Peavey Studio Pro 40 dead no signs of life. Where to start ?

[Zappacat]

I'm trying to fix a friends Peavey Studio Pro 40.  It shows no signs of life when plugged in.  I pulled out the chassis and checked the fuse (is there only one?) and voltage levels coming into the amp.  Those were OK.  Now I'm looking at the power transformer.  The numbers on the transformer are 705-18712 .  A google search on this number yields nothing.  Am I looking in the right direction to start at the PT transformer from this point.  Is it safe to check power transformer voltages without a load on them?

Peavey parts no longer carries the transformer but a substitute transformer number of 705-18762 was given to me at a price of $ 42.99.  Google search for 705-18762 part number yields nothing.  If the problem turns out to be the transformer can I get something that will do the job elsewhere for less ?  Can anyone tell me transformer specific requirements based on the schematics I posted in the schematics section.  It may sound like I'm cheap but the guy I'm trying to fix this for is even more broke than I am right now and I can't have this situation end up COSTING me money like these things sometimes do.

Thanks in advance.

[J M Fahey]

1) Amp unplugged. Turn it on, grab the power cord, measure resistance across ths hot and neutral power plug pins. Do you have continuity?
2) Plugged amp, power on. What AC voltage do you measure from secondary center tap (R/Y) to each Red wire?
3) What DC voltage do you measure across C48 and C49?

[Zappacat]

1) Amp unplugged. Turn it on, grab the power cord, measure resistance across ths hot and neutral power plug pins. Do you have continuity?
2) Plugged amp, power on. What AC voltage do you measure from secondary center tap (R/Y) to each Red wire?
3) What DC voltage do you measure across C48 and C49?

1) No continuity with respect to question 1.
2) I'm getting 0.00 AC voltage for both secondary center taps (should this be +- 36 volts?).   
3) I'm still trying to figure this out.

Thanks

[Enzo]

Well, first, those part numbers are Peaevy part numbers, so you are not going to find specs on them online.  $42.99 sounds very reasonable to me for that transformer.

The transformer works or it doesn;t.  The mains wiring in the amp works or it doesn;t.  SO look for the power transformer primary wires.  Most Peavey amps have connectors for this.  PLug the amp in and turn on the power switch.  Is there mains voltage between those primary wires or not.  If there is, then it is getting to the transformer, if there is not mains voltage there, then the mains wiring is bad.

Unplug the amp from the wall.  Disconnect the primary wires of the transformer from the mains wiring in the amp.  Use an ohm meter to measure continuity - resistance - through the primary.  it will either have some fairly low resistance, or it will be open.

The idea behind JM's test was that if you measure resistance between the mains plug prongs with the amp power switch ON, then your meter is measuring the resistance up one side of the mains cord, through the fuse and switch, through the transformer primary and back down the other mains cord wire.  If the transformer winding is intact, you would be measuring its resistance through the powr cord.

Since you seem to get an open that way, you need to determine if it is the transformer primary that is open or the mains wiring to it.

Yes, you can apply power to the transformer with the secondaries unloaded.

If the primary is OK AND getting voltage, then see what is on the secondary wires.

[Zappacat]

I'm getting 1.8 volts AC on EACH of the primary wires going to the transformer which are blue and black.  Maybe I'm not measuring the voltage properly.  With the power on I put one probe on either the blue or black wire going to the primary but where should I have the other probe?  Should it be connected to the main chassis ground or somewhere else?

[phatt]

Hi Zappa,
Chassis is ground ,,no use measuring that
What ever colour wires come from the power cord and which also run to the Power Tr's *Primary* winding.
Measure the mains voltage on those two wires,, Remember this is the mains supply side which can kill!!

*With power off and unplugged* A visual Check for fuse, Pwr switch and *Earth to Chassis*
Ground/Earth is normally Green or green /yellow,,, so obviously the other two wires will have AC mains potential between them.
While the powers off follow the continuity tests already mentioned.
Phil.

