• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Why Do Outputs Blow?

I think another contributing factor is the old heatsink transfer grease drying out. Despite the close contact, the mica insulator doesn't transmit heat that well, hence the need for the grease. It's a simple matter to just R&R the outputs and put fresh grease in there.

Lee.
 
dr*audio said:
If it's properly functioning that means no failed components and no oscillating, so not probable, but anything is possible.
Click to expand...

Thanks that's what I thought. The amp in question was fully functional with a 90% rebuild. It gets to my buyer's house he claims he hooks it with no load and one channel goes ka-fluey. I get it back and find several blown outputs. My guess is there was some fibbing on the buyer's part and he shorted the speaker wires.
 
leesonic said:
I think another contributing factor is the old heat sink transfer grease drying out. Despite the close contact, the mica insulator doesn't transmit heat that well, hence the need for the grease. It's a simple matter to just R&R the outputs and put fresh grease in there.

Lee.
Click to expand...

A very good point. I'm getting a little older and have to replace the oils in my hands with the O'K****'s hand crème that I get at HD, in the small green jar. There should probably be a list of "first things to do" before powering on a 'new' amp/receiver. Replacing heat grease and trimmer resistors are now on my list.
.


motorstereo said:
Thanks that's what I thought. The amp in question was fully functional with a 90% rebuild. It gets to my buyer's house he claims he hooks it with no load and one channel goes ka-fluey. I get it back and find several blown outputs. My guess is there was some fibbing on the buyer's part and he shorted the speaker wires.
Click to expand...

Now you have to take a video of all functions operating, as well as two dozen pics to document S/Ns and cosmetic conditions before you sell your gear. You should probably get several witnesses to verify the pics & video.
 
ghazzer said:
Now you have to take a video of all functions operating, as well as two dozen pics to document S/Ns and cosmetic conditions before you sell your gear. You should probably get several witnesses to verify the pics & video.
Click to expand...

Apparently my still pics of it working weren't enough. Just another one of many reasons I shy away from shipping.
 
Update:

I got some of the grunt work done this morning. Checked all of the resistors on the output boards. I found this on the main amp board.

picture.php


Those resistors (R724 & R728) are toast. The Alpha 440 manual shows them as 820Ω.

Pulled all of the e-caps and pulled and tested all 50 transistors. Q724(2sc1845) and Q718(2sb718) tested bad. I'll need to find a sub for the 2sb718s.

The drivers all tested fine on my Peak Atlas. Don't know if that's good enough.

I'd like to find originals or subs for;

2sc2336

2sa1006

2sb718

2sd758

If anyone has good substitutes, I'd love to hear them. B+D has them all except the 2sd758 but their shipping calculator shows $37.50 as the cheapest option to Canada.:wtf:
 
Is there a reason I couldn't use a 160v transistor in place of a 200v?



I turns out I can no longer get 2sb718/2sd758 but I can get the lower voltage 2sb717/2sd757. The specs look the same except for the lower voltage. Is it possible these transistors would ever see more than 160v? I don't know if it matters but my mains average about 125v.
 
Last edited:
Would it be possible for an output to blow with the amp just idling? Ex Amp was replaced functionally but never removed from the rack nor disconnected for power just sitting there idling. (This was how I found mine with a couple blown outputs)

Also building on the topic of blowing outputs, is it possible to damage an output so that its bandwidth is reduced or it causes distortion?
 
so_ein_pech said:
why not go up to 300V?

2SA1381/2SC3503 look reasonable to me
Click to expand...

I don't understand transistor specs nearly enough to make a decision on my own. From what I can tell these look good but what do I know? To my way of thinking(likely flawed), the 2sb717/2sd757 would perform excactly like the originals as long as their max voltage isn't exceeded?
 
I did some digging around and got some of the specs writen down of the recommendations I've received from others. I have to start somewhere, I guess.

These are the ones I'm trying to find subs for.

2SB718 200V-50mA-1.25W-B=60-ft=140MHz-TO-202 case (the mounting tab does not need to carry current)

If a 160V is enough, this is a no brainer. They share the same data sheet. How do I find that out?

2SB717 160V-50mA-1.25W-B=60-ft=140MHz-TO-202 case

If not, this looks pretty good to me.

2SA1381 300V-100mA-1.2W-B=100-ft=150MHz-TO-126 case

These don't seem to be very similar.

MJE5852G 450V-8A-50W-B=15-ft=?-TO-220 case

MJE15033 250V-8A-50W-B=10-ft=30 TO-220-3 case

NTE 39 300V-500mA-20W-B=30-240-ft=?-TO-126 case

Where do I go from here?:scratch2:
 
Last edited:
He does not have a scope, yet. Hopefully he has a friend nearby with a scope and can take his overheated patient for a visit and a check-up.
 
I once had a fellow bringing me (repeatedly) an amplifier that was all blown up in the power amp with all the outputs toast. After I fixed it for him about 3 times (same thing each time) I asked him if he was using it for loud parties or something? He told me he was driving 150W floodlight bulbs in a homemade "color organ".

Yeah, that's gonna work. It was lucrative for me, as he had plenty of money. I don't remember just how many times I "fixed it", but I hated to see the amp abused like that.

His comment. "It only blows up when I turn it up so the lights are "real bright". Yeah....

Dave :music:
 
dr*audio said:
Do you have an oscilloscope? If so, check the output to see if the amp is oscillating.
Click to expand...


I am actively searching for one at the moment. Would 20 MHz be sufficient for this and in general, or should I be looking for something in the 50-100 MHz range?
 
On average, the human range of hearing is considered to be ~20 Hz to ~20KHz, so 20MHz would be 1,000 times the hearing range that a young person might hear. Your gear might produce some harmonics, but 20MHz should more than cover any that might be of interest.

I lucked on to a TEK 468 that is very similar to the scopes I sometimes used at work, so I am familiar with the style. Many of the available scopes today are digital, and can present a text interpretation of a signal as "subtitles". I can see where it would be very useful to see a sine wave on the scope and read "Freq =1,234 KHz, Amplitude = 4.5V," etc at the bottom of the screen.
 
So I have this amp now, Sriskie brought it last weekend. The bias is now under control, turns out the thermistors in the bias circuit were way above their 312 ohm cold rating, one measured 495 and 510. I installed new tonight, bias dialed in like a new amp, stable at 17mv. I then proceeded to perform the 2nd step which is the current limiter, just as I got the right dialed in the left blew. Negative 8a amp fuse puked and the outputs blew, also took out q717. Back to replacing parts- i'll do the outputs and diodes- there's really nothing left to replace????
 
I had an interesting issue with a very vintage Technics su3602 that I will work on shortly - there was a problem powering it up with the DBT and it wasnt happy. Checking all outputs revealed one had blown - nothing else further upstream had gone. never had this issue before - there are usually related components that have also been fried. The one output blown in isolation makes me wonder if a shorted speaker cable was the culprit.....
 
You must log in or register to reply here.
Back
Top Bottom