OT Diagnosis affirmation

[Mr_bibbles]

Hey All-

Was wondering if someone could help me verify my results of my first advanced OT test.

Amp: AC15-
Symptoms: "choked" sound, fluctuating (but always low) output, ugly distortion. my bandmate occasionally uses an extension cab and is very bad about switching between 8-16 ohms so I suspected the OT a bit earlier than I normally would have.

Normal tests showed continuity being ok between secondaries / primaries / shorts to ground as according to R.G.'s excellent debugger:
Output Transformers

As per directions for the advanced tests, I removed ALL secondaries and primaries from the amplifier and hooked up the 8 ohm tap to one half of the AC filament line. The 8 ohm tap measured 3.3v AC, which would indicate healthy OT?? Let me know if I've missed anything, and thanks in advance.

[nickb]

Did you have a light bulb limiter in circuit? If so, how bright was it?
Did you check the other windings per the instructions? What are the readings?

[Mr_bibbles]

EDIT- there was a lightbulb limiter, but it measured 2.5VAC and the bulb was dim so I ran the full voltage to get the above results.


B+ Primary to Plate 1 - 130ohms
B+ Primary to Plate 2 - 130ohms
Plate 1 to plate 2 - 260 ohms

Secondaries to Primaries - OL all around

8ohm to 16ohm Secondaries - .3 ohms

All leads to chassis- OL

HOWEVER... There is a black ground wire coming out of the OT, and the secondaries both measure .5 Ohms resistance to it (while the black wire is disconnected form it's chassis ground). Does this mean that the secondaries have shorted to the transformer Chassis??? I may have missed this on the simple tests.

[nickb]

It sounds like the black wire is the start of the secondary winding. With a dim bulb, I'd say your OPT is fine.

[Enzo]

Secondaries will always measure very low resistance, they are after all just pieces of thick wire wrapped arounf the iron core. 8 ohms, 4 ohm is impedance, not resistance.

[The Dude]

I agree. The OT seems fine. FWIW, I have had several AC15's in the shop. Most every one of them had only a bad preamp tube. If you haven't tried yet, try some tube substitution. As I said, I haven't found much else that goes wrong with them.

[Mr_bibbles]

Cool thanks everyone. That's what I was assuming, I almost never suspect OT's but like I said my bandmate takes very poor care of switching his Output Impedance. I have swapped every tube, checked for cold solder, tested diodes and a bunch of other discreet parts across the board. Time to remove the big caps I guess. I'll keep up with y'all to let you know what happened.

[pdf64]

When was the amp made?
I've found a lot of failing solder joints on Marshall built Voxes from the 90s.
They're all but impossible to see without visual magnification and strong light.
Try re-soldering all solder joints on the pcbs and see if that helps.
I wish Marshall would specify through-hole plating on their boards.

[Enzo]

Running a 8 ohm cab in the 16 ohm tap or vice versa won't harm the OT in the slightest. The worst it might do is stress the power tubes a little, and I wouldn;t worry about that myself.

[Tom Phillips]

... Time to remove the big caps I guess...

Nope. I say that the next step would be to measure the plate and cathode voltage of each stage of the amp. Especially the phase inverter. Post the results and that will give clues to support a diagnosis.

[Mr_bibbles]

Nope. I say that the next step would be to measure the plate and cathode voltage of each stage of the amp. Especially the phase inverter. Post the results and that will give clues to support a diagnosis.

Also noticed another symptom, when I turn the gain down to about 10% I can hear a hard cutoff and a very soft pop.

V1
Plates
Pin 1 - 159v
Pin 6 - 160.6v
Cathodes
Pin 3 - 1.2v
Pin 8 - Edit: 1.2v

V2
Pin 1 - 218v
Pin 6 - 238v

Pin 3 - 50v
Pin 8 - 50v

v3
Pin 1 50 mv
Pin 2 fluxuates between 60-100mv
Pin 3 11v
pin 7 348v
Pin 9 345v
V4
Pin 1 18mv
Pin 2 Fluxuates between 60-100mv
Pin 3 11v
Pin 7 348v
Pin 9 345v

[Tom Phillips]

It appears that the second triode (Pins 6,7 & 8) of Vi is not working properly.
Please post the exact schematic you are referencing (the one that matches your version of the AC-15) so we can be sure that we all are discussing with respect to the same reference points. Reference the questions in post #8.

[Mr_bibbles]

Thanks Tom. Can you explain briefly what healthy triode voltages would look like for V1b and why?? Just Curious...

I'll start checking the components in that stage and let you know what I find.

Schem:
Vox AC15CC Schematics - Get 'em here! | Telecaster Guitar Forum

[Tom Phillips]

OK. So you have an AC15CC.
V1 Pin 8 is the cathode of the input stage triode. The voltage at that point is listed as 1.2V dc on the schematic and you are reading basically nothing. This indicates that the V1B triode is not conducting. You have plate voltage on pin 6 so the first things I would try are:
1) A different tube
2) Check that R3 is not open
3) Check that C6 is not shorted
4) With the power off and caps drained verify that you read ~1500 Ohms resistance between V1 Pin 8 and chassis ground.
5) Edit: Verify that both V1 heaters are on. I.e. that you can visually see two distinct heaters glowing inside the tube.
Report back and we will go from there.

[Mr_bibbles]

Looks like I bungled those measurements, I was recording for pin 9 instead of 8 (the words "Octal Socket" came into my head for some reason...). Pin 8 checks out at 1.2v

Just to be sure I performed the checks as you described, nothing is out of place. It sounds like a failing cap of some sort based on previous experiences, and when you hit a large note the sound will dissappear completely after fading for a second. Any other ideas where to start checking??