SVT 4 Pro DI issue

[lowell]

Channel A no worky. Channel B is good.

I see that J23 has the primaries of both DI transformers in parallel on pins 1 and 2 of J23. This being the case I am suspecting Channel A transformer. Connection of both DI transformers seem good as I am getting a DC resistance reading across pins 2 and 3 of both xlr outs of 325ohms. This tells me that the meter is seeing the secondaries on both channels. I also checked the cap and resistor across both secondaries as well. What do those of you that are familiar w/ this repair think? See pg.2 of the attached schematic.

[Enzo]

I think you should look at the schematic again. I don;t see the transformer primaries in parallel across pins 1 and 2. I see pin 2 (ground) wired to both, but the other end of each gets its own pin. T1 goes to pin 1 of J23, and ultimately takes signal "A/wet line out." T2 is fed from pin 7 of J23, and is "B/dry line out." Unplug J23, now the transformers are isolated. Measure continuity from pin2 to pin 1, and then from pin 2 to pin 7. Either one open? I'd bet they are OK.

Now apply a signal to the amp. Scope or otherwise monitor pins 6 and 8 of J17. They are the source of those signals.

And across the connector to J16, pin 6 signal comes from TP9 - anything there? - and then a couple JFETs select the source. See if all that is there. And the pin 8 signal appears to come from the lione out level control. And without me looking too much deeper, I'd be checking out all those Pre/post and Mono/stereo switch contacts.

[lowell]

Good call... my bad in reading the primary connection. It really pays to take a deep breath and slow down sometimes eh? Thanks I'll report back with results.

[lowell]

Okay so there is indeed signal on pin 8 of J17. So the issue is AFTER that. NOW, that being said, I cannot for the life of me find J22/J23 "header". NOR can I find T1 or T2. Are there 2 versions of this amp? I do not see any isolation transformers for the DI outs. I followed the signal from pin 8 of J17 and it went to a 470ohm resistor, then to a 1meg resistor finally ending up at the input to a DIP opamp...?? I'm lost.

[Enzo]

J22 should be immediately behind J10, the Channel B line out jack. J23 is on the small breakaway board with the transformers. The trransformers are on their own little piece of board with nothing else. There are a couple different schematic sets, but they both agree on this section. The transformers little board would be mounted with a couple screws somewhere on the chassis wall or floor.

Of course if someone has removed the little transformer board, then your DIs won;t be working...

I may have led us astray. J17 is the header on the jack board, while corresponding J16 is on the other board. I was confused by the numbering of the pins. I didn;t realize they did not number them pin for pin, and the reverse pin number order confused me. SO it is not pin 8 to pin 8. Pins 6 and 8 on the jack board connect to pins 9 and 11 on J16, both of which come from wipers on the line out control.

I am afraid I cannot find any 470 ohm resistors, nor can I follow the traces from pin8 - including the "pin 8" from the wrong end - to any resistors matching your description./

[lowell]

Enzo,
Thanks for the help. So I've attached a pic of the DI jack breakaway board. There are no transformers on it as you can see... and I cannot find any board with transformers on it in the amp... J17 is the white connector on the left. The only things I see that slightly resemble transformers are what I believe to be the output protection relays... as they are near the output jacks and there are inductors physically right next to them. Again could it be that the DIs on this are opamp driven? Not sure if that is possible, but as I said I traced the XLR DI jack back to an opamp... U7.

[Enzo]

Of course it is possible. The transformers would be recognizable if they were there. This is a different version.

WHat are the board numbers on that board? Looks like a paper sticker covering them. 539-02?

I can't find the -02 version, all I have is the -04. I looked through the other SVT series drawings, and it is not from one of them.

[MarkusBass]

So I've attached a pic of the DI jack breakaway board. There are no transformers on it as you can see... and I cannot find any board with transformers on it in the amp... J17 is the white connector on the left. The only things I see that slightly resemble transformers are what I believe to be the output protection relays... as they are near the output jacks and there are inductors physically right next to them. Again could it be that the DIs on this are opamp driven? Not sure if that is possible, but as I said I traced the XLR DI jack back to an opamp... U7.

Lowell,

The transformers in SVT4Pro are not on the DI board. If you look at the schematic, you see that they are connected to the board with J23 connector. They are attached to the enclosure in kind of metal can. Please make a picture of the amp and I will show you where the transformers are. Or just follow the wires that were connected to the board. I never heard of another, transformerless version of this amp. Unless someone has just removed them and shorted the connector.

Mark

[Enzo]

Markus, look at the photo of his board, it doesn't have a J23, it is rectangular, without that cutaway area the transformers live on in the more modern revision the schematics cover. COmpare his photo with the layout drawing in the SVT4Pro schematic set. His board number appears to be an -02 while the schematics are all for -04 boards.

[MarkusBass]

OK, I have to read the thread again. It seemed to me that in the first post Lowell was checking resistance of the primary windings of the transformers and we find out there there are no transformers in this amp . This is a version that I haven't seen. I think I have somewhere this board - I will check what version it is.

Mark

[lowell]

Markus good point about my initial post. I thought because of my DC resistance reading that my meter was seeing the primaries of the transformers. I was wrong. Okay so we know this is an odd version. I'll report back tomorrow when it's in front of me with the board #.

[Enzo]

At some point we transitioned from working on the schematic to working on the amp.

I looked through the various SVT-ish amps looking for one with a solid state DI drive, figuring the circuits might be similar. Most all of them use transformers. I got out my older SLM service disks, but the SVT4 stuff was the same as we already found.

[MarkusBass]

I have version 4. It's different from the one posted by Lowell.


What about providing signal from a generator and check it in the circuit with oscilloscope? You have one channel working and the other not. You can compare one channel with the other. Should be easy.

Mark

[lowell]

Board # is # 06A539-02

Markus I will do just that if I cannot get a schem from Ampeg. I just sent 'em an email.

[lowell]

Here it is for you guys since ya don't have it.

edit: actually nevermind this is still wrong. I'm gonna email them again.