Test bulb is on. Need help with BrianGT's LM3875.

maurycy · 26th September 2007, 12:36 AM

I just completed the BrianGT's LM3875 Rev C. One PSU board, two channels powered by 42V CT. The PSU board has only 4 diodes and provides sort of correct voltage. I have measured V+ to PG+ and got ~33.5V (I have analog meter that might not be that accurate). Same on V- to PG- (also ~33.5V). Everything looks OK so far.

Now today, I have connected the amp boards using three wires going out from PSU (V+, PG+ and V-). They go to V-, PG+ and V- on amp boards (respectively). When I turn the whole system on thru the 100W test bulb is on (big thanks to AndrewT). I have turned the system immediately off to prevent any damage.

What should I test now? Please mind that I am complete noob so if you compose your reply, please try to stick to the labels from Brian's board as I might not know all the specific terms

My connection layout was posted here:
Connection layout
as part of this thread: Connection thread
The changes I have made to this layout are:
1. G from power socket is connected to the chase next to the socket (not together with CHG from amp board) - as pointed in the other thread
2. Pink wires from PG- on PSU to PG- on amp boards are gone - as suggested in the other thread

Any help is appreciated.

DigitalJunkie · 26th September 2007, 01:49 AM

Quote:

I have connected the amp boards using three wires going out from PSU (V+, PG+ and V-). They go to V-, PG+ and V- on amp boards (respectively)

Izzat a typo?

maurycy · 26th September 2007, 01:57 AM

Yes, it is. Should be:
They go to V+, PG+ and V- on amp boards (respectively).
Can't edit my original post.

Nordic · 26th September 2007, 06:33 AM

Does your transformer have 2 seperate secondaries, or does it just have a split one (likely only 3 wires comming out of it)?

maurycy · 26th September 2007, 12:03 PM

My transformer has just three wires 21V-0-21V.

AndrewT · 26th September 2007, 06:16 PM

Hi,
nothing to do with this light on problem, but you have three different star grounds in this version of the amplifier.
Star ground as labeled has CHGL +CHGR + Safety Earth connected to it.
PSU PG+ has PG+L & PG+R connected to it.
PSU PG- has PG-L & PG-R connected to it.
Then the signal grounds on the PCBs have pot ground and RCA chained via pot ground to the respective LSG & RSG.
That is a recipe for confusion when you come to debugging your hum problems.

Nordic · 26th September 2007, 06:19 PM

oh of course yes, otherwise he would have bigger problems... its just that 50% of the problems we see is because people don't get the rectifiers right for their transformer setup......

AndrewT · 26th September 2007, 06:22 PM

Hi,
how long did you leave the amp powered up with the bulb lit?

The bulb should flash very briefly if only the rectifier is connected to the transformer.
The flash should dim progressively, over two or three seconds, to a dimglow or completely off with smoothing connected to the rectifiers.
With one amp connected the delay may take a second or so longer and with both amps another second or so longer.

Disconnect the transformer from all three rectifier connections and tell us what the bulb does at switch on.

I'm away out, back later.

AndrewT · 26th September 2007, 11:06 PM

Hi,
just noticed post 1 notes that didn't sink in first reading.
My post 6: ignore comment about star ground with Safety Earth attached. But are CHG L & CHG R connected to chassis?

Also ignore extra star ground labelled PG-. It has transformer centre tap only feeding via shorting link to PG+

maurycy · 26th September 2007, 11:42 PM

CHG L and CHG R are connected to the chassis.

PG- on the PSU is not connected to the amp boards. This is the wire from my connections diagram that I have never used.

For the first try, I had the amp powered for less than 2 seconds. I tried it now plugged in for about 10 seconds (or more) and the light did not go off. It is bright light (not dimmed) and I am using 100W bulb.

When I disconnect the transformer from the PSU board and turn the amp on, the light stays off. Does not even blink.

Hope I have answered all your questions AndrewT.

