Digital Designs D4.90 in protection

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Have a Digital Designs D4.90 that shows the red protection LED when power is applied. The amp uses the TDA8954J chip as outputs and I have 2 new ones here in case they are needed. I removed the rectifiers to isolate the power supply but still only get the red protection LED. The PS driver board uses a SG3525AP. There was no gate drive to the PS FETS so I went digging around the driver board. I found a shorted MMBTA06 at Q508 and the 47 ohm resistor that fed +12v to VCC of The SG3525AP. After removing and reinstalling the driver board a few times, the gate drive out of the IC started to work so I resoldered it in that exact position, and it has gate output out every time remote is applied now but the protection LED is still illuminated.

The voltages coming out of the SG3525AP pins 11 and 14 are not equal, one is slightly higher than the other. With PS FETS installed, they heat up rapidly, the current draw increases greatly, and then the transformer buzzes before I quickly cut off the supply power. The rectifiers are still out so it shouldn't be anything in the output section, and it does this even with different FETs. I am posting pics of the amp board, pS driver board, and scope shots of the PS gate drive.

Any idea what to check next?
 

Attachments

  • IMG_20201022_172902__01.jpg
    IMG_20201022_172902__01.jpg
    556.5 KB · Views: 74
  • IMG_20201022_172934.jpg
    IMG_20201022_172934.jpg
    987.6 KB · Views: 73
  • IMG_20201022_172745.jpg
    IMG_20201022_172745.jpg
    921.1 KB · Views: 68
The capacitor is to be connecting the gate pad and the ground, or +12v pad?

For some reason, the red trace for probe 1 is shown higher than the yellow trace for probe 2 even though the arrows for both are lined up at 0.0.

The voltages on pin 13 is 9.57, and pin 15 is 9.57 also. The gate voltages are 3.38 for one bank, and 3.59 for the other at the gate pads. Pin11 is 3.65, and pin 14 is 3.87 at the SG3525.
 
Last edited:
If I lower the arrow for the red trace/probe 1 to -.02 divs, it will line up with the yellow trace.

The attachment is the gate drive with a 10nf capacitor loaded into each bank, gate to source. This is the most recent test performed, I deleted and replaced a previous attachment.
 

Attachments

  • IMG_20201022_202148.jpg
    IMG_20201022_202148.jpg
    980.1 KB · Views: 56
Last edited:
Generally, if it's a drive problem, the drive signals are crossing above 2v. These appear to be OK.

What is the component that's adjacent to D511?

It's possible that the transformer is shorted. Most shorts where there is no evidence of burned windings are intermittent. Power up through a current limiter and twist the transformer to see if the current draw changes.
 
I assume that is a thermistor next to D511. It is location TH501.

I powered it up with a single FET in each bank. The transformer squealed and whined almost the whole time it was powered up, and twisting it only changed the volume and pitch of the noise. The mains limiter bulbs remained brightly lit, and the two tightly clamped FETs got very hot.
 
The secondary windings have no effect on the excessive current draw. Pulling the primary windings will eliminate the current draw.
Is it feasible to try sticking in a transformer from a scrap board, or to try to unwrap this one enough to hopefully find a short?

Is it possible that something on the driver board is causing this?

I reinstalled the rectifiers to see if the amp would turn on, and it did. The white light that DD uses lit up, but the red protection LED remained illuminated also. As the amp powered up, there is a momentary spike of around+2vDC on each speaker output terminal. Should I attempt to find the cause, and rectify the protection light issue as it might lead me to the cause of this problem?
 
There are only eight connections for this transformer, four primary and four secondary. I don't think the transformer is bad as I replaced it with another from a parts board with exactly the same result. In fact, the original transformer is back in the amp and silent right now, but the amp is drawing a lot of current at idle. I will list my most recent findings:

PS FET/gate resistor: I experimented with different FETs in the power supply to try to see if it would make a difference. I currently have 1 CS50N with 47ohm gate resistors in each bank. During experimentation this combination had the transformer the quietest. Original was IRF1404 with 10ohm gate resistors. I assume that I will end up having to go back to the original specs.

When I got the amp it wouldn't turn on, just the red protection LED was on. I was able to get it powering up after changing a few driver board components. After experimenting with the gate resistors, the amp would not turn on again, just red protection LED. While troubleshooting, I plugged the RCA from the deck into the amp, and the amp turned on every time with remote power. I discovered +6.7v on the amp's RCA shields which I traced to the PS driver board. On the driver board, I lifted one side of Q517, and the amp turns on and off with remote, AND the red protection LED turns off when the white LED turns on at power up. All four channels play clean audio, but the mains bulbs are still glowing brightly, and the rail voltage is unbalanced. Currently it is at +24v / -20v. It was offset the whole time I have been working on it but the + voltage has been as high as +29v at one point.
 
Amp powers up using a mains limiter, however the bulbs glow warmly. I had to lift Q517 to allow the amp to power up, and it had the side effect of disengaging the protection LED. However, right now, the amp powers up with Q517 reinstalled but with the protection LED illuminated. It was working like this before and then just stopped powering up until I discovered the RCA issue. There is +1.39v on the RCAs right now which may have reduced the voltage on Pin 10 of the SG3525 to +0.48v which might be why it is powering up. Previously, I had +1.09v on pin10 when it wouldn't power up.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.