Jump to content
peniku8

Rob's Amp tests

Recommended Posts

I investigated dielectric absorption quite a long time ago when I was 17 so my memory is a bit fuzzy.  I built a precision high dynamic range current mirror and monitored the charge/leakage current over the period of hours when the capacitor was connected to a voltage source with some series resistance.  I also had access to a volt meter with exceptionally high input impedance for monitoring the capacitor voltage.  I found that my capacitors fitted a model of a capacitor in parallel with another capacitor with a very high impedance in series with it that represented the long term leakage trend and the dielectric absorption. 

Static electricity can kill you if you use it to charge a large capacitor ;)

Share this post


Link to post
Share on other sites

I somehow feel like this is bad design or something isn't working as it should. The 4 transistors I replaced in the PSU, which all broke after about 2 minutes of 4KW output generate a humongous amount of heat even in idle.

The heatsink seen below reaches 80°C (176°F) in idle and SOMEHOW I measure over 1KV from heatsink to ground and when I touch it with the thermo sensor I see small sparks. What is going on here??

fWzrDr9.jpg

 

uNp7W4e.jpg

 

Share this post


Link to post
Share on other sites

What confused me even more was the seemingly random choice of output transistors, as two different models were actually used in the amp.

One output channel has an even distribution of both types, while the other one has 5 of one type and 9 of the other. Both are the NPN type.

https://www.onsemi.com/pub/Collateral/NJW0281-D.PDF

https://www.onsemi.com/pub/Collateral/NJW21193-D.PDF

Looks like the 21193 is a bit better. I'll solder two from the broken pcb to the working pcb to "upgrade" the FP-13000 to Fp-14000 on the output stage. It might make a slight difference into lower impedances. 14 vs 16 transistors is still a decent upgrade.

Share this post


Link to post
Share on other sites

Seems like this amp is haunted by Hoffmann's Law. I'm not sure why but the modifications I made (wich was adding the two missing resistors and transistors) broke the amp stage. The fuses melted so I removed the components again but it's still inoperational. Welp, now the amp doesn't work at all anymore, since both output stages are broken. I'm guessing that it's a relatively minor thing, but I'm not the one to track down a broken component and I'm fairly certain that getting it fixed will be more expensive than the 150$ for the replacement board.

Share this post


Link to post
Share on other sites

This is so annoying. I was just way too curious to see if I could improve the amp's performance by adding those components and now I caused 150$ damage again.

Well, heres more random info on the PSU: when both amp stages are disconnected, the PSU hold over 200V for an hour. I used a light bulb to quickly drain it. When it's down to 0.1V, it takes about 10 seconds to climb back to 2V. I immediately connected the 230V light bulb after I powered the amp off, when the PSU was still at around 350V from positive to negative. It was a 30W light bulb or smth, it lit up brightly for a second and the light faded completely after some 20 seconds. Very convenient way when you're working on this kind of stuff.

  • Like 1

Share this post


Link to post
Share on other sites

The 1kv to ground sounds very weird as even if the heatsink was live on purpose you wouldn't expect 1kV to be generated anywhere in the amp.  Mix of output transistors is a very bad idea as they won't share current effectively.  On the plus side this is making me very certain I don't want one of these amps in my life :)

  • Like 1

Share this post


Link to post
Share on other sites
21 minutes ago, kipman725 said:

The 1kv to ground sounds very weird as even if the heatsink was live on purpose you wouldn't expect 1kV to be generated anywhere in the amp.  Mix of output transistors is a very bad idea as they won't share current effectively.  On the plus side this is making me very certain I don't want one of these amps in my life :)

I ordered two replacement modules, will see if the current ones are the same. This amp is 7 years old after all.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...