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Author Topic: Low resistance on output caps + voltage imbalance.  (Read 1324 times)

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Offline cabcabaret

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Low resistance on output caps + voltage imbalance.
« on: June 03, 2017, 04:31:35 AM »
Hello,

The attached pic summarises the readings. This is incorrect see post #4 below!

I built this in 2013 with the 240V VAC (i'm in the UK). All the checks passed first time then. About a year ago it started giving no volume unless I turned it off and wobbled the 12AU7 tube. Sure enough turning it over found an iffy solder joint which had come loose.

After fixing it (and inspecting all of the joints and reflowing any discoloured ones) I did a resistance check and found that the resistance on terminals to the output capacitors and the RCA jacks was wrong, but no shorts. See attached table.

As I knew the amp was previously working and no shorts were evident I went ahead with the voltage checks too. These show several incorrect readings, mainly overvoltage in terminal 7, yet not in terminal 9.

I don't know what to make of it! Any help would be greatly appreciated!

imgur.com/a/A014Z

« Last Edit: June 11, 2017, 01:39:14 AM by cabcabaret »

Offline cabcabaret

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Re: Low resistance on output caps + voltage imbalance.
« Reply #1 on: June 04, 2017, 12:15:20 AM »
OK I think I've found the problem. I put connected some cheap headphones and there was only sound in the left channel, but touching the small tube it flicked in and out.

Flipped over the board and now one of the LEDs was out. I found I could push it back in and voltages returned to normal, except for terminals 15 & 21 which remained at 200 & 220.

It seems to have been caused by me soldering the LEDs without enough slack, as Doc B points out in the sticky thread.

I've ordered some new HLMP-6000 LEDs and it should work, not sure about #15 being 200V though?
« Last Edit: June 07, 2017, 07:55:49 AM by cabcabaret »

Offline cabcabaret

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Re: Low resistance on output caps + voltage imbalance.
« Reply #2 on: June 07, 2017, 07:59:18 AM »
I didn't find the problem. I bought some new HLMP-6000 LEDs and soldered them according to advice and tested the resistances to find they are the same. When powered on both LEDs light up, but when testing the voltages I blow the a8 LED when I touch it with the multimeter.

I've blown 2 LEDs now, since I didn;t realise what happened to the first. The result is nothing the right channel.

After replacing the LED the voltages on 1 & 7 are now correct but the rest remain unchanged (see attached)

What to do? I guess I need to rule out damaged components but where to start?
« Last Edit: June 07, 2017, 08:09:34 AM by cabcabaret »

Offline Paul Birkeland

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Re: Low resistance on output caps + voltage imbalance.
« Reply #3 on: June 08, 2017, 09:02:34 AM »
The LED's on the 9 pin socket influence the voltages at terminals 1 and 5, which are correct in the image you posted.

I'm not sure why you have an expected voltage of 85V at terminal 15.  Your kit/manual are a bit old, but that is a mistake.  15 will be lower than 21 and higher than 13 and around 175-220V, so no problems there.

The 0V at B3 is the root of the problem.

You have appropriate voltage at the plate and the grid of the right channel on the 6080, but no cathode voltage.  I would strongly suggest posting some photos of your build if possible, but also you'll want to look at both ends of the 3K resistor that feeds B3, as well as the legs of the cap that sits between 6 and 7.  It is almost certain that something is disconnected and not allowing that half of the 6080 to draw current properly.

The good news is that you can leave the 12AU7 and its parts alone, as it isn't the problem.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline cabcabaret

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Re: Low resistance on output caps + voltage imbalance.
« Reply #4 on: June 11, 2017, 01:38:01 AM »
Thank you for your reply, much appreciated.

Yes I'm afraid #15 and b3 were typos in my list above!

After reading your post I revisited everything, and think I was basically blowing LEDs since a8 doesn't need a reading!

I added another LED on a8 and checked the voltages again and everything was fine.

However the resistance checks show exactly the same results. Is this a problem?

All in all I made a mess out of this by not being as organised as I should have! I've just tested it with headphones and it sounds great.

« Last Edit: June 11, 2017, 02:39:34 AM by cabcabaret »

Offline Paul Birkeland

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Re: Low resistance on output caps + voltage imbalance.
« Reply #5 on: June 12, 2017, 12:38:36 PM »
If you're not certain, you can post photos of your build.

At terminals 7 and 9, generally you are supposed to only see the resistance between each 3K resistor and ground, and most meters do that.  Unfortunately, there's those 100uF caps between 6 and 7, as well as 9 and 10 that are grounded through some 2.49K resistors, and your meter may somehow be able to also detect that resistance. 

If the 100uF caps are in backwards, they can short and cause the resistance readings that you see, but then those 2.49K resistors on the headphone jack will heat up and explode in a very short time and the voltages will be off as well (you'll get DC voltage at 6 and 10 that's much higher than 0V).

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man