Sunn Concert Lead - hum

[lowell]

schematic is at this thread:
concert lead

I have the "Old" version of the PA in this amp. So there is hum at the speaker. I have narrowed it down to the PA as the preamp out has no hum when the PA is disconnected (opened the shorting jack at "power amp in"). There is 2 Vdc at the speaker jack and 5 Vac at the speaker jack. This makes sense because the main rails are +25.2/-23.7 respectively measured from each rail to the speaker jack - ~2 Vdc of offset. These rails are +22.7/-25vdc referenced to ground. These measurements are all without a load connected. I replaced the single main filter cap C1, checked power transistors and driver transistors for shorts, and checked Vbe voltages on everything and they all seem to be in order. I've also checked ground at TP34. All ballast resistors are good as well. So I'm stuck here. Not sure why there is ~2v of DC offset and/or why there is 5 Vac on speaker out.

[Jazz P Bass]

There are other voltages on the power supply.
Have you checked/ recapped them.

[loudthud]

The four output transistors in these amps need to be matched for the lowest DC offset but small differences in the resistors can also cause offset. The driver transformers in these amp are prone to developing shorts between windings, but that usually causes much more offset. Remove the two wires from each winding (one winding at a time) and check resistance to ground. It should be infinite.

I suggest you first remove all the output transistors and check for offset. That will tell you if the bias networks and driver transformer is ok. Then install the power transistors and swap them around for lowest offset. Note that Q2's collector is grounded, sometimes it won't have a mica washer.

[gbono]

Note that Q2's collector is grounded, sometimes it won't have a mica washer.

I never noticed this - is TP 33 (or is it 35??) a connection to R307?? The 6-7 of these amps I've worked on always had an insulator on Q2.

[lowell]

Anyone know what wattage that 15v zener is? CR306 (on the "old" schematic). I disconnected the wire connecting this supply to the preamp boards and the voltage on that zener is -14.7vdc. So I'm thinking that zener may be failing. Even if so would this slightly low supply really cause the offset in the PA?

Voltages on the CR305 rectifier supply are a bit low it seems. 32v, 27v, 21v.

But I guess I need to remove the output transistors first per LTs suggestion.

[loudthud]

The 33 in a circle on the schematic next to R307 is not really a test point, it is a connection to the circuit board. In the amp that I have here it's connected to a black wire that disappears into the wiring harness.

[loudthud]

Anyone know what wattage that 15v zener is? CR306 (on the "old" schematic). I disconnected the wire connecting this supply to the preamp boards and the voltage on that zener is -14.7vdc. So I'm thinking that zener may be failing. Even if so would this slightly low supply really cause the offset in the PA?

Voltages on the CR305 rectifier supply are a bit low it seems. 32v, 27v, 21v.

But I guess I need to remove the output transistors first per LTs suggestion.

My amp has a 1N4744 zener. It's 15V 1W. It shouldn't affect the output offset.

[lowell]

I pulled all the power transistors and now the main rails are -24.8/+23.2v. There is now -.8mv on the speaker jack. This clue you in? On the "old" schematic there aren't listed voltages. On the "lead pwr" schematic these main rails should be +/- 34vdc. Not sure if the older versions had the same voltages or not. Anyone know?

Also the 3rd prong is broken off on the power plug, however I had alligator clipped to the chassis and to the wall earth earlier during troubleshooting and it didn't seem to have any effect.

[gbono]

Did you measure the preamp output to see how clean it is?
I believe there were service notes for updates to the output board though I don't believe I remember a specific noise fix.

[loudthud]

The older version uses a floating power supply with the output transistors configured as a bridge, except one side of the bridge (and thus the speaker) is grounded. The advantage of this circuit is that two transistors are always in series with the load so they do not see the excessive voltage that causes second breakdown. The floating power supply runs about 45V so with no signal the rails should be about +/- 22.5V.

The second version, after about 1975, was a more conventional parallel totem pole with +/- 34V rails. Both versions use transformer drive to the power transistors and nearly the same driver stage. On the old version the power transformer has 8 wires, on the new version just 5 wires.

[J M Fahey]

Anyone know what wattage that 15v zener is? CR306 (on the "old" schematic). I disconnected the wire connecting this supply to the preamp boards and the voltage on that zener is -14.7vdc. So I'm thinking that zener may be failing.

So they STOLE you 0.3V??
Robbers !!!!
I'd sue them or at least complain to the BBB.

Next time I go shopping I'll carry a ruler with me and measure all Zeners one by one, they won't cheat me easily

[lowell]

Ok I sense some sarcasm. haha. So I guess this isn't a big deal then. I always thought if the voltage on a zener is less than it's specified voltage that it isn't functioning properly. But I guess .3vdc isn't a whole lot.

[lowell]

The preamp output doesn't have this hum as specified in a previous post. Thanks for those figures LT.

Could the bias adjust trimmer pot be causing this? Maybe it's not biased correctly? I don't see any notes on how to set the bias.

[lowell]

How does one determine the correct bias for a solid state PA? I guess this is a transformer coupled output so it's different from more modern designs. Do I check the datasheets for these driver transistors and set them to the Vbe "on" voltage?

[Enzo]

You scope the output and adjust for the crossover distortion to just disappear.