Hi!
I have an old bass amplifier of a brand called TALMUS, all it says is "Compact Amplifier". It is probably from the 80s and lately is having the following problem.
As soon as a chord is played with the power volume and preamp at some average levels the output protection circuit kicks in and a safety relay opens disconnecting the speaker. I have tested the output offset voltage and it is fairly 40mV with no signal in but as soon as the signal comes in the offset rises pretty high up to 3V-3.5Vdc 😱.
I have not found an schematic for this amp, so far I've seen it is based in 4 NPN power transistors (2N3055) driven one branch by a TIP41 and the other one by a TIP42 which mounts a pot (bias pot probably) with no effect in the offset problem. I have checked all those transistor and they are ok. So I am trying to figure out where th problem might be, may be these old caps are leaking too much or might have some DC feedback etc
Any hints will be appreciate. I am trying to draw down the circuit, but if anyone got a similar experience or can suggest causes why the offset increases so much I will appreciate too!
I have an old bass amplifier of a brand called TALMUS, all it says is "Compact Amplifier". It is probably from the 80s and lately is having the following problem.
As soon as a chord is played with the power volume and preamp at some average levels the output protection circuit kicks in and a safety relay opens disconnecting the speaker. I have tested the output offset voltage and it is fairly 40mV with no signal in but as soon as the signal comes in the offset rises pretty high up to 3V-3.5Vdc 😱.
I have not found an schematic for this amp, so far I've seen it is based in 4 NPN power transistors (2N3055) driven one branch by a TIP41 and the other one by a TIP42 which mounts a pot (bias pot probably) with no effect in the offset problem. I have checked all those transistor and they are ok. So I am trying to figure out where th problem might be, may be these old caps are leaking too much or might have some DC feedback etc
Any hints will be appreciate. I am trying to draw down the circuit, but if anyone got a similar experience or can suggest causes why the offset increases so much I will appreciate too!
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Bias pots should not affect offset.
Do you have a scope, and understand its use?
First, see if it does this with no load on the output. If the amp puts out a good clean signal with no load, but acts this way WITH a load, the problem is likely that the output stage cannot deliver any current. My mental image is that the amp is able to deflect the output one direction but not the other into your speaker. The net effect of that is an average DC offset. it is just a thought.
If you can scope it, look for any clipping or missing parts of waveforms, with and without a load.
If the output stage is suspect then I would be checking all the output and driver transistors, as well as the associated resistors. You must have some fractional ohm power resistors as ballasts for your output transistors.
Do you have a scope, and understand its use?
First, see if it does this with no load on the output. If the amp puts out a good clean signal with no load, but acts this way WITH a load, the problem is likely that the output stage cannot deliver any current. My mental image is that the amp is able to deflect the output one direction but not the other into your speaker. The net effect of that is an average DC offset. it is just a thought.
If you can scope it, look for any clipping or missing parts of waveforms, with and without a load.
If the output stage is suspect then I would be checking all the output and driver transistors, as well as the associated resistors. You must have some fractional ohm power resistors as ballasts for your output transistors.
Hi Enzo, thanks for the reply. I tested the unit with load and with out and basically the effect is the same but for the DC offset rising slower without load also up to 3Vdc and more. Even without load the security relay opens sometimes.
The signal (sine) is reproduced correctly until is amplified way too much and clips evenly. What I have notice when it clips for this reason is that the negative part of the waveform takes more time to recover its shape.
The first thing I checked were the power transistor and the driving ones and found no faulty one so far.
In reference to the ballast power resistors (two for the + branch connected to the emitters and two for the - branch connected to the cathodes), in the visual inspection I noticed one of them is a little bit crack (in the - branch, may be is related to the recovery time mentioned before). The four of them are marked as 0.5Ohm and read 0.8Ohm but for the cracked one wich reads 1.1Ohm. This cracked resistor shows obviously a bigger dc voltage drop always than the other three even more with no load. Anyway, all the ballasts power resistor show an important dc voltage drop among 0,5mV and 0,8mV show I guess the problem might come from the switching or even the power transistors. I am not sure how dc can be produced in this switching topology I'll try to get some info about it.
The signal (sine) is reproduced correctly until is amplified way too much and clips evenly. What I have notice when it clips for this reason is that the negative part of the waveform takes more time to recover its shape.
The first thing I checked were the power transistor and the driving ones and found no faulty one so far.
In reference to the ballast power resistors (two for the + branch connected to the emitters and two for the - branch connected to the cathodes), in the visual inspection I noticed one of them is a little bit crack (in the - branch, may be is related to the recovery time mentioned before). The four of them are marked as 0.5Ohm and read 0.8Ohm but for the cracked one wich reads 1.1Ohm. This cracked resistor shows obviously a bigger dc voltage drop always than the other three even more with no load. Anyway, all the ballasts power resistor show an important dc voltage drop among 0,5mV and 0,8mV show I guess the problem might come from the switching or even the power transistors. I am not sure how dc can be produced in this switching topology I'll try to get some info about it.
What switching topology?, it doesn't sound like it's a switching (class D) amp, not from the 80's, and not with 2N3055's.
Hi Nigel. Well, transitors are not conducting continuosly, they are switched on and off based in input signal, kind off a push-pull topology. That is switching but do not confuse it with high frequency switching in a "sampling" enviroment.
Any hints about how this DC is being produced?
Any hints about how this DC is being produced?
The Company which manufactures the Talmus brand of gear still exists and is still manufacturing amps under the Talmus and Musicon brand names.
Here is their details:
Musicson Sl (spain) - Company Profile
That page has a "Contact Us" link which you might try for schematics etc.
Cheers,
Ian
Here is their details:
Musicson Sl (spain) - Company Profile
That page has a "Contact Us" link which you might try for schematics etc.
Cheers,
Ian
Why do you imagine that?, it just sounds like a standard linear analogue amplifier, no 'switching' involved.
As a DC coupled amplifier any fault in the amp is likely to cause a DC offset, as it's fairly small it could be something near the front end? - but you really need to obtain (or draw out) a schematic. Amplifiers are usually pretty similar, so you tend to have a good idea before you start what it's going to be like.
Do you have a decent picture of the top of the board?.
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