Pundit
Post by papahemi on Feb 16, 2020 15:06:59 GMT 12
I'm looking for a bit of help here, I noticed a sparking at power on on my APPJ FU32 amplifier and this afternoon i investigated further. To my alarm this is what I found:  The capacitor is labelled CX02 on the circuit board, but I have no idea of its value as the values have been blown off and I cannot find a circuit diagram for this amplifier. Any ideas?
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by jon on Feb 16, 2020 15:33:25 GMT 12
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Pundit
Post by papahemi on Feb 16, 2020 15:58:29 GMT 12
Thanks Jon, I bought it from China and don't know if it wil be that easy to get assistance as it would be from the DL thinktank. I found this schematic on DIYaudio, posted also by pcan (who you linked to) of the actual power supply: ![]()  from that it look like a X2 0.1uf - does that seem correct?
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by jon on Feb 16, 2020 16:44:19 GMT 12
Um...
Why am I seeing RT1 printed there?
I suggest you have a look for the other CX02.
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Post by jon on Feb 16, 2020 16:45:00 GMT 12
That RT1 is in your pic, by the way, on the PCB.
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Post by Graham on Feb 16, 2020 16:54:14 GMT 12
The device with its guts blown out is mounted on the circuit board where it is printed RT1. RT1 on the schematic is listed as a 7 ohm resister 
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Pundit
Post by papahemi on Feb 16, 2020 17:08:16 GMT 12
I see now - it doesn't look like a 7 ohm resister. I think I will have to dismantle it for a closer look.
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by jon on Feb 16, 2020 17:14:14 GMT 12
Rather be safe than sorry
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Post by colinf on Feb 16, 2020 20:37:31 GMT 12
Can you take a photo of the underside of the board? I need to see how it’s wired to be able to identify it properly. Definitely don’t want to get the identity wrong at mains power! If it is CX02 it’s likely a mains suppression capacitor, 0.1uf value. Element14 would have them. I doubt it’s that though as the mains suppression cap looks like it is the yellow block component next to it. That should say 0.1uf on it, or 104, or 100n, 250Vac. If it’s RT1 it’s an inrush current limiter thermistor. Since it’s blown it might be due to the rectifier after it having shorted, or even the main switching transistor having shorted. It could also be due to just having had a mains voltage spike. Be careful!
AMR-iFi R&D
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Pundit
Post by papahemi on Feb 17, 2020 10:04:36 GMT 12
Can you take a photo of the underside of the board? I need to see how it’s wired to be able to identify it properly. Definitely don’t want to get the identity wrong at mains power! If it is CX02 it’s likely a mains suppression capacitor, 0.1uf value. Element14 would have them. I doubt it’s that though as the mains suppression cap looks like it is the yellow block component next to it. That should say 0.1uf on it, or 104, or 100n, 250Vac. If it’s RT1 it’s an inrush current limiter thermistor. Since it’s blown it might be due to the rectifier after it having shorted, or even the main switching transistor having shorted. It could also be due to just having had a mains voltage spike. Be careful! Here are the board pics:  Board underside - slight burn mark bottom left shows where component located.  Birds eye. Yellow blocks are indeed CX01 & CX02. RT1 has both sides blown off.  Further shot showing white glob over IC01. Shouldn't be an issue?
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by Owen Y on Feb 17, 2020 10:54:29 GMT 12
(Will await @colif's advice but....) I'd say not good for the heatsink to have material over it, esp over the IC itself & its pins. It's a big heatsink so is dissipating quite a bit of PSU heat.
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Post by colinf on Feb 17, 2020 20:53:40 GMT 12
Ok thanks. It’s RT1, an inrush current suppressor. It’s shown as 7 ohms on the hand drawn circuit which is probably the value they measured while it’s off and cold. When you switch it on the resistance reduces. Need to be careful here in recommending a replacement, it looks like there is no fuse on the circuit board so it needs to act as the fuse as well, should something go wrong. I’ll get back here shortly! Is there a fuse on the mains socket, or nearby? BTW the white silicone on the heatsink shouldn’t be anything to worry about, they most likely constructed it that way to secure it and/or insulate the ic pins.
AMR-iFi R&D
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Post by colinf on Feb 17, 2020 22:12:37 GMT 12
Try one from RS NZ (nz.rs-online.com), part number 516-7811.
AMR-iFi R&D
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Pundit
Post by papahemi on Feb 18, 2020 7:42:03 GMT 12
Thanks Colin. The IEC socket is fused. Will order part today.
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by colinf on Feb 18, 2020 19:04:45 GMT 12
Let us know how you get on!
AMR-iFi R&D
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Pundit
Post by papahemi on Feb 19, 2020 15:19:02 GMT 12
Big ups to rs-online, part ordered yesterday was delivered (to Gizzy, from Sydney) within 24hrs! Replacement slightly larger than original - can only be good - sturdier.  celebrated with BB King and the Underground Kingz and it sounds good. I noticed that while this wee thing was out of the system, replaced by the AR A-04 that the sound of the two are quite different. I wonder how much is tube v SS and how much is class A v class AB. The wee amp is a bit underpowered for the main rig, but is ideal for a desktop system as pictured. Thanks to all for the assistance.
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by colinf on Feb 19, 2020 19:39:13 GMT 12
Good it’s working again. There would be a few things going on with the AR vs the valves. The negative feedback transistor circuit in the AR gives lower overall distortion but the crossover (switching) distortion made by a transistor class AB output stage results in a little high order harmonic distortion at low levels. The technique of negative feedback can be a compromise when it comes to reducing switching distortion in class AB or class B output stages. You can hear it as slight image instability and a smidge of harshness. Also, the harmonics of transistors themselves is different to that of a valve. The valves, although they have higher distortion without negative feedback in the circuit they’re in, the crossover distortion is non-existent being run in class A, so would sound smoother with more stable imaging. The typical harmonics made by a valve are more pleasing to the ear so you can get away with more of it before the ear complains.
AMR-iFi R&D
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Post by jon on Feb 20, 2020 7:31:25 GMT 12
Great to hear you have it back up and running!
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Post by Owen Y on Feb 20, 2020 8:15:42 GMT 12
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Pundit
Post by papahemi on Feb 20, 2020 15:46:07 GMT 12
Yes - but as always when you play with something other issues arise - I really need to get off my arse and sort out a power amp that will sit well in my system. I just hum and ha between JLH, ACA and gainclone.
We are what we pretend to be, so we must be careful about what we pretend to be.
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Post by colinf on Feb 20, 2020 20:00:58 GMT 12
The JLH circuit looks interesting. The original was designed in 1969 and an updated version in 1996. I’d bring it up to 2020 and use currently available transistors. I’d also add the input jfet to increase the input impedance. Here is the 1996 circuit (with the error as described): 
AMR-iFi R&D
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