Please Help with a difficult Ampeg SVT PRO 4 repair

[rickybocker]

I am currently on my 3rd attempt to fix my ampeg svt pro 4. I do have a Degree in Electronics Tech from a my local community college/tradeschool and I am currently working on my Bachelor's in Electronics at a 4 year University, so I know safety and some general knowledge. I really love working on amps and other musical electronics and have had success with tubes amps and smaller repairs before, but I am still learning to try to apply the classroom theory to teach myself more involved amp repair. My old Ampeg SVT pro 4 failed and I am really trying to use the repair as a learning tool.

On a side note if anyone has any suggestions on literature or training courses I can take to further my knowledge please let me know.

The Ampeg Pro 4 has me stumped. It is by far the most complicated amp I have ever worked on. The first failure saw R118 and R119(the biasing resistors for Q102 PNP transitor) burnt to a crisp on the power amp section. O102 qnd Q103 was open Q105 was shorted. The board was scorched and the PCB trace melted by the R118 and R119. R11 was also open leading from bridger rectifier to the R118/R119.

Several of the Power Mosfets(Q110-Q119) were shorted or open.. failure varied...the gate resistors to thso corresponding FETS were open and burnt. The zener diodes D114 and D115 in the gate drive rails shorted.

I was advised by a lab tech at my school to repair what I knew was defective then check the test points. I replaced all the FETS with matched pairs, all the gate resistors, and other general transistors I found to be defective through the power ouput section. I turned the amp on with No gain No volume and still on mute. The amp burned R118 and R119 again as soon as the relay tried to turn on after power up.

I feel like I cant even get to the point of checking any of the test point until I get the parts to stop burning. This is only happening to the Power Amp A side of the output and B is still operating correctly. And it seems like there is some short I am missing maybe on the high side of the gate drive section(were the N Channel FEts are) but why would the push pull amp before the gate drive be burning out?

Below is a link to the schematics
AMPEG SVT-4PRO SCH Service Manual free download, schematics, eeprom, repair info for electronics

Im just stumped as to how I should proceed...Is there something further back I am missing??

Thanks for any help anyone can give me.

[diydidi]

Welcome! Just my two cents, but check/replace the bias trimmer too.

[rickybocker]

Welcome! Just my two cents, but check/replace the bias trimmer too.

I checked the bias pot while still in circuit for shorts/opens and compared it to channel B's pot since the amp has two independently stereo sides that are mirror images of the other. It seemed to be ok but I will pull it out and check it again. Im assuming there has to be a short for this large amount of current that is melting resistors. Even the flameproof resistors burned. As soon as that relay tries to kick on...parts start burning

Thanks for the lead let me confirm my understanding... that bias pot just sets the crossover point on the push pull amp then that signal gets past to the power fets to be boosted up.

[J M Fahey]

Please post yourself the schematic here as an attachment, so we just click and open it, without going through Elektrotanya protocol every time.

Even better, "print" the power amp schematic page as a .gif or .png so it opens within the browser instead of in Acrobat and upload it here.

And don't worry, that amplifier is quite repairable

Start by building a lamp bulb limiter

Light Bulb Limiter

[olddawg]

If you have access to one, a variac with an amp meter is much preferable to a lightbulb limiter. Depending on how it the circuit is set up, you might want to jumper the protection relays if you are using a variac. Do not do your initial test with a load. Since you have a stereo amp with a working channel, you can check for discrepancies as you bring the voltage up SLOWLY and watch the current draw. A lightbulb limiter is better than nothing, but it is not what most bench techs would use in a shop environment. Since you are a serious student with a degree, you might as well do it right if you can.

[oc disorder]

SVT 4 Pro Pwr.PDF

[Jazz P Bass]

Agree, never hook up a load on the output until you know that the output section is stable.

The main voltages have got to be stable.
That includes the high voltage V+ & V- and also the +15 & -15 volt supplies.

There are a number of very good test point voltages listed (rare indeed).

Try to keep in mind that the output section itself is a balancing act between the two high voltage supplies.
And being a solid state amplifier, with massive feedback, it will not take much in the way of one blown component to upset the section.
If you have one half of the section slamming on (Vdc on the output) the polarity of the voltage can help in deciding what is wrong.