[J M Fahey]

1) AMP UNPLUGGED !!!!
Measure resistance from Black to Blue, post result.
2) Multimeter on "beep" position, short probes to check they beep, then one probe on black wire, other to power plug, one blade should beep. Mark it somehow (tape/marker/nail paint).
3) repeat with one probe always on blue wire; trace back the connection until you get to the other, unmarked blade; that's to say, touch in order:
both pins of power switch , one should always beep, the other follow on/off switching;then leave it on, proceed to thermal switch mounted on heat sink, both pins should beep; then touch both pins of fuse holder, both should beep; then touch the unmarker power plug blade, it should beep; finally apply one probe to each power plug blade, it may not beep (depends on your multimeter) but it must show continuity.
4) Wherever the beep stops along the way, that's where you lose power.
5) It all depends on first having continuity across black/blue wires, otherwise we are wasting time.

[Zappacat]

1) AMP UNPLUGGED !!!!
Measure resistance from Black to Blue, post result.
2) Multimeter on "beep" position, short probes to check they beep, then one probe on black wire, other to power plug, one blade should beep. Mark it somehow (tape/marker/nail paint).
3) repeat with one probe always on blue wire; trace back the connection until you get to the other, unmarked blade; that's to say, touch in order:
both pins of power switch , one should always beep, the other follow on/off switching;then leave it on, proceed to thermal switch mounted on heat sink, both pins should beep; then touch both pins of fuse holder, both should beep; then touch the unmarker power plug blade, it should beep; finally apply one probe to each power plug blade, it may not beep (depends on your multimeter) but it must show continuity.
4) Wherever the beep stops along the way, that's where you lose power.
5) It all depends on first having continuity across black/blue wires, otherwise we are wasting time.

1) 9.6 ohms
2) done
3) neither pin of power switch yields continuity with blue primary transformer wire
4) still working on this
5) Do the black/blue primary transformer wires need to be disconnected(desoldered in this case) from the circuit to test continuity, resistance, etc...?

[Enzo]

This may sound more complex than it really is.  All we want to do at this point is determine if the power transformer primary winding is open or intact.

Imagine you have a table lamp.  A mains plug and cord, a switch, a socket for the bulb.  Imagine we know a good bulb had 100 ohms resistance.  If we put the bulb in the socket, unplugged the lamp from the wall outlet, we ought to be able to measure the resistance of the bulb itself out at the end of the power cord.  If you measure resistance between the prongs of the power cord with a meter and you get the 100 ohms, then we know the bulb is OK.  If we get an open reading, then either the bulb is burnt out or the switch and wiring is bad.   At that point we could unscrew the bulb and measure it direct - it is OK or it is open.

We are doing the same thing here, except instead of a light bulb, there is a transformer winding at the inner end of the power cord.


Imagine we plugged out table lamp into the wall and turned on the switch.  If we remove the bulb, we can set our meter on AC volts and probe the wall of the socket and the little bump in the center.   We expect to find 120VAC between the socket wall and the center bump.   Just so here.  If we unplug the transformer primary, there should be 120VAC (or whatever your mains voltage is) between the two wires it was connected to.  If that voltage is not getting there, then the transformer has nothing to run on.  If it IS getting there, then the transformer is suspect.

[Zappacat]

I've got 120 volts all the way to the fuse.  

What are the two blue components in the pic behind the fuse?  I'm thinking there should be continuity between the first one and the trailing fuse lead but there is none.

So the black wire coming into the wall turns into yellow after the power switch.  I'm getting 120V all the way through the heat sensor and the fuse.  What should the white wire(I have three wires from mains black, white, green) be reading for voltage ?

[phatt]

The white wire would normally go to the other end of trans Pri?

Coming from the black mains wire through switch> turns into Yellow wire which runs through Thermal sensor back to FUSE>
The Fuse > (left side is Yellow wire)
From the *righthand side* of fuse follow the tracks back Through to the transformer,,
which WILL ulltimately leed back to the *White wire in power cord*

You need a pen and paper drawing the circuit as you go untill you arrive at the trans windings
\which looks like it maybe blue and black Going Down through the hole.

( I guess the other 3 wires comeing up *Red- Red/Yellow stripe-  Red* are the *Secondary* wires.)

Don't assume anything,, Visual checking as well as using the continuity testing.
Don't concern yourself to much with blue things,, likely mains suppression caps,, usually wired across the primary switch.
You need to develop the ability to work from schematics ,, not just rely on ever changing colour coding on some obscure PCB.
Phil.

