Hi Glen!
The cap on the base of Q401 (C401) is supposed to delay the closing of the relays on power up, but there is no provision to open the relays on power down. The circuit has to bleed out for the relays to open.
The relay circuit is pretty simplistic, and SAE should have done it better. All you can do make sure that the offset adjust is as close to zero as possible so that when the contacts open and close the offset is as low as possible (with the .1µf caps across the relay contacts, seems SAE knew that thump might be a problem, but did nothing to address it).
The amp doesn't have a lot of caps...replace them with modern equivalents. Should be able to get it done with less than $15 worth of caps.
The cap on the base of Q401 (C401) is supposed to delay the closing of the relays on power up, but there is no provision to open the relays on power down. The circuit has to bleed out for the relays to open.
The relay circuit is pretty simplistic, and SAE should have done it better. All you can do make sure that the offset adjust is as close to zero as possible so that when the contacts open and close the offset is as low as possible (with the .1µf caps across the relay contacts, seems SAE knew that thump might be a problem, but did nothing to address it).
The amp doesn't have a lot of caps...replace them with modern equivalents. Should be able to get it done with less than $15 worth of caps.
As a relay driver, the MPSU06 may be replaced by the MPSW06. This is the one watt version of the TO92 plastic package, the MPSA06. The free-air rating of the MPSU06 with its metal tab TO152 package is also one watt.
The relay driver circuit has room for improvement.
If you power it directly off the transformer secondary with a half wave rectifier and a 220µF 100V cap, and add a discharge diode for C401 in parallel with R402, the relay will drop out right now on power down. R403 and R407 should be changed to about 120K and 68K. The reason being if one channel dumps to the positive rail, and the other dumps to the negative rail, the relay will not trip with equal value resistors in the integrator.
The relay driver circuit has room for improvement.
If you power it directly off the transformer secondary with a half wave rectifier and a 220µF 100V cap, and add a discharge diode for C401 in parallel with R402, the relay will drop out right now on power down. R403 and R407 should be changed to about 120K and 68K. The reason being if one channel dumps to the positive rail, and the other dumps to the negative rail, the relay will not trip with equal value resistors in the integrator.
This is worth the modification. I have had a number of 2400's that have come to me with both channels blown sky high. Its not hard to do and it will help safe guard your speakers.
For those that aren't sitting on several hundred out of stock transistors the information is very valuable. My advise is to purchase enough to keep your 2400 running.
I momentarily posted another circuit, till I realized that the current limiting resistor is in the emitter of the Q401. Grrr....
This ought to work...just add the two diodes and the power resistor and the cap. Should pull the base down ASAP and turn the transistor off when powered down. Might have to play with the cap value...22µf might be too big. 10µf maybe...
This ought to work...just add the two diodes and the power resistor and the cap. Should pull the base down ASAP and turn the transistor off when powered down. Might have to play with the cap value...22µf might be too big. 10µf maybe...
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EchoWars said:I momentarily posted another circuit, till I realized that the current limiting resistor is in the emitter of the Q401. Grrr....
This ought to work...just add the two diodes and the power resistor and the cap. Should pull the base down ASAP and turn the transistor off when powered down. Might have to play with the cap value...22µf might be too big. 10µf maybe...
I will get the parts and try this. Thanks.
Glen
Worth a shot...
Note that the point of connection of the first 1N4004 is not at the filter caps, but directly from the output of the transformer. This is critical, so that as soon as the switch is turned off, the voltage is lost (as opposed to the voltage on the big 16,800µf 75V caps, which takes forever to bleed off).
The amp also has a separate 6.3V winding for the lamps, which would have been great to use, except with the limiting resistor in the emitter of the relay driver transistor the base will be at too high a voltage for that. So you're stuck with getting the voltage from the high-voltage / high-current winding.
Note that the point of connection of the first 1N4004 is not at the filter caps, but directly from the output of the transformer. This is critical, so that as soon as the switch is turned off, the voltage is lost (as opposed to the voltage on the big 16,800µf 75V caps, which takes forever to bleed off).
The amp also has a separate 6.3V winding for the lamps, which would have been great to use, except with the limiting resistor in the emitter of the relay driver transistor the base will be at too high a voltage for that. So you're stuck with getting the voltage from the high-voltage / high-current winding.
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