Toroid Taps question

[ldervish]

Trying to determine if the toroid is bad or not in a 2002 Ampeg SVT-3 Pro, the question is about the secondary taps. I got it as "non-working, not worth repair expense". The toroid is one of the most expensive bits so I thought I would verify it first. No power has been applied.

Schem shows the following secondary taps order: Red(6) to Grn(2) is a winding, then Grn(4) to Red(1) is the next winding. (The number in parenthesis is the plug pin.)The pcb ties both Grns, (2) & (4), to ground. The Reds are each rectified to supply +/- 100vdc.

The taps pull from the toroid in exactly that order i.e. (6) (2) (4) (1), and they insert into the plug as numbered.

When I check continuity via the plug however, I read .4 ohms from Red(6) to Grn(4), and .4 ohms from Grn(2) to Red(1) and no continuity (6) to (2) or (4) to (1), or (2) to (4).

So not knowing how windings are created/tapped on a toroid, I'm wondering if one winding is supposed to be complete before the next winding begins/ends, and so on, or are the Grn leads switched in the plug or what the story is? If they are switched it won't matter to the circuit since they tie together. I'm thinking the toroid is good but don't know how else to verify without powering up.

[Jazz P Bass]

I would test what you have found.

Red (6) to Grn (4) & Grn (2) to Red (1).
If you have a Vac reading & they are both the same, then the schematic is confusing the issue.

[Enzo]

I think the schematic is clear. it shows two red to green secondaries. What the rest of the circuit does with them after the wires leave the transformer does nothing to change that. So I expect them to be the same as each other, and I don;t expect them to be connected together - that is to say there should be no continuity from one to the other. UNLESS you plug it in and the circuit wiring makes connections.

Some thoughts about your approach. It may have been tagged not worth fixing, but the transformer is stil the LEAST likely part to be bad. Yes, they can fail, but not often. Also, it is VERY difficult to diagnose a transformer with a hand meter. You measured 0.4 ohms per secondary. Did you compensate for the resistance of your meter leads? Even so, if you have a couple shorted turns in the transformer, your meter would not know the difference. About all a meter will tell you is if a winding is open or if it is shorted to another winding or to frame.

A note about a toroid transformer. it may look different, but a transformer is a transformer, and windings won;t look any different on a schematic.


You want to test your transformer? Unplug that 6-pin connector - so now all the secondary circuits are disconnected. Hopefully you left the primary wiring in place. Now look up "light bulb limiter" and make one and use it. Power up your amp (which is not just a transformer) through the bulb limiter. If the thing lights up bright, your transformer is bad, if it stays dim, the tranny is likely fine.

If I see a not worth fixing, first thing I do is check all the output transistors for shorts. Then those red transformer wires go through the connector and wind up at the main rectifier bridge, so check it for shorts on all four sides. If those rwos of MOSFETs are blown out, someone may have decided not to put the money into the amp. A guy quotes the owner $200-250 and he decides to bail.


But do make the bulb thing and tell us the results.

[ldervish]

OK, I've got to go to the store for light limiter parts…be back soon.

The bridge rectifier checks out OK.

This guy was quoted $400, reason enough to bail, and one reason I was looking at the xfmr as a possibility since they are very expensive on this amp.

[Enzo]

Don't get me wrong, I do hope you will test just the transformer, but expensive repair estimates are often largely labor expense.

[ldervish]

But do make the bulb thing and tell us the results.

No worries Enzo, I'm here to learn.

With the light bulb limiter in place and the secondary connector pulled, I get nothing.

If I re-connect the secondary I get a steady bright light.

[Enzo]

What do you mean "nothing?" If the bulb stays dim or dark with the connector pulled, then your transformer is not shorted. That is a good sign. In that condition, you could probe the loose connector (the wires from the transformer in other words) pins for AC voltage. I expect about 45vAC between the red and green wires in each case.


But the fact the bulb lights up when the connector is restored tells me the voltages are there. otherwise there would be nothing to cause the bulb to light.


And that bright bulb with it connected brings us right back to looking for shorted output transistors, shorted bridge rectifiers, less likely but possible shorted filter cap.

[ldervish]

By 'nothing' I meant a dark bulb.
The two red/green pairs each read 52vac and the org/org pair reads 249vac.
I don't know if I did this correctly or not but all 8 mosfets read at most a couple ohms S to D in circuit, and 3 read .5 ohms

[Enzo]

Transformer is fine, start looking for blown power amp.

[Jazz P Bass]

I don't know if I did this correctly or not but all 8 mosfets read at most a couple ohms S to D in circuit, and 3 read .5 ohms

You have a major short on the mosfets.

I would take them out one at a time until the low resistance reading, D to S, is removed.
Be aware that there are two sides to the output circuit, so work on one side at a time.

Basically speaking, the mosfets should be high resistance, D to S.

Schematic link: COOL SCHEMATICS

[ldervish]

There is nothing I can spot visually that would indicate a failure so I'll pull the board and check the mosfets. One side (IRFP9240's) has been worked on before, those 4 are all different. I'll start there and report what I find.

FWIW my little Conar tube tester says the 12AU7 is bad, and R32 (Q7 source R) kind of crackles when I probe it, like maybe it has a bad solder joint, so I'll look closer at that when I get the board out.

[Jazz P Bass]

[QUOTE=ldervish;324933]There is nothing I can spot visually that would indicate a failure so I'll pull the board and check the mosfets. QUOTE]
Ahh, if it was oh so simple.

Actually, the best (as far as repairing) faults are those that do Not show a failure.
When the failure is obvious it is harder still to find the stressed parts.

The 12AU7 is a common failure.

[ldervish]

Here's what I found for out-of-circuit resistances D to S:
Q7: 2.8R
Q8: short
Q9: open (no reading at all)
Q10: short
Q11: 636k
Q12: 12M
Q13: short
Q14: 6.26M

[Jazz P Bass]

Might as well simply replace all of them.

Get them from Mouser or Digikey & the chances are real good that they will match.
IRFP240
IRFP9240

Check/ replace Q2, 3 & 4 & all associated resistors.

And you have already found the 12AU7 is a dud.

[ldervish]

Mouser no longer stocks the Fairchild, only 2 Vishay models. The 'in stock' *PBF devices are roughly 1/4 the price of the 'on order' model. I'm not in any particular hurry so I'm wondering if it's worth waiting for the more expensive ones? Are they more likely to be matched, or higher quality?