Silvertone 1484 with blown capacitor

[misterc57]

My next project is a 1484 I just bought with one 100uf 450 V blown capacitor. Originally these (4) caps were 100uf 150 volt caps, someone replaced them with 450 V caps.

I have a Sprage Atom 150uf 150V cap that I can use for replacement. Any issue with the voltage not matching the other 3 caps? I would assume this is ok but want to check.

I also wonder what could have caused this cap to blow. I found one bad tube, a 6CG7. All other tubes are good. The fuse is not blown. Anything else to check out before replacing and turning the amp on? I will bring up slow and monitor the current.

Thank you. MC

[Enzo]

Is it blown, or is it just that the plastic sheath has split open due top the heat in the amp?

[Justin Thomas]

Enzo,

I see a split in the rubber top piece, where the "vent hole" is - it's cracked, too, between the lead and vent hole... easiest to see in picture 2.

Justin

[g1]

Try it with your cap, but monitor the voltage on the new cap as you bring up the voltage. It's possible with modern grid voltage the 150V is being exceeded. That could be why someone put in higher voltage caps.

Justin: I think that may be a shadow, as it appears to extend below the cap?

[misterc57]

The cap does have a crack in it as Justin described. Here is a better pic with the cap out. Also this caps adjacent diode is shot.

I am curious as to what could have caused the failure. Did the diode cause the cap to fail, or the other way around, or something else.

[52 Bill]

I am curious as to what could have caused the failure. Did the diode cause the cap to fail, or the other way around, or something else.

I'm not sure which cap that one is, but some of those silvertones use voltage doubler circuits to get the high voltage for the power supply. If the wrong diode goes bad, it could place ac across the cap, which would cause it to heat up and pop like that.

[olddawg]

I'm not sure which cap that one is, but some of those silvertones use voltage doubler circuits to get the high voltage for the power supply. If the wrong diode goes bad, it could place ac across the cap, which would cause it to heat up and pop like that.

Pretty sure the 1484 has a doubler circuit for the B+. I have one untouched with all the original tubes waiting in my pile waiting to restore. Still has the original AC cord which is cracked and shorting. I would have fixed it long ago but the speaker cab has water damage.

[misterc57]

I'm not sure which cap that one is, but some of those silvertones use voltage doubler circuits to get the high voltage for the power supply. If the wrong diode goes bad, it could place ac across the cap, which would cause it to heat up and pop like that.

That cap is one of C26, C27, C28, C29. I am also going to replace all 4 diodes (D1, D2, D3, D4), I guess these are the rectifier diodes.

[52 Bill]

That cap is one of C26, C27, C28, C29. I am also going to replace all 4 diodes (D1, D2, D3, D4), I guess these are the rectifier diodes.

Yes, they are the high voltage rectifier diodes.

And if you look at the schematic that you posted, if any of the four dioeds gets leaky or shorts there is ac from the transformer secondary placed directly across a filter cap.

[misterc57]

I replaced the one bad cap with a 100uf 150V cap I had. Also replaced the 4 rectifier diodes with 1N4007. Amp is working now! I measure about 106 VDC on the plus side of the new cap.

I noticed that when the amp is on stand-by, I can hear a bit of low volume crackly guitar with the guitar volume at 10. Is this worth investigating?

[olddawg]

I replaced the one bad cap with a 100uf 150V cap I had. Also replaced the 4 rectifier diodes with 1N4007. Amp is working now! I measure about 106 VDC on the plus side of the new cap.

I noticed that when the amp is on stand-by, I can hear a bit of low volume crackly guitar with the guitar volume at 10. Is this worth investigating?

The standby circuit works differently on these amps. It doesn't simply open the B+ as I remember.

[Justin Thomas]

Looks to me like it shorts the PI outputs together? Kinda like a crossline MV, right? So if the PI signals aren't perfectly balanced, something squeaks through? Just asking...

Justin