Fuse and Protection

[trem]

I looked at my copy of Merlins Power Supply book, but i do not see this stuff mentioned.....

The bass player in a friends band said his
Ampeg SVT3 Pro quit working.
I checked it out...fuses are good, but the preamp tubes do not light or get warm.
The fuses both measure good/short.
If you look at the schem....... there is C1 and C2 right at the IEC input. I guess C2 is in the "Death Cap" position.
I do not get anything coming out the PT Secondary.

Right where the 120 VAC comes into the amp.....
I put my meter on Capacitance.
C2 = 3.2 nF
C1 = 0.005 nF

When i use my Ohm Meter:
C1 to Hot of IEC is a short. The other side of C1 to Neutral is Open.
C2 to ground is a short...the other side of C2 to Neutral is Open.

1.What is C1 doing between the Hot and Neutral.?
2. What are these caps exactly...they have a "funny" nomenclature. On the schematic where there is usually a voltage rating for the cap, it says UL/VDE
Thank You

Sorry.....i forgot to link the schem:
http://el34world.com/charts/Schemati...VT-3%20pro.pdf

[Jazz P Bass]

" When i use my Ohm Meter:
C1 to Hot of IEC is a short. The other side of C1 to Neutral is Open.
C2 to ground is a short...the other side of C2 to Neutral is Open."

The way you are saying this is very confusing.

They are not , technically, death caps.

Back to Basics: What are Y-Capacitors? | Vicor PowerBlog

If you do not have any secondary voltage, do you have primary and secondary resistance?

An open on either will cause that.

Ampeg SVT3 Pro Pwr.pdf

[eschertron]

+1 to Ohming out the PT both sides.
Also: Is the power switch broken? Where do you lose line voltage?

[Enzo]

C1 is from hot to neutral, that is across the incoming mains...C2 is from neutral to chassis.

They are not the cause of your problem. If C2 opens or shorts, you will never know it. If C1 shorts, your wall outlet breaker will pop. If C1 opens, you will never know it. But they are wired to the mains, so if I read you right, you lack continuity from the main connector to the caps? The convenience outlet is wired across the mains, when the amp is plugged into the wall, does the convenience outlet work? plug a table lamp into it or something to see.

Look at the main IEC connector - the thing your power cord plugs into. is the solder cracked around its pins?

[trem]

Let me check out some of these suggestions, the basic tranny readings, and i will get back to you. This amp is very hard to access compared to a Turret or Eyelet hand-wired board.
I pulled the switch on the Hot Side.....it is working properly. I think the AC is dying before the switch. It is very hard to see anything on the solder side of this PCB.
Yeah...the convenience jack...it only reads 65 VAC.

I linked to the schem i forgot in my OP.....sorry about that.

[trem]

OK, correct me if i am wrong, but i must have an issue at the IEC and PCB connection.
There is no connectivity between the IEC Neutral and J18.
There is a connection between the Convenience Neutral and J18 because there is a jumper between those two points.
But from IEC Neutral to J18, there is only a PCB Trace that i cannot See/Access from this side of the board.

[Enzo]

IEC neutral needs to get to J17, the power switch. It is a solder connection, a trace, and a spade post soldered in. Not much there to troubleshoot. Expose the board and check the solder. I mention the IEC connector because it is the one place getting most mechanical stress. Check its solder.

[trem]

Thanks Enzo -
Right, that is what i figured. Any of you guys ever work on this amp.?
It looks to be a Major PITA. It is all one board big board, with lots of connections.....Molex, Spade, The Fan, Heat Sink, 1/4" Jacks, etc
It would seem Ampeg did not plan on this thing being taken apart very often.

[Enzo]

No one designs an amp to be taken apart often. That implies the amp is not very reliable. Some amps are more inconvenient than others, just a fact of life.

ALWAYS make up a chart or drawing or take GOOD CLEAR pictures, so you can get all the wires back where they belong. Most times I find all I have to do is free the board enough to flip it over, many of the wires can stay connected. I don't have to completely remove the board and move it to another room.

We have a bit of shop philosophy, the Tao of the Ten Screws. The technician is faced with something to take apart, "But it has TEN SCREWS, for god's sake." At that point he has the choice of bitching and moaning about it and then taking out the ten screws, OR he can just take out the ten screws and get on with his life.

[Tom Phillips]

...
ALWAYS make up a chart or drawing or take GOOD CLEAR pictures, so you can get all the wires back where they belong...

I am adding this post to emphasize Enzo's point. There is often a feeling that you will remember the connections when it's re-assembly time. There is also often an optimistic expectation that you will be re-assembling the amp in a few minutes after a quick fix. Neither of these assumptions are good practice. Another advantage of making a good chart and/or taking pictures is that you can keep them in your files for use the day someone brings you an amp that has previously been messed with and already has disconnected or mis-connected wires. Sometimes individual cable assemblies are even missing. Such nightmares come to mind regarding basket case Marshall DSL*** amps.
Cheers,
Tom

[trem]

Yes...i will heed that advice.....i will be marking, drawing, pictures, etc etc.
Thanks Again

[Jazz P Bass]

I like to make a quick basic drawing of the board & the connectors & where they go.

Then, after I mark them on the drawing (JXXX) I remove them from the board.

[trem]

I have not worked on anywhere near the number of amps that most of you guys have.....and most of what i have seen has been your typical 1960's Fender Marshall Vox, hand-wired stuff.
These "modern" PCB amps that use A Lot of jumper wires with the crimp on, spade connectors always worry me. They often seem to be a Very Tight fit...where i have to Yank/Pull each one with a pair of pliers. It almost feels like you are going to break the PCB, or tear the lug connector right out of the PCB if you know what i mean.....or maybe F Up the wire or spade itself. Just not fond of them at this point in my glorious career.
In the past i have had good luck with taking a fine tip Sharpie Pen and writing the number of the connector on the plastic spade insulator.
ANYWAY.....i am trying something different with this Ampeg. There are LOTS of wires involved with the Power Tranny and Rectifier. Both of those (the PT is toroidal) are attached with a single bolt through the middle. So i have removed that, and will pull the PCB with the PT and Rect attached, and so do not need to deal with all those slip-on connectors.
Either way, this is a PITA to perform what might be a very simple job. Cannot imagine what Ampeg might charge a guy to do this. No doubt they have a few tricks, and have experience on their side.....but it must be a pretty big job for them as well.?
But Anyway.....about those two, blue caps...C1 and C2.....is there something "special" about these.?
With the lack of voltage rating, and instead the UL/VDE stamp where we usually see a voltage number.

Thank You

[Tom Phillips]

...about those two, blue caps...C1 and C2.....is there something "special" about these.?...

Yes. See the first link that JPB included in post #2 of this thread.

[Enzo]

When I see something like this, I usually free the board from the rear panel, and remove any screws to the bottom panel. I set the chassis on end, transformer down, preamp tubes top, and I see maybe three molex connectors with red and black wires, which I pull off. I leave all the transformer wiring in place, and let the board flop out towards me. I can access the far side.