Ampeg SVT-400T Repair Progress

[washburnbassman]

Hi Folks,

Repairing a 1994 SVT-400T (Nirvana/Novoselic bass tone machine). Its giving me some grief, so thought to post some progress and pics. In general it works, but has low volume, cut outs, and a non-functioning EQ board.

This unit is way beyond economical repair, I'm just going to do it myself because I want to use the SVT soon for recording some early 90's bass tones, and they are getting harder to find in working condition.

Schematics used

[Invalid or Expired Link Removed]
Test points

Troubleshooting

Per other posts on this board, bridging the effects loop with a patch cable yields no improvements here, so it's likely just old and dying components, maybe even a bad transformer.

General Pics



EQ Board

EQ board is dead. Replaced 3 transistors and JRC 4558, looks like someone else tried to fix this before. Still not working. Audibly pops and then buzzes when EQ is engaged. P4 pull switch looks to be good.


Power Section

Volume output is very low. Amp cuts out frequently. Need to check filter caps.


Will keep thread updated,

WBBM

 

[agedhorse]

Do you have a scope and understand how the circuits are supposed to work?

Can you calculate the expected voltages and compare with measured voltages?

Can you trace the signal and compare with the expected signal?

Transformer? Maybe 1 in 1000 odds at best.

Filter caps? Maybe 1 in 100 odds at best.

You need to correctly identify the actual cause of the fault(s) before you will be able to properly repair your amp.

In general, for an amp like that, a QUALIFIED tech should be able to repair it properly within about 2 hours.

 

[washburnbassman]

Yes
Yes
Yes
Fair Point
Fair Point

I'd honestly have a tech come out to the studio if we weren't in the times we're in, but as it stands It'll just be a project for me to work on.

Do you have a scope and understand how the circuits are supposed to work?

Can you calculate the expected voltages and compare with measured voltages?

Can you trace the signal and compare with the expected signal?

Transformer? Maybe 1 in 1000 odds at best.

Filter caps? Maybe 1 in 100 odds at best.

You need to correctly identify the actual cause of the fault(s) before you will be able to properly repair your amp.

In general, for an amp like that, a QUALIFIED tech should be able to repair it properly within about 2 hours.

 

[washburnbassman]

Sorry I should also add, in case I gave the wrong impression. I have not been just randomly replacing parts These chips/components are blown, and measured as such per schematic. I'm not a qualified tech, but getting a qualified tech right now isn't possible.

Do you have a scope and understand how the circuits are supposed to work?

Can you calculate the expected voltages and compare with measured voltages?

Can you trace the signal and compare with the expected signal?

Transformer? Maybe 1 in 1000 odds at best.

Filter caps? Maybe 1 in 100 odds at best.

You need to correctly identify the actual cause of the fault(s) before you will be able to properly repair your amp.

In general, for an amp like that, a QUALIFIED tech should be able to repair it properly within about 2 hours.

 

[washburnbassman]

Quick Update on this one.

On testing the line out only, the sounds is perfect with no drop outs, so worth trying if you are trying to figure out if its the preamp or poweramp. I recorded it today with no problems.

I'll still continue with the repair, but at least I know it's likely just the power amplifier section.

The EQ however, still doesn't work for either mode.

 

[washburnbassman]

Update. Had some time today and made some progress.

Tested EQ board today using Fluke MM. D11 diode is blown. Pin 1 of JRC 4558 Op Amp tests @ 13.9 volts, but when the EQ switch is engaged it does not flip to -12v per schematic test points. It switches to - 0.2v. Will replace diode and recheck.

 

[beans-on-toast]

Try an instrument patch cable between the fx send and fx receive. Then try patching preamp out to power amp in, one channel at a time.

To test the power amp, try plugging another preamp output (or your bass) directly into the power amp input jack, one channel at a time.

Try turning the EQ ON/OFF with a footswitch. If you don't have one, plug in an instrument cable and short the tip to the sleeve with a wire at the other end.

 

[washburnbassman]

Thanks for the tip on the foot switch I’d forgotten I could engage it there. The patch cable trick didnt help but I like the idea of the power amp test, will try that out

 

[washburnbassman]

Quick update. Removed Zener D11 from the board and tested it with Fluke 87 Diode mode again. It is shorted. Now that it is removed, the green LED lights when the EQ knob is pulled, but EQ is obviously still dead. Once the Zener is replaced, I'll update.

 

[washburnbassman]

This amp's EQ section has been repaired. The (now blue) D11 Zener 1N746A diode was causing the EQ problem.

With these units, you will need to ground the board when you are working on it to complete the circuit and be able to test components. I grounded it by soldering wires to the ground points of the board, and then winding them around the screws connecting to the chassis. it also makes it much easier to test your work.

You can hear this exact amp DI'd without the EQ section here


Pics below

 

[washburnbassman]

This amps power section has been repaired it sounds like brand new driving a marshall 1960A cab.

I replaced the power caps, but i also replaced pretty much every other cap aswell, i dont want to be goinng back into this unit for a long while.

I also replaced 2 shorted transistors.

I will list the power caps used in another post.

 

[washburnbassman]

At a minimum you should replace the power caps below (about$20 for 6).

