| Electronics > Repair |
| Ampeg PF350 - IRS20957s no oscillation |
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| Mojoampguy:
Still No oscillation on the IRS20957s! I have again replaced both Mosfets irfb4227’s and IRS20957s purchased from Mouser ( so definitely Not fakes). No change. CSD voltage sat at 3.2v. Probably due to elusive fault. See attached file with measured voltages on all pins. Note values in () are relative to Vss (-73v). I subsequently took a tip from Chris56000 to pull up CSD to VDD via 10k resistor. No joy. CSD does rise to 4.5v but needs to exceed +5v to start oscillation. This leads me to believe that the device is protecting itself. Points I notice are 1. VB- Vs = 7.5v - This below is Undervoltage threshold. Needs to be between 10-14v. I do not know why as I have confirmed 100K resistor from +73v rail is in spec and connects to all appropriate points. 2. CSH voltage is 1v. Again too low. At this stage I seek further help. It’s driving me mad!!!! |
| Chris56000:
Afternoon Dave! . . .I would replace all of the ES1D gate circuit diodes and the '4148s associated with them, but it might be worth removing the '20957 and the MOSFETs again, and making sure that you've not got some hidden or latent PCB defect, possibly associated with your previous attempts – this i.c. can sense a fault on its external circuit quicker than you can see it with a meter or oscilloscope! . . .A few meter checks with the i.c. and MOSFETs removed might show up something o/c, low or high resistance, or s/c that the i.c. has detected as a fault – I agree with you that it has gone into an internal protection mode! . . .There are any number of faults that.can occur on a circuit like this that can't be drawn on a diagram, and it is very possible something has cropped up associated with your previous work that's consistently been missed! My late Dad always used to say "check and re–check the work you've done!" Chris Williams |
| jboy32:
Hi Dave and Chris I followed closely your discussion on this situation. I have exactly the same situation on an AMPEG PF 350. I have already fixed the SNPM part with Q1 an Q2 burnt. + some other diodes and resistors around. I'm not a tech amp but that is my hobby :) and have some electronic skills. On the Power side, Q10 and Q13 were shorted, so I change them but again they go to shortcut. So I have been looking to IRS20957s and changed Q10 and Q13 it and got the same results. For the moment Q10 an Q13 are off the board until I understand why they are burning. Without them the amp power up without the Fault Light with sound on the headphone. I have also +/- 74Vdc on the rail where based on the schematic I should get +/- 55Vdc. I was thinking the same as without any load or because IRS20957s do not start to oscillate that is why I'm getting a high voltage ... On Pin 2 of IRS20957s voltage is not stable. I read some other topics related to IRS20957s but without real fix. I noticed on my amp based on the revision that +VCC_12 coming on pin 11 of IRS20957s is not produced as what I have in the schematic. I have not U7 that seems to regulate voltage from a floating +18Vdc to 12Vdc ... I measured this and having +14,97VCC referenced to -74Vdv. In my case also I have a DC voltage on the output 2,7 Vdc (GND ref), I measured with and without IRS20957s. I replaced 2 times IRS20957s but still the same situation. So that is not good. I add the voltages measured on IRS20957s if that can help to investigate without Q10 and Q13 installed. Did you make some progress there ? Many thanks. |
| Mojoampguy:
Hi JBoy32, You appear to be in the same position as me. Still not resolved. If we finally find a resolution, we will post details here for others to benefit from. |
| jboy32:
Hi Mojoampguy, for sure if we fixe it we will update a solution here :) I was thinking to investigate also on the rail +/- 74vdc vs +/- 55 on the original schematic. I fixed the SMPS by replacing what was burnt or without the right value but maybe I missed something around. I was also looking to maybe change the diodes D32/D33 ES1D and D34 ER1D. |
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