Ampeg SVT 4-Pro repair help

[defunkmike]

I have an AMpeg SVT 4-Pro that overheated and fried out part of the power board. I have replaced the damaged components and soldering in jumpers where the board was damaged. The unit powers on and I can hear the relays click shortly after, like normal. There is no signal being passed to the power amp section. On either Amp A or B all I get out is Static Crackle. I do however have Bass signal passing through the tuner out. Any suggestions on what I'm missing?

[blindboybenton]

Hi

we all need more info on what you did and whats now going on to help.

What components did you change ? Get the schematic out and check the voltages.and post it on here

BBB

[Jazz P Bass]

[QUOTE=defunkmike;359838] There is no signal being passed to the power amp section. On either Amp A or B all I get out is Static Crackle. I do however have Bass signal passing through the tuner out. QUOTE]


The Tuner Out is one opamp away from the input jack.

You should prove that the signal at least reaches the power amp.
On the 42800 schematic (Power Amp) on page one, on the far right.
Points A Hi & A Lo.
Points B Hi & B Lo.

[Enzo]

Make sure the switch is in the MONO setting, and not BRIDGE.

There are POWER AMP IN jacks for each power amp, plug you signal into those to test the power amps directly.

Since you had to rebuild power amp circuits, are all the power rails still present in the power amp?

[defunkmike]

Well I have only .4 volts on the rails (TP18, TP19, TP26, TP27). Of course they should be 15v so I fear I have a bad transformer. Also TP14 is .11v and not the 61v my schematic states. Thoughts, suggestions?

[Enzo]

TP18,19 are AC voltages, while TP26,27 are DC voltages. That is one thing. TP14 is also AC, so any chance your meter was set to the wrong types of voltages for these readings? Will your meter still read about 9vDC on the DC scale measuring a 9v battery? If you are in the USA, do you measure about 120vAC on the AC scale at the wall outlet? Just checking your meter.


Bad transformers are really quite rare.

[nevetslab]

If you have the PCB assembly lifted out of the chassis, I'd take a close look at the power supply bridge rectifier for starters. On these Ampeg SVT4-Pro and others in that line, more often than not I'm finding solder fractures to be the main culprit. As seen here:



The first image is quite obvious, while the second one, at first glance, one only sees one obvious fracture. Actually all four rectifier leads have thin radial fractures present. I've found similar fractures on the LV rectifier board, and sometimes a failed IC regulator. I've yet to have a bad Pwr Xfmr on any of these. As seen in my 'avatar', that's what I use when I go hunting for solder fractures.....high intensity light and good magnification, adjustable to set good working distances.

I'd go thru your main PCB & LV boards slowly and carefully to see if you're dealing with the effects of long term use & road-vibration induced lead fractures. Rear panel of these Ampeg PCB's are poorly supported, and jacks' solder terminals are likewise victims of radial fractures on the solder joints. As Enzo mentioned, once you have your power supply voltages up and running, use of the Power Amp input jack quickly isolates where your signal problems are (preamp or power amp section).

Also, the front panel handles are traditionally loose. I've replaced all the original #10-24 PHMS with Socket-cap screws, split-lock & flat washers, and add a little dab of Loctite onto the threads. No more loose handles/panels to promote further vibration-induced part-mounting problems and their solder joint fractures.

[defunkmike]

Ok, I've checked the joints and my meter. Voltages look fine on TP 18,19,26 and 27. =!%v and -15v right off of IC4 and IC5 now but still getting static output on both amps A&B. Preamp output works fine. Noise still exists if I disconnect the preamp completely. Sending a signal directly into either power amp makes no difference. Still have no sound from source signal just static/distorted swirling sound.

[nevetslab]

If I understand correctly, your preamp is now functioning normally....meaning you can plug signal into the input jack and get output from the Preamp A & B Output jacks on the rear panel, with all the preamp functions working. Just can't pass signal thru the Power Amps, and are only getting noise. That tells me you have restored your bipolar supply lines (+/- 15V & +/- 22V unreg) to all the circuits in the amp/preamp.

As Enzo pointed out, TP's 26 & 27 are DC voltages....and TP's 18 & 19 are AC voltages. You wouldn't see any appreciable DC voltages at TP 18 or 19, nor anything but low mV DC voltages (10mV or less typically) on the outputs of any of the signal Op Amps thruout the preamp and amp circuits. Beware of the tube circuits, though, as you do have high DC voltages present on the plates and Cathode Follower circuit before they are AC coupled to the op amps circuits. TP's 25 thru TP 28 are the bipolar supplies, before and after the IC regulator circuits.

So, you're now trouble-shooting the power amps. Assuming the relay's DC Protect function is still working on both channels, the fact that you're hearing noise and do hear the relays click in when you power up does indicate you probably don't have serious DC levels sitting on your outputs....but you do need to check.

