Morning, I’ve been looking at the schematics of B&K and other mosfet architecture amps and it seems each new design had more mosfets. My AV5000 S2 has 4, the AV125.5 that replaced it had 6 and my old REF7250 had 10 per channel. That means to perform the same repair I’ve done to the AV5000 on a REF7250 would be a total of 50 mosfets, for 300 solder joints! Is the increase a matter of making the mosfets work less by splitting the amplification between more components or is there an increased quality of sound by additional power transistors? Or both?
Just wondering why you want to replace all these mosfets????
I have not looked at your references, but there are lots of reasons that different designs use different # of components. Was there a cost or board size constraint, voltage and power specs of the individual devices, availability, various performance based trade offs like PSRR or linearity or bandwidth. Too many reasons to enumerate.
I have not looked at your references, but there are lots of reasons that different designs use different # of components. Was there a cost or board size constraint, voltage and power specs of the individual devices, availability, various performance based trade offs like PSRR or linearity or bandwidth. Too many reasons to enumerate.
YOU have been looking at them.
Upload them HERE for us to see them.
But you haven´t, so why ask?
Repairing this amp has been a summer project that hopefully has come to a successfully finish. The engineer in me just wondered what differences there were between the AV5000 and other designs. I noticed the additional mosfets and as a still an electronics noob, wondered if it was a sound quality, power distribution, or something else for the increasing numbers. Also, years ago I had a B&K, REF7250 go BANG in a very big way. A repairmen estimated the repair would be at least $600 in parts not including labor. At the time I also wondered why he said it would include ordering parts from the UK. Having just replaced 20 mosfets, with Exicons (from the UK!) I have a much better understanding of what might have been involved fixing the REF7250. Attached are the AV5000, AV125.5, and REF7250 service manuals.
Usually if a amplifier is designed for more power.
You have to parallel more output devices for more current.
So multiple devices share the total current.
I belief the amplifiers shown
2 of the models are around 125 watts per channel
And the larger 5000II amplifier is around 200 watts @ 8 ohms and around 375 @ 4 ohms.
So to have a successful amplifier which can dissipate all the heat and carry the current
of 375 watts.
You would have to share that current among multiple devices.
Which is why you are seeing more mosfets in the higher power amplifier.
You have to parallel more output devices for more current.
So multiple devices share the total current.
I belief the amplifiers shown
2 of the models are around 125 watts per channel
And the larger 5000II amplifier is around 200 watts @ 8 ohms and around 375 @ 4 ohms.
So to have a successful amplifier which can dissipate all the heat and carry the current
of 375 watts.
You would have to share that current among multiple devices.
Which is why you are seeing more mosfets in the higher power amplifier.
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Unless the output devices are damaged or shorted
there would be absolutely no reason to replace all the output devices on all the channels.
Also just guessing they would all need replacing is a very expensive assumption.
A few simple measurements will confirm if the mosfets are ok or not.
and save a incredible cost and labor to not replace unneeded expensive parts.
there would be absolutely no reason to replace all the output devices on all the channels.
Also just guessing they would all need replacing is a very expensive assumption.
A few simple measurements will confirm if the mosfets are ok or not.
and save a incredible cost and labor to not replace unneeded expensive parts.
Yeah why would you replace all the FETs on all the channels?
(Why you would want to fix this amp is another question)
(Why you would want to fix this amp is another question)
Why would I want to repair this amp?
(Besides possible any masochistic issues!)
This AV5000 fell into my hands due to an Ebay deal that went bad. The seller claimed it was factory and had never been repaired. But when I received it a tech and I found evidence of major repairs. Like all the original Hitachi mosfets had been replaced with inferior components that resulted in not having the expected sound quality. I sent a detailed email (with pictures!) to the seller and PayPal requesting return and refund. To the credit of the seller he returned all payments, but then told me to keep the amp! So I had a B&K, AV5000 S2 with issues, but nothing invested. I decided to set a budget for any repairs (not including my time!) and with a lot of help from the members of this forum and the FB, Vintage Audio and Tech group I was able to get the amp back to original condition.
(Besides possible any masochistic issues!)
This AV5000 fell into my hands due to an Ebay deal that went bad. The seller claimed it was factory and had never been repaired. But when I received it a tech and I found evidence of major repairs. Like all the original Hitachi mosfets had been replaced with inferior components that resulted in not having the expected sound quality. I sent a detailed email (with pictures!) to the seller and PayPal requesting return and refund. To the credit of the seller he returned all payments, but then told me to keep the amp! So I had a B&K, AV5000 S2 with issues, but nothing invested. I decided to set a budget for any repairs (not including my time!) and with a lot of help from the members of this forum and the FB, Vintage Audio and Tech group I was able to get the amp back to original condition.
Initial diagnosis we found one of channel 5’s SJ162s was totally dead and both of the K16Hs that were installed as replacements for the SK1058s were nowhere near spec of the originals. Since all 10 of the SK1058s were replaced with K16Hs and despite being labeled as SK162s, none were even close to spec, it just made the best sense to just replace all the “Brand Xs” with Exicons mosfets. Testing after the mosfet replacement, channel 4 would just go quiet after extended playing. Nothing dramatic and scary, no bangs or crackles just go quiet. Testing its caps found 4 tested good, but had questionable ESR and Vloss values. This included the V+, V-, 100uF, 100V filters (I’m learning! LOL) we thought might be shorting open as their temperature rose. Replacing them seems to have fixed the issue.
The bottom line is, for less than $100, I now have a B&K, AV5000 S2 amp that’s totally back to factory and a paper trail to prove it! While I agree, the time invested is valuable, I’m marking it as “education” since I’ve learned a ton on this project. Besides, it was a fun and challenging summer project that kept me out of trouble! Is there anything more dangerous than a bored engineer, with tools! LOL
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OK, I understand!
I was thinking "how did all the FETs burn" but hadn't considered the scenario of some dodgy tech having replaced them all with wrong/counterfeit parts lol.
I was thinking "how did all the FETs burn" but hadn't considered the scenario of some dodgy tech having replaced them all with wrong/counterfeit parts lol.
First several post repairs fire ups of the amp, my stomach felt like it was "half a worm"!
I have to admit I'm still a bit anxious about this amp. I've had it running for a couple of hours connected to a passive pre (no electronics to kill if something goes horribly wrong!) and a pair sacrificial lamb speakers. So far its run perfectly, making the cheapo Fisher, SST-992s sound better than they should. I know eventually I’ll need to install it into my system, but is it just a matter of time before you gain total confidence the amp and in your work?
A good trick with multi mosfet outputs is to get it working with 2 mosfets first.
Its cheaper to replace 2 blown mosfets than 20 !
A mosfet usually blows because something else has gone wrong like a driver or the bias circuit. Or someone has shorted the output or grossly overdriven the amp.
Or put too low ohm speaker on the output.
Or a speaker has failed short.
Its cheaper to replace 2 blown mosfets than 20 !
A mosfet usually blows because something else has gone wrong like a driver or the bias circuit. Or someone has shorted the output or grossly overdriven the amp.
Or put too low ohm speaker on the output.
Or a speaker has failed short.