18 Watt Tmb frying 2.7 k 5 watt dropping resistor

[strongjay912]

I built this 18 watt tmb from a mojotone kit and this is my second attempt after the first one, it did the same thing I scrapped it, tore it down and ordered new filter caps and rectifier tube, rewired it all but it's the same problem. When I hit the standby switch to on position the 2.7 k 5 watt resistor on the first filter cap smokes and fries itself before I can take any voltage readings. Any help is appreciated.

[tubeswell]

I built this 18 watt tmb from a mojotone kit and this is my second attempt after the first one, it did the same thing I scrapped it, tore it down and ordered new filter caps and rectifier tube, rewired it all but it's the same problem. When I hit the standby switch to on position the 2.7 k 5 watt resistor on the first filter cap smokes and fries itself before I can take any voltage readings. Any help is appreciated.

There is a short to ground after the 2k7 supply resistor (causing the full HT voltage at that point to be dumped across the resistor). Could either be the next filter cap is shorted, or could be a lurking solder blob or untrimmed lead behind the board touching the chassis, or something like that.

[strongjay912]

Understood I will check for shorts to ground some things I have done to no avail is changed the 6bq5s several different ones as well as the 12ax7s,tried an alternate PT with 290 VAC because the one supplied to me is putting out 320 Vac, New filter caps mostly because I thought the first build this happened to hurt the first pair terminally. Is it possible to have a bad output transformer doing this. Thanks for the help

[tubeswell]

Is it possible to have a bad output transformer doing this. Thanks for the help

The HT voltage for the OT primary winding is (normally) sourced from the reservoir cap, so if the OT primary was shorted to ground, we'd expect to see the amp mains fuse blowing (instead of the 2k7 frying)


(Edit: I'm assuming the OT is positioned to be sourced from the reservoir cap. If you know otherwise, please upload a schematic of your build, so we can all save ourselves from unnecessary guesswork)

[uki]

That exposed shielded cable going to under the board is suspicious.

[strongjay912]

Layout

[strongjay912]

Should pins 4 and 5 on Power tube where green wire from PT comes in for filaments voltage be grounded, I might have found the problem if that's the case. The shielded wires lead connected to the 68k resistor under the grid board is grounded only after the resistor is this a no no. The other shielded wire to pin 2 of V1 is not grounded at the pin only when you probe after the 68 k resistor connected to the lo input leg. Thanks again

[strongjay912]

Schematic

[shooter]

NO POWER wait 15 min
pull ALL tubes
OHM each PS tap to ground


POST results


(assumes new 2.7k before measuring)

[strongjay912]

Green 0.0 Green 0.0 Green Yellow CT 0.0
Red 85.2 Red 81.6
Yellow OL White OL
Yellow White OL
Black OL Black White OL
Not sure but continuity exists when green yellow is center tapped to ground so the filament heaters get grounded at pins 4 and 5 of Tubes? But no other continuity exists from any other center taps when checked but only when all leads are disconnected is there continuity. Is there a short inside my xfmr it's a Heyboer or does this Continuity always exist when the green yellow lead is center tapped heck there's continuity on all other leads but when there grounded there is none except for the green wires there is.

[shooter]

Read last post;

OHM each PS tap to ground

Hint:   there are 4, there are no transformers at those nodes

[strongjay912]

I think your saying to replace the smoked out 2.7 k (R) by the way the build calls for a 2 watt but I used a 5 watt and that didn't help, then check for (R) at HV center tap, 6.3 center tap, 6.3 Y/W CT at rectifier and the 4th tap I'm not sure where you mean but is it at the south side of the 150 ohm Cathode resistor. Sorry but I'm at kindergarten level at this and Thanks again any help is appreciated.

[tubeswell]

'Ohm each PS tap to ground' = use your R-meter to measure the resistance to ground at each PS (filter cap) node