Model 18 Scope Troubleshooting
- Thread starter Fhamre
- Start date
monkboughtlunch
Super Member
Is the selector switch a "break before make?" If so, do all inputs need to be grounded to a common ground? Is the FM grounded differently than the other inputs on the selector switch (which don't pop)?
monkboughtlunch
Super Member
Found this article. Thoughts?
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Source: http://www.zen22142.zen.co.uk/Design/dcthump.htm
Removing the DC Thump in Audio Circuits
by Andy Collinson
Overview
You may all be familiar with this sound effect. You listen to music through an amplifier, then switch the source to a different input and hear a loud click or "thump" in the speaker system. Not all equipment is affected. Some high end audio systems are not affected, but intercoms,switching systems, and multi-channel systems are often affected.
The Cause
Most audio outputs have some standing DC voltage which is separated by a capacitor. The capacitor will block this DC voltage while allowing the AC or audio to pass. Audio components using transformer coupling are not affected. The input to an amplifier may have a capacitive input or just a volume control. When switching inputs, charge is transferred through the switch as the charge on the capacitors reach equilibrium. This takes only a fraction of a second and the sound you hear is the annoying "DC" thump. A typical circuit for audio switching is shown below. This is just an example.
At input 1, there is a standing dc voltage on the emitter of the transistor in system A. This is isolated by the 47u capacitor. The capacitor will either be discharged or partly charged. When the rotary switch is turned to position 1 a click will be heard as the capacitor charges through the switch and volume control on the following amplifier. A similar condition appears at switch position 3. System A represents part of a preamp while system B represents the output of an amplifier. What can be done to prevent this?
A Solution
One possible solution is shown above. The rotary switch is part of the right hand circuit. The switch contacts now have a dc path to ground. When the switch position is moved there is no charging or discharging of capacitors, as these have already been charged by the inclusion of the new resistors. The potential on the switch contacts is 0 volts, whilst the opposite site of the capacitor plates are at the potential of the preceding stage. I have shown different resistor values as each system has different coupling capacitors. The resistors can be soldered at the remote amplifier switch, alternatively they could be included at the output of the remote audio systems.
========================================
Source: http://www.zen22142.zen.co.uk/Design/dcthump.htm
Removing the DC Thump in Audio Circuits
by Andy Collinson
Overview
You may all be familiar with this sound effect. You listen to music through an amplifier, then switch the source to a different input and hear a loud click or "thump" in the speaker system. Not all equipment is affected. Some high end audio systems are not affected, but intercoms,switching systems, and multi-channel systems are often affected.
The Cause
Most audio outputs have some standing DC voltage which is separated by a capacitor. The capacitor will block this DC voltage while allowing the AC or audio to pass. Audio components using transformer coupling are not affected. The input to an amplifier may have a capacitive input or just a volume control. When switching inputs, charge is transferred through the switch as the charge on the capacitors reach equilibrium. This takes only a fraction of a second and the sound you hear is the annoying "DC" thump. A typical circuit for audio switching is shown below. This is just an example.
At input 1, there is a standing dc voltage on the emitter of the transistor in system A. This is isolated by the 47u capacitor. The capacitor will either be discharged or partly charged. When the rotary switch is turned to position 1 a click will be heard as the capacitor charges through the switch and volume control on the following amplifier. A similar condition appears at switch position 3. System A represents part of a preamp while system B represents the output of an amplifier. What can be done to prevent this?
A Solution
One possible solution is shown above. The rotary switch is part of the right hand circuit. The switch contacts now have a dc path to ground. When the switch position is moved there is no charging or discharging of capacitors, as these have already been charged by the inclusion of the new resistors. The potential on the switch contacts is 0 volts, whilst the opposite site of the capacitor plates are at the potential of the preceding stage. I have shown different resistor values as each system has different coupling capacitors. The resistors can be soldered at the remote amplifier switch, alternatively they could be included at the output of the remote audio systems.
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The super easy fix....
Just jumper out the +200V on the switch. The only reason that the 200V is switched is to turn the neon lamps on and off. The FM section is allways live anyway. I just tried it and it works like a champ.
This also shows that it's not on the audio switching side so the solution above would have no impact. This is good because it means that there is no DC sitting on the output of the tuner.
