The Push-Pull Sziklai Pair design is used the vintage Marantz Model 18 and is a classic from the late ’60s. I recently picked one up and I am in the process of trying to restore it. I have encountered a problem in the Left channel power output stage.

The problem I am having is exactly how the bias circuit (comprised of Q2) works to provide the correct Q point for the output stage. Exactly how does this transistor perform the function of setting the desired bias for the amplifier? The Marantz service manual states that the voltage across R43 should be set to 80mv without an AF signal. I have had problems in trying to set it to the desired level and experienced failures in both of the power transistors (Q13/Q14) while attempting this adjustment. It appears to me that the way the R27 is connected is a potential failure mode. R27 is a wire wound pot and there exists a possibility that during an adjustment the resistance could drop to zero between the Base and Emitter of Q2. This would put Q2 in the cutoff but is this condition which would lead to an overcurrent condition in Q13/Q14 causing them to fail? It would help considerably in the troubleshooting of this circuit if someone could explain the best way to approach how to determine which part of this circuit might be causing the problem.

I have attached the schematic for the power output section and the associated driver circuit; left channel only. I have tested Q2 and R27; the diode stack (CR18) was only showing a 1.7v drop and I replaced it with 3 x 1N914s; after installation, this turned out to be 1.99 volts. Posts about this problem indicate that it should be around 2.1 to 2.2. I don’t know if I need to select 3 new 1N914s and try to achieve a higher very drop closer to the 2.1 value.

 

You're right - that's not clever.
First, are you absolutely certain that CR16 is three diodes in series? I know it's drawn that way, but I would approach it with some scepticism. Is it attached to the heatsink to sense the temperature?
It's a sort-of Vbe multiplier, but not quite - it sets the bias to V(CR16)+Vbe, and as you vary the current through CR16 it varies the voltage a little.
It would be better with a resistor in series with the pot. Look where the pot is set on the good channel, and take a guess what resistance it sets, halve it, and put it in series with the pot before you adjust it. That way, it might be a bit under-biassed but it won't blow up.

 

CR18 is a small package about 14mm x 14mm x 3mm. My tester shows that it’s Silicon and has a Vf of 2.038V and it was mounted next to R27 and in contact with the main heat sink. Several posts on audiokarma.org indicate that it could be replaced with 3x 1N4148 diodes. The original Q2 had an Hfe of 50 and I have tried others (recommended subs) and it did not help.

 

You're probably correct, but better to be sure. If the original one gives the same reading as the one in the other channel don't replace it.
Will the amplifier run at minimum bias (with the rheostat set to maximum resistance)? If not, get to that stage before trying to set the bias.

 

The Push-Pull Sziklai Pair design is used the vintage Marantz Model 18 and is a classic from the late ’60s. I recently picked one up and I am in the process of trying to restore it. I have encountered a problem in the Left channel power output stage.

The problem I am having is exactly how the bias circuit (comprised of Q2) works to provide the correct Q point for the output stage. Exactly how does this transistor perform the function of setting the desired bias for the amplifier? The Marantz service manual states that the voltage across R43 should be set to 80mv without an AF signal. I have had problems in trying to set it to the desired level and experienced failures in both of the power transistors (Q13/Q14) while attempting this adjustment. It appears to me that the way the R27 is connected is a potential failure mode. R27 is a wire wound pot and there exists a possibility that during an adjustment the resistance could drop to zero between the Base and Emitter of Q2. This would put Q2 in the cutoff but is this condition which would lead to an overcurrent condition in Q13/Q14 causing them to fail? It would help considerably in the troubleshooting of this circuit if someone could explain the best way to approach how to determine which part of this circuit might be causing the problem.

I have attached the schematic for the power output section and the associated driver circuit; left channel only. I have tested Q2 and R27; the diode stack (CR18) was only showing a 1.7v drop and I replaced it with 3 x 1N914s; after installation, this turned out to be 1.99 volts. Posts about this problem indicate that it should be around 2.1 to 2.2. I don’t know if I need to select 3 new 1N914s and try to achieve a higher very drop closer to the 2.1 value.View attachment 293059View attachment 293060

Hello there,

What you would really do here is analyze the entire output drive section and see what goes. Since you probably dont want to do that, why dont you use a simulator. You can get answers that way that can be very definitive and help solve your problem.
I am not sure you have to worry about the pot shorting out, I think you have to worry about the pot arm becoming disconnected from the resistive element which is a typical problem with pots in power circuits. This implies a resistor in parallel. That's if i understand your problem correctly.