Both the high side & the low side turning on at the same time is a recipe for smoke.

By the massive amount of damage this amp has incurred, I would suspect that it was turned on more than once in the failed state.
You have your hands full on this one.
Dot your I's and cross your T's.
All it takes is one missing copper trace, open resistor or failed semiconductor.

[J M Fahey]

Here's the editable, Browser friendly image:

[J M Fahey]

Part 2:

[J M Fahey]

Sorry, looks like Tinypic site downsampled my images

Here they are as attachments:

[J M Fahey]

I am really trying to use the repair as a learning tool.

For now be happy to plain repair it (and even that is a tall order), even if it means "painting by the numbers".

The Ampeg Pro 4 has me stumped. It is by far the most complicated amp I have ever worked on.

You bet .
Even for experienced Techs it's a beast to be approached with respect.

The first failure saw R118 and R119(the biasing resistors for Q102 PNP transitor) burnt to a crisp on the power amp section.

So Q102 is fried too.

O102 qnd Q103 was open Q105 was shorted.

In general I'm against shotgunning and always advise against it, but in this particular case, and given the general massacre, I'd replace Q102 to Q106 straight away, and just in case, IC1 too (not IC101).

The board was scorched and the PCB trace melted by the R118 and R119. R11 was also open leading from bridger rectifier to the R118/R119.

OK, scratch and repair as needed.
Let's split this repair in 2 parts, since, thanks God, power amp can be split in 2 functional blocks too: drive and power boards.
First we repair drive which is the conceptually more complex part, and later the Power Block, which is basically all muscle and no brain.

Start by disconnecting power block from both V+ and V- rails and from drive board, I guess there's a connector for that.

Then you must bridge drive A-hi and A-lo terminals (at the drive board) with 2 100 ohms resistors in series, and return the center point of them to the "(from) Output A" node, meaning the joint of R109/110/112 , so output is referred to input , closing the loop.

Several of the Power Mosfets(Q110-Q119) were shorted or open.. failure varied...the gate resistors to thso corresponding FETS were open and burnt. The zener diodes D114 and D115 in the gate drive rails shorted.

We'll deal with that later

I was advised by a lab tech at my school to repair what I knew was defective then check the test points. I replaced all the FETS with matched pairs, all the gate resistors, and other general transistors I found to be defective through the power ouput section. I turned the amp on with No gain No volume and still on mute. The amp burned R118 and R119 again as soon as the relay tried to turn on after power up.

I feel like I cant even get to the point of checking any of the test point until I get the parts to stop burning. This is only happening to the Power Amp A side of the output and B is still operating correctly. And it seems like there is some short I am missing maybe on the high side of the gate drive section(were the N Channel FEts are) but why would the push pull amp before the gate drive be burning out?

Follow first steps suggested above.

Just replace what's asked for and post here, without turning it on yet , I might have forgotten some detail, let me think a little more about it.

Build the bulb limiter and get 2 small bulbs, a 25W and a 40W one, besides the customary 100W one , because we are dealing with low power drive circuit first.

If you like to see it this way, think of them as very advanced, fast, current limited "automatic variacs" they will let enough voltage pass as amp can stand ... without surpassing a pre-set current limit

And they react way faster than my half dead brain thinking "WTF ????? " and sending the order to my arm and hand to lower Variac setting.

Rather than a diagnostic tool, I see Variacs as confirmation that amp has been repaired.

Ane we are not there yet.

[diydidi]

If you have access to one, a variac with an amp meter is much preferable to a lightbulb limiter. Depending on how it the circuit is set up, you might want to jumper the protection relays if you are using a variac. Do not do your initial test with a load. Since you have a stereo amp with a working channel, you can check for discrepancies as you bring the voltage up SLOWLY and watch the current draw. A lightbulb limiter is better than nothing, but it is not what most bench techs would use in a shop environment. Since you are a serious student with a degree, you might as well do it right if you can.

I use a bulb limiter and a variac with amp meter at the same time. Just feels safer to me, and gives ma a better visual indication. When all looks safe I bypass the bulbs.

[nevetslab]

I agree with JM Fahey's approach, making a functional high voltage amp out of the front end, AFTER replacing Q102-thru Q105, and perhaps Q106 & Q107 if at all questionable. olddawg's suggestion of using a variac with ammeter highly recommended, as it allows watching the circuit come on as you bring up the mains, hopefully preventing further failures.

