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Old 7th November 2007, 11:01 PM   #11
grjr is offline grjr  United States
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Well I plan on putting them in sockets so if it turns out there is a problem it won't be difficult to swap them out. Thanks for all your help Perry.
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Old 24th November 2007, 02:15 AM   #12
grjr is offline grjr  United States
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Ok, so I finally got the time to replace the BJTs that were causing the TL494 to shut down and the amp powers up fine now without excessive current draw. As I stated in my first post, the original zeners for the OPAMP power supply went bad. The power supply for the OPAMPs is made up of two 1.2k ohm resistors and two 1N4744A zeners. I haven't yet put in the OPAMPs because I want to make sure nothing else is wrong that may cause problems. I measured the voltage across the resistors to be 31.7 V and the diodes to be 15.5 V. I calculated the power dissipated in each resistor to be ~840mW and in each diode to be ~410mW. These parts are running relatively hot to the touch, should I be worried about them failing? I'm guessing it's a common problem for the preamp PS to go bad on these things.
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Old 24th November 2007, 03:21 AM   #13
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If you used 1 watt resistors, the components are being run below their maximum power ratings and should survive.

You probably noticed that the board showed signs of damage/discoloration due to the heat from the components. This is common in many of the amplifiers that use this type of regulator. When I see this this type if damage, I generally use a larger resistor and install it with more clearance above the board. For the zeners, I mount them under the board (between the board and sink), leave the leads long and bend the diodes over so the leads are almost parallel with the board. A piece of closed cell foam (placed between the board and the diode) or the spring tension in the leads (if you don't bend them too far) will keep the diode in contact with the sink. Applying a bit of heatsink compound to the diode will help transfer heat to the sink.

Having longer leads on both components will reduce the heat stress on the board. The larger resistors will run cooler due to the increased surface area and the diodes will run cooler because they're on the heatsink.

There's no point in changing what you have. The regulators will probably hold up for years.
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Old 25th November 2007, 03:20 AM   #14
grjr is offline grjr  United States
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So I finished putting the amplifier back together and it plays music. I still have to put the zeners on the underside on the PCB as you've suggested and replace some output transistors but other than that it's pretty much done. Some of the BJTs in the preamplifier section are running warm but nothing I would consider hot, the drivers for the output transistors are running cool though. I'm thinking about maybe putting two zener pairs in the OPAMP power supply instead of the single pair that it has right now as I'd like to reduce the load on each individual zener. Also I am thinking about putting higher wattage resistors in place of the originals or putting resistors in parallel to reduce the load on the individual resistors. Currently the resistors run hot and I can smell that they are running hot, though they haven't discolored. When resistors fail they normally increase in resistance or open up correct? I'd like to prevent an OPAMP PS failure that could take out the OPAMPs I've replaced. Thanks for all the help Perry, you really know your stuff
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Old 25th November 2007, 05:31 AM   #15
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If you put the zener on the sink, you won't need to use 2 per location.

When resistors fail, they generally open but some resistors that run hot for a long time can go down slightly in resistance. I've never seen a resistor fail shorted.
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