As a side note, when testing higher power circuits it is customary to set the input power levels to a lower value to help prevent blowing out parts. This would mean instead of applying plus and minus 36v you apply plus and minus 10v or maybe a little lower or a little higher. You can also try putting fuses in series with the transistor collectors and completely ignore the old saying, "When you put a fuse in series with a transistor the transistor protects the fuse not the other way around", because that is not always true. For quiescent adjustment you probably only need 1 amp through the transistors, maybe even less, and they probably dont blow right away either they probably blow due to heating, however fast that might be. This means the fuse might protect the transistor. Of course it should be the fastest type fuse you can get. You also have to be aware that the final adjustment will have to be with no fuses, or higher rated fuses to handle the actual max operating current. This would come after you solve any failure problems. Also, after you are done testing and have the solution you should replace the output transistors with new units just in case there was any minor damage internally due to the testing that does not show up yet as a fully blown unit. Minor internal damage causes what is usually referred to as a "slow" failure because it takes more time but even under normal operating conditions the unit could fail prematurely after that.

Note: When lowering the power supply voltages for testing you only have to lower the supplies to the power stages. You can also try placing a small value power resistor in series with each supply, like 1 Ohm, 2 Ohms, 5 Ohms, even 10 Ohms, to keep the max current lower for testing.

This is quite an amplifier, wow. More parts than i have seen in any project in a long time. Very interesting though.

 

You're probably correct, but better to be sure. If the original one gives the same reading as the one in the other channel don't replace it.
Will the amplifier run at minimum bias (with the rheostat set to maximum resistance)? If not, get to that stage before trying to set the bias.

 

Just got through measureing the Vf on the Right Channel diode pack and it measured very close to the same value as the left at 2.039 volts. I will be putting it back in the circuit and using the original Q2 as a starting point to check the bias values again.

 

Hello there,

What you would really do here is analyze the entire output drive section and see what goes. Since you probably dont want to do that, why dont you use a simulator. You can get answers that way that can be very definitive and help solve your problem.
I am not sure you have to worry about the pot shorting out, I think you have to worry about the pot arm becoming disconnected from the resistive element which is a typical problem with pots in power circuits. This implies a resistor in parallel. That's if i understand your problem correctly.

As a side note, when testing higher power circuits it is customary to set the input power levels to a lower value to help prevent blowing out parts. This would mean instead of applying plus and minus 36v you apply plus and minus 10v or maybe a little lower or a little higher. You can also try putting fuses in series with the transistor collectors and completely ignore the old saying, "When you put a fuse in series with a transistor the transistor protects the fuse not the other way around", because that is not always true. For quiescent adjustment you probably only need 1 amp through the transistors, maybe even less, and they probably dont blow right away either they probably blow due to heating, however fast that might be. This means the fuse might protect the transistor. Of course it should be the fastest type fuse you can get. You also have to be aware that the final adjustment will have to be with no fuses, or higher rated fuses to handle the actual max operating current. This would come after you solve any failure problems. Also, after you are done testing and have the solution you should replace the output transistors with new units just in case there was any minor damage internally due to the testing that does not show up yet as a fully blown unit. Minor internal damage causes what is usually referred to as a "slow" failure because it takes more time but even under normal operating conditions the unit could fail prematurely after that.

Note: When lowering the power supply voltages for testing you only have to lower the supplies to the power stages. You can also try placing a small value power resistor in series with each supply, like 1 Ohm, 2 Ohms, 5 Ohms, even 10 Ohms, to keep the max current lower for testing.

This is quite an amplifier, wow. More parts than i have seen in any project in a long time. Very interesting though.

 

Thanks for the tips. I have download LTSpice and will have to try doing what you suggested. I have been limiting the voltage/current during testing with a variac and dim blub.

 

Thanks for the tips. I have download LTSpice and will have to try doing what you suggested. I have been limiting the voltage/current during testing with a variac and dim blub.

Beware of circuit where the bias varies with the power supply voltage! Guess what happens if you set it up with the power supply lower than the normal running voltage! (I'm not too sure what sets the VAS current)

 

As I was making changes I would alway bring the amp up slowly at first and then I would remove the dim blub and just bring it up with the variac to full line voltage and then check the final results.

 

To all of you who have offered up suggestions to help with the restoration of the Marantz 18; thank you very much.
Finally after replacing Q9/10 and Q13/14 and putting the original CR18 and Q2 back in; the unit is operating within acceptable limits.

 

I once repaired a discrete transistor H H Scott amplifier with a similar issue by also replacing one of the driver transistors. Apparently it had developed leakage, although it tested correctly on a transistor tester.