i think i know why this happens.
if you read 0,9V via R19 this means 0,9V / 0,47R = 1,91A
1,91A x 24V = 45,9 W per device
if i remember correct i read somewhere you should stop at 35 W per device.
So you should try to lower the bias. R19 at 0,4V - 0,5V
Use a pot at R27 and try to trim the bias.
look at your heatsinks that you not exceed 55C°
if you read 0,9V via R19 this means 0,9V / 0,47R = 1,91A
1,91A x 24V = 45,9 W per device
if i remember correct i read somewhere you should stop at 35 W per device.
So you should try to lower the bias. R19 at 0,4V - 0,5V
Use a pot at R27 and try to trim the bias.
look at your heatsinks that you not exceed 55C°
Turn the transformer, attache chopped leads to terminal block, leads from terminal block to destination. You may have to move block, if more turning is required. I see you have DIY store chassis, or some other chassis with perforated floor pan...moving T. Block around will be easy.
Russellc
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I checked all the solder joints, all the resistor values, and I replaced Q3 and Q8 just in case.
I turned down R27 until there was no current across R19 then I slowly turned it up until it was about 0.15V. Even at such a low current, Q8 heated up until it was too hot to touch after about 10 seconds. Q5, Q6, Q7 were not hot.
I measured the voltage at the drain and source pins of Q8 and they were -25V relative to ground.
I really cannot figure out why Q8 is heating up so quickly.
Are there any voltages or currents I can check to diagnose this?
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Yes, I'm working on my kitchen counter!
By the way, these heat sinks were used for my Aleph 30 which uses MOSFET devices per channel.
Hi Headshot, I thought someone more knowledgeable than myself would chime in before now. I would take a good look at the circuit board bottom and top with a good magnifying glass and check for good joints and possible a short circuit with solder flowing where it should not. There is a chance that there is a problem with your insulator that is grounding your mosfet. Have you tried replacing the insulator? If you do not have any extras swap them and see what happens. If there is a piece of metal piercing through the insulator grounding the transistor it would do what you say is happening. Check the heatsink surface under the transistor with your finger for a raised piece of metal. I hope this helps, persevere.
David
David
I solved it!
The problem was actually kind of obvious.
The metal of the heatsink where I tapped the hole was slightly raised underneath Q8. Even when the MOSFET was tightly screwed down, because of the metal that was raised up by the tapping, it was not flush against the surface of the heatsink. This caused it to heat up very quickly. I sanded it down slightly but this wasn't enough since I have another hole underneath the device from an earlier failed tap hole. I had to move the MOSFET nearby on the heatsink and made sure the surface was flat. I had to extend the leads though. Eventually I'll get a better enclosure.
Thanks everyone for your help!
The problem was actually kind of obvious.
The metal of the heatsink where I tapped the hole was slightly raised underneath Q8. Even when the MOSFET was tightly screwed down, because of the metal that was raised up by the tapping, it was not flush against the surface of the heatsink. This caused it to heat up very quickly. I sanded it down slightly but this wasn't enough since I have another hole underneath the device from an earlier failed tap hole. I had to move the MOSFET nearby on the heatsink and made sure the surface was flat. I had to extend the leads though. Eventually I'll get a better enclosure.
Thanks everyone for your help!
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Aaaargh!!! I started to stuff my boards and it seems that I made a mistake: when soldering the 3W resistors, I forgot to leave some space between two of them and the PCB. So I re-warmed the solder joint and used small tweezers to seperate the resistors from the board. The result is on the picture below: a small part of the covering cracked.
Do you I have to change these resistors? (I didn't order spare parts
)
An externally hosted image should be here but it was not working when we last tested it.
Do you I have to change these resistors? (I didn't order spare parts
)
Aaaargh!!! Do you I have to change these resistors? (I didn't order spare parts
Hi Folkdeath95
I could send for free & quickly two of this 0.47 R 3 W Panasonic
shine new resistors from my spare to your home in small letter.
If interested write me your postal adress in private message PM section.
Have a nice day
Attachments
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Hi guys,
Im in the process of building an Aleph J , somewhere in the coming day's i'm planning on soldering the power wirings on the Aleph boards. To do the things the right way I have a question about the ground on the Aleph board, on the pictures in the building guide i see a bridge connection between the gnd and the -in, am i seeing this correct?
this is a very thin wiring, why is this?
btw, great building guide 6l6
Im in the process of building an Aleph J , somewhere in the coming day's i'm planning on soldering the power wirings on the Aleph boards. To do the things the right way I have a question about the ground on the Aleph board, on the pictures in the building guide i see a bridge connection between the gnd and the -in, am i seeing this correct?
this is a very thin wiring, why is this?
btw, great building guide 6l6