There are two last components that I am going to try anyway. C4 and C5. You will find them at the input of the AC to the receiver. They are used for safety and interference suppression. I don't think they are bad but I have seen them go bad in other equipment.
Since we went down the path of playing with the LDR I am going to replace mine, it never worked anyway. I know that, with enough light, I get full muting. Rather than reinvent the circuit I am going to try a couple different photocell and lamp combinations. In particular I am going to switch the lamp for a green version. This is because the photocell is more sensitive to green than orange light.
Just jumper out the +200V on the switch. The only reason that the 200V is switched is to turn the neon lamps on and off. The FM section is allways live anyway. I just tried it and it works like a champ.
This also shows that it's not on the audio switching side so the solution above would have no impact. This is good because it means that there is no DC sitting on the output of the tuner.
There are two last components that I am going to try anyway. C4 and C5. You will find them at the input of the AC to the receiver. They are used for safety and interference suppression. I don't think they are bad but I have seen them go bad in other equipment.
Since we went down the path of playing with the LDR I am going to replace mine, it never worked anyway. I know that, with enough light, I get full muting. Rather than reinvent the circuit I am going to try a couple different photocell and lamp combinations. In particular I am going to switch the lamp for a green version. This is because the photocell is more sensitive to green than orange light.
monkboughtlunch
Super Member
If one jumpers out the +200v on the switch will the neon lamps no longer function?
monkboughtlunch
Super Member
if the station is not tuned in, how does the stereo light behave -- does it just stay on all the time?
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monkboughtlunch
Super Member
Going back to the scope, mine is erratic. The scope will disappear every hour or so. If I power down the unit, the trace will reappear in the center of the dial and if I power up quickly the scope image returns. Has anyone experienced this?
Is the TIP47 transistor the same spec transistor that Marantz used in the original build? Is this the ideal transistor to use as a replacement?
The selector switch has two sections, front and back. The back section has a set of orange wires. One contact has a single orange wire, this carries the +200V from the power supply. The other contact has two orange wires, one wire is routed to the oscillator and MPX boards. Just place a wire across the two contacts. This leaves the power to the neon lamp circuit on all the time but the input switching will still work exactly like it should.
The stereo light works as it normally would when you tune a station, if it's off tune it will go out.
bktheking
Gitter Done!
There is no way of knowing the spec as marantz didn't publish the part number, only marantz part numbers. I suspect the tip47 is over spec'd for the application, you have nothing to worry about. Since your scope is misbehaving and those 4 transistors are known to fail your best bet is to replace them and see how it fairs out, they are cheap to purchase and take about 10 minutes to install.
monkboughtlunch
Super Member
Are there any disadvantages of jumpering? Would the LDRs always be "on" and thus have shorter life?
The lamp in the muting LDR is only on when "off tune" so I don't think that one is any problem. LDR602 should be jumpered out. LDR601 would be on when the tuner is in stereo.
The reality is that the lifetime of the neon tube is very long and if it did fail you can make a new LDR for under 2 bucks.
monkboughtlunch
Super Member
Agreed. Will be interesting to see how bktheking's selector switch is wired since his unit doesn't pop like ours.
By the way, how easily can the FM stereo neon indicator light be replaced? Mine still works but rapidly flickers for a while after start up. I wonder if that is a sign it is failing. When I cleaned my dial glass, I tried to unfasten the stereo indicator to take a closer look but it wouldn't budge. I didn't want to force it for fear of breaking it, so I gave up. Does that orange plastic lens and black lid on the dial side unscrew or just pop off from the rest of the wire assembly behind the dial? And to put a new NE2H lamp in, does it require soldering? Is there a resistor built-in to the stereo indicator wire assembly?
By the way, how easily can the FM stereo neon indicator light be replaced? Mine still works but rapidly flickers for a while after start up. I wonder if that is a sign it is failing. When I cleaned my dial glass, I tried to unfasten the stereo indicator to take a closer look but it wouldn't budge. I didn't want to force it for fear of breaking it, so I gave up. Does that orange plastic lens and black lid on the dial side unscrew or just pop off from the rest of the wire assembly behind the dial? And to put a new NE2H lamp in, does it require soldering? Is there a resistor built-in to the stereo indicator wire assembly?
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