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Old 14th July 2012, 07:36 AM   #81
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Quote:
Originally Posted by ncc View Post
It's been a while and I've not finished my Fetzila due to lack of time and it did take sometimes for me to collect all needed parts.

A few day ago, I started building the amp. Yesterday, after I finished soldering everything on the PCBs. I switched on the power, I use a variac and set the (main) input voltage at half (110V)(the transformer secondary voltage is only 12V AC) to see if something go wrong and actualy something did go wrong (I shoud have used a bulb in serie with the amp as someone advised on this topic). After I turned the power on, the toroid hummed loudly and it seemed that I got a short somewhere since I saw smoke come out below the PCB, from the position I came out, It seemed to come from the metal oxide resistors which are mounted underboard, it happended to both channels.
I turned off the power immediately to prevent further damage. I guess that I did something wrong. After that I tested the fuses on PCB and they all intact.

Attached here are the pics of my PCB. Coud you please take a look at them to see if I put everything in correct order.
As you could see, in the power supply section picture. I did not mount the diodes optimally as I soldered them with the metal tab facing in instead of facing out but I only realised it after soldering them in positions. Actually, I tried to separate them and not to let their metal tab to touch each other.

Thank you in advance.
ncc
Your rectifier diodes are soldered the wrong way the plastic case of the diodes must face the inside see pic.
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File Type: jpg DSCN4938.JPG (949.2 KB, 448 views)

Last edited by meanman1964; 14th July 2012 at 07:40 AM.
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Old 14th July 2012, 11:46 AM   #82
unmibh is offline unmibh  Bosnia and Herzegovina
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The diode should work either way, as long as the metal tab do not touch the any of the two beside it.

ncc, have you use the mica/kapton insulator when you mount your amp to the heatsink?

rgs
Rom
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Old 14th July 2012, 12:16 PM   #83
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ncc, one question did you've to drill any holes for the rectifiers diodes?
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Old 14th July 2012, 01:49 PM   #84
ncc is offline ncc
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Quote:
Originally Posted by unmibh View Post
The diode should work either way, as long as the metal tab do not touch the any of the two beside it.

ncc, have you use the mica/kapton insulator when you mount your amp to the heatsink?

rgs
Rom
Yes, I did use mica insulator when mounting the amp to the heatsink

Ncc
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Old 14th July 2012, 01:53 PM   #85
ncc is offline ncc
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Quote:
Originally Posted by meanman1964 View Post
ncc, one question did you've to drill any holes for the rectifiers diodes?
Hi,

No, I did not drill any holes, I use the holes for rectifier diodes that are on the PCB, although it is a bit difficult to mount the diodes using those holes

Ncc
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Old 14th July 2012, 06:56 PM   #86
unmibh is offline unmibh  Bosnia and Herzegovina
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Ncc,

Check your out transistor with meter and make sure the correct type is on proper position. If they are good then, try checking all the semis, after that I don't see why your amp behave as you mention. I will leave it to Hugh and the others to make suggestion.

rgs
Rom
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Old 14th July 2012, 08:18 PM   #87
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Also check with a magnifying class for solder shorts.The first time I powered up my second channel I did have the same problem with one of the resistor underneath the board.
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Old 14th July 2012, 11:35 PM   #88
AKSA is offline AKSA  Australia
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Ncc,

I've just seen this now.

I'm sorry you are in difficulties!

When you have huge current flows like this, routinely check the following:

1. You have the correct output devices mounted, 1058 on left, 162 on right, not wrong way around.
2. Secondary #1 connected to LEFT side, positive rail, and Secondary #2 connected to RIGHT side, negative rail.
3. Diodes D1 and D2 correct way around.
4. Bias string connections all correct; D4, D5, D6 and P1. Check all these vital parts for continuity.

The problem is 95% likely to be in the output stage. The rectifier diodes have a common cathode at the center terminal, so can go in the board either way, tab forward, or tab rearward. Doesn't matter at all!

This is a ridiculously simple power amplifier and should not be too difficult to get going.

Good luck!

Thanks guys for helping NCC, he is being very patient, careful and calm!

Cheers,

Hugh
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Old 15th July 2012, 10:57 AM   #89
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Hi Hugh and everyone,

After reading your advices, I dismounted the PCBs from the heatsinks and carefully checked everything and I could not find anything wrong:

- Output devices mounted correctly. I don't have 2sk1058/2sj162 so I use what I have on hand which is BUZZ900/905 pair, BUZZ900 on left, BUZ905 on right of the PCB
- Diodes D1 and D2 orientation is correct
- Bias string D4, D5, D6 connections all correct

I could not find anything abnormal, so mounted one channel on heatsink to test. I used a variac to increase voltage from about 50V to 220V and things seemed to be ok this time, I could set the bias and voltage across bootstrap resistor and output offset to about 5mv. Then I was able to do the same to the other channel (one channel at a time).

I puzzled me because I did not know what was wrong and I did not correct anything then everything seem to be fine.

After that I put everything back to the chassis then power on again and I got the same short like before. It took sometime for me to find out the problem:
I use only one transformer with two 25V secondary windings. Secondary #1 connected to LEFT side, positive rail of left channel but I inadvertently connect it to the RIGHT side negative rail of right channel, the same apply for the Secondary #2, that's should be the problem.

After correcting the problem, I could power two channels with out any problem. The amp is making beautiful music right now.

Thanks Hugh and everyone for helping

However as an inexperienced Diyer I do not understand why it has happended like that. If I power only one channel at a time it's fine but if I power two channels at the same time with my original wiring then I get a short.

I should be very grateful if you could explain it for me.

Thanks,
ncc
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Old 16th July 2012, 08:06 AM   #90
AKSA is offline AKSA  Australia
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Hi NCC,

Since you write (and presumably speak!) English about 1000 times better than I do Vietnamese, it will be a pleasure.

A single transformer has two secondaries.

One secondary must be wired for POSITIVE rails on each module, and the other secondary must be wired for NEGATIVE rails on each module.

If one secondary supplies both a pos and a neg rail, then the negative of one output will be the positive of the other, and this will make for a dead short at the common, ground connection.

This is a real trap using two, full wave rectifiers for each module, one for pos, one for neg.

It delivers much better sound quality, but requires careful wiring. This is the reason I urge people to use two transformers, each of 160VA, in the instruction to avoid this problem.

Happy listening! It is a late night, half sleepy amplifier, don't you think?

Cheers,

Hugh
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