| Electronics > Repair |
| Ampeg PF350 - IRS20957s no oscillation |
| (1/7) > >> |
| Mojoampguy:
I have repaired the SMPS in this class D bass amp. Unit powers up, fault light goes out, but no audio output. This is due to IRS20957s gate driver NOT starting up oscillation. All voltages look good, except for CSD which sits at 3.2v. It need to be greater than 5v to start up. External mute circuits are not active,. I’m stuck. Schematic is attached. IRS20957s is U8 in power amp section. I have changed this device 3 times already. All connecting components appear good. |
| Audiorepair:
Where did you buy the 3 IRS devices? |
| Mojoampguy:
Bought from Farnell components in Uk. Reputable supplier. I realise there are fakes about. |
| Chris56000:
Hi! . . .Thank you for providing the schematics and layouts – it helps us enormously when diagnosing a fault like this! . . .Apart from some strange Asian semiconductor type nos, it's relatively straightforward to diagnose this! . . .First of all, is the delayed power–on reset releasing the "MUTE" line? The +55 V main h.t. supply is connected to a reset circuit consisting of C127, R86 and R89, the voltage at the positive terminal of C127 should be about 5.7 V and Q9 should be conducting with its base approx. 0.6 V. . . .If Q9 is not being turned on, check the +55 V h.t. supply is reaching that voltage, and replace C127, Z2 and Q9 if you're in any doubt about these components – a BZX84–C5V1 will do for Z2 if the package matches! There is also a 5 V "P_MUTE line needed to turn ON transistor Q11 to release the "MUTE" line once the main power supply circuit has started up – this comes from the 5 V "Reference Voltage" output of the '3525 SMPS controller in the power supply circuit – if the '3525 isn't running correctly, Q11 will not turn ON and the "MUTE" line will remain low – this is a safety protection feature designed to turn OFF the drive to the Class D power MOSFETs until the power supply is running correctly! . . .Also make sure you have no extraneous DC at the mid–point of the Class D output stage due to defective MOSFETs or a low or missing ± 55 V h.t. supply – problems here will trip the power muting circuit via the d.c. protection transistors Q15, Q16 and Q17 in the power amplifier circuit! . . .There are also three temperature sensors in your amplifier that need a quick check – TS1 is a normally closed bimetallic thermal switch fitted to the main heatsinks, an PTC thermistor TH2 in the temperature protection circuit, approx. 50 Ω at room temperature , and a DO220 package thermal device TH3 called an "AIRPAX 67L050" which should be a short–circuit cold – this becomes high resistance when its tab reaches 50°C or greater, when any of these sense an over–temperature fault, Q12 in the power supply circuit turns ON, shutting off the main '3525 SMPS controller by way of the '555 monostable U4 which trips the "/SHUTDN" input at pin 10 of the '3525! . . .Therefore, the first thing to do is check both the +55 V, –55V and the ±15 V supplies as all these are sensed, followed by the power–on–mute circuit, then the output stage itself for DC faults, then the over–temperature protection, roughly in that order! Chris Williams |
| Mojoampguy:
Lots of great input there Chris. I check all this when the Grandaughter baby sitting duties allow??? One thing I did not mention yet. +/-15v in spec. The HT voltages are measuring +/-73v at the moment, but this May again be due to the IRS20957s not oscillating bringing o/p Mosfet load into play. Also, I have struggled with this for months on and off, your suggestions are well worth checking. I can’t even remember what I’ve checked previously, at this stage. Thanks again for your input. Dave |
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