Punch 150s/Power 550s

[Perry Babin]

Re-post the voltage on the 4 transistors in post 32 but post it for all 3 legs of each transistor.

 

[neela30]

Q220
1 44.2
2 44.2
3 44.2

Q221
1 15.3
2 43.3
3 44.2

Q222
1 -13.9
2 -42.3
3 -42.8

Q223
1 -44.1
2 -43.4
3 -42.8

 

[neela30]

The fets I relpaced seem to have blown and the output on the right channel is now +44V instead of -40V?

 

[Perry Babin]

Did you use the numbering scheme shown in the following datasheet:
http://datasheet.octopart.com/MMBTA06LT1G-ON-Semiconductor-datasheet-8326744.pdf

You'll have to replace the blown parts before you can do anything else. Clamp them down before applying power.

 

[neela30]

My mistake - Corrected

Q220
3 44.2
1 44.2
2 44.2

Q221
3 15.3
1 43.3
2 44.2

Q222
3 -13.9
1 -42.3
2 -42.8

Q223
3 -44.1
1 -43.4
2 -42.8

All fets were clamped tightly.

 

[Perry Babin]

Were these taken with new (no blown) FETs?

 

[neela30]

These were taken with the newly installed fets that seem to have burned immediately. The IRF540(Q225) burned open and the 9540(Q224) appears shorted again.

 

[Perry Babin]

Did you confirm that the 540 was, in fact, not functioning as it should?

Did you confirm that the 9540 was indeed shorted?

To make any sort of suggestions, I need definitive information.

 

[neela30]

I pulled the fets from the board and checked resistance per your page and they show OL in all 3 tests. My diode capable meter seems to have been borrowed/misplaced so I could not complete that test at this time.

 

[Perry Babin]

Did you check another 540 the same way with the same meter and get the same results?

 

[neela30]

I pulled all of the 9540/540 on that side. 540s check out but I did find that one of the other 9540 is fused (Q219).

 

[neela30]

Soldered all known good parts in and the DC voltage is gone off the output.

 

[neela30]

Used a spare 9540 from the 150 repair and it sounds good with the exception of the crossover switch. You can fiddle with it and the output changes volume like the switch is dirty. Can these be easily cleaned?

 

[Perry Babin]

They can sometimes be cleaned. Use any type of contact cleaner that's safe for plastic and flood the switch. The stuff Radio Shack sells is decent. Then work it back and forth 50-100 times.

 

[neela30]

Thanks for all of your help Perry!

One more question... I am replacing the 9540s and I am thinking of replacing the 540s on the right channel. Will the 77mOhm 540s work or do I need to go to a different FET?

 

[Perry Babin]

Any IRF540 will probably work except the IRF540N. I'd suggest that you use the IRF540 (no suffix).

 

[neela30]

The right channel blew again in the 550. I haven't removed any parts as of yet.

I had installed this in my car replacing the 150 I had previously repaired. I noticed immediately the amps sounded different. The 150 has good response to all types of low frequencies. The 550 sounded "mushy" as resonant bass was loud but drums just didn't kick. It also ran a bit warmer than the 150 - mainly on the right channel.

Could this condition be caused by an op amp or a weak transistor? Could the switch have damaged anything while it was getting a poor connection?

 

[Perry Babin]

If the switch caused a loss of signal to the right channel, that could have caused it to run hotter than normal.

When you get it repaired again, test each channel (either one at a time or into two independent loads but not bridged) on the bench for at least 20 minutes and listen to make sure they sound the same.

 

[neela30]

I swapped in a known good 9540 and got it going again. The output on the left channel is crystal clear. The right is a different story.

The best way I know how to describe how audio sounds on the right channel is that it is very similar to listening to a partially magnetized cassette in that some sound fades in and out and others seem to have an exaggerated stereo effect?

 

[Perry Babin]

Does it play consistently if you move the crossover selector switch to other positions?