#124 started

[norburybrook]

once again short of time as I'm away first thing in the morning...............

so.

No sound at all when I plugged in.

B+ voltages are

1; 422vDc
2; 420
3; 413
4; 361
5; 354

I'm getting 349vDC on V1 plates and 354 on v2 plates


what's going on? I'm stumped.....


Marcus

[martin manning]

Shooting from the hip in the dark, missing a ground?

[norburybrook]

Ok ,

I've got high voltages on both V1 and V2 plates and zero voltage on the cathodes.

Also I have 230v on one side of the phase inverter and 410 on the other. I've checked both resistors and they're 110k and 120k respectively. the high voltage is coming from the 120k


where would the missing earth be?

I've checked continuity and everything is connected properly. The cathodes bypass caps and resistors all check out with the ground.



Marcus

[martin manning]

0V on V1 and V2 cathodes with high voltage on the plates probably means that there is an open circuit between the cathode and ground. What is the resistance to ground from the cathode pins? Anything like Rk? The grids should be at 0V. The PI problem is something similar. What is voltage on pins 3 and 8? Should be the same... is it?

[norburybrook]

Martin,

as I said there's zero voltage showing on both V1 and V2 cathodes. Pins 3 and 8 showing 0v


I'm away from the studio now until Tuesday.


It's my Birthday on the 9th so I'm going away this weekend to celebrate.


I'm also picking up a new guitar, a present from my partner. Its a OOO size handmade acoustic with spruce top and rosewood back and sides, made by a Luthier in the south on England


I'll resurrect this thread on Tuesday.

I hate it when after all the time and effort things don't work!!!

I'm sure I'll learn another valuable lesson though


Marcus

[martin manning]

Have a happy birthday, and enjoy the new guitar! The problem has to be something pretty basic... it'll get sorted quickly I'm sure.

[norburybrook]

thanks Martin, I hope so....never had a build that had ZERO sound and bad voltages from the get go!!!!!!!!!!!!!


I'm picking up our Lurcher Willow tomorrow as well , she's been with my in laws for the last week.


It's strange how not having the dog at home feels so weird!!!



M

[larsvictor]

Happy birthday, Marcus!

Had a similar thing with my build. It turnded out to be a bad connection between the tube pins and the tube socket plugs. Had to replace the sockets.

Can you see the heater filaments in V1 glowing ? Could be a simple problem with the heater wiring on V1. Same with one half of the PI. No heating - no current.

Where do you measure the voltages ? Directly on the sockets or on the mainboard ? If you measure at the legs of the resistors (Rp and Rk) you´ll have to check the continuity to the socketpins.

Lars

[larsvictor]

sorry, douple post.

[JazzGuitarGimp]

Happy birthday, Marcus!

Had a similar thing with my build. It turnded out to be a bad connection between the tube pins and the tube socket plugs. Had to replace the sockets.

Can you see the heater filaments in V1 glowing ? Could be a simple problem with the heater wiring on V1. Same with one half of the PI. No heating - no current.

Where do you measure the voltages ? Directly on the sockets or on the mainboard ? If you measure at the legs of the resistors (Rp and Rk) you´ll have to check the continuity to the socketpins.

Lars

Also, keep in mind a 12AX7 has two heaters; one for each triode. A casual glance might reveal a glowing filiment but a closer inspection might reveal only one filiment is lit. And, HAPPY BIRTHDAY!!!

[norburybrook]

morning all.

Thanks for the birthday wished, it's actually on Monday 9th but I'm away for the weekend.


I've been taking the voltage measurements from the valve pins.

I have good continuity between the board and the plates and cathodes of all the pre amp valves.

there seems to be heater action on the pre amp valves , however I did initially think they didn't seem to be glowing as bright as I remembered (it's been a while since my last ODS build)

I did a continuity check with the heater wiring all the way along and that's fine.


OK, I'll be back on the bench on Tuesday.........another year older!!!! fortunately not deeper in debt


Marcus

[rogb]

Happy birthday buddy!

It sounds like a classic "ground missed out somewhere". Check those cathode grounds real careful.Been there, done that. Not forgetting to check the FET load substitution is correct.

Go right through the whole amp from input to spkr out in sections, with a printed layout, ticking off each wire and connection and component value, reflow any suspect solder joints, you will find it!

[martin manning]

I've been taking the voltage measurements from the valve pins.

Good, but keep in mind that if the socket-pin contact is bad you won't catch that unless you test at the actual pin wire (difficult to do with noval tubes).

I have good continuity between the board and the plates and cathodes of all the pre amp valves.

Good.

there seems to be heater action on the pre amp valves , however I did initially think they didn't seem to be glowing as bright as I remembered (it's been a while since my last ODS build)

The brightness depends on how much of the heater wire is exposed above the top of the cathode tube, which can vary from one valve to the next.

I did a continuity check with the heater wiring all the way along and that's fine.

Since the heaters are in a "ladder" arrangement and the heater and winding resistances are very low this must be done with all the valves removed.

0V on the cathodes of V1 and V2 makes me suspect the connection from the common ground on the board to the input jack ground lug. That is why I suggested measuring the resistance to ground from the cathodes, which should be ~Rk.

[Sven]

Due to the fact that there are various schemes and layouts for an original pre-skyline D-124 model (as the one being presented here), I wish to clarify issues with potentiometers, to make sure they are proper values (Ohms), proper tapers (logarithmic a.k.a. audio or linear) and very importantly, which pots and which pins on pots are to be connected to the rest of the circuit by coaxial cables? Kindly assist if you wish. Thanks. Sven

POTENTIOMETERS, VALUES, TAPER AND COAX. CABLES

Volume - 1M logarithmic (aka audio) - one coax. cable on wiper (pin 2)

Treble - 250k linear - no coax. cables?

Middle - 100k logarithmic - no coax. cables

Bass - 250k logarithmic - is there coax. cable on wiper (2)?

Level - 250k logarithmic - are there