SVT-4 Pro output relay

[lowell]

just got an svt-4 pro on my bench. one of the output relays are doing their job. Has anyone worked on one of these? I have not and am not sure how in depth the repair will be. I assume that there is a problem w/a component somewhere hence the relay protecting the DC to the speaker. Also, if the rear left side is tapped lightly w/ my hand it affects the relay and seemingly the fan too. There is a grey box mounted in the back left, what is it? If some has experience w/ these any advice will be appreciated.

[Enzo]

I assume

Never do that. Either know, or find out, but don't assume.

I... assume... you mean that one channel relay never comes on to connect the speakers, yes?

Either the relay system works and the amp channel has a problem, or the channel is fine and the relay system has one.

The relay is controlled by a thermal sensor circuit as well as output problem detectors and a power up timer too.

I don't know what the gray box is, can you post a photo or at least give diensions? You aren't referring to the heat sink assembly are you?

You can check on the other side of the relay for DC on the output. All those power transistors have .47 ohm 5w resistors to the output buss. Slap your meter on one of those. Is there DC there or not? A few millivolts wont matter.

Uf tapping the amp makes a difference, then some connection is loose. I'd inspect all the solder carefully, with particular attention to larger things like cement power resistors, big filter caps, anything larger. Bigger items can vibrate mor and break their solder.

[lowell]

Enzo,
Yes... assuming I know... makes an ass out of u and me, but really just me. In assuming I meant that was my hypothesis and that I would further investigate before coming to any conclusions.
The little grey box is about 1.25"X3" and there is a set of wores connecting to the output assembly/pcb. know what it might be? anywho, thanks for help I'll check those things...

OK, there is no DC or a couple mv's on the ballast resistors. Both the +80v and -80v supplies seem to be ok and are on the collectors. Also, tapping things is no longer having any affect... there is NO output. Oh do you have a schematic for this amp?

[mykey]

just got an svt-4 pro on my bench. one of the output relays are doing their job. Has anyone worked on one of these? I have not and am not sure how in depth the repair will be. I assume that there is a problem w/a component somewhere hence the relay protecting the DC to the speaker. Also, if the rear left side is tapped lightly w/ my hand it affects the relay and seemingly the fan too. There is a grey box mounted in the back left, what is it? If some has experience w/ these any advice will be appreciated.

another crap crate amp labeled as an ampeg.

[lowell]

hmmm... what a helpful post...?

[mykey]

hmmm... what a helpful post...?

quite helpful because it illustrates that this amp is neither a real Ampeg or a real SVT. It is a CRATE amp with the name "ampeg" stuck on it.
And you are quite right, the protection circuit is detecting DC from heat damaged output transistors. A problem that affects a great number of these amps, like 100%.
Replace the bad output transistors and take out the thermostat control so that the fan runs 100% of the time at top speed. then it could work for a while until something else fails.
OK forgot to tell you about the defective photocell limiter circuit, that fails too. could be that it's just the photocell module.
Would someone tell SLM that heat RISES?
OK I admit it, the jokes getting old. you bought a Crate amp and now you are seeing why Crate amps have a reputation for being the least reliable* in the entire industry. Better luck next amp.

* so unreliable in fact that SLM had to buy the rights to the ampeg name, and relabel crate amps as: "ampeg". Otherwise, sales were rather down, so to speak. But is there any difference between a crate and a new ampeg? None that I am aware of besides the new logo stuck on the cabinet, and nicer grill cloth. NOTE: a real SVT weighs 85 pounds and is the industry standard workhorse bass amp. Now you know the difference.

[Enzo]

Hey Mykey, either help or get the fuck out of the way.

The man just told you he measured at most a couple of mv on the channel output, pay attention. That is not a DC offset, and the relay is not detecting any therefore. His output transistors are not heating up, and show no sign of damage.

SOmething is wrong in his circuit, but doesn't seem to be related to the bullshit you continue to shovel.

[Mark Black]

Enzo throws down! (Bravo!)

In addition to the components Enzo has pointed out for solder checking I would also like to mention the multipin connector headers, as I often find those solders cracked (and not just in Ampegs).

Probably doesn't help that they are a "square peg in a round hole".

[MarkusBass]

The little grey box is about 1.25"X3" and there is a set of wores connecting to the output assembly/pcb. know what it might be? anywho

Lowell,

This box contains two transformers for balanced line output. Just open it and check the connections inside.
I actually have simmilar Ampeg on my bench. It's in slightly worse condition - there is DC on the output (disconnected by DC protection relay) and some signs of smoke. But I'm quite busy at the moment - don't have time to look at it. I assume it's original Ampeg.

