SVT 7 pro no speaker output

[lowell]

Please note that you haven't specified Vdd voltage for U17 so it's difficult to say whether it's too low or not. Before you start desoldering U300 I would check diodes in the power amp. Have you checked (with magnifying glass) how surrounding components are soldered? I would also look (very carefully) at HI and LO outputs of both U300 and U17. Can you post a photo of how U300 is soldered?

Mark

Thanks guys. I have no idea how I forgot to list Vdd for both chips. Sorry. U300 is 4.2vdc and U17 is ~10vdc.

I'll check out the other stuff later today.

[Jazz P Bass]

Thanks guys. I have no idea how I forgot to list Vdd for both chips. Sorry. U300 is 4.2vdc and U17 is ~10vdc.

I'll check out the other stuff later today.

There is a problem with U300.
VDD shhould be 10Vdc.
(there is an internal 10V zener in the IC)(20955 datasheet/page 9)
So either the voltage leading in to pin #1 is low or the IC itself is loading down the VDD.
(the internal zener may be bad)
The 15Vdc path is through R110 (2.2K) C142 to ground (10uf/25V) to U300/pin 1.
If R110 checks good (2.2K) you could try lifting C142 (10/25).
That cap "may" be loading down the VDD.
If not, then U300 is suspect.
You could also check the zener with your DVM.
Pin 1 to pin 4.
Compare it to U17.

[MarkusBass]

If you decide to replace IRS20955, please note that this IC is not recommended for new designs (it is not recommended in general) and should be replaced by IRS20957. You may read this: http://www.irf.com/technical-info/appnotes/an-1141.pdf . In this case both ICs should be replaced.

Mark

[Jazz P Bass]

re: IRS20957 / replaces IRS20955
So they said what that applies to this repair?
Do not let Vdt pin 8 exceed 15 volts?

[MarkusBass]

I'm not sure who you mean by "them". Ampeg support? The change is recommended by IR. You may ask Ampeg support but they usually do not answer to such detailed technical questions. IRS20955 can still be purchased but I heard several complaints on noise caused by the IC. It's even mentioned in the application note. They (IR) also mention improved (corrected) narrow puls behavior. But the replacement may require some additional checks so maybe Lowell is not going to change the chip. Also, he hasn't check all the voltages required by the IC. By measuring the voltage drop on R110 (2k2) it is possible to calculate current consumption. This could answer whether the internal Zener diode is dead, or there are other voltages missing.
BTW, isn't the DT voltage referenced to COM? What is this voltage in this amp?

Mark

[Jazz P Bass]

re: IRS20957 / replaces IRS20955
So they said what that applies to this repair?
Do not let Vdt pin 8 exceed 15 volts?

Sorry Markus.
"They" as in International Rectifier.
I was refering to the new datasheet 20957. (soto voce)
That the Vdt voltage must be 15Vdc or less is the what I saw that pertains to this amp. (as far as what Lowell should (can) check.
The other 2 changed items are cool but do not need to be tested for an assured value.

[MarkusBass]

Sorry Markus.
I was refering to the new datasheet 20957. (soto voce)
That the Vdt voltage must be 15Vdc or less is the what I saw that pertains to this amp. (as far as what Lowell should (can) check.

Please note that Vdt in this amp is (should be) set by voltage divider R108/R109 between -43V and -55V rails. But R108 is labeled as EMPTY so Vdt voltage is exactly equal to -55V. And the voltage is measured relative to -55V so in this case it is equal 0V. So there is no danger that it will be higher than 15V. As the difference between -43V and -55V is 12V, with any R108 resistor the voltage cannot be higher than 12V. I wonder why is it set in this way.
The problem that Lowell has is that the amp is not working correctly. Usually, in such cases I don't start with modifications, but first I bring the amp to the state it was before the failure. So maybe IRS20955 would be better in this case.
I wonder what Ampeg support says about replacing the IC. Could you ask them? You have a shorter distance

Mark

[Jazz P Bass]

The IRS2097S is cheaper

[lowell]

Those gate drive chips have about 10 different kinds of protection, all of which shut them down. I'd check that all supply rails are present, and none of the power MOSFETs is blown.

Is that Vb really -18V?

Sorry Steve... no it's +18v. Good catch!

[lowell]

Ok so diodes check out fine with diode test on meter. I unsoldered C142 (on pin1) on U300 and the voltage was still low and the same value of 4.2v. I cannot check the HI and LO pins as they are hidden under the heatsink or whatever those metal u-shaped things are. I have attached a couple pictures of what it looks like inside. As you'll see it's a PITA to locate components that are connected to the pins. JPB the zener from pin1 to pin4 tests good with my meter.

It looks as though I'll be needing to replace U300. I'm quite scared at the thought of this as it's proving to be next to impossible to work on these SMD components. At least with the equipement I have. I do have a desoldering gun and a good iron w/ a very small tip. I cannot even seem to get C142 back on the board.

[lowell]

So I guess I'm left with replacing the 20955. I feel quite unqualified in this repair. I fear desoldering the IC and soldering in a new one. SMD is a new thing to me. I've done a bit of research on repairing SMD. One site suggests a solder that has a lower melting point, and to use flux to help hold the components while they are soldered in. I would REALLY appreciate some advice on this, otherwise I'm gonna send the customer to someone more qualified, which is rather not do since I feel like a may be close to fixing this amp. I've certainly learned a lot about class D.

[Jazz P Bass]

Lowell, from the photos that you posted it appears that you will be working in a well.
It is one thing to walk you through replacing a SMD opamp on a wide open board.
This is another animal.
With all due respect, the customer deserves a properly repaired amplifier.
I would bag it.

[MarkusBass]

One site suggests a solder that has a lower melting point, and to use flux to help hold the components while they are soldered in.

If you don't have specialized tools, you may use this: Chip Quik: Easy removal of surface mounted devices
Soldering the IC back is easier since every lead can be soldered separately.

Mark

[lowell]

Thanks Markus. I also found this video which is quite helpful. However, I cannot find ANY irs20955 ic online. Anyone know where to get these?

YouTube - Surface Mount Soldering 101

[Jazz P Bass]

IRS20955SPBF | Arrow Electronics Components Search

Cool video.
Thanks.