Ampeg SVT 5Pro Schematic

[Bassboy49]

I purchased an SVT 5 Pro from an auction site two months ago. It has two symptons. I am a player, not a repairman. One, when first turning the amp on, the amp turns itself on and off four or five times until the fan "warms" up. Once the fan stays on, the amp works fine, with one possible exception. Secondly, the bass tone controls on both the clean and overdrive channels do not seem to have much effect on the bass tone. I have tried another SVT 5 Pro in a store, and the bass tone control had the same response as mine. May be characterisic of this amp. Possibly a schematic might help me. Thanks in advance.

[captntasty]

A quick search didn't really find much - but this:




For amplifier support or any questions concerning owner’s manuals or schematics please contact the LOUD Technologies Inc, please contact technicalsupport@loudtechinc.com .

Technical Support, Service and Parts departments are available Monday-Friday, 10am-Noon & 1pm-3pm Pacific Time

Without a schematic I can't be sure but I am working on a powered mixer right now and it has a thermal cutoff (thermal fuse) mounted on one of the large transistors to protect from overheating - it would reset once it gets back below the cutoff temperature. I wonder if your SVT has something similar? Could indicate a larger problem though - why is it overheating? Just a thought - could be a number of things....

And if you didn't get one of these...

SVT-5 Pro Manual

What is your setup? Multiple cabs or only one?

[Jazz P Bass]

Ampeg SVT 5 Pro

When you turn on the Ampeg SVT 5 Pro amp, if it is cold the fan turns a few revolutions and then stops. When heat is sensed on the heatsink it will spin, progressively faster with increased heat.
The clicking is the protection circuit kicking off the output relays.
It is either faulty or doing its function: sensing Vdc on the output.
Which can mean a bad soldering job from the factory, a component(s) in the output section has (is) failed, the power supply is not outputing the correct voltages, a connector is on wrong.............
Do both of them click.
How do you have the amp set up. Stereo, Bridged, BiAmped.
I would take it to a tech.

As to the bass control: 20db @ 40Hz.

From The manual:
6. ULTRA HI: This switch, when depressed,
increases the high frequency output by 7dB at
15kHz.
7. ULTRA LO: This switch, when depressed,
increases the low frequency output by 5dB at 30Hz.
8. BASS: This control provides an adjustment
range of 20dB at 40Hz.
9. MID: This control provides an adjustment range
21dB at 300Hz.
10. TREBLE: This control provides an adjustment
range of 18dB at 10kHz.

[chipprogr]

Im looking at the schem and i have some questions for you. When you say it turns itself on-off a few times, are you seeing the power light flicker as well or are yo only hearing the relay inside click in and out? The fan works because the power is on, not the other way around. The fan circuit is sensed by a LM35 , looks like a to-92 small transistor, but its a temperature sensor.

Please tell us more about the observed things happening when the power switch is 1st turned on please -THanks-Eric

[Bassboy49]

OK. Thanks guys for the quick response. Right now I have the amp on my saw table plugged in with the cover off. If it's been sitting for awhile, when I turn it on, the green channel light on the right side and the red mute light on the left side come on (I have the mute switch depressed, even though I don't have any speakers connected right now). They stay on for about a minute until the amp warms up and the fan attempts to turn on. Then the amp shuts off for about five seconds, turns itself back on again, until the fan comes on again, then repeats this four or five times until the fan stays on. Then the amp runs fine (except the bass tone is practically nonexistent). I usually set it up in the biamp mode with a jumper from the HiOutput to the Amp B plug-in (as suggested by the manual). I run two speaker cabinets, a full-range and a woofer. I use a BagEnd D10-XD as the full range and a BagEnd S-18D as the lowend speaker. I have switched the 4 ohm speaker from the BagEnd 18 to an 8 ohm speaker so that the cabinet resistances match. After the amp stays on, I have played it with both speakers set up for over a half hour without any problems. I have a gig I will use it on on Thursday night. See what happens in real world conditions. I have previously owned an SVT 4 Pro for 8 years and used a similar biamp setup with the same cabinets. Much deeper, fatter bass tone from that amp (never used the Equalizer Chaneel, though it worked fine). Otherwise, with the exception of the Overdrive Channel and the Octave effect, almost the same amp.
I did ask a friend who works as a technician at the local music store. He said it was probably a bad power transistor or connection. He suggested replacing all the power transistors and resoldering the main board.
Finally (at last), the amp came with a Sovtek 12AX7. I have tried a Tung Sol 5751 in it's place. The symptoms were a little different. Instead of taking four or five attempts for the fan tp get up and running, with the Tung Sol it took ten or eleven attempts. Thanks again from the hinterland (North Dakota).

[chipprogr]

does the green and red light go out and then back on when it does this cycle stuff? AND, for this amp, trust me, replacing ALL the outputs is not even close to the right answer here.

