voltages and if something is not right you can always back down without damaging new parts. Having worked on these fishers, you'd never want to risk just changing shorted outputs. Even a slight problem in predriver would cause an immediate destruction of the outputs if fully powered.
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sharkbyt48



Joined: 10 Nov 2010
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PostPosted: Sat Nov 13, 2010 12:31 am    Post subject: Reply with quote

vtech wrote:
When it comes to blown audio output circuit, there are no safe bets in anything. especially dealing w/ discrete components . If this used a STK module you could be pretty safe in assuming a good fix.
Unless you are willing to gamble with new outputs and if I were you, I would NOT do that. It can get expensive quick. The three legged white resistor is the emitter resistor and almost always goes when outputs short out. Why not at least test the associated components with a meter? Transistor(s),diodes, pots, etc.
Hopefully you still have an identical good channel/components that you can use as reference to compare to. Another way is to utilize an isolation transformer where you can bring up the AC slow as you monitor the dc voltages and if something is not right you can always back down without damaging new parts. Having worked on these fishers, you'd never want to risk just changing shorted outputs. Even a slight problem in predriver would cause an immediate destruction of the outputs if fully powered.



I really really appriciate you input and help this one has fuses on the output board. which probally saved this unit. i checked a bunch of things and went back and tried changing the components from the other channel and tired it and it works. one other thing i'm not sure how important it is but the bias , how important is it and what do i need to do to adjust it if needed?
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sharkbyt48



Joined: 10 Nov 2010
Posts: 6

PostPosted: Sat Nov 13, 2010 12:33 am    Post subject: Reply with quote

[quote="sharkbyt48"]
vtech wrote:
When it comes to blown audio output circuit, there are no safe bets in anything. especially dealing w/ discrete components . If this used a STK module you could be pretty safe in assuming a good fix.
Unless you are willing to gamble with new outputs and if I were you, I would NOT do that. It can get expensive quick. The three legged white resistor is the emitter resistor and almost always goes when outputs short out. Why not at least test the associated components with a meter? Transistor(s),diodes, pots, etc.
Hopefully you still have an identical good channel/components that you can use as reference to compare to. Another way is to utilize an isolation transformer where you can bring up the AC slow as you monitor the dc voltages and if something is not right you can always back down without damaging new parts. Having worked on these fishers, you'd never want to risk just changing shorted outputs. Even a slight problem in predriver would cause an immediate destruction of the outputs if fully powered.



I really really appriciate you input and help this one has fuses on the output board. which probally saved this unit. i checked a bunch of things and went back and tried changing the components from the other channel and tired it and it works. one other thing i'm not sure how important it is but the bias , how important is it and what do i need to do to adjust it if needed? also the emmiter resistor, can i use single wire wound resistors in place of the tri-wire wound resistor?
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vtech



Joined: 08 May 2006
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Location: USA

PostPosted: Sun Nov 14, 2010 5:29 am    Post subject: Reply with quote

Bias is very important. Some are preadjusted, most have some sort of an adjustment (small potentiometer). In an amplifier, you need some level of bias control at idle (no signal condition). it differs in each amp but if i remember right, there were about 10-30 ma. You'd need an ampmeter and usually done on two test points that almost always are across the emitter resistors.
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sharkbyt48



Joined: 10 Nov 2010
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PostPosted: Mon Nov 15, 2010 12:37 am    Post subject: Reply with quote

I downloaded a manual for a fisher ca873 it looks to be a urope one its was talking about measuing voltage somwhere between 7-15 millivolts.
and what was your opinion with using single wire wound resistors in place of the emitter resistor?
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vtech



Joined: 08 May 2006
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Location: USA

PostPosted: Mon Nov 15, 2010 10:40 pm    Post subject: Reply with quote

A lot of those series were similar. As far as the emitter resistor, while you may get away with using a wirewound, it is not recommended. Reason being that the ceramic package is specifically designed to be non-inductive thereby preventing possible oscillation that will immediately kill an amp. You can get them from MCM or other suppliers.
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