How did I blow this fisher BA6000

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srinath

Member
2010-02-12 4:09 pm
I have this fisher BA6000 with the stk0100ii power amp chip.

That amp came to me with a dead stk chip, I pulled the good chip and put it in he other ba6000 I had and sold that.

So now I needed 2 stk0100ii's which I got and installed in this amp.
It worked for about 2 hours at about around 1/3rd-1/2 volume. Then I saw 1 meter was not working, so I tapped like you knock on a door :smash: on the case just on top of that meter. I heard a pop in the speakers and the amp is dead.

The stk's have been removed and the safety relay clicks. There is no obviously burnt spots in the board. I can replace the stk's but is that all there is to it ?

This is the schematic. I have a feeling one or more of you guys would knock this out of the park like you did with my nad meter pegging issue.

Thanks guys.
Srinath.
 

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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
You don't say who you dealt with to replace those "hybrids". If they were pulls from scrapped amplifiers, it's likely they were marginal or already blown in one transistor, as you describe above. There is no real guarantee that pulls are fully functional and many that have been bought over the last 10 years or so, to nurse old amplifiers back to life, have to be returned for credit - not really a viable way to do business.

If the power supply rail voltages are correct and remain within acceptable limits under load, only thermal runaway or any DC above the correct marked bias level at R52,62/R51,61 would cause failure of these Darlington output stages (which is all the Hybrid modules are) unless they are driving a significantly heavy load or there is a short at the output. The protection circuits themselves are simple current limiters (Q25,26/23,24) and DC speaker protection (Q27-31), for both channels) so it is just a matter of time for this amplifier to fail due to unusual speaker loads, faults, intermittent shorts etc.

Check the voltages marked on the schematic of the upper channel. If there are significant differences in DC readings at the output (R95,96), for instance, it will be a failed module as you say. Then go on to compare DC measurements between relevant areas of the channels as they are shown - I can't read them on this attachment, perhaps others can. If everything up to the hybrids is OK within a few % and they were correctly fastened to the heatsinks, you only have the hybrids themselves or the output wiring and load to blame. There are other possibilities, but without other indications, this is more likely, I think.
 
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srinath

Member
2010-02-12 4:09 pm
These stk0100ii's were not used pulls, these are new replicas. I have had enough BS with these, 1 of 3 came in blown, then I noticed soldering and desoldering maybe 1-2 times gets their legs to break off ... This amp needs to be killed. In nay case I will look for what you suggested, though a bad stk means I will not be buying any more - I may change the guts to one of em stk4046 kits off ebay, or something else. The problem is that its 57vdc. I cant really use it for an lm3886 or other amp.
Thanks so much, I knew I'd get my answer ... just may be not what I wanted.
Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
I think I am going to go onto something else, I would probably try a stk4046 kit off ebay in this chassis. Cos the solder traces in both channels are begining to go, the safety relay has a melt spot in it, and several things are not on par.
Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
Yea I know right. I am hoping to even have the meters working.
First however, I would rather try my hand @ this el cheapo Onkyo TX27 I got ... so I can get some momentum going. The nice trick would be to just eliminate the power amp section, but leave the tuner, the switching and everything else intact ... I am even thinking of how I can leave the protect mechanism in there. That amp basically fried a few resistors in the output in 1 channel, without frying the chip or anything else.
The relay clicks, the amp makes sound in 1 channel good but scratchy and crappy in the other.
Tomorow I will try swapping the 2 at the amp board to see what happens - if it switches sides.

Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
Ha ha, I fixed the tx27 without using a chip amp, it just had a bad 4843 chip, which I had bought many moons ago ... swapped it in and it works.
So I guess I am working on this BA600 as the first "surgical gut and replace"

Anyway I have 4 pairs of inputs coming into the power amp board, 2 from the DC/subsonic, and the other 2 - no idea where.

But the meters are run off the output caps. So presumably I can keep those working.
Then the safety relay could be left in circuit, but I seem to have 2 pairs of outputs - I think from the A and B switches.

