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Old 27th December 2016, 10:15 AM   #1
Carlosraj is offline Carlosraj  United Kingdom
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Hi

A friend brought this amp, which has all its 4 power transistors shorted. He brought 2 new filter caps (10000uf/50V), 4 new power transistors (2 x MJ21193G) and (2 x MJ21194G). This amp originally uses 2 x 6800uf/50V caps, 2 x 2SD426 and 2 x 2SB556. Is it OK to use this 10000uf caps in this amp and what are the consequences. Please advice.

Thanks

Carlos
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Old 27th December 2016, 05:38 PM   #2
Ian Finch is offline Ian Finch  Australia
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Without knowing much more about the amplifier's design than the 40W/8R specification, it would be a wild guess what capacitance is necessary for proper function, let alone how much excess capacitance it could handle before blowing any fuses. However, 10,000uF is nominally only 1/3 higher than 6,800uF and should not lead to problems, so long as other circuits for features like soft start and protection relays aren't adversely affected - assuming any exist. If there are timing issues with more capacitance, you might have to reconsider.
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Last edited by Ian Finch; 27th December 2016 at 05:47 PM.
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Old 27th December 2016, 07:13 PM   #3
Welcome is offline Welcome  France
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Zero issues. Go ahead.
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Old 28th December 2016, 10:22 AM   #4
Carlosraj is offline Carlosraj  United Kingdom
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Quote:
Originally Posted by Ian Finch View Post
Without knowing much more about the amplifier's design than the 40W/8R specification, it would be a wild guess what capacitance is necessary for proper function, let alone how much excess capacitance it could handle before blowing any fuses. However, 10,000uF is nominally only 1/3 higher than 6,800uF and should not lead to problems, so long as other circuits for features like soft start and protection relays aren't adversely affected - assuming any exist. If there are timing issues with more capacitance, you might have to reconsider.
Thanks, I have not replaced the caps yet, but I have replaced the power transistors on one channel and the amp seems to work OK. I have yet another problem on the other channel. There is a silicon varistor connected to the bias which has blown out on the other channel, which is why I did not connect the other channel. On the component is written "64B" but on the schematic it is "STV4H". This part (diode) is totally not available. Can this component be replaced with some other component. I read online, some suggesting that it can be replaced by connected 4 " IN4148" diodes in series. Does anyone has worked on this before. I need some advice on this . Please assist.

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Carlos
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Old 28th December 2016, 04:10 PM   #5
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Yes, 4 1N4148 in series is the way to go. No other way to fix it.
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Old 29th December 2016, 04:09 AM   #6
Ian Finch is offline Ian Finch  Australia
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4 x1N4148 is the obvious way to do the repair with through-hole components but you still have to thermally couple them to the heatsink - not as easy as it might seem. SMD type diodes on a tiny piece of 2.5 mm pitch matrix board would be a good method and easier if your eyesight and soldering tools are any good. Otherwise, you could use ordinary leaded diodes with standard veroboard, if soldered with slight pressure on the assembly to ensure the diodes lie uniformly flat and will all be in contact with the heatsink. The STV4 and similar diode strings just have a bronze tag that is bolted to the heastsink, which makes it all so easy.

You could also just solder the 4 diodes side by side, in alternate directions so that you have a compact, square array with all diodes connected A to K in series. If they lie flat enough with light pressure, you can clip any open assembly like that to the heatsink with an improvised strip of steel, thinner than the pic below which is just for illustration. It should be drilled for the same attachment bolt that I assume was used previously. Don't forget to add some heat-proof insulation between the metal clip and the diodes - a short to the leads there would not be good.
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Old 29th December 2016, 10:09 AM   #7
Carlosraj is offline Carlosraj  United Kingdom
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Quote:
Originally Posted by Ian Finch View Post
4 x1N4148 is the obvious way to do the repair with through-hole components but you still have to thermally couple them to the heatsink - not as easy as it might seem. SMD type diodes on a tiny piece of 2.5 mm pitch matrix board would be a good method and easier if your eyesight and soldering tools are any good. Otherwise, you could use ordinary leaded diodes with standard veroboard, if soldered with slight pressure on the assembly to ensure the diodes lie uniformly flat and will all be in contact with the heatsink. The STV4 and similar diode strings just have a bronze tag that is bolted to the heastsink, which makes it all so easy.

You could also just solder the 4 diodes side by side, in alternate directions so that you have a compact, square array with all diodes connected A to K in series. If they lie flat enough with light pressure, you can clip any open assembly like that to the heatsink with an improvised strip of steel, thinner than the pic below which is just for illustration. It should be drilled for the same attachment bolt that I assume was used previously. Don't forget to add some heat-proof insulation between the metal clip and the diodes - a short to the leads there would not be good.
Thanks, Welcome and Ian for your advice. I will do as advised.

Carlos
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Old 30th December 2016, 10:12 AM   #8
Carlosraj is offline Carlosraj  United Kingdom
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Originally Posted by Carlosraj View Post
Thanks, Welcome and Ian for your advice. I will do as advised.

Carlos
Hi

I connected the four IN4148 diodes in series, have not mount the diodes on the heatsink yet because I am looking for the suitable material but placed them very close to the heatsink. Both channels are now working, but the driver transistors and power transistors on the IN4148 side are getting very hot in just within about a minute the amp is ON. I tried turning down the bias voltage to minimum but no difference. I had to turn off the amp, because afraid the power transistors might burn. Can adding one or more diodes make any difference. I need some advice.

Thanks

Carlos
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Old 30th December 2016, 03:04 PM   #9
Ian Finch is offline Ian Finch  Australia
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Originally Posted by Carlosraj View Post
......Both channels are now working, but the driver transistors and power transistors on the IN4148 side are getting very hot in just within about a minute the amp is ON..... Can adding one or more diodes make any difference. I need some advice.....
Quite likely but the diodes aren't the only factor in controlling the bias voltage and heating of the output stage. The original part could have failed due to this problem too. Before looking elsewhere, check the voltage across the STV4 and compare it with your diode array in the other channel, as the amplifier warms up. Compare the bias current measurements at the output transistors, as per the service manual bias adjustment method too.

Obviously, stop before it overheats again, as you won't learn much from dead parts. We're trying to see how the bias voltages track so you need to compare either channel several times with as little delay as possible. Take care not to short anything in haste with the probes though. Let us know if there were any glaring differences.
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Old 30th December 2016, 03:43 PM   #10
sgrossklass is offline sgrossklass  Germany
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Check the bias pot, too. These have a few decades on them, and intermittency would overbias the outputs.
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