Hi. I got an old amp from the mid 70's that I'm trying to fix. The fuses were gone and so some tiny bulbs which illuminate the front. The switches need deep cleaning in order to get some sound of it.
The thing sounds and pots work ok, but it distorts very badly. At low volumes is ok, but it starts to distort pretty soon.
It uses a couple of BD131 and BD132 as output stage transistors. The voltages are odd. Here they are:
BD131 (NPN)
BD132 (PNP)
The amp is fed with 32v DC. It's a 2x12w at 8ohms.
My knowledge is quite limited, but assuming this is a class AB complementary amp, base voltages are crazy. And if I'm not wrong, we should be expecting a lower emitter voltage as well. Something around 16v.
There are a few resistors that are pretty hot. You can touch them for about two seconds, then it hurts. Couple of them are "tanned". They read the correct value, but you can see they suffered at some point. There's one of them that drops the voltage to 21v to 0.7v...
I replaced the output transistors and all electrolytic caps with no luck. Where should I be looking at? I'm lost
There's a trimmer right at the supply voltage. It's set so the voltage drops to around 19v. At max. it drops the voltage to 16v or so, no more.
There's obviously something very wrong. Transistors are biased at really high voltages. I can't understand what's causing such levels. It happens to both channels. Right now I'm focusing just in one of them since they behave the same.
There's no schematic so I'm going to make a sketch of output stage core. The core are the couple BD131 & BD132, a MC150 which feeds the BD131 and a SC107B. The latter is handling 32v at one of its legs...
I appreciate any help, thanks.
The thing sounds and pots work ok, but it distorts very badly. At low volumes is ok, but it starts to distort pretty soon.
It uses a couple of BD131 and BD132 as output stage transistors. The voltages are odd. Here they are:
BD131 (NPN)
- 21v
- 32v
- 21v
BD132 (PNP)
- 21v
- 0v
- 20v
The amp is fed with 32v DC. It's a 2x12w at 8ohms.
My knowledge is quite limited, but assuming this is a class AB complementary amp, base voltages are crazy. And if I'm not wrong, we should be expecting a lower emitter voltage as well. Something around 16v.
There are a few resistors that are pretty hot. You can touch them for about two seconds, then it hurts. Couple of them are "tanned". They read the correct value, but you can see they suffered at some point. There's one of them that drops the voltage to 21v to 0.7v...
I replaced the output transistors and all electrolytic caps with no luck. Where should I be looking at? I'm lost
There's a trimmer right at the supply voltage. It's set so the voltage drops to around 19v. At max. it drops the voltage to 16v or so, no more.
There's obviously something very wrong. Transistors are biased at really high voltages. I can't understand what's causing such levels. It happens to both channels. Right now I'm focusing just in one of them since they behave the same.
There's no schematic so I'm going to make a sketch of output stage core. The core are the couple BD131 & BD132, a MC150 which feeds the BD131 and a SC107B. The latter is handling 32v at one of its legs...
I appreciate any help, thanks.
Last edited:
Well the PNP collector is missing the -32V supply. So I'd start with the main supply components+wiring at the very least.
Perhaps there is no manufacturer identification and I doubt there is a negative supply. As the OP says, for 12W/8 Ohm, there will be a single 32V supply and the output will be DC isolated via a large capacitor. This is possibly similar to a standard Philips design of the 1960s.
Last edited:
I suspect this amp has a unipolar supply, but could be wrong. The supply voltage would be appropriate for a 10 - 12W amplifier.
The bias pot should be set so that the output sits at mid supply. (Is there an output coupling cap?)
A schematic would really help. Coupling caps on the input of the amplifier replaced?
If you like to measure on a defective transistor amp it is important to disable the feedback loop. With feedback loop the amp will try to fix a defect and therefore it is more difficult to pinpoint where the error originates from.
The manufacturer is called Bettor. A spanish brand that worked with Dual in the past. The took their turntables and made amps to put them together. Sadly, there's no schematic.
Yes, it has a unipolar power supply. The big filter cap is 4700uF.
The output caps are 1000uF. Replaced.
Input caps are 4.7uf and were also replaced.
Without a schematic I'm not sure I can identify the feedback loop and disable it properly. I haven't done that before.
I'm sketching a part of the circuit.
Yes, it has a unipolar power supply. The big filter cap is 4700uF.
The output caps are 1000uF. Replaced.
Input caps are 4.7uf and were also replaced.
Without a schematic I'm not sure I can identify the feedback loop and disable it properly. I haven't done that before.
I'm sketching a part of the circuit.
