Originals are Toshiba trenchfets, these have positive tempco much like HEXfets but lower treshold voltages, and no these are NOT lateral FETs, this myth keeps popping up over and over. Amongst other things the structure of the lateral FEt die puts the source on th center pin and cooling tab. Attempting to solder these in will destroy them instantly.
Yes, 33mv across either side (so centre pin to either of the others).
How does that value compare with recommended value for the original devices ?
Thanks ilimzn, I kept seeing the reference to "Laterals" but could see that the pin outs didn't agree for those.
Its normal if you can be sure the bias current is correct as per manufacturer recommendations and that it stays correct i.e it doesn't wander upwards.
It doesn't matter what transistors etc are used. Watts is amps time volts and so the heat output is the same.
Just for completeness I would say that the amp should ideally be checked for stabilty with a scope to make sure its not oscillating but I doubt that very much.
Lots of things in this amp get hot and it's actually 95% of the reason for failures of these amps.
Make sure to check bias after it has warmed up properly and re-adjust if necessary. The bias reading will vary some at start-up and needs a rather long period to stabilize properly.
Also, I forgot to mention, and i assume you have tested this in the previous steps, check that the bias servo transistor (the one on the heatsink between the MOSFETs) tests ok.
Make sure to check bias after it has warmed up properly and re-adjust if necessary. The bias reading will vary some at start-up and needs a rather long period to stabilize properly.
Also, I forgot to mention, and i assume you have tested this in the previous steps, check that the bias servo transistor (the one on the heatsink between the MOSFETs) tests ok.
I suppose its reassuring that, it's supposed to get hot at idle, it was just the transformer and heatsink that were the most worrying as I've not seen heat to that extent at idle in a class ab before.
Re failures, what else should be looked at, replaced while the lids off?
And bias current what is the optimal value for the irfp pairs?
The amps a 105 not U model.
Thanks
Chris
Re failures, what else should be looked at, replaced while the lids off?
And bias current what is the optimal value for the irfp pairs?
The amps a 105 not U model.
Thanks
Chris
The driver board and tube board should be carefully cleaned and best resoldered. There are a number of components there that get quite hot and the result is a lot of dry joints (I had transistors literally fall out of the board on one of these amps!). Careful!!! The pins of the tube connectors can 'cook' the board making it very brittle, heat from the tubes tends to do the same thing with the connectors from the tube board to the driver board. Proceed with caution!
Tens Years later....
So this is user; Rinsbag started the thread way back. I finally fixed it. I think.
All the interest and information shared over the years.
It wasn't until recently I found myself ready to try this fix again. Im using counterfiet 2SJ115 and 2SK405's. I cant even remember buying them....I was able to get it to bias though. Didn't have it on for too long though over maybe 10 minutes. I noticed the driver board was getting hot. Gonna take another close look. If they stop working I have the other IRFPs.
Just curious anyone modded this to work with 2SC3281 and 2SA1302's? I have some that's why? And incase my fix is short lived....
tbc
So this is user; Rinsbag started the thread way back. I finally fixed it. I think.
All the interest and information shared over the years.
It wasn't until recently I found myself ready to try this fix again. Im using counterfiet 2SJ115 and 2SK405's. I cant even remember buying them....I was able to get it to bias though. Didn't have it on for too long though over maybe 10 minutes. I noticed the driver board was getting hot. Gonna take another close look. If they stop working I have the other IRFPs.
Just curious anyone modded this to work with 2SC3281 and 2SA1302's? I have some that's why? And incase my fix is short lived....
tbc
possible but this is a pain...the lat fets are driven from buffers which can not supply needed current to a bjt output, besides you have to do a darlington output to be able to do that...
i tried the irf640 and the 9640 and it worked, although i did not like the results
perhaps because i was not biasing it enough...
I need to wake up this old thread since I fried my LV105u this weekend.
Painted the summer house and needed some music, so the old stereo was installed in the garden.
After a few hours of operation it started to sound bad and one could smell the expensive smoke (even outdoor).
Retired this one and installed the spare one instead.
The burnt one was bought modified with the IRFP transistors, is that still the best way to go ?
Painted the summer house and needed some music, so the old stereo was installed in the garden.
After a few hours of operation it started to sound bad and one could smell the expensive smoke (even outdoor).
Retired this one and installed the spare one instead.
The burnt one was bought modified with the IRFP transistors, is that still the best way to go ?
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