Can someone tell me if I can just slap an amp board off ebay - a 4046 kit and have it work as a cap coupled amp ?
I may pull the output off the pins to the stk0100ii chip, as well as the B+ etc, and use the whole board with the stk4046 board in place of the 0100ii.
Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
easy for you to say

If the rest of the amplifier is good, voltage amp ok and protection circuits ok, it might be possible to knock together your own discrete Darlington output stages on the existing heatsink. How hard is four transistors and a few resistors?

Easy for you to say ... I cant tell a transistor from a quadratic differential (just to channel Baljeet from Phineas and Ferb).

Anyway about the BA6000 - I am thinking of checking the amp and putting in a L20 amp in that chassis - but if you have a way to put in discrete transistors which would put out ~100 watts with a pin to pin type cross - I would love to try that.

That amp's main amp board has a bit of problems - the solder traces are bad in spots near the stk chips. But not unusable.

The onkyo tx27 - its a 30 watt amp, I dont think the chintzy power supply leg with the 27V ac supply can take any more than that.

Cool.
Srinath.
 
The schematic is pretty awful, but it looks as if the power meters are really just looking at the output voltage from the power amplifier. So they'd connect to any amp really. The peak LED's appear to be driven by a comparator type circuit monitoring the voltage drop over one of the output emitter resistors (eg R77).

What may be confusing is that there are two sets of power supplies, +/-58V for the output stage and +/- 66V for the input stage. That's used to get the maximum drive from the output transistors. You could easily ignore that on a replacement amp and just use the 58V supplies.

That sort of power level is out of range for chip amps really. So you'd be looking for a circuit that's at least 2 pairs of output transistors per channel, something advertised for +/- 55v supplies - the extra few volts wont make much difference.

The protection circuit is nothing special - it is just a DC detect circuit. Plenty of kits to do that, or roll your own. Maybe reuse the existing relay if it's in good condition.

You can't use the STK4046 in the way youre thinking - the 4046 is a complete power amplifier that takes a line level input, and the STK0100 is just an output stage. As Ian points out, you COULD make a replacement for the STK0100 fairly easily - the datasheet shows it's equivalent circuit, it is a triple darlington type output stage.
 

srinath

Member
2010-02-12 4:09 pm
I was going to use this stk4046 kit -
STK4046V Two Channel Class AB Audio Power Amplifier IC 120 Watt RMS Liquidation | eBay

Pull the 58v into its Vcc and Vee, etc, and feed the input from the tone amp etc etc into the input, and sent the output to the outputs from the stk
Essentially that was going to just be the stk0100ii - something like that.
I also have a L20 kit I was thinking of using doing that same thing -

Another issue I have is the inputs from the tone board - there is 4, I am in the process of looking to see what is the final one, I think the thing has 1 location just before it goes to the power amp chip - I will find it soon I hope.

Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
Looking @ the circuit of the amp: stk0100ii - 0 and 1 get audio input. Pins 3 and 8 produce the output which is merged into 1 with resistors etc and sent to the protect circuit.
4, 5, 6, 7 are essentially not required. They are externally capacitored - internally ... no idea.
2 and 9 get B+ and B-.

Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
So say I found a complimentary transistor pair like the BD139/140 - something that works with 58 v dc and can produce massive gain - something like 35 X or so -

Would that work ?

If so - I should feed the audio signal to the emitter, the B+ to base and output from the collector right ? I know it as being backwards - but is that the right type of backwards ?

If it wont work - and it probably wont, please suggest something before I send a nice 20lb transformer in a beautiful power amp case to an early grave ...

Cool.
Srinath.
 

srinath

Member
2010-02-12 4:09 pm
OK I got back to fiddling with this amp.
With the stk0100ii removed the amp makes good sound in the left channel and scratchy weak sound in the right channel.
The same chip as above in the right channel, I only have 1 good stk - and I only get weak and dirty sound in the right and nothing in the left.
I will go back to looking over the schematics. But anyone point me to a possible problem ?

Thanks.
Srinath.
 
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