No sound after changing resistor and rect diodes (rookie alert)

[fkitch]

Rookie, it’s my first (repair) project, apologies for missing the obvious, something I am totally sure I’m doing.

 It’s a Italian Sound Big 25 RT amp from the 70s you guys already helped me with

Ok, the 270k resistor in the pic burnt, not sure why honestly, while I was messing around with a different tank  trying to figure out why the reverb is not working (trem works ok BTW).

One of the diodes also was damaged (melted) by the resistor itself.
I changed both diodes to 1N5408 and put a new 270k resistor like the one in the pic (metal oxide)

Result, 18V after the 270k resistor (I read 330v from the diodes) and no sound.

Any tips on next diagnose / potential fix would be appreciated!
Original pdf schematic:

https://www.freestompboxes.org/viewtopic.php?t=30169



Thank you!!
Fernando

[sluckey]

That resistor can't possibly be 270K. Must be an error on the schematic. 270Ω or 1KΩ is more likely. What resistance does your burnt resistor read?

[fkitch]

Thanks Sluckey
It was so calcinated I just threw it away
But I do have a good picture of it, I attach it here

[sluckey]

I suggest using a 270Ω 10W resistor.

[fkitch]

I suggest using a 270Ω 10W resistor.

Ok, finally found some time to go on
I changed the resistor to a 270 ohm 10W and I have sound again!! Thanks Sluckey!

Bad news, now there’s a fast ticking noise (heard through the speakers) with random static noise too), even without a guitar plugged in.
It takes a few seconds for the ticking to appear after switching on the amp and its volume and frequency are not affected by any of the amp controls.
It just disappears when I remove both v1 and v2 (clean and reverb channel 1st tubes)

After the resistor burnt I had to change the rect. diodes as well and had to be ‘creative’ in terms of wiring positioning (one diode crosses above the other) , not sure if that’s the issue but moving them slightly did not make a difference

Chopsticking didn’t show any potential faulty components or wiring
The new resistor is of those white cemented type large ones, vertical leads, not axial

[sluckey]

There are 11 triodes being supplied by one B+ filter cap. I'm surprised it doesn't play "The Star Spangled Banner" all by itself.   

[fkitch]

There are 11 triodes being supplied by one B+ filter cap. I'm surprised it doesn't play "The Star Spangled Banner" all by itself.   

Give me time, I’ll make it play it, Hendrix style, and of course end up in flames to end the show
Ok , I just realised that, thanks to my creative repositioning of things, some components now cross over the trem intensity pot wires…
I’ll start there

[Williamblake]

Just out of curiosity, what type of reverb tank is in that amp? What resistance do you measure across the tanks input? (There maybe high voltage present, you would have to drain the filter caps / B+ caps / HT caps to do that).

[Esquirefreak]

I serviced an old FBT amp a couple of years ago. They were made in Italy, but clearly inspired by Marshall amps. All Mullard, ISKRA and Philips components. Looking a lot like the trem channel of a Marshall 1974

On the schematic, however, all cap values was marked as "K" rather than Farad/F. Like 22k rather than 22nF.

I suspect that some details were lost in translation when "borrowing" circuits.

/Max

[PRR]

> Like 22k rather than 22nF.

Some days, "all caps are in pFd".

22 K pFd is 22nF.

[fkitch]

There are 11 triodes being supplied by one B+ filter cap. I'm surprised it doesn't play "The Star Spangled Banner" all by itself.   

Ticking solved! It was a bad solder work on one of the new diodes.
 About the filter caps, there’s another 32uf one but it’s not in the schematic…

Just out of curiosity, what type of reverb tank is in that amp? What resistance do you measure across the tanks input? (There maybe high voltage present, you would have to drain the filter caps / B+ caps / HT caps to do that).

Thanks for the HV reminder! back to the reverb not working:
When activated by the footswitch if I turn the reverb level up all it does is add white-ish noise, the higher the reverb level the higher the noise.
Already tried different tubes, and already changed the electrolytic caps , no change in behaviour. Also reflowed solder joints
The tank itself (picture attached)
Small, around 18x8cm. No brand just says ‘PAT. NO. 2982819’
Resistance at the side that has a resistor:9.96 kOhm
Resistance at the side without resistor: 290 Ohm
Resistance across both positive terminals: 10.24 kOhm

I serviced an old FBT amp a couple of years ago. They were made in Italy, but clearly inspired by Marshall amps. All Mullard, ISKRA and Philips components. Looking a lot like the trem channel of a Marshall 1974

On the schematic, however, all cap values was marked as "K" rather than Farad/F. Like 22k rather than 22nF.

I suspect that some details were lost in translation when "borrowing" circuits.

/Max

I thought it was based on a Fender Super Reverb , but no idea honestly. Changing the clean channel to be more Marshall sounding could be an option for a next phase!

> Like 22k rather than 22nF.

Some days, "all caps are in pFd".

22 K pFd is 22nF.

Correct!

Thanks all for your inputs!