I think your model is described in the following thread
Restauracion Bettor EF-5 Dual 1214 | Foros de Electronica
Cheers
Restauracion Bettor EF-5 Dual 1214 | Foros de Electronica
Cheers
Attachments
Nice little retro amp🙂
Replace electrolytic capacitors. This can give immediate results. You can replace the output transistors Q5,Q6 with Darlington BDW93\94, TIP122 / 127, 142/147 or similar. You can take the Chinese TO-220. I bought inexpensive Chinese 2sk772 / 2sd882 with Hfe = 300. R4 sets the voltage at the emitters of the output transistors 1/2 from the supply. R7 sets the current of the output transistors.
With another power supply and heatsinks, it can be converted into a Hood1969 circuit 🙂 But this is optional.
Replace electrolytic capacitors. This can give immediate results. You can replace the output transistors Q5,Q6 with Darlington BDW93\94, TIP122 / 127, 142/147 or similar. You can take the Chinese TO-220. I bought inexpensive Chinese 2sk772 / 2sd882 with Hfe = 300. R4 sets the voltage at the emitters of the output transistors 1/2 from the supply. R7 sets the current of the output transistors.
With another power supply and heatsinks, it can be converted into a Hood1969 circuit 🙂 But this is optional.
Last edited:
Mine is the Bettor EF225 like this one:
Bettor EF-275 con dual 1225. (1a Parte). - YouTube
The schematics looks similar. Although mine has only one trimmer per channel.
Bettor EF-275 con dual 1225. (1a Parte). - YouTube
The schematics looks similar. Although mine has only one trimmer per channel.
Wow, I was way off on my assuming what this design is. 😱
[OP reports Bettor EF225]
I haven't seen a turntable / amp combo since I was kid; pretty cool item to restore.
Axial electrolytic capacitors can be replaced with conventional ones.
Instead of bulbs, use colored LEDs with a resistor of about 1k to limit the current (brightness).
The output transistors can be soldered out and checked with a transistor tester. Without turning on the power, you can check on the spot with a multimeter in diode mode - there should not be 0 or a lot. Many types of transistors (complementary pairs) can be used. The main requirement is a maximum current gain and a current of at least 3A. You can use darlington, but you have to change the bias.
Spend half an hour of your time sketching a diagram looking at the board's conductors and external connections. Check the quality of the rations and the integrity of the conductors at the same time. Make a list of capacitors - values and sizes. This will make your purchase easier. You don't need audiophile 🙂
Instead of bulbs, use colored LEDs with a resistor of about 1k to limit the current (brightness).
The output transistors can be soldered out and checked with a transistor tester. Without turning on the power, you can check on the spot with a multimeter in diode mode - there should not be 0 or a lot. Many types of transistors (complementary pairs) can be used. The main requirement is a maximum current gain and a current of at least 3A. You can use darlington, but you have to change the bias.
Spend half an hour of your time sketching a diagram looking at the board's conductors and external connections. Check the quality of the rations and the integrity of the conductors at the same time. Make a list of capacitors - values and sizes. This will make your purchase easier. You don't need audiophile 🙂
Actually the base voltages don't say much. What is important is the Vbe, the voltage between base and emitter. That determines the transistor bias.
The only one that seems wrong is the 0V on the second BD132.
A circuit diagram is definitely needed, everything else is groping around in the dark.
Jan
The only one that seems wrong is the 0V on the second BD132.
A circuit diagram is definitely needed, everything else is groping around in the dark.
Jan
At the output of EF1, therefore the collector npn + 32v, and the collector pnp 0v (ground)
Measure voltages with respect to ground (0), in addition to B-E transistors (1-3), indicating the polarity.
Measure voltages with respect to ground (0), in addition to B-E transistors (1-3), indicating the polarity.
Last edited:
The midpoint voltage is 20v, which is more than half (16v).
Take the above diagram as a basis, compare with the PCB and draw your actual one.
Take the above diagram as a basis, compare with the PCB and draw your actual one.
Last edited:
I made a schematic. The input is not complete though, I have to spend more time tracing it. But there's only one SC109b involved, so it shouldn't change a lot.
The output transistors are ok. I desoldered them and did the diode test. All of them read ok. I also tried new couple of transistors and still the same problem.
By measure B-E voltage do you mean one lead to base and the other one to emitter? All measures I have ever done were always to ground.
Many thanks.
The output transistors are ok. I desoldered them and did the diode test. All of them read ok. I also tried new couple of transistors and still the same problem.
By measure B-E voltage do you mean one lead to base and the other one to emitter? All measures I have ever done were always to ground.
Many thanks.
Attachments
20W Audio Amplifier Using ECN149-ECP149 Pair Transistor
Short description. Not necessarily a complete match 🙁
Attention! Carry out measurements with a connected speaker.
1976. The usual Philips / Siemens / Dual scheme.
Last edited: