I hope this topic has not been dealt previously, in this case I apologize.
I am faced with a personal challenge, I am not a repairer but I have completed studies in electronic engineering, and I can say that I can manage with applied electronics.
I especially like to resurrect vintage objects.
Now I'm struggling with a Hitachi power amplifier, the HMA-7500 MK II, which has mosfet power amps in push-pull. The amplifier was previously grossly remodeled by a scoundrel.
I only managed to get one of the channels back in order, but I found some absurd failures and had to replace the pre-finals, the VAS and even the double op amp of the DC servo loop. Many bjts and most of the fuse resistor were blowed.
Now I have to replace the power MOSFETs, but their availability is a problem (they cost a lot and I am never sure they are not counterfeit items), it is the 2SK134-2SJ49 pair.
The thing I wanted to do is replace them (even if only for testing) with a modern pair of IRFP240-IRFP9240, which however have a higher gate-to-source threshold voltage (2-4V Vs 0.15-1.45V) and cannot go into that light conduction that allows the passage of the idle current (drain current now is zero).
I would think of modifying the R755-R766 and R760-R759 resistors with the aim to drive the modern pair with a right voltage. (please see at pic below)
What do you think ? Any ideas / suggestions?
Thanks a lot in advance
porzione-schema-Hitachi — ImgBB
I am faced with a personal challenge, I am not a repairer but I have completed studies in electronic engineering, and I can say that I can manage with applied electronics.
I especially like to resurrect vintage objects.
Now I'm struggling with a Hitachi power amplifier, the HMA-7500 MK II, which has mosfet power amps in push-pull. The amplifier was previously grossly remodeled by a scoundrel.
I only managed to get one of the channels back in order, but I found some absurd failures and had to replace the pre-finals, the VAS and even the double op amp of the DC servo loop. Many bjts and most of the fuse resistor were blowed.
Now I have to replace the power MOSFETs, but their availability is a problem (they cost a lot and I am never sure they are not counterfeit items), it is the 2SK134-2SJ49 pair.
The thing I wanted to do is replace them (even if only for testing) with a modern pair of IRFP240-IRFP9240, which however have a higher gate-to-source threshold voltage (2-4V Vs 0.15-1.45V) and cannot go into that light conduction that allows the passage of the idle current (drain current now is zero).
I would think of modifying the R755-R766 and R760-R759 resistors with the aim to drive the modern pair with a right voltage. (please see at pic below)
What do you think ? Any ideas / suggestions?
Thanks a lot in advance
porzione-schema-Hitachi — ImgBB
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The Exicon/Profusion lateral FET's would be ideal replacements. These are current new production lateral FET's.
Exicon Lateral MOSFETS - Our Product Range
The design is not suitable for vertical FET's such as the IRFP types.
Use of a bulb tester during repair would protect the FET's from overcurrent damage.
Exicon Lateral MOSFETS - Our Product Range
The design is not suitable for vertical FET's such as the IRFP types.
Use of a bulb tester during repair would protect the FET's from overcurrent damage.
They should be direct replacements for the Hitachi devices. There are two types available, 8A or 16A which are double die devices (two in parallel in the same package).
I would suggest you stick to the original design concept and use the single 8A devices.
A bulb tester is much better than a fuse for fault finding and initial powering up. Set the bias to minimum on both channels before applying power.
I would suggest you stick to the original design concept and use the single 8A devices.
A bulb tester is much better than a fuse for fault finding and initial powering up. Set the bias to minimum on both channels before applying power.
Great ! The G to D cutoff voltage is quite the same ! Thanks a lot !
I'm wondering if them are usually used to replace the hitachi pair. Isn't it ?
p.s. For trials I put a small 0.5 Amp glass fuse on both drains.
They are lateral mosfets in your amp so need replacing with the Exicon ones.
I have converted lateral amps to IRFP240/9240 by changing the bias circuit but its a few components and could mess up your pcb.
And you'd have thermal stability to worry about or at least test for. Laterals don't have that issue.
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