[J M Fahey]

Hi Zappacat.
1) Calm down, we are bumping against the problem all time, yet brushing it aside and going on, not paying attention to it.
Let's review what we have done so far:

1) AMP UNPLUGGED !!!!
Measure resistance from Black to Blue, post result.
1) 9.6 ohms <--Reasonable, the primary is not open.

2) Multimeter on "beep" position, short probes to check they beep, then one probe on black wire, other to power plug, one blade should beep. Mark it somehow (tape/marker/nail paint). <--Good, one side of primary to one blade of plug OK

3) repeat with one probe always on blue wire; trace back the connection until you get to the other, unmarked blade; that's to say, touch in order:
both pins of power switch , one should always beep, the other follow on/off switching;then leave it on, proceed to thermal switch mounted on heat sink, both pins should beep; then touch both pins of fuse holder, both should beep; then touch the unmarker power plug blade, it should beep; finally apply one probe to each power plug blade, it may not beep (depends on your multimeter) but it must show continuity.
3) neither pin of power switch yields continuity with blue primary transformer wire  <--BAD, we lost continuity here.
HOW does the blue wire connect to the power switch? Follow it visually, as was stated by Enzo and Phatt. Electrons travel along a metal path, whether it is a piece of wire, a PCB track , a connector, a tab, etc.
WITH AMP STILL UNPLUGGED put one probe on the blue wire (how do you do it? does it have some kind of push terminal on its end?) and visually follow the path towards the switch, checking along it with the other probe (beep)
WHERE do you loose the beep?
Following your picture, AMP UNPLUGGED,

touch one beeper probe to the unmarked power plug blade (the marked one already checked good), in order touch your other probe to:
Black power wire soldered to power switch.
Other leg of the power switch. Switch it on/off; does it work? Leave it on.
Follow the yellow wire soldered there. It gets lost somewhere in the middle: Is it spliced there, hidden behind something? The picture is dark there.
Follow it to the thermal protector attached to the heat sink. Touch the terminal where it is soldered.
Then touch the other terminal. Do we still have a beep ?
Follow the other yellow wire, which goes to the open fuse holder (I HATE them if power line connected, major death hazard) Still beeping?.
Touch the other end of the fuse holder. Still beeping?
Follow the path (PCB track?) to the Blue transformer wire.
It's the only path not visible in the picture. Check for cracked track, solder, terminal, whatever that joins the fuse holder to the blue wire.
That's it, there's NOTHING else in that circuit.
If you get beeps along the way, power MUST reach the power transformer primary, which we already know is not open.
If you lose beeps somewhere, take a good hard look as to why that happens.

4) Wherever the beep stops along the way, that's where you lose power.
4) still working on this

5) It all depends on first having continuity across black/blue wires, otherwise we are wasting time.
5) Do the black/blue primary transformer wires need to be disconnected(desoldered in this case) from the circuit to test continuity, resistance, etc...? <-- No, but the amp must be UNPLUGGED

[Zappacat]

Thanks guys.  With your help I fixed this.  The solder point going from the fuse to blue primary wire had disintegrated.  There was just a hole left there.

I can't remember which speaker lead went to what post on the speaker connections.  Does it matter?  There's a blue and white wire coming from the amp chassis for the speaker connections.  One of the speaker terminals have a red dot on it.  Any clues as to which wire goes where?

[J M Fahey]

Hi zappacat.
Tell your avatar that his glasses look cool, but are not worth sh*t.
How many times did you  pass along that solder pad, actually bumping into it every time?
32744 times?
Not much less than that.
Oh well, now it's working, good !!
The speaker wire colours baffle me, but just on an absolutely unfounded hunch, blue might be minus/hot/black, and white might be plus/hot/red.
Anyway it will be happy the other way; only important if you run a couple of these little amps daisy chained .
Good luck.

[Enzo]

Peavey usually uses blue and yellow for its main speaker leads, and yellow is the positive lead.  I would assume the blue to be negative here as well.  With the speaker not connected, check which wire is closest to ground.  That would be the negative wire.