That 47K must put the current into the uA regime I doubt the Ebers-Moll sort of stuff applies in this sort of illegitimate area of operation. Tried a 6.2V 1W zener Vz 6.22V (initially then rising) @ 78mA
96 percent of Vz @ 7.4mA 95% @ 810uA 92% @ 102uA 77% @ 9.6uA 66% @ 2.6uA (4.12V)I'd forgotten to at least try the PA and its ok up to +/-25V dc on a bench PS , 63V rated caps, so less than the 80V of that schematic. The PA is certainly different to that schematic, pair of those 5 pin SAP devices with intimate thermally connected sensing diodes
I thought startup was a bit quiet, somewhere along the way I've managed to short G-S on the high side IRF740. The other one changes state on DVM "diode" test. Shorted turns on the switch mode transformer ? Change C68, perhaps punch through ?
A dropper will go in with changed 1uF and new sacrificial IRF740 tomorrow
1KHz RLC testing of the Tx feedback coil .039mH 240V tap 2.37mH to gnd 230V tap 2.22mH 100V 1.8mH looks reasonableAlong the way that Zener3 in the 200+220V zener chain is probably 6.2V (at low uA level) and the 2N7002 (SMD topcode mark 782 R) functions as it should
On Thu, 17 May 2012 16:42:56 +0100, "N_Cook" put finger to keyboard and composed:
AISI, the 2N7002 FET should be off in normal operation, so I think you're wasting your time in this area. If you're concerned about the "Drive IC's" Enable pin, then disconnect D37.
- Franc Zabkar
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(at
should
I agree, there is another associted point. There is no jumper configuration in that area for 100/120V countries so must be completely inoperative , in mormal operation, in those countries
I suppose for this amp only used in 240V land, then replacement SMPS driver powerFETs only need to be rated 600V/10A and for such an amp only in
100/110/120V land then 300V/20A ratingI've acquired a new IR2153 and desoldered the original unkown driver and as the Tx primaries are some distance from the driver IC, soldered in a turned pin socket. The old one has Thailand and R3 mould marks on the underside and perhaps the second line on the top starts with a 7 and the third line ends with a 5 . I'm assuming there was a top line that is well ground out. Will power up with a couple of droppers in there with 60 percent mains , still set on 240V jumper. I can now compare old and putative replacement
A bit further forward I suppose. Exactly the same, for 2153 and original, audible low level oscillator noise , 10 seconds or so after switch off presumably coming down from ultrasonic and dropping in pitch over a couple of minutes while there is HV in the main caps. And +/-0.5V on the main rail caps transfered across the Tx from the startup pulses, instead of the +/-60V or so . But no sustained drive. Next is some monitor of the Tx primary feedback route to the driver and somehow monitor the osc f, perhaps change the SM cap is easier, the SMRs measure ok. Any tricks for fooling the driver into thinking there is more feedback than actual, a battery wired-in?
On Fri, 18 May 2012 08:49:52 +0100, "N_Cook" put finger to keyboard and composed:
JP1 in the bridge rectifier area configures the unit for 240VAC operation when open, and 120VAC when closed. In the latter case the circuit becomes a voltage doubler.
Therefore Vdc = 350VDC in both configurations.
- Franc Zabkar
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On Fri, 18 May 2012 16:12:40 +0100, "N_Cook" put finger to keyboard and composed:
I would have done a bit more testing before I opted for the 2153.
For example, I would have disabled the "Driver IC5" by shorting the drain and source pins of T24 with a blob of solder (assuming a GDS pinout). This would allow the Vcc capacitor (C52) to charge to the IC's internal zener voltage. You could then compare this voltage against the datasheet.
If the IC is oscillating, then this frequency may also provide a clue to the chip's identity.
- Franc Zabkar
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On Fri, 18 May 2012 16:12:40 +0100, "N_Cook" put finger to keyboard and composed:
I'm having some difficulty understanding your plan, but first allow me to explain how I think the circuit works. If I'm wrong, someone will hopefully correct me.
Firstly, I would not attempt to substitute a battery for the "feedback" voltage. This will defeat the IC's overload protection. If you are going to do this, at least disconnect the 1uF capacitor (C68). Also, instead of a battery, you could add several 270K feed resistors between Vdc and Zener3. Then monitor the IC's Hi and Lo outputs. That should at least give you some confidence in the IC.
As for how the IC works, I believe it would have a start mode and a run mode. In the start mode it would draw a very small current, probably less than 1mA. This would allow the Vcc capacitor (C52) to charge. When Vcc exceeds the undervoltage lockout threshold, the IC switches to run mode. It then attempts to kickstart the supply by pulsing the MOSFETs. In run mode the IC would draw more current than can be provided by R78, in which case C52 would begin to discharge. If a regenerated supply cannot be developed from a secondary transformer winding via D34, C69, D21, and C70, then C52 will discharge to below the lockout threshold, causing the IC to revert to start mode. C52 then charges once again, and so on.
Therefore, if there is an overload on the secondaries, the regenerated supply never eventuates, and the IC hiccups. Some IC's will count the number of hiccups and shut down completely if there are too many attempts.
- Franc Zabkar
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I am getting very inconsistent poor to bad ESR with electroC69 so will change that and C70.
The 2153 has Thailand in a dimple on the rear of the package and an empty dimple on the diagonal, this structure the same as the ground off one, minus the R 3 . I wonder how many IR 8pin , that pinning packages , were available in 2008 and a year or two before. Unfortunately I don't like using isolation transformers and scopes on SMPS to narrow down a bit more, if I regularly dealt with SMPS failures then perhaps it would be different. If this one wasn't a lot of difficult to probe SM around the DIP8 SMPS driiver ,hemmed in by heatsinks and large caps then may have been different
On Sat, 19 May 2012 09:10:14 +0100, "N_Cook" put finger to keyboard and composed:
Those capacitors are a common reason for failure to start. Sorry, I assumed you would have checked them.
- Franc Zabkar
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driver
So that 220/220V zener chain is perhaps more for the possibility of someone jumpering JP1 to 100V and plugging in to 240V and instead of 350V dc trying to get to 700V dc
Have got inundated with more normal stuff and this Mark will have to wait a few days
I now see someone else has been here before and ground off IC5
music-electronics-forum.com/t10881/
What is the difference between IR2153 and IR21531? The original has a diode between Vcc and VB so perhaps whether 2153 or 21531 or any other IR it should be the D variant or adding a high voltage diode externally
On Sat, 19 May 2012 11:29:12 +0100, "N_Cook" put finger to keyboard and composed:
Ah, that would make sense. I was wondering why there was a 275V MOV across A-N, since that voltage would be below the overvoltage threshold for the zener chain.
- Franc Zabkar
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On Sat, 19 May 2012 13:52:58 +0100, "N_Cook" put finger to keyboard and composed:
Did you see the following post in that thread?
==================================================================== Have you noticed that pins 1 and 4 (Vcc and COM) of IC5 are reversed on the schematic (if this is IR21531)? ====================================================================
I noticed on the one i'm working on. I was hoping it was an error in the schematic but it is not.
I verifyed continuity to ground at pin 1.
So the IR21531 is not a drop in replacement as far as I can tell. I have looked at just about every self oscilating half bridge drive they sell and nothing that has pins 1 and 4 swapped. ====================================================================
- Franc Zabkar
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On Sun, 20 May 2012 09:31:43 +1000, Franc Zabkar put finger to keyboard and composed:
Sorry, I should continued reading. It appears that they are indeed reversed.
- Franc Zabkar
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