Hi,
I have an old amplifier from somewhere in the 1980s, a Montarbo model 164.
It used to work fine as a bass guitar amplifier years ago. Then it got moved from one house to the next and sat on the attic for many years. The attic is insulated, so although there will be some temperature variations, it will never be freezing cold nor scorching hot.
The amplifier powers up, but produces no sound.
I found the circuit diagram in a huge document of Montarbo circuit diagrams on archive.org [1], the correct diagram attached for your convenience.
I was able to identify that the NPN output transistor T4 is short-circuited. I measured it out-of-circuit and it is short across all three pin combinations: base-emitter, base-collector, emitter-collector. This transistor is of type TIP 33c, which is no longer produced. Its PNP complement T3 (which measures fine) is TIP 34c.
The power rail was still correct (+42V, -42V and +15V, -15V). I also measured the diodes, the T1 and T2 transistors (in-circuit) and a number of capacitors and resistors, although not exhaustively. There are no visual problem signs like burn marks or similar.
My questions are:
1. With which transistor should I replace the TIP 33c?
2. Should the complement TIP 34c also be replaced, if so with what?
3. Is it reasonable to expect the faulty T4 to be broken due to age? Or do you have a suggestion of what to check in this circuit that could explain T4 to go short-circuit?
4. There is one potentiometer on the output circuit, between pins 6 and 7 (both marked 'bias') of the LM391 IC (marked 'x2'). The LM391 datasheet says "Adjust to set bias current in the output stage." . The manual annotation on the circuit diagram indicates the bias current (Courant Repos, in French) is 6mA, which more or less corresponds to the official indication on the first page of the full pdf [1], which specifies 5.5mA for this circuit.
Am I correct that I should measure the voltage drop across one of the 0.18 ohm resistors, then tune the potentiometer until I read a voltage of about 1mV (0.18 ohm x 5.5 mA) ? (or measure across both 0.18 ohm resistors and tune to 2mV).
5. Finally, is there anything to do with respect to maintaining 0V on the speaker terminal, or will this automatically be the case once the bias current is set correctly?
Thanks a lot for your advice!
Patrick
[1] https://archive.org/details/montarbo-service-1970-2000/mode/1up (page 70)
I have an old amplifier from somewhere in the 1980s, a Montarbo model 164.
It used to work fine as a bass guitar amplifier years ago. Then it got moved from one house to the next and sat on the attic for many years. The attic is insulated, so although there will be some temperature variations, it will never be freezing cold nor scorching hot.
The amplifier powers up, but produces no sound.
I found the circuit diagram in a huge document of Montarbo circuit diagrams on archive.org [1], the correct diagram attached for your convenience.
I was able to identify that the NPN output transistor T4 is short-circuited. I measured it out-of-circuit and it is short across all three pin combinations: base-emitter, base-collector, emitter-collector. This transistor is of type TIP 33c, which is no longer produced. Its PNP complement T3 (which measures fine) is TIP 34c.
The power rail was still correct (+42V, -42V and +15V, -15V). I also measured the diodes, the T1 and T2 transistors (in-circuit) and a number of capacitors and resistors, although not exhaustively. There are no visual problem signs like burn marks or similar.
My questions are:
1. With which transistor should I replace the TIP 33c?
2. Should the complement TIP 34c also be replaced, if so with what?
3. Is it reasonable to expect the faulty T4 to be broken due to age? Or do you have a suggestion of what to check in this circuit that could explain T4 to go short-circuit?
4. There is one potentiometer on the output circuit, between pins 6 and 7 (both marked 'bias') of the LM391 IC (marked 'x2'). The LM391 datasheet says "Adjust to set bias current in the output stage." . The manual annotation on the circuit diagram indicates the bias current (Courant Repos, in French) is 6mA, which more or less corresponds to the official indication on the first page of the full pdf [1], which specifies 5.5mA for this circuit.
Am I correct that I should measure the voltage drop across one of the 0.18 ohm resistors, then tune the potentiometer until I read a voltage of about 1mV (0.18 ohm x 5.5 mA) ? (or measure across both 0.18 ohm resistors and tune to 2mV).
5. Finally, is there anything to do with respect to maintaining 0V on the speaker terminal, or will this automatically be the case once the bias current is set correctly?
Thanks a lot for your advice!
Patrick
[1] https://archive.org/details/montarbo-service-1970-2000/mode/1up (page 70)
I would replace it with a Tip142G as the originals are obsolescent.
https://uk.rs-online.com/web/c/semi...ctedNavigation=attributes.Package_Type=TO-247
The Tip34c should also be replaced with a Tip147G as it may have been damaged.
T4 will most likely have failed due to misbiasing or overload; a shorted loudspeaker connection.
https://uk.rs-online.com/web/c/semi...ctedNavigation=attributes.Package_Type=TO-247
The Tip34c should also be replaced with a Tip147G as it may have been damaged.
T4 will most likely have failed due to misbiasing or overload; a shorted loudspeaker connection.
TIP142/7 are darlingtons, the originals are NOT. The are the output of a CFP and that’s the last place you’d want them unless it was designed that way. Use TIP35/6C, which are the next size up of the same thing. They are not the most rugged solution, but neither were the originals.
It would also work well with regular audio outputs, like MJW21193/4. Definite improvement in ruggedness over TIP anything. The fast ones may or may not be a stability risk. One could try C5200N/A1943N. The L (TO-264) cased versions would work, but for sure W or N case (TO-247 or -3P) would fit.
It would also work well with regular audio outputs, like MJW21193/4. Definite improvement in ruggedness over TIP anything. The fast ones may or may not be a stability risk. One could try C5200N/A1943N. The L (TO-264) cased versions would work, but for sure W or N case (TO-247 or -3P) would fit.