Marek

[lowell]

Ha! Lovin' it Enzo. Thanks for help guys will check these tomorrow. Although it looks like quite a process to get in there, we'll see how much time I have.

[jeffects]

I am having the same problem with an SVT 4 Pro only with both channels. Was there a solution to this?

[nevetslab]

SVT4-Pro with both relays open

If I understand you, jumping in to an older thread with your Ampeg SVT4-Pro, your amp powers up but the output relays aren't pulled in. This may be expensive, if both relay's DC sensing circuits have inhibited the relays from closing. If you have no DCV at all on the rear panel outputs, instead of some level in the +/- mV range, you need to probe at the two output busses, which is where the 0.47 ohm 5W ballast resistors reside between the two heat sinks. If those are also at 0V, and you have the nominal supply voltages present, shut down, drain off the suppliy voltages and have a look at the one area you have access to while the main PCB assembly is still in the chassis. Unfortunately, you can't get at both ends of any of the ballast resistors, as they're tucked under the heat sinks. PCB assembly has to come out to get at everything. Methodical work in order to get it all back together correctly.



My last round with one of the SVT4-Pro amps had one channel whose output relay was open on Ch B. I saw one of the emitter resistors cracked open, a good indicator that since the other channel was running, I had open gate and source resistors on all the Ch B MosFET's. (sure enough, total shorted MosFETS...all ten of them) From the front side of the heat sink, you have access to the 47 ohm gate resistors of both channels....N-Ch side. If all those are measuring open circuit, your're in for an expensive ride. These amps need reasonably close matching on the N-Ch and P-Channel MosFET's (IRFP 240's && IRFP 9240's). I finally began buying in bulk, and am matching and batching them with success, but that's a topic for later.

A more common problem on SVT4-Pros are no signal passing thru the preamp output/power amp input jacks, or no DC on the supplies due to solder fractures on the Bridge Rectifier, rear panel phone jacks, etc.



I run into more solder fracture problems, being the cause of no output than any other ailment in our rental depot's gear. PCB interconnect headers are a common place to find fractures. The rear panel of the SVT4-Pro is supported by panel components, the mistaken theory is they work better as PCB support brackets, so no need for actual mechanical support hardware. Keeps me employed, theories like that!

[jeffects]

Hey nevetslab, thank you for your very detailed response. A little backstory on this amp. It came to me pretty fried up with noticeable burnt components. Most of the burning was around R119, R12 and R11. I ended up replacing those components along with 10 of the output mosfets and their corresponding emitter (source) resistors. Right now the amp powers up just fine. When I feed a 1k sine into it I can probe right before the relays (R173 & R273) and get a perfect signal with no DC offset. On the B channel I can manually trip the relay and get that same perfect signal on the output. On the A channel I get a nasty looking signal when I manually trip the relay although it's fine when probed from R173. Any suggestions on where to go next? Thanks again. It sounds like you know this amp well.

[nevetslab]

When you say you can manually trip the relay.....do you mean the relay for Ch B isn't pulling in at power-up? It sounds like with no DC offset on either channel (low mV range), your output is back to being stable. Either side of R173, having the RF choke in parallel with R173, the signal should be the same. Or is it not passing thru the output relay contacts to the output jack(s). Under load of some sort, open circuit? It's been a while since I've had any faulty relays in these amps. I can't recall if after un-soldering one, if you can pry it open or not, to deal with pitting on the contacts. Solder fractures in the relay connections as well as the output jack terminals can render dodgy looking signals. I recently had to de-solder and re-solder a driver tube socket on an Ashdown BTA 400 to get it working right, with perfect-looking solder joints on the board (lead-free solder again), so sometimes, just humoring logic and re-working connections can restore normal operation.

The relay driver/DC sense circuits are pretty straight-forward.....comparator circuit with a common neg DC reference signal applied to the (+) terminal of each to drive the PNP relay driver into conduction and pull in the relay, unless there's sufficient DC level on the output to prevent the comparator from toggling to neg output. If in doubt, lift the amp output side of R169 and R270 and manually connect either/both to a variable DC source to see that each trips at the same DC levels above/below ground.

[jeffects]

The relays are like the photo above. I can manually push the contact with a wooden stick. Neither relay is pulling in at power-up.