[Bassboy49]

Yes, both lights go out every time the fan tries to turn on and the amp shuts itself off. They both come on again when the amp turns itself on a few seconds later. They stay on (unless I deselect the mute switch) once the fan turns on, and the amp and the fan stay on. Thanks again.

[chipprogr]

Youve got a low voltage power supply problem. The tube draws power from the same 15 volt power supply as the rest of the preamp and fan circuit. When you put in the different tube, its heater drew more power and its only connection is to the +15 volt line, not -15. You have an intermittant, possibly bad solder joint that the initial current draw is causing it to go open, as it cools, it reconnects then goes open again. You MAY also have a failed LM7815 regulator, very possible. Grab a meter and see if you have a constant ~22 volts on the left pin of the LM7815 regulator and 15 volts on the right pin. center pin is a ground connection, no voltage there. If your +15 is bad AND you have the 22 there, your regulator is bad. i think even radio shack carries these.

If not the regulator- check the following:

Check your fuse #3 and #4. near c31,32. these are the fuses for the low voltage PS. Make sure they are good and seated properly. 1st check J30 , its a 3 pin connector, it supplies the necessary AC voltage to the low volt power supply. Next look at the voltage regulators LM7915, LM7815 (IC #4,5) and see if any of the 3 legs on them are bad solder jooints or maybe broken off at the circuit board. Pry on the legs gently and see if they are broke loose from the PCB or component.

Good Luck-Keep us informed- Eric

[Jazz P Bass]

Ampeg SVT

Yeah, I would nix the transistor replacement.
Just curious as to what happens at startup if you have no tube installed.
I have a feeling it is a bad solder joint (job)

[Bassboy49]

Thanks again guys for the very detailed assistance. Some may be over my head.
To Chipprogr: I do have a voltmeter and have done some testing as a vending tech at work, but not at this fine a level. I believe I have located IC4 and IC5 on the main circuit board, between the fan and the heat sink. However, I'm not sure where to place my test connectors, I do not see any test points marked.
To Jazz P Bass: I did try turning the amp on without the tube. Everything was the same (I did not have an instrument plugged in or speakers plugged in), except...the fan didn't come on. I waited about twice as long for the fan to kick in and nothing happened, just the same two lights on the front panel.
Thanks again, I probably won't have time to fool with it today, but, plan on using it tonight.
Get the lawn chairs out it's 30 above!

[Bassboy49]

Tried the amp without the tube. Seemed the same, two lights on the front panel. But the fan didn't kick in. I waited twice as long as usual. I did not have any instruments or speakers plugged in. Thanks for the help.

[Jazz P Bass]

SVT 5 pro

O/k. I am not clear at all on this.
With the tube in the amp goes into protect (relays un- click) & at the same time the leds on the front panel go out. Correct?
Now when you tried it without the tube in, the leds stayed lit.
You said nothing about the relays clicking or not.
Did they ?
If not, return to the previous post from ChipProg about a power supply issue.

[Bassboy49]

OK. I tried it again. This time I turned the power on with the tube in. The amp warmed up for about 2 minutes and then the fan started. Once the fan started to spin, the amp turned itself off. I can hear a clicking sound when the amp goes on or off. The amp cycled on and off until the fan continued to run, then the amp stayed on. During this cycling process, the two leds (clean channel, mute) stayed on until the fan tried to come on, then the amp shut itself off. Just prior to the shutdown happening, the orange limit led also lights up for just a second. The amp stays off for about 15 seconds, turns itself on again, stays on until the fan starts, then the process starts over again. After several attempts (this time it was 8 times) the fan stays on and the amp, for all intents, is normal. Then, I turned the power off, removed the tube, and turned the power back on. This time the amp turned on, the fan turned on and both stayed on. I did not have an instrument or speakers plugged in. I did hear the one click when I turned the amp on. Thanks again.

[Jazz P Bass]

AVT 5

The speaker relays will click ON when you turn on the amp.
It is the slight double click you here that means the coil dropped out (off).
That is Protect mode.
What I see is either the tube is faulty and pulling something down, or the voltages associated with the tube are failing.
Check your voltages aroung the tube without a tube in. See which one fails when a tube is in.
See ChipProg above.

[Bassboy49]

OK. Thanks again for all your guidance. I have progressed through all your suggestions; the fuses check out, the J30 pin is seated and has continuity from the transformer, the tube socket has voltage on pins 4, 8 and 9 when the amp is on,voltage regulator IC4 has the negative 22 on the #1 pin and the -15 (varies with fan speed) on the #3pin. Now, the culprit, the voltage regulator IC5 does not have any voltage on either pin. The regulator appears to be properly connected. However, to check the solder points and/or remove the regulator, I will have to take the entire amp apart to get the main board out. Keep you posted and, thanks again. (Blizzard coming this weekend, should give me some spare time to get this worked out.)