Check my Lm3915 vumeter circuit

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JAY X

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Here is a link to a YouTube video where you can see the behaviour of the led vumeter.


Any help appreciated!!
Jay x
 

ruffrecords

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Hi!

Good and bad news.
It works, but in a strange way. I have built this circuit around bc547/557 transistors, instead of 2n3904/3906.

There are two strings of 5 leds each. The first one lights up to a level, and when this level is surpassed, the next string of leds light up. It does so, but alternatively, maybe because the transistors are not switching as fast. So there are moments where the lower led string is off when upper led string is on.
That is odd. Are you rectifying the audio first or are you applying it directly to the LM3915?

Edit: Now I have seen your video. It looks like Q2 is not operating correctly. It should be creating a constant current source for the first string.

Cheers

Ian
 
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JAY X

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I revised my board and the schematic. I found one trace to be cut on my pcb, but this did not change the vumeter behaviour.

Besides, there is something not clear in the schematic.: The collector of Q2 seems to be connected to V+, but this is not very clear, because the dot representing a connection is very small or not clear.

Jay x
 

ruffrecords

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I revised my board and the schematic. I found one trace to be cut on my pcb, but this did not change the vumeter behaviour.

Besides, there is something not clear in the schematic.: The collector of Q2 seems to be connected to V+, but this is not very clear, because the dot representing a connection is very small or not clear.

Jay x
No, Q2 collector goes straight to pin 15 and nowhere else. it is the output of the replacement constant current souec for the first string. I agree the schematic is not exactly clear but Q2 collector is definitely NOT connected to V+

Cheers

Ian
 

JAY X

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Ok!. I think I got it!. I missed the connection between the 470R resistor and anode Led L1. I will check tomorrow again...
 

JAY X

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Hi!

I corrected the missed conection, but go figure...Now the two separated linear power supplies (+/-16v and +48v), ceased working!! O.M.G.!!:LOL:

I tested the two toroidal transformers (30VA 18-0-18), and both primaries and secondaries are ok, they measure resistance, but once I power up, nothing comes out from them at the secondary winding. I measured AC-AC and AC-0v, and nothing.... not a single volt... At primaries they both receive 230v AC. Maybe it is time to get a proper 500 series rack.:sneaky:

Jay x
 

ruffrecords

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Hi!

I corrected the missed conection, but go figure...Now the two separated linear power supplies (+/-16v and +48v), ceased working!! O.M.G.!!:LOL:

I tested the two toroidal transformers (30VA 18-0-18), and both primaries and secondaries are ok, they measure resistance, but once I power up, nothing comes out from them at the secondary winding. I measured AC-AC and AC-0v, and nothing.... not a single volt... At primaries they both receive 230v AC. Maybe it is time to get a proper 500 series rack.:sneaky:

Jay x
Are you still runing the 3915 from only one of the rails?

Cheers

Ian
 

JAY X

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R3 to q1 base to led 6. This is ok. The Lm3915 still works on single rail voltage. +16v/gnd. And the buffer rectifier TL072 is NOT ac coupled.
Right now I have no clue why it does not work as expected.
 
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JAY X

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Hi!

Today i discovered the original schematic by R.M.marston. in that schematic, pin9 is connected to pin11.( Cathode of led9). I will try.
 

JAY X

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The leds still work as two separated segments. This is not normal. Maybe the transistors?
 

ruffrecords

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The leds still work as two separated segments. This is not normal. Maybe the transistors?
If the higher string works OK but when it is on the lower string does not work then yes there is something wrong with the transistors.Q1 turns on the extra current source and Q2 is the current source. Q2 supplies current to the lower string when the higher string works so this looks to be the part of the circuit that is not working. I just found MArtson's book on the web. Great stuff in there.

Cheers

ian
 

JAY X

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I checked both transistor out of the circuit and both vbe voltage seems ok, about 0,74 on bc547 and 0,715 on bc557.
 

JAY X

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I changed both transistors, but still no luck! 😝.

I was thinking...what if i feed the two separated led strings directly with 16v, without any transistor?. Each led string accounts a voltage drop of about 2,2v x 5 leds: 11v. Well within the supply rail.
There was a siemens vumeter ic UAA180, that worked this way..🤔
 

ruffrecords

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Another fail!!😝
The key is to get Q2 to take over the constant current source duties when you get in the higher string. You need to to some basic tests to find out if it is working. Feed in a fixed level so that one or more LEDs in the second string are illuminated. The check what is going on with Q1 and Q2. Q1 should be hard on so its collector should be close to the +ve rail. Current should be flowing through the 4K7 resistor and the two diodes so that Q2 base sits at about 1.3V. So there should be about 0.65V across the 220R resistor R5 meaning about 3mA is flowing through Q2 and the first string of LEDs. Maybe 3mA is too low?

Cheers

Ian
 

JAY X

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Hi Ian!

I changed the transistors, for mpsa 42/92.then I made the measurements as you suggested me:

Hi string led measurements:

Q1 vc: 15,4v
Q1 vb: 1,3v
Q2 vb: 1,3v
Q2 ve: 0,73v
Q2 vc: 6,4v

Perhaps changing R5 220r to 100R (7,3ma), may help.

I also made measurements with only low string and hi/lo string.

Hi/lo led strings measurements:

Q1 vc: 7,2v
Q1 vb: 0,5v
Q2 ve : 0,3v
Q2 vc: 5,5v
Q2 vb: 0,5v

Low string led measurements:

Q1 vc: 0v
Q1 vb: 0v
Q2 vb: 0v
Q2 ve: 0v
Q2 vc: 5,6v
 

JAY X

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¡Solved!

I changed the 220R resistor for 130R.
See the result in the vídeo. Also, the current draw in the +v rail of the preamp with 8 leds lit up is about 110ma. Well within the 130ma limits of the 500 series specs.

Thanks a lot Ian!!
Jay x.
 
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