Phase Linear 400 Amplifier Upgrade to WOPL

[laatsch55]

Yep , right channel definately exhibits some non-linear behavior. Can you tell a difference in a listening test??

 

[NavLinear]

Ok - time for my short story "What Happens When You're not Paying Attention".

Once upon a time in a lab far far away - ahhhhhhh kinda corny beginning isn't it?

Anyways - I have some of my stereo and test equipment at work as I didn't want to compete with the animals at home and using the dining room table dosn't pass the wife check - besides more than one cat has been shocked while being curious so until I get a bench set up at home I'll do my tests at work. Of course all testing is done after work and at lunch.

One evening last week before leaving to tend to the horses I wanted to spend a half hour or so conducting tests with the amp under load. I had my notes scattered about so I grabbed one and looked at what input voltage was required to get approximately 10 watts across 8 ohms. I punched in the numbers on the HP 8903A and was getting ready to take some numbers when I smelled HOT. You know - not the let the smoke out hot but just HOT - like something was getting toasty and you should do something about it - like NOW!!!

I looked up at the scope and realized that the amplifier output trace was way past full scale so I dialed the gain down while I looked at the Fluke multimeter and saw a fairly big number and that didn't seem right - way too big of a number for 10 watts. Now this all happened over a couple of seconds and it was late and ... uh-oh.

I saw a sine wave on the o'scope and it wasn't normal but in that instant my mind must have a little numb so it didn't sink in that the amplifier was way into clipping. Now I should have known better but the HOT smell was strong so what did I do? Well I glanced at the PL output meter while I placed my hand on the loads to see if they were hot. Does anyone know what the normal response time is when !HOT! is felt at the hand - the signal goes to the brain - the brain process that's F$#king HOT - and then sends a return signal to arm to move hand from !HOT! . I don't know what the answer is but it was way too long. Those damn resistors get bloody hot when driven hard. So I pulled the hand from said HOT source at the same time I was dialing down the input source.

Everything turned out well - no damage to the amp or loads as I got to it quickly. Just a sore hand. The voltage I dialed in was the voltage that should have been across the load and this is a 26dB gain amp so.... The meter that I glanced at indicated a voltage across the load of approximately 59 Vrms - I think this amp goes into clipping in the mid to upper 40's.

Just to clarify - no animals were hurt during these tests.

This is my story and I have no doubt that there will be more - I just hope it dosn't include HOT!

 

[NavLinear]

Yep , right channel definately exhibits some non-linear behavior. Can you tell a difference in a listening test??

I'll get it set up this week and give it a listening test.

 

[laatsch55]

Hmmmm.... sounds awfully familiar......

 

[mlucitt]

Excellent load test of load test resistors - pass!

 

[NavLinear]

Hmmmm.... sounds awfully familiar......

So what was the diagnosis?

 

[NavLinear]

Excellent load test of load test resistors - pass!

It made quite the hand warmer. I forgot to mention that I did - quickly and delicately - touch the heat sink and it was doing its job well!

 

[Pure_Brew]

I remember when I had an adcom 555, and I messed with some lil trimmers inside the amp for some reason, think they were bias adjustments? I dunno. Anyway I remember kicking back and listening to tunes, smelled that HOT smell. I looked up and I saw big wafting heat waves above the amp! I undid, whatever I did lol.

 

[Skratch]

Ok - time for my short story "What Happens When You're not Paying Attention".

Once upon a time in a lab far far away - ahhhhhhh kinda corny beginning isn't it?

Anyways - I have some of my stereo and test equipment at work as I didn't want to compete with the animals at home and using the dining room table dosn't pass the wife check - besides more than one cat has been shocked while being curious so until I get a bench set up at home I'll do my tests at work. Of course all testing is done after work and at lunch.

One evening last week before leaving to tend to the horses I wanted to spend a half hour or so conducting tests with the amp under load. I had my notes scattered about so I grabbed one and looked at what input voltage was required to get approximately 10 watts across 8 ohms. I punched in the numbers on the HP 8903A and was getting ready to take some numbers when I smelled HOT. You know - not the let the smoke out hot but just HOT - like something was getting toasty and you should do something about it - like NOW!!!

I looked up at the scope and realized that the amplifier output trace was way past full scale so I dialed the gain down while I looked at the Fluke multimeter and saw a fairly big number and that didn't seem right - way too big of a number for 10 watts. Now this all happened over a couple of seconds and it was late and ... uh-oh.

I saw a sine wave on the o'scope and it wasn't normal but in that instant my mind must have a little numb so it didn't sink in that the amplifier was way into clipping. Now I should have known better but the HOT smell was strong so what did I do? Well I glanced at the PL output meter while I placed my hand on the loads to see if they were hot. Does anyone know what the normal response time is when !HOT! is felt at the hand - the signal goes to the brain - the brain process that's F$#king HOT - and then sends a return signal to arm to move hand from !HOT! . I don't know what the answer is but it was way too long. Those damn resistors get bloody hot when driven hard. So I pulled the hand from said HOT source at the same time I was dialing down the input source.

