Vox AC15 TBX - burnt resistor cause?

[Enzo]

the earth connection for the OT sec and the speaker ONLY.

On THIS amp only, but on other amps the speakers are usually grounded right to chassis, and if that chassis is hot, plugging two amps into a speaker or plugging a signal cord into the speaker jack - trust me it happens all the time - can connect that hot chassis to earth through this resistor and cap.

[nevetslab]

While I don't have a large library folder of Vox AC15's and AV30's, I have three variations with regards to the output transformer low side connection: Grounded to circuit common, grounded to chassis/earth, not grounded at all, and grounded thru the RC network, that we're discussing. It looks like the RF network appeared in the late 90's or early 2000's, as I didn't find any in the early 90's or before.

This amp is in operation in Italy or nearby. Not sure if that factors in. And, as stated by Intripped in post # 11 "Yes, the burnt resistor is R303 on the schematic in post #1; W302 & W301 are hot and common of the OT secondary. W304 is the connection to chassis/earth. So this res + cap network is the earth connection for the OT sec and the speaker ONLY."

[Intripped]

mmm... probably I should have used underlined or bold instead of CAPSLOCK, because my intention was just to highlight those words and not to shout...
Anyway, I hope you take me well, I'm not here to be arrogant or whatever, I just wanted to clear the confusion around the specific amp I'm servicing.

So in this specific amp there are 2 RC networks:
One at the input jacks (with also the two diodes, as shown in the preamp schematic);
The other on the speaker and line-out jacks PCB (The burnt resistor was on this network).

Now,
I've eventually tried the amp and initially everything was ok, but while I was playing, the tremolo stopped working; the reverb also seems to be too weak, maybe V2 has problems, I don't know at the moment.
Tomorrow I will do some more testing and measures.

I'll keep you informed, thanks guys!

[Intripped]

still have that weak reverb issue, with a bit of hum with BLEND pot at max.
and the tremolo also is not 100% working:

- if i set the SPEED pot at max, the tremolo disappears; it only comes back if turn back a bit the SPEED pot, and flip the switch on the footswitch jack (i've already cleaned the contacts of the jack's switches and it hasn't solved the problem).
Flipping the jack's switch isn't always necessary: if i keep turning the DEPTH, BLEND and also BLEND pots, the tremolo sometimes comes back.
tried to change V2, but the problem is still the same.

i measured all resistors and re-checked all the solder joints - everything ok.

i also measured some voltages, you can see them in the attachments

[g1]

V3 is driving the reverb tank, have you tried changing it?
V2b is the trem circuit. If it is stalling, it could be a leaky cap problem. C9, 8, 15, 19, 18, could all cause this kind of issue.

[Intripped]

Hello, sorry for the delay, but in these days I have to deal with a bad flu.

Thanks G1, I solved the stalling tremolo issue: C19 was the culprit.

I've tried changing V3, but the reverb is still weak. I also asked the owner, and he told me that it has always been like this.
I don't know, maybe it's supposed to work this way...

[g1]

Is it the original reverb tank? What is the part number printed on it?

[Intripped]

I think that the reverb tank is original, it's an Accutronics made in the USA tank, but I haven't found the usual Accutronics code on it.
There are 2 codes punched in the metal:
- pat. 2.982.819
- 3.106.610

And an ink stamped code: 40 323 96

I have disassembled the tank and looked inside, It seems to be a type 4 (https://www.amplifiedparts.com/tech-...specifications)

I have also measured the DC resistance of the coils:
INPUT is 33 Ohm
OUTPUT is 204 Ohm
You can find a table with DC resistance here, just scroll down the page a bit.

So I think the code could be 4CBxxxx or 4DBxxxx

[Intripped]

The reverb tank should be 4DB2C1D (found it here)
I think that the tank I have is original, or correct at least.

I'm now triple-checking and testing all the components in the reverb path.