Hello.

A friend of mine has this "Cerwin Vega Bass Turbocharger DB-10B" unit, it's meant to be coupled between a signal source and an amplifier, then it boosts the bass notes approximately at 30 hz, 5 dB or 10dB chosen by a switch.

But now this unit only has a lot of noise, when it's connected all i get it 50 hz sound to the amplifier.

I figured i could try to fix this, it doesn't have too many components on the inside.
I do not have the schematics.

What i have done:

Tested all diodes in circuit, seems to be ok.
Tested the small transistor out of circuit seems ok.
Replaced all four IC, opamps

The two voltage regulator transistors i am a little curios about, my transistor checker will only say damaged or unknown part when i tested them out of circuit. But checking them with a dmm doesn't indicate a short, or open circuit.
If i apply 12V DC to them and measure the output voltage, one showed 0.5V, and the other one showed 10.5V, but i guess they are two different parts, so i might have wired incorrectly to check them the correct way, i am insecure about my method

What do you guys think could be the culprint?

It is driven by a 15V AC power supply.

 

Hello,

The input must be AC to have a double powersupply from a single voltage.
The input circuit may look like this:


Feeding the input with DC will only have one active.

Bertus

 

Welcome back!

There is no need to remove the regulators from the board in order to test them. Leave them soldered on the board.

The 3-terminal regulators are 7815 for +15VDC output and 7915 for -15VDC output.
Hence the power input ought to be about 40VAC. Show us the label on the power adapter being used.

All four ICs are TL072 dual FET opamps.



Pin- 4 would be -15VDC and pin-8 is +15VDC.

You already have a DSO138 hobby oscilloscope. Use this to measure the -15VDC and +15VDC supply rails, each with respect to GND.

 

I think a power supply has rectifiers that produce 100Hz, not the 50Hz from electrical wiring that is all around you.

Maybe the 50Hz hum is caused by a cable problem. Audio connecting cables are shielded so that they are not an antenna that picks up interference. If the shield on a cable is missing or broken then there will be lots of hum.

Does the amplifier produce the 50Hz hum when it has no input cable connected?
Does the 50Hz occur when the the turbocharger device has no input cable but is connected to the amplifier input with a good shielded audio cable?

 

Welcome back!

Thank you MrChips
There is no need to remove the regulators from the board in order to test them. Leave them soldered on the board.

Okey, so i left them on the board.

The unit is supposed to have 16VAC power supply.
Some correction; I thought at first the power supply was 15VAC, but it has been used with a 12VAC power supply because 16VAC is not so easy to find, and has been working with this for some years.
This power supply actually provides 14.10VAC, measured when connected.

Measured with DMM
PIN4: -5.24V
PIN8: +15.5V

I connected the scope, but when i ground the probe, the screen freezes
Tried some more but now i can't get no trace anymore... need to have a look at it

 

I think a power supply has rectifiers that produce 100Hz, not the 50Hz from electrical wiring that is all around you.

Maybe the 50Hz hum is caused by a cable problem. Audio connecting cables are shielded so that they are not an antenna that picks up interference. If the shield on a cable is missing or broken then there will be lots of hum.

Does the amplifier produce the 50Hz hum when it has no input cable connected?
Does the 50Hz occur when the the turbocharger device has no input cable but is connected to the amplifier input with a good shielded audio cable?

This device has a green LED who will light up when the device gets enough input/output.
This LED is now constantly lit, hence the hum signal output from it.

So what i'm saying, this device puts out the hum on both channels without any input now, it's a high pitch 50 hz

 

That does not make any sense to me.
7815 is a +15VDC regulator. It needs about +18VDC input.
7915 is a -15VDC regulator. It needs about -18VDC input.
I don't see where they are deriving 36VDC from 16VAC input.

 

So what i'm saying, this device puts out the hum on both channels without any input now, it's a high pitch 50 hz

50Hz is a very low frequency hum, a high pitch is a high frequency squeak that is thousands of Hz.
Does it produce a hum or a squeak?

 

50Hz is a very low frequency hum, a high pitch is a high frequency squeak that is thousands of Hz.
Does it produce a hum or a squeak?

I know what you mean, i might be bad at explaining, but it's 50 hz with a noise.

See video:
Noise - YouTube

 

That does not make any sense to me.
7815 is a +15VDC regulator. It needs about +18VDC input.
7915 is a -15VDC regulator. It needs about -18VDC input.
I don't see where they are deriving 36VDC from 16VAC input.

These regulators, seems like they work with even lower voltage since they are put in this circuit?

 

It could be one of the OPAMP Issue

 

It could be one of the OPAMP Issue

I have edited the first post in what i've done.
I have actually replaced all four opamp's with new ones, no difference

 

So it could be a faulty regulator or a diode

 

These regulators, seems like they work with even lower voltage since they are put in this circuit?

No such thing.
The only possibility I can see is that they are not 15V regulators.

Edit: Another possibility is someone installed the wrong regulators instead of 7805 and 7905 regulators.

 

No such thing.
The only possibility I can see is that they are not 15V regulators.

Edit: Another possibility is someone installed the wrong regulators instead of 7805 and 7905 regulators.

Strange.
I actually have one working unit of the same type, i went and got it.
It has the same type regulators as in this one.

Now i can compare values measured

 

The two voltage regulator transistors i am a little curios about, my transistor checker will only say damaged or unknown part when i tested them out of circuit. But checking them with a dmm doesn't indicate a short, or open circuit.
If i apply 12V DC to them and measure the output voltage, one showed 0.5V, and the other one showed 10.5V, but i guess they are two different parts, so i might have wired incorrectly to check them the correct way, i am insecure about my method

Just to follow up on your initial post.

1) Even though the 3-terminal voltage regulators look like power transistors, you cannot test these with a transistor tester. Simply, voltage regulators are not transistors.

2) You cannot test a 3-terminal voltage regulator with an ohmmeter.

3) You cannot simply test 3-terminal voltage regulators with just a DC power supply and a voltmeter as you have shown. The regulators are likely to oscillate because they need proper HF filter capacitors in the circuit hook up.

4) The pinouts of 3-terminal regulators are different for different part numbers. Never assume that the pinouts are the same. You need to look up the datasheet for each device.

 

A 7815 needs at least a 17VDC input then it produces a regulated 15VDC output.
The 7915 is the same but uses a negative minimum -17VDC input voltage and produces a regulated -15VDC output.
If the inputs are less than 17VDC then their voltage regulation fails.

Your input voltages are too low for them or they have the wrong voltage number.

 

I don't see where they are deriving 36VDC from 16VAC input.

The peak value of a sine wave is 1.414 x the RMS value; peak to peak is double that. So, 16V AC and a half-wave rectifier gets you about 22.6V DC, closer to 22V if you subtract the diode drop.

 

Post #5 shows the measurements.
The positive 15V regulator works fine but the negative 15V regulator has a problem causing an output of only -5.24V.

 

Just to follow up on your initial post.

1) Even though the 3-terminal voltage regulators look like power transistors, you cannot test these with a transistor tester. Simply, voltage regulators are not transistors.

2) You cannot test a 3-terminal voltage regulator with an ohmmeter.

3) You cannot simply test 3-terminal voltage regulators with just a DC power supply and a voltmeter as you have shown. The regulators are likely to oscillate because they need proper HF filter capacitors in the circuit hook up.

4) The pinouts of 3-terminal regulators are different for different part numbers. Never assume that the pinouts are the same. You need to look up the datasheet for each device.

Ok, thanks for this info.
I actually checked the pinout before testing ;-)