Everything turned out well - no damage to the amp or loads as I got to it quickly. Just a sore hand. The voltage I dialed in was the voltage that should have been across the load and this is a 26dB gain amp so.... The meter that I glanced at indicated a voltage across the load of approximately 59 Vrms - I think this amp goes into clipping in the mid to upper 40's.

Just to clarify - no animals were hurt during these tests.

This is my story and I have no doubt that there will be more - I just hope it dosn't include HOT!

Dennis, The smell you experienced was the oil coming from the resistors that probably never got that hot in their life. A PLWO 400 at full power will heat up a 200 watt load instantly

 

[laatsch55]

Dennis, The smell you experienced was the oil coming from the resistors that probably never got that hot in their life. A PLWO 400 at full power will heat up a 200 watt load instantly.

I beg your pardon---- Those are the same loads that took 760 watts for a few seconds when I also hit the wrong button on the AP. They had been smoked previously, it was an experienced smoker.

 

[laatsch55]

So what was the diagnosis?

Near as I can tell Dennis it's a case of PEBCABP. I know because I also have pushed the X10 button instead of the X1 button.

Oh , you meant what's wrong with the amp?? Old and tired, chuck the board throw in Joe's and forget about it for the next 30 years!!


PEBCABP---- Problem Exists Between Chair And Button Panel

 

[NavLinear]

Near as I can tell Dennis it's a case of PEBCABP. I know because I also have pushed the X10 button instead of the X1 button.

Oh , you meant what's wrong with the amp?? Old and tired, chuck the board throw in Joe's and forget about it for the next 30 years!!


PEBCABP---- Problem Exists Between Chair And Button Panel

Now you've got me LMAO!

Yeah - old board will be coming out but I did warm up the load resistors again but this time on purpose. I was testing the amp at clipping and almost completed testing the right channel and then nothing. It took me a few minutes to figure out what went south but with a a 20 mV input driving the amp the scope showed only a positive excursion and this was on both channels. Had to be something common and I almost probed the amp but turned it off - used the cap zap and pulled the fuse. Sure enough that was it - probably weaken it with the PEBCABP moment.

I may have some fuses at home - I remember getting some many years ago and I might even know where they are. If not it looks like Newark carries them.

View attachment AGX 5 Fast Acting Fuse.pdf

 

[NavLinear]

Dennis, I don't know if it will make a difference but bias on a pl 14 board which is what I'm assuming you have should be taken with positive on the base bus bar and negative on the common tie point for the emitter resistors. Ya might try that and see if there is a difference.

I missed this post last night. Yes - it is a PL 14 board. I'll check this out after the fuse is replaced - but probably a moot point as the new board is going in.

 

[NavLinear]

Dennis, The smell you experienced was the oil coming from the resistors that probably never got that hot in their life. A PLWO 400 at full power will heat up a 200 watt load instantly.

I beg your pardon---- Those are the same loads that took 760 watts for a few seconds when I also hit the wrong button on the AP. They had been smoked previously, it was an experienced smoker.

Well - actually there were two virgins. I replaced the two that were somewhat damaged (see above) so the newbies still had smoke in 'em.

 

[laatsch55]

I missed this post last night. Yes - it is a PL 14 board. I'll check this out after the fuse is replaced - but probably a moot point as the new board is going in.

No it's really not moot, there's something naggin at me about the way you measured bias........,something important....just can't put my finger on it yet...

 

[laatsch55]

Well - actually there were two virgins. I replaced the two that were somewhat damaged (see above) so the newbies still had smoke in 'em.[/QUOTE

Somewhat damaged?? Like pushed halfway out the case?//

 

[laatsch55]

Now you've got me LMAO!

Yeah - old board will be coming out but I did warm up the load resistors again but this time on purpose. I was testing the amp at clipping and almost completed testing the right channel and then nothing. It took me a few minutes to figure out what went south but with a a 20 mV input driving the amp the scope showed only a positive excursion and this was on both channels. Had to be something common and I almost probed the amp but turned it off - used the cap zap and pulled the fuse. Sure enough that was it - probably weaken it with the PEBCABP moment.

I may have some fuses at home - I remember getting some many years ago and I might even know where they are. If not it looks like Newark carries them.

View attachment 6995

Get a lot of em. When sine wave testing 5's will go frequently. I had to go to 10's when maxing out a WOPL 1000 at 4 ohms.

 

[NavLinear]

Well - actually there were two virgins. I replaced the two that were somewhat damaged (see above) so the newbies still had smoke in 'em.[/QUOTE

Somewhat damaged?? Like pushed halfway out the case?//

Didn't want to let the cat out of the bag - but now that you mention it I'd say more like "taking half a dump" or "sqeezing off mid way while pinching a loaf".

 

[NavLinear]

No it's really not moot, there's something naggin at me about the way you measured bias........,something important....just can't put my finger on it yet...

Ok - I'll give it a closer look. I did follow the PL Service Manual but will double check to see if there's more than one procedure. There are multiple schematics so I wouldn't be suprised if there's more than one test procedure. Hey - this is fun though.

 

[laatsch55]

Oh yeah , smokers are always a gas!!!