Group Buy Interest?

[antichef]

As a data point, I swapped the BC457/BC447 in Q4/Q5 for BC461/BC441 transistors on the two EQ boards that have inductors in one of my ACMP81s, and the hum dropped dramatically. (was that zmix / Chuck Zwicky? so many threads on so many boards, I've lost track - anyway, thanks whoever you are!)

I may try it on the other 81, but that's the one that I might want to send back for other reasons, so I'm on the fence as to whether to mod it.

 

[tarnationsauce2]

As a data point, I swapped the BC457/BC447 in Q4/Q5 for BC461/BC441 transistors on the two EQ boards that have inductors in one of my ACMP81s, and the hum dropped dramatically. (was that zmix / Chuck Zwicky? so many threads on so many boards, I've lost track - anyway, thanks whoever you are!)

I may try it on the other 81, but that's the one that I might want to send back for other reasons, so I'm on the fence as to whether to mod it.

Where did you source some BC461/BC441's at?

 

[antichef]

Where did you source some BC461/BC441's at?

http://www.newark.com

 

[jasonbmerrill]

whoa what the heck is this? I smelled something, opened up the case, and found this:

now, everything is fine, its working:

http://shup.com/Shup/101770/73.flac

however, that concerned me lol...

 

[antichef]

whoa what the heck is this? I smelled something, opened up the case, and found this:

That's a resistor, minus some smoke. Not sure what that one does.

 

[mshilarious]

Given the size and value, I'd guess that's somewhere in the power supply, perhaps between filter caps or something. I would replace that with the same rating, or perhaps higher seeing what happened to it . . . 1W at least . . .

That burning down the studio scenario is starting to look more likely

 

[antichef]

That burning down the studio scenario is starting to look more likely

That is indeed a spooky picture

 

[crazydoc]

whoa what the heck is this? I smelled something, opened up the case, and found this:

now, everything is fine, its working:

http://shup.com/Shup/101770/73.flac

however, that concerned me lol...

That's the emitter resistor for the output stage power transistor (2N3055) - I wouldn't run the unit like that, or you risk blowing the transistor and possibly other components. I don't know why it blew, unless something upstream biased the transistor on too hard. Maybe 0.5W isn't robust enough, try a higher wattage (though that risks blowing something upstream if there's a problem there). Here's the germain part of the schematic:

 

[jasonbmerrill]

so... can i get one of these @ radioshack? How hard is it to put in?

 

[mshilarious]

so... can i get one of these @ radioshack? How hard is it to put in?

Maybe; it's a common value. It's an easy swap if it's easy to get the PCB off. If not, you can clip the leads back to the holes and solder the new leads from the top.

 

[crazydoc]

so... can i get one of these @ radioshack? How hard is it to put in?

Probably - try a 1watt 47ohm resistor. If they don't have a 1watt one, you can try a 1/2 watt one again, or piggyback two 100ohm ones in parallel. Can you solder? If so, you can either remove the old one and replace it, or cut it off its leads and solder the new one to the remaining leads (don't have to remove the pc board that way.)

 

[mshilarious]

That's the emitter resistor for the output stage power transistor (2N3055) - I wouldn't run the unit like that, or you risk blowing the transistor and possibly other components. I don't know why it blew, unless something upstream biased the transistor on too hard. Maybe 0.5W isn't robust enough, try a higher wattage (though that risks blowing something upstream if there's a problem there). Here's the germain part of the schematic:

Is 7W1 a trimpot? That would seem to set bias for the output stage . . . dunno what it is supposed to be though . . . jason, while you're at Rat Shack, pick up their $14 multimeter. Once you replace the resistor (or before), measure the voltage across it. If that's much more than 4V, then the resistor's rating is exceeded (4V^2/47=0.34W). 7W1 probably would need to be adjusted until that is a safe reading, but I'd wait for more expert confirmation before you go screwing with it . . .

 

[jasonbmerrill]

im not gonna screw with anything for a while... i might just take it to get repaired and modded locally.

i know nothing about this stuff.

