Crescendo 17k

[caraudioguy2100]

Original power supply fets are IRFP7530

Does anyone know of a good replacement for them ?

I can find some stock of the original part number but either they don’t have enough in stock or I have to buy a large quantity.

 

[Perry Babin]

https://www.newark.com/infineon/irfp7530pbf/mosfet-n-channel-60v-195a-to-247ac/dp/43X7227?CMP=AFC-OP

Newark has about 100 in US stock.

 

[caraudioguy2100]

The drivers are a1220 and c2690 are there good replacements for these ? Everywhere I check says the A1220 is obsolete

 

[Perry Babin]

PS drivers? BD139/140?

 

[caraudioguy2100]

For the gate resistors does it matter if I use 1% or 5% tolorance ?

I do not know the original tolerance of the resistors since this amp has been repaired before by someone else and there are 1% and 5% and both 1/8 and 1/4 watt resistors .

Appears someone used what they had on hand when repairing it the first time

 

[Perry Babin]

It generally doesn't matter but if you're unsure, use the 1%.

 

[caraudioguy2100]

Replaced all the drivers here is what I get for drive do these look good or do I have an issue ?

Scope set to 5us 5volts/div

 

[Perry Babin]

It mostly looks OK but the low part of the drive signal is a bit higher above ground than I'd expect. That's likely from not centering the trace before checking the signal.

 

[caraudioguy2100]

The trace was centered on the screen before taking the pic

 

[Perry Babin]

Well, if the FETs aren't heating at idle, I can't say definitively that there is a problem that needs to be resolved.

 

[caraudioguy2100]

I gotta pull all the fets back out of the amp since it draws excessive current with them in circuit . So something isn’t right with the drive or something is shorted causing the power supply to draw current

 

[Perry Babin]

That seems like a lot of work. Does it get better with the rectifiers out of the amp?

The drive was a bit off of what I'd expect but unless the FETs have a VERY low threshold, I don't think the drive is the problem.

What's the frequency on pin 9 or 10 (measured with a multimeter)?

 

[caraudioguy2100]

I can’t get a frequency reading with my meter for some reason .

I removed all of the fets from the power supply

I noticed the drivers get red hot after a min they get to hot to touch them

 

[Perry Babin]

place the black meter probe on pin 7 (if the IC is a 494).

What's the resistance (no power applied) between the emitters of the drivers to ground?

 

[caraudioguy2100]

Some of the BD140’s read 0.486K ohms while others read 10 ohms

And the Bd139’s I can’t get a reading on them my meter won’t measure them for some reason

 

[caraudioguy2100]

I rechecked them and I get 0.468 on both 139,140 in a few banks of drivers

The other banks of drivers I get 10 ohms

 

[Perry Babin]

I don't understand. The emitters of the 140s are directly connected to the emitters of the 139s. Why can't you read the 139s?

10 ohms would appear that the pulldown resistors are 10 ohms.

What's the resistance across the gate and source of the FETs?

What's the value of the gate resistors?

 

[caraudioguy2100]

See post #16

Gate resistors are 10 ohms

From gate to source on some I get 0.467K ohms Others I get 10 ohms

I think some of the drivers are shorted should I pull the drivers ?

 

[Perry Babin]

Are you sure that you don't have any solder bridges?

You could pull the drivers reading low and see if anything changes with the board readings.

 

[caraudioguy2100]

Positive I have no solder bridges I will pull the drivers that read low and post pack