Which rectifier diodes to replace

[indoubt]

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 )


Usually I'm quite carefull but I was eager to have it fixed before going to bed. (haastige spoed is zelden goed)

[indoubt]

I changed the rectifiers, the problem remains, I think a trace of the PSU PCB is gone. It is not visible but I have no other explanation after changing the rectifiers.

Has anyone experienced that before? Is there a way to measure the PCB?

[Nuuk]

Is there a way to measure the PCB?

Just use the continuity tester on your meter and check between each component connections. There are not many on the GC so it shouldn't take long.

However, you may have damaged caps and/or chips!

[Jay]

I cannot understand how the chip will survive that accident

[indoubt]

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.

[Upupa Epops]

Which tapping have this pot - A or B ( log or linear ) ?

[indoubt]

It's a Noble 25K stereo log pot with 4 pins

[Upupa Epops]

Are you sure, that pin 4 is tap ( for loudness ) ? In which pins you can measure resistance 25 kOhm ?

[indoubt]

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.

[Peter Daniel]

It seems like you wired the pot incorrectly. The output should be taken from the wiper, which is pin 3, if you start counting from most outside pin (that is not supposed to be used).