 

[crazydoc]

Is 7W1 a trimpot? That would seem to set bias for the output stage . . . dunno what it is supposed to be though . . . jason, while you're at Rat Shack, pick up their $14 multimeter. Once you replace the resistor (or before), measure the voltage across it. If that's much more than 4V, then the resistor's rating is exceeded (4V^2/47=0.34W). 7W1 probably would need to be adjusted until that is a safe reading, but I'd wait for more expert confirmation before you go screwing with it . . .

Yes. It's factory adjusted with a drop of glue (whatever "factory adjusted" would mean in this case. )

Sounds like he'll do the right thing and get somebody to look at it.

Too bad this is all turning out this way, especially for non-electronics folks.

 

[jasonbmerrill]

there was no glue on that pot on mine. I dont want to touch it, however.

The way i see it, its still very worth it.

 

[tarnationsauce2]

Jason, I found that holding the gain switch in the "pop" position will make that resistor get VERY HOT quite fast. I suggest replacing it with a higher wattage rating resistor.

The bias being way off could possibly cause the problem too.

See my post here: http://www.prodigy-pro.com/diy/index.php?topic=27791.msg376956#msg376956

Here is the schematic:

 

[jasonbmerrill]

what if i just turn the bias down?

will it be ok then?

seriously i dont know much so dont rape me for these comments

 

[rodabod]

Long story short, pushing sideways on connector 5C21 made the hum almost completely disappear. (I think it completely disappeared at one point, but I can't reproduce that.)

I checked the connectors to make sure that none of them were failing to make good contact when the board was used. I didn't find any problems.

That leaves two possibilities that I can think of:

1. The placement of the board lines in parallel (and or in proximity to the transformer) is responsible.

2. Adding a negligible amount of resistance (a few inches of wire) between these two boards keeps some part of the circuit from oscillating.

Yikes. Anyway, with a set of wires in place of that board, it no longer makes any change to the sound when I move wires around inside the case, and there is no longer that bizarre behavior where the hum gets suddenly louder near either end of a pot.

Good work. Does the hum start quite abruptly when turning the pots? If so, this sounds quite likely to be oscillation to me. It can also suddenly stop and start if you manipulate the wiring depending on where it lies in the circuit (as you suggest, you could be adding resistance somewhere).

Maybe the transistor swap might be a good idea before trying other options. That said, it might be worth trying star-grounding each of the boards and lifting their current ground connections.

 

[antichef]

there was no glue on that pot on mine. I dont want to touch it, however.

The way i see it, its still very worth it.

I'll have to look at mine - reminds me of my other 81, which is missing a brass screw from the middle of one of the inductors - its like each one of these units had its own little fab/QC adventure, which makes for a lot of discussion topics

BTW, I don't know much (if any) more than Jason, but I have a multimeter and I'm too stupid to take my units anywhere -- when you all are talking about measuring the voltage across 7R8, this would be with the power on?

 

[mshilarious]

I'll have to look at mine - reminds me of my other 81, which is missing a brass screw from the middle of one of the inductors - it's like each one of these units had it's own little fab/QC adventure, which makes for a lot of discussion topics

BTW, I don't know much (if any) more than Jason, but I have a multimeter and I'm too stupid to take my units anywhere -- when you all are talking about measuring the voltage across 7R8, this would be with the power on?

Yes, power on. Otherwise there is no voltage!

Since the resistor problem seems to be related to the pop problem on the switch, this could be a reliability problem with every unit. Resistors take time to heat up, even if their rating is exceeded, so it would be best to switch past the pop as quickly as possible. It would also be good to replace the part with 1W or even higher. If anyone is brave (and has appropriate safety equipment . . . glasses and an extinguisher handy!), you could measure the voltage with the switch held in the pop/buzz position, that would give you an idea of how much power is being dissipated. The secondary problem is that current might stress other components as well. Hopefully somebody at prodigy can comment on that.

The issue of whether bias was truly set is interesting. Obviously, if your unit had a power connector that was not connected, how did they set bias? It's possible to order trimpots set at a particular value from the factory, so maybe that's what they did?