If I presume you're hearing the same thing on both channels, and neither are passing signal, you're likely to be looking for a common problem to both channels. IC1 is common to both power amp's front end....it drives the voltage gain discrete stage of each power amp. Also common to both channels is the Limit control line that controls the JFET switches Q101, Q201, though that only enables the Vactecs to function, and don't normally shunt the feedback resistors R104 & R204 to the point of not allowing signal to pass on to IC1 (common to both power amps).

I'd check DC levels on the outputs of all your op amps in the two power amp stages, along with verifying you have 'nominal' equal/opposite voltage levels in the power amp stages (following your schematics), yielding near 0V at the output buss of each power amp stage. You indicated you had gone thru and made repairs to restore the power amps from the failures you found. It sounds like that task isn't complete, but at least you're past the high current failure issues, and now looking for the pesky hidden culprit preventing signal to pass from Power Input jack thru the Outputs. I'd also look to see when you have signal applied to the power amps, that you have signal before the relays

And, of course, follow your input signal thru the op amp stages of the power amp's front end, and thru the voltage gain stages to the MosFET drivers. Somewhere you're loosing signal.

I presume you've made a set of extender leads so you can safely operate the amp module(s) outside of the chassis, allowing you to look on the bottom side of the power amp assembly. It is a pain to work on. I have one in the shop now that has never been serviced, so later today or tomorrow, I'll have one on the bench, going thru it doing preventative maintenance work.

[defunkmike]

Tricky little thing this is. Yes, TP voltages look good now and relay's are clicking in. Yes I do have preamp signal out, I can run into a different power amp with no issues.
I did replace some IC's. Prior to IC replacement I had no signal pass through, just static noise on amp A and B. The noise was a little bit different on each amp.
I replaced IC1 (5532), I still had noise but it was "different", still no signal.
I then replaced IC101 (TL074) and Amp A was then silent, still with no signal. I then replace IC201 and IC3 (TL074's). Now I have static noise on both channels that is different than before but identical on each amp. Still looking for common component that is taunting me.

[nevetslab]

Yes, tricky, and not so user-friendly to get at signal test points. From your previous measurements, I gathered your experience with troubleshooting is limited, and from what you can access on from the rear side of the back heat sink, there's a lot of circuit points that are inaccessible without making the amp a bit dangerous. You can get in as far as the front end of each power amp, up to IC1A & B without any real problems....just make sure nothing on the top PCB that gets removed shorts to the chassis. Insulating the top rear panel PCB would insure that, after removal and left dangling out the back, as seen in the photos below:



Since you had the power amp assembly out of the chassis before, I'm assuming you checked all the semiconductors for shorts and proper junction readings (remembering NPN's and PNP's require flipping your probe leads). I don't recall if you stated what parts HAD failed when you began your forum thread. It could be you have resistors that have opened, for all I know. It is a challenge, but it's also a great teaching tool for you, if you go in with eyes wide open.

You can also remove that small Transformer Module on the left rear wall, just to get it out of the way. The Power Supply assembly is, however, very much IN the way. When I've had to go in that far and have the amp powered up, I remove the amp assembly from the chassis, have the PCB Assembly well insulated, and I have extender leads to get the AC mains Primary and Secondary leads to the boards' connections, and have a lot of space around the amp...standing the chassis up on its' side so the leads will reach. It is also quite dangerous in that configuration!

With the amp module still installed, you do have access to the front end of both Power Amps (just forward from the Power Amp input jacks where IC 101, IC201 & IC1 reside), and at least up to the Gate Drive busses for Ch A, which can be found at the junctions of D107 Anode/R127 (pos drive buss) and D108 cathode/R130 (neg drive buss. Access to the output ahead of the relay is between the two heat sinks, where the 0.47 ohm 5W resistors live and sum together. You will want to follow the signal, injected at the Power Amp input jacks on the rear panel, and for both channels, can get at least as far as the outputs if IC1, and all that's ahead of that. The LV Power Supply assembly is really in the way to get at drive busses for Ch B.

I'd go thru and see what you can find using the TP chart, and also what you find on the output pins of the Op Amps...first without no signal (DC Level), then with signal injected. I don't know what you have to test with besides a multimeter. If you have a scope and signal generator, you should be able to find where you have lost signal. It can be done without. Test Tone from a small mixer works just fine, if one is available. I'd start with about 100mV @ 1kHz for starters at the Power Amp Input jack for both Ch A & Ch B, in STEREO mode. Make sure you HAVE signal from your source before plugging it in, and then that you still have it on the input jack..that can be measured on the top of the input jacks...Gnd is closest to the chassis wall, Tip is your signal, and will be the rear terminal of the jack.

On the power amp schematic, Ampeg only lists the Test Points for Ch A. They are also usable for Ch B...gotta use the component placement map to find each, and, of course, be extremely careful NOT to short adjacent pins together on components while searching. If you have the PCB assembly OUT of the Chassis, be Extremely careful to have the bottom of the PCB insulated, as there's live AC Mains present.