SVT-II-P dead...

[boroman]

I have SVT-IIp 1U rack preamp (quite rare but a good one for the bass!) that I bought "dead". It's completely stock, no mods, no burnt resistors, no /leaked/bad caps... Found 2 suspicious diodes, but unsoldered they were okay. Put back in place.

I don't have any AC on between fuses. Those pins with wires go to power on/off switch. I jumpered them on this pic to remove switch from the circuit. There's correct wall AC between those pics, but still no on the fuses (fuses re good = continuity etc). Solder joints all good... am I missing something here?

(attaching schematic too)

[Indyryder02]

Between fuses?
do you have voltage on the secondary side of the transformer?
have you checked the test points? Filaments etc.

[g1]

There should not be any voltage between the 2 fuses. They are connected at the J10 end. You should see your wall voltage across C53.

[boroman]

It appears PT is shot.
I contacted Ampeg about transformer forr this one - they dont have the part and they dont have any schematics/parameteres of this PT.
So it seems this unit is useless and can't be repaired if we dont know the parmetres of PT?

[Indyryder02]

It appears PT is shot.
I contacted Ampeg about transformer forr this one - they dont have the part and they dont have any schematics/parameteres of this PT.
So it seems this unit is useless and can't be repaired if we dont know the parmetres of PT?

How did you come to this conclusion? Transformers are commonly misdiagnosed as faulty is why I ask.

have you made the previously suggested tests?

[boroman]

How did you come to this conclusion? Transformers are commonly misdiagnosed as faulty is why I ask.

have you made the previously suggested tests?

Hey, thanks, But what tests...? Maybe I overlooked something

[Indyryder02]

Hey, thanks, But what tests...? Maybe I overlooked something

There’s a few suggestions above..
do you have any secondary voltages on the other side of the pt?
there’s several test points on the schematic. Check your voltages there and work back.

[DrGonz78]

When testing for a bad transformer we need remove secondary taps. I typically would use a light bulb limiter or watch my current on the ammeter. If no large current flow is present it still could mean that there is no voltage through the primary due to a thermal fuse failure. I am thinking you are suspicious of the latter. Have you measured the resistance of the primary side of the PT? I don’t think I have read anything in your posts to confirm what is going on here. Excuse me in advance if I missed something from your post.

[The Dude]

What is the measured resistance of the primary winding?

[boroman]

Very sorry about the little information (I'm rarely in the studio)
So, it's 2Ohms between pins 1-3 and (about the same) 2.2Ohms between pins 2-4

[Helmholtz]

What DCR do you measure between pin 1 and pin 4?

[boroman]

What DCR do you measure between pin 1 and pin 4?

I have no knowledge how to measure it (?)

[DrGonz78]

I have no knowledge how to measure it (?)

Helmholtz is asking for a resistance reading across those two points. I think he is specifying that you’re DC powered multimeter is only capable of measuring the DCR much like a pickup or a speaker. When you measure the resistance of an 8 ohm speaker it is actually only DC Resistance. It is not accounting for the operational state where heat and frequency come into play. We all know this when trying to determine the impedance of an output transformer. The DCR reading is only a crude measurement comparably and you have to use a power supply to measure transformer windings to understand the ratios between the primary and secondary.

[boroman]

Helmholtz is asking for a resistance reading across those two points. I think he is specifying that you’re DC powered multimeter is only capable of measuring the DCR much like a pickup or a speaker. When you measure the resistance of an 8 ohm speaker it is actually only DC Resistance. It is not accounting for the operational state where heat and frequency come into play. We all know this when trying to determine the impedance of an output transformer. The DCR reading is only a crude measurement comparably and you have to use a power supply to measure transformer windings to understand the ratios between the primary and secondary.

Resistance betrween 1-4 is "OL" on the meter, wall voltage present between those points too, btw

[Helmholtz]

An Ohmmeter reads DCR (= DC-resistance).

AFAIR, your mains voltage is 230V.
For 230V the 2 primaries of the PT are wired in series via switch S5, connecting pins 2 and 3.
Means that there must be continuity between pin 1 and pin 4 and the DCR should be the sum of the two primary windings.

As you don't have continuity, there's no primary current and no secondary voltage.
Check switch S5 and its wiring.