Which rectifier diodes to replace

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Ok, so the unthinkable happened, I accidentally changed the V+ and V- (and PG+ and PG-) and of course heard a crackle and saw smoke comming from the chips.

After changing the wires and putting in a new fuse at least the wires seems to be ok now.

Transformer is ok, 25V AC on both secondaries, V- is ok with approx 35V but the V+ is not with close to zero.

I'guess I'll have to change diode 5,6,7&8 or is it possible that only one of the dodes is bad. I cannot see any burnmarks on the diodes (actually the smoke was on the chips)

Any possibility that the chips survived or is it better to change them anyway (hey, at least they are burned in:hot: )


Usually I'm quite carefull but I was eager to have it fixed before going to bed. (haastige spoed is zelden goed)
 
As I had a full duplicate set I just changed all parts except the resistors (after measuring whether they were still ok) Also pcb's from both the PSU and the amplifiers are replaced. I think the damage is only to the chips and to the psu board but it was quicker to exchange now and find out the wrong parts(chips?) later.

This time I took my time and measured voltages during every step of the process (yes, should have done that in the first place)

After checking how the amplifier reacted under load (8 ohm 17w resistors at the output) I connected two simple loudspeakers. The amp showed no signes of oscilation or high DC on the output.

There are two remaining issues:
1: with the volume pot entirely closed you can still hear very soft music.
2: There is a big jump in the volume. Until approx 25% (of pot turn) the volume is very soft, at approx 25% the volume takes a big jump and is quite loud immediately. It is not possible to select the volume levels in between. I'm using the Noble pot supplied with the kit chassis. I don't think the pot is wired wrong, I measured the pins and connected pin 1 to ground, pin two to output and pin 3 to input. Pin 4 is not used


Is there an explanation and solution for this volume jump?
Can the amp be made entirely quite when the pot is closed?

Except for the volume jump the amp sounds fine, although not better than my surround receiver (Sony STRDB 1080QS) but it just started playing.
 
Measuring between pin 2 & 3 gives approx 25 kohm and when turning it diminishes to 0-1k so I thought this was the right connection for input and output. (also from other postings.

I will measure the others again (forgot what they were) but I gues I have to disconnect the pot to measure properly and that means taking the amp apart (partly)

I remember now a rapid diminish from 25 to 4k when turning the pot while measuring prior to installation but thought the connection of the probes were bad.

Would it help if I put another resistor in front of the input? that will diminish the total volume but will give me a more usable range for volume control for now.

I searched the net if i could find this pot with proper designationof the pins but was not able to.

Nevertheless there are not too many possibilities to connect the pot wrong but if there are any users of the same pot I would like to hear how they connected the pot.

After running 20 hours it beats my HT receiver in sound quality so we shall see how far we can get with this baby.
 
Noble pot pin layout, looking at the shaft ( so the shaft in front), pins on top

1 2 3 4

| | | |
----------------------
| |
| |
| O |
| |
| |
----------------------

Pin 1 is closest to the edge
Pin 3 is connected to the slider

pin 1 is not used, Pin 2 is input, pin 3 is output and pin 4 is connected to ground?

Is this correct?
 
The amp is running perfectly now. I found out that the initial pin designation was ok but just before soldering I turned the pot and soldered the wires wrong. So the pin designation of a couple of post back is ok(like Peter confirms). I should have marked them clearly in stead of relying on memory when it is way past midnight or very early in the morning.

The amp is sounding more and more decent. I do not know where the discussions from weak bass have ever come from but that's certainly not the case in my situation. (might be if you have very hard-to-drive, low impedance speakers)

Thanks a lot to all that have contributed and especially to Peter and Brian.
 
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