Sunn (Fender) SR8520P Noisy AC driven fan

[DrGonz78]

So I have this old Sunn PA unit that I fixed years ago and I want to gift it to someone for Xmas. However, there is still an issue with the AC driven cooling fan to deal with first. I have another AC type fan laying around somewhere in the shop that might work as a replacement. But does anyone have some info on what it would take to fix this one up to make it less noisy? Been searching online already and have yet to find any good starting place. Thanks in advance for any advice on this matter.

[DrGonz78]

The noise is almost like a wobble or something. So I pulled it out and noticed that there was a round backing plate not completely flush. Gave it a nice whack or two with a rubber mallet and that seems to have improved the wobble sound. However, the fan is still pretty loud but is that just something to expect with these type of fans? The fan does not look like something that could be taken apart. There is a cover where the AC wires run up into the unit and it looks like a small spot where you could flush a bit of lubricant into the thing. However, I am hesitant to try that since it might not be a good thing to do. Can you lubricate these types of fans?

[Enzo]

Noisy fan? As in wind noise or as in the bearings make noise?

As to wind noise, they make quieter fans, but they often do that by simply moving less air. On larger Peavey amps with fans, they put a large resistor in series with the fan to slow it down. Then a thermal switch on the heat sink shorts across teh resistor when it all gets hot, thus bringing the fan up to full speed.

But really, no one records with this mixer, it is for live sound, so really how many dance bar acts are ruined by a noisy fan?

But worn bushings can chatter. There is probably a label/sticker on the stationary hub, usually with the voltage and current. Peel that off to expose the innards. You will probably see the end of the shaft. A drop of very light oil on it quiets many fans.

I googled "quiet noisy fan" and found this quickly:
Quiet noisy computer fans with a drop of oil - Page 5034842 - TechRepublic


Fans tend to be 120vAC or low voltage DC. After that size is the deal. Make sure to have the air flow direction the right way.

[DrGonz78]

Okay thanks Enzo that helps to know that it can be lubricated. I figured so but I have dealt mostly with brushless DC fans and not AC bearing types. The sticker is on the side of the fan case and not on a hub part. I think that round thing that I hammered down on is the hub but it does not look like it comes apart happily. I will take a picture tomorrow so I can show you guys what it looks like and where I thought might be a spot to lubricate. Thanks again for the quick feedback.

[Enzo]

Not the hub on the spinning fan, the hub meaning the central part of the stationary frame. The motor. No little hole plug covering the shaft end?

AC, DC brushless, they all have the motor in the middle and a shaft for the fan blade assembly.

[DrGonz78]

So I tried to lift up the hub hole plug but it does not want to budge. In fact, trying to get the plug hole off has now revealed that the fan will spin good for a few rotations then it gets stiff for a few spins and then back to good again. Makes sense now that tapping it with a mallet might have aligned something making it spin better. I have a attached some photos to show what this thing looks like. Also, the second photo is a thought to inject lubricant into a tiny opening but I am not sure that will be sufficient or safe to do in that location. Any thoughts?

[Enzo]

Safe? You are about to replace the fan because it doesn;t work well. What does safe mean? if the lube doesn;t work, you are no worse off than you were.

[DrGonz78]

I meant safe in respect to putting oil somewhere it is not supposed go into. I would think the middle cap part is supposed to be removed to get into the shaft part. That outlining area is all I can reach at this point and I am not sure what's in there. I would guess it is the bearings or something but that is a guess. Also, I meant safe as in oil splatter across AC driven motor parts that could cause a problem.

[Enzo]

Its just oil, they don't get hot enough to ignite anything. Imagine overspray from Deoxit. Doesn't hurt the circuits.

If you google the term I wrote above, the video I linked and lots of other images show what is inside. All fans are more or less the same, just a motor, and some have a small bit of circuitry.

[DrGonz78]

Okay I added a drop or two of some oil into the fan and I think it might hold up well. Either way I got that other fan too. I tend to over think these things but only that I can learn more. Thanks for the help Enzo.

Another thing just occurred to me... The PA is missing a reverb tank. According to the schematic it is a 4BB3S1D type tank. The 5th character is an "S" but would expect A-D. Anyone know what the "S" indicates?

[Enzo]

I would think it was a typo. That character is the one specifying the connectors. SO unless something weird is going on with connections...

[g1]

4BB3C1D seems to be a common tank.

[DrGonz78]

I was banking on replacing it with a 8BB2C1B since it is all that I have on hand. So type 8 will increase Input Impedance as well as Output Impedance a bit. Medium decay instead of long. And I will have to mount it horizontal open side down instead of vertical. Honestly looking in the amp I don't see holes mounting a 17" size reverb tank. In fact, there are four hole marks that look like a type 1 or 8 sized short tank.

[Enzo]

If the drive impedance is off a notch, often as not it is no big. And the mounting direction mostly matters in combos where ther is high ambient sound level. Inside a box mixer, I don't think it matters much. The difference is which holes they hang the four short corner springs from.

[DrGonz78]

Well got the new tank into the PA and it works great!! The reverb circuit was something that I repaired, in theory, years ago and I had not gotten to test that till just now. I remember how many lessons this amp taught me and now gifting it to a loved one will be such a reward for the time spent repairing the unit. Also, I shot just a tiny bit more lubricant into the fan and it really got much quieter after that. Thanks for the feedback Enzo and G1 too.