As Jazz P-Bass suggests, any copper trace damage and you've got real problems. I've had trace breakage on the solder pads of Q012 & Q105 before, very mechanically unstable, and you already indicated burnt traces in that voltage gain stage.

Defeating the output stage on the SVT4-Pro isn't so simple. Lifting up the source end of the gate resistors, and lifting up either end of the 0.47 ohm/5W source resistors of Q110 thru Q119 effectively takes the MosFET stage out of circuit. Little room in between the two heat sinks to get at them though. Maybe be easier to just remove Q110-Q119. Yup....not a service-friendly amp.

I've made myself a set of AC Mains extender jumpers (male/female 1/4" insulated fast-on 12" cables) so you can operate the main power amp PCB out of the chassis...with the chassis standing on it's power xfmr side, allowing it to connect the secondary wires to the PCB assy. and, of course, a large insulated surface underneath the PCB assy!!

If the high voltage 'line amp' does power up without failing, you may also want to lift the two zener's D114, D117 cathodes, so they're not in circuit. The current limiter is out of circuit by the 100 ohm connection from Q102/Q105's collectors. I was going to suggest tying those collectors together, and using around a 1k resistor to tie it to the feedback node at R109, R110 & RT112.

Don't load that circuit...run it open load and see that it's working on it's own. Then, you've got something to compare to Ch B, as far as driving open circuit goes.

If you don't have the PCB layout drawing, I have it here.

1600 Watt Power Amp PCB (428XXPWA_0).pdf

[olddawg]

For now be happy to plain repair it (and even that is a tall order), even if it means "painting by the numbers".


You bet .
Even for experienced Techs it's a beast to be approached with respect.

So Q102 is fried too.


In general I'm against shotgunning and always advise against it, but in this particular case, and given the general massacre, I'd replace Q102 to Q106 straight away, and just in case, IC1 too (not IC101).


OK, scratch and repair as needed.
Let's split this repair in 2 parts, since, thanks God, power amp can be split in 2 functional blocks too: drive and power boards.
First we repair drive which is the conceptually more complex part, and later the Power Block, which is basically all muscle and no brain.

Start by disconnecting power block from both V+ and V- rails and from drive board, I guess there's a connector for that.

Then you must bridge drive A-hi and A-lo terminals (at the drive board) with 2 100 ohms resistors in series, and return the center point of them to the "(from) Output A" node, meaning the joint of R109/110/112 , so output is referred to input , closing the loop.



We'll deal with that later



Follow first steps suggested above.

Just replace what's asked for and post here, without turning it on yet , I might have forgotten some detail, let me think a little more about it.

Build the bulb limiter and get 2 small bulbs, a 25W and a 40W one, besides the customary 100W one , because we are dealing with low power drive circuit first.

If you like to see it this way, think of them as very advanced, fast, current limited "automatic variacs" they will let enough voltage pass as amp can stand ... without surpassing a pre-set current limit

And they react way faster than my half dead brain thinking "WTF ????? " and sending the order to my arm and hand to lower Variac setting.

Rather than a diagnostic tool, I see Variacs as confirmation that amp has been repaired.

Ane we are not there yet.

I repaired thousands of hi powered amps over 25 years as an authorized tech for a large number of manufactures and never used a light bulb limiter once.... Ever. Brands like Yamaha would pull your authorization if they saw one in the shop. It is a useful tool if you don't have a variac and a scope. But whatever works for you. There is more than one way to Egypt, lol.

[hodgy]

Hi Rickybocker. Working for Ampeg, I have repaired a dozen or more of these. Some folks on here may disagree with me, but here's what I do when fixing these. For starters, all of the Mosfets. It looks like you have shorted parts on both the + and - side, so you might as well do all 10. When the mosfets go, in more cases than not these are destroyed as well:
10 x .47ohm (the big guys) on the source
10 x 1.5k ohm on the gate
10 x 47 ohm on the gate
Start there, in addition to the stuff in the biasing circuit that you've already seen. I've seen Q106 and Q107 be all wonky even though it bugs out OK. My record was 76 components replaced on one of these bringing it back to life.
And get a variac and ammeter!