| Electronics > Repair |
| Ampeg PF350 - IRS20957s no oscillation |
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| jboy32:
Good afternoon I followed your recommendations by sitting an IRFB4227 on the HO. So getting all new and replaced component in place. In parallel, I was thinking that it could be good to reduce the input voltage of the amp, I have a VARIAC (0Vac to 300Vac) and moved from 235Vdc to 190Vdc main input so, I have +/- 56 VDC like the original schematic, and other low voltages are pretty closed to what is expected. You know what ? it works ! ;D I have 9,31 stable dc voltage on pin 2 CSD and 2Vdc on Pin3 with 400Khz PWM ! :-+ with a load of 8R connected to the output and no input signal. I plugged into a CAB ang got a nice sound, for the moment I didn't go forward, because I don't know if it is because of the low input voltage that finally adjust rails on +/- 56Vdc or I did something else. I'm just afraid that if I move from 190Vac to 235Vac so getting +/- 75Vdc on rails that could destroy again the power amp part. If that is the case we need to investigate on the SMPS power supply. That in my case was also burnt at the beginning, and I replaced some components there. From the power amp part the only thing I changed : - IRF20957s x1 - IRFB2742 x2 - C61 with another 10uF capacitor If only an Ampeg tech could confirm that rails voltage on this release should be +/- 55Vdc or +/- 75Vdc with REV H that could help to investigate on the SMPS. Julien |
| jboy32:
Good morning, Update on the situation, I did some tests yesterday evening by increasing progressively main input voltage so increasing in parallel rail voltage from: 190Vac Main input -> +/- 56Vdc rail Working and powering off then on = ok 200Vac Main input -> +/- 59Vdc rail Working and powering off then on = ok 210Vac Main input -> +/- 62Vdc rail Working but when powering off then on -> one IRFB4227 (Q10) shortcut I replaced again Q10 IRFB4227 and it is working by reducing main input voltage to 190Vac. So it makes me thinking that we need to get +/- 55 Vdc on the rails. Possible investigations : U1 : KA3525A (I already checked all components around they are all good) T1-1 : Transformer for rails voltages damaged ? D16 : MUR1620CT damaged ? D19 : MUR1620CTR damaged ? |
| Vicus:
There is no feedback on the +-55V converter so a reason for an overvoltage could be a different switching frequency so I would check the passive on the oscillator section of the 3525. Also maybe a partially shorted primary could change the transformer ratio? |
| Chris56000:
Update on the one I'm attempting to fault–find for Member Dave :– a) It appears the the power supply as originally designed gives out ±73 V despite the available circuit being clearly labelled ±55 V ! I ordered a ±55 V power supply from AliExpress and it was a considerable time in arriving and then when it arrived it gave out ±68 V despite being marked "55 V" on both the box it came in and the PSU transformer, so that was returned with a note to the supplier, who eventually supplied a replacement that did give the ±55 V marked on it, which I verified with a reliable meter! b) After the shenanigans over the wrong voltage test psu being supplied the '20057 STILL didn't oblige me, despite supplying ±55 V to it – I have to come to the conclusion that this is some sort of undocumented revision to the design, which I did notice was marked "Rev H!" . . .I have had a most courteous tootle with the owner, and if it doesn't oblige me by next Friday I will reassemble the unit and return it to him, FOC post paid – there is obviously a serious latent defect in this "Rev H" design that causes repeated failure that another Member has run into! . . .I will do my best to provide an updated "Rev. H" schematic covering these PCBs before I post the one I'm attempting back to the OP! Chris Williams |
| jboy32:
Hi Chris thanks for the update, I was thinking the same, +/- 75 Vdc seems to be the design of the Revision H. I have seen on different forums people facing issue with REV H and getting +/- 75 Vdc ... It seems so that the issue is more on the power output side. I have tried multiple investigations with no real success. I'm now thinking to replace the full PSU + power with one from AliExpress. I did order one unit, but the output is noisy and the fan also very noisy ... :-\ The problem is that there are lot of controls, limit / peak ... that needs to be set with the proper voltage on the preamp side. The best deal might be to just replace the power amp side with a unit having the same characteristics, powered with the +/- 75 VDC of the Ampeg. I never investigated on the preamp side could that have an issue on the power side, I don't know. I post the 3 schematics revision I found if that can help some others. |
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