Mine were the originals from the early 90s and while they tested fine for capacitance (i.e around 3381uF), they were no longer capable of keeping stable high power output (loudness) at their age and were the core problem with the amp.

There is a hot glue that was used in these amps to keep the caps from vibration, i did not replace this as i dont intended ever gigging this amp or abusing it on a world tour.

The power caps are :

Link Removed

Replacing these would likely resolve the issue I encountered (low and farty output) . Anything past that is up to you

i.e These are the main pre-amp caps

Link Removed

The rest are small microfarad 16v to 50v caps that you can find easily.

I used entirely RADIAL replacements. AXIAL leads werent actually necessary, and it was much easier to find high quailty radial caps.

The transistors are readily available too, but dont replace them unless you absolutely have to, or they have tested as bad, do the power caps first if you are just looking for a quick fix.

 

[agedhorse]

Or test and verify that there actually is a problem, identify the actual cause of the problem, and replace the part(s) that is causing the problem.

 

[washburnbassman]

Pearls of wisdom from agedhorse here folks. Pearls of wisdom.

If you are an SVT400T owner with an EQ board problem followed by a power board problem, I encourage you all to scroll up to some of the other helpful and informative posts he has submitted in relation to the SVT400T in this thread.

Oh that’s right. There are none.

 

[Geri O]

Pearls of wisdom from agedhorse here folks. Pearls of wisdom.

If you are an SVT400T owner with an EQ board problem followed by a power board problem, I encourage you all to scroll up to some of the other helpful and informative posts he has submitted in relation to the SVT400T in this thread.

Oh that’s right. There are none.

Oh, there are plenty, but being that it's not what you want to see/hear, you in fact will not.

You got lucky going the long way around. And probably spent more than you need to. Oh, that's right. It's yer money....

None of that is actually a problem, until you start throwing dirt at one who is willing to help those that will listen.

Willy-nilly throwing out technical information that may not be correctly understood and could get the untrained person hurt or killed is really bad for the knowledgable tech's reputation. Not to mention legal ramifications of such activity.

As the old saying goes, "you know just enough to be dangerous"...

 

[washburnbassman]

If me going through my fixes isn't useful content, and isn't even recommended, given the dangers involved, then I'm happy to remove it.

agedhorse seemed to not recommend me trying to fix this myself from the get go, which is fair enough, and his posts kind of reflected that.

I don't mind deleting the thread, I'll just save it for my own learnings.

Also, I felt like agedhorse just kind of came in and dumped on the thread for the sake of it. I am a member in many electronics repair forums, with other folks who work for major equipment manufacturers.

This is one of the only forums where I got this kind of attitude from the get go. By the time I got here, I had already received guidance on measuring signal paths etc from other (more helpful) folks.

@Geri O IMO, I wouldn't classify agedhorses responses as having tremendous value to a person looking to teach themselves audio device repair. He just seemed cranky and unhelpful. I listen to smart people all of the time, thats how I've learned to fix everything I've fixed so far.

 

[dbase]

I was expecting this ... but sadly no entertainment today. Glad you got it working for you.

 

[washburnbassman]

I was expecting this ... but sadly no entertainment today. Glad you got it working for you.
View attachment 3868244

extra credit for back to the future, and it certainly could have turned out like this, and almost did a couple of times.

As Geri said, I probably got lucky, but I have fixed a lot of radios, so pre-amp/power amp work wasn't foreign to me. I did very specifically track down the Zener diode problem using the schematic and a scope, and I only replaced that diode on that board (even though I purchased other spare parts).

For the caps, I just decided to do what I do on my old radios, and do a full cap job regardless.

This isn't a particularly desirable amp, so it was just some practice for me, and was beyond economical repair.

 

[washburnbassman]

Can a mod add the text below to the top of the first post (if we're not deleting the post, which I'm also fine with).

DISCLAIMER : You can electrocute yourself with mains voltage working on this amp. If you are not used to opening and working on radios or other mains voltage devices, then do not attempt the repair below.

 

[agedhorse]

IMO, it's important to present the information both conceptually as well as factually.

Since I am a design engineer IN THIS INDUSTRY, with an EE, and have written numerous service and technical training manuals addressing many of the common myths that I see with service personnel that do not really understand as much at they think, I am sharing this experience with those who want to learn more about how amp repair is done in a qualified service shop.

A qualified, knowledgeable service tech will approach a repair by applying diagnostics, verifying the fault(s), identifying the cause(s) and then correcting the faults that have been identified. This would include replacing parts that test bad. They will also understand the necessary safety precautions that go along with servicing line voltage equipment.

As a follow up, they will then test after the repair to verify the performance and at the same tone check for parts that do not meet the performance spec.

Just going in and replacing parts is not something I would consider as being skilled, and if charging a customer for such work might be considered dishonest, deceitful, or even fraudulent.

Unfortunately, this is all too common, and along with the additional labor from such an inefficient (and generally less effective) way of attempting repairs, is one factor in driving g up the repair cost to the customer.

This IS helpful and informative information to those who are interested in learning and understanding. For those who already know it all, perhaps this information is not as interesting.