Ancient 3M SP-15 Amplifier needs replacement - need a diagnostician!

[daveman]

Greetings and salutations!

New to the forum, but not the business. That said, if I'm in the wrong place, please point me in the right direction.

THAT said, I need help! I was asked to service a restaurant speaker system where the sound cuts out. I assumed it was a 70v chain but that was before I took a look at this thing. I can't find any trace of this model amplifier anywhere on the internet so I'm turning to forums for an expert in the field.

Below are the photos of the back of this unit. I took two because the first washed out. Between them you can see clearly the various wires and what they are labeled. I'm beyond confused on replacing this thing at this point. Any suggestions are welcome!

 

[daveman]

closest i can come up with is this...

 

[daveman]

Tried tuner cleaner and it still crackles and cuts out at the desired volume. The potentiometer knob is too tight a shaft to get any CRC in there. So my follow-up question, if i remove the pot, what do i replace it with? are they pretty much interchangeable? only information i can find on the internet is about guitar pots. I am fine with a soldering gun, just not the science, so I apologize in advance for ignorance on the subject.

 

[daveman]

Last attempt, then moving on. The POT itself, can't I just replace it? All the research I've done only describes linear and audio taper pots of varying resistances. This one has 3 leads, thus mono. What is a safe replacement pot for a device such as this?

 

[ImcLoud]

Why not just replace this unit? I would just throw in an Atlas AA60 {or aa35 depending on how much power they need, I am sure that old sp-15 is around 15-20 watts hence the model number} and be done with it, hook it up wire for wire and call it a day, that thing is obviously old and no longer in working order.. Although I would first hook a load directly to it and make sure it isn't an external wiring issue causing the problem... Sometimes the time/materials it takes to repair something doesn't warrant replacement but sometimes it does and not knowing all the ins and outs of replacing a pot would make me lean towards replacement coupled with the age of the unit and its lack of available resources...
You can buy a used atlas aa35 for under $40, I have an aa60 that I don't use, somewhere but it would take me a month to find it.. heres one on ebay that looks good... Atlas Sound AA Series AA35 Mixer Amplifier 612079182954 | eBay

another option is to buy a used sp15 identical to what you are removing and keep their old one for parts, BUT you are getting another very old unit... heres one for sale but for a little more money I would get an atlas http://www.ebay.com/itm/VTG-AMPLIFIER-SP-15-3M-Working-VG-/221498687444?pt=Vintage_Electronics_R2&hash=item339259bfd4
good luck, sorry no one replied sooner, we have a good pro on here {BMX} and he must have missed this post...

 

[Rickster71]

Have you had the cover off the unit?
Check to see if it's full of dust.
The transformer has a built-in thermostat that shuts it down when it gets too hot.

 

[BoredSysAdmin]

Check it for leaking caps -replacing caps is cheap and relatively easy with basic knowledge in soldering
This looks like a PA amp, not HT one. Maybe even 70v - Try to reach out to TLS Guy he might be able to help you

 

[daveman]

Caps are clean, inside is clean. it sits on a sheltered shelf. The unit powers the speakers fine on all settings except the desired volume probably due to it being on that same settings for so many years. That said, I'd be happy to swap out the unit for the atlas sound unit but I'm confused on the "wire for wire" swap when the old one has that secondary cable ran to the mute/ground/low-z terminals because those atlas units seem to be lacking those terminals. I would imagine all it needs is the 25v out and nothing else, am I right? The second set of wires seems out of place as if it interfaced an old intercom or something. Please advise!

 

[ImcLoud]

the atlas is going to wire the same, common and 25v, then wire your mute wires to the remote mute terminals on the atlas, then the grnd to the grnd on the atlas the your inputs to the input, it will wire exactly the same way... the low z is going to wire to the terminals marked "input +-" on the atlas, it wil lbe wire for wire, every pa system I have seen is wired this way, 3m labeled the inputs "low z" but its your +- inputs coupled with the ground.... Pretty easy and you can score the altas aa35's on ebay..
take a closer look at the terminals on the atlas... AtlasSound.com

 

[TLS Guy]

Last attempt, then moving on. The POT itself, can't I just replace it? All the research I've done only describes linear and audio taper pots of varying resistances. This one has 3 leads, thus mono. What is a safe replacement pot for a device such as this?

Disconnect the pot, and then take an ohm meter and measure the resistance between the two outside connectors. That will give you the value.

Now order a pot of the same value, you will need audio taper, also known as log. Volume pots are never linear.

That is worth a shot. If that does not fix it replace the unit.

 

[daveman]

I'll go ahead and win the aa35 auction and do a swap... can't really argue with that price. Thanks everyone!

 

[ImcLoud]

you are very welcome, good luck

PS search ebay there are a few on there for good prices...

 

[daveman]

trying the one for $35 shipped. will know in two days

 

[daveman]

Just to wrap this up, I went with the Atlas Sound AA35. It puts a very good sound through the entire building. I went line-in to 25v out with the common and it worked like a charm. Thanks everyone!

 

[ImcLoud]

You are very welcome, thanks for the update, I have used the aa35 and have one around here somewhere... Glad to hear it worked out for you...