26th September 2007, 12:36 AM	#1
maurycy diyAudio Member Join Date: Dec 2005 Location: NJ	Test bulb is on. Need help with BrianGT's LM3875. I just completed the BrianGT's LM3875 Rev C. One PSU board, two channels powered by 42V CT. The PSU board has only 4 diodes and provides sort of correct voltage. I have measured V+ to PG+ and got ~33.5V (I have analog meter that might not be that accurate). Same on V- to PG- (also ~33.5V). Everything looks OK so far. Now today, I have connected the amp boards using three wires going out from PSU (V+, PG+ and V-). They go to V-, PG+ and V- on amp boards (respectively). When I turn the whole system on thru the 100W test bulb is on (big thanks to AndrewT). I have turned the system immediately off to prevent any damage. What should I test now? Please mind that I am complete noob so if you compose your reply, please try to stick to the labels from Brian's board as I might not know all the specific terms My connection layout was posted here: Connection layout as part of this thread: Connection thread The changes I have made to this layout are: 1. G from power socket is connected to the chase next to the socket (not together with CHG from amp board) - as pointed in the other thread 2. Pink wires from PG- on PSU to PG- on amp boards are gone - as suggested in the other thread Any help is appreciated.

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26th September 2007, 01:57 AM	#3
maurycy diyAudio Member Join Date: Dec 2005 Location: NJ	Yes, it is. Should be: They go to V+, PG+ and V- on amp boards (respectively). Can't edit my original post.

26th September 2007, 06:33 AM	#4
Nordic diyAudio Member Join Date: Sep 2005	Does your transformer have 2 seperate secondaries, or does it just have a split one (likely only 3 wires comming out of it)?

26th September 2007, 12:03 PM	#5
maurycy diyAudio Member Join Date: Dec 2005 Location: NJ	My transformer has just three wires 21V-0-21V.

26th September 2007, 06:16 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, nothing to do with this light on problem, but you have three different star grounds in this version of the amplifier. Star ground as labeled has CHGL +CHGR + Safety Earth connected to it. PSU PG+ has PG+L & PG+R connected to it. PSU PG- has PG-L & PG-R connected to it. Then the signal grounds on the PCBs have pot ground and RCA chained via pot ground to the respective LSG & RSG. That is a recipe for confusion when you come to debugging your hum problems.

26th September 2007, 06:19 PM	#7
Nordic diyAudio Member Join Date: Sep 2005	oh of course yes, otherwise he would have bigger problems... its just that 50% of the problems we see is because people don't get the rectifiers right for their transformer setup......

26th September 2007, 06:22 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, how long did you leave the amp powered up with the bulb lit? The bulb should flash very briefly if only the rectifier is connected to the transformer. The flash should dim progressively, over two or three seconds, to a dimglow or completely off with smoothing connected to the rectifiers. With one amp connected the delay may take a second or so longer and with both amps another second or so longer. Disconnect the transformer from all three rectifier connections and tell us what the bulb does at switch on. I'm away out, back later.

26th September 2007, 11:06 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, just noticed post 1 notes that didn't sink in first reading. My post 6: ignore comment about star ground with Safety Earth attached. But are CHG L & CHG R connected to chassis? Also ignore extra star ground labelled PG-. It has transformer centre tap only feeding via shorting link to PG+

26th September 2007, 11:42 PM	#10
maurycy diyAudio Member Join Date: Dec 2005 Location: NJ	CHG L and CHG R are connected to the chassis. PG- on the PSU is not connected to the amp boards. This is the wire from my connections diagram that I have never used. For the first try, I had the amp powered for less than 2 seconds. I tried it now plugged in for about 10 seconds (or more) and the light did not go off. It is bright light (not dimmed) and I am using 100W bulb. When I disconnect the transformer from the PSU board and turn the amp on, the light stays off. Does not even blink. Hope I have answered all your questions AndrewT.