SAE 2400 turn on/turn off pop

[Glen B]

I have an SAE 2400 with a pop in the speakers immediately upon turn on and about 5 seconds after turn off. What parts should I be taking a look at on the protection board ? I have attached an image of the relevant section of the schematic.

[EchoWars]

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.

[Glen B]

Echowars,

I already replaced all of the caps. I will try getting the DC offset as close to zero as possible. Thanks.

Glen

[EchoWars]

Q401 should not turn on for a few seconds after power up...maybe it's shorted?

Are the relays OK? If they are stuck closed, that might also cause problems.

[6BG6GA]

I have done a number of 2400's in the past and recommend replacement of transistors as well as the caps on the relay board.

Mr. James Bongiorno had his head someplace else when he designed the amplifier. The relay circuit is among its many flaws.

[Glen B]

Originally posted by burnedfingers
I have done a number of 2400's in the past and recommend replacement of transistors as well as the caps on the relay board.

Mr. James Bongiorno had his head someplace else when he designed the amplifier. The relay circuit is among its many flaws.

Do you know of a readily available replacement for Q401/11-0080A ? MPSU06, S1375 seem to be discontinued. Not even the NTE replacement NTE210 is available.

Glen

[djk]

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.

[Glen B]

Originally posted by djk
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.

This is great information. Mouser has the MPSW06 in stock, along with the other transistors on the relay board. Thanks.

Glen

[6BG6GA]

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.

[Glen B]

Originally posted by burnedfingers


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.

Burnedfingers,
I have a few questions just to be clear. The diode goes across R402, correct ? Is it anode side to the base of Q401 ? Any specific diode ? Is a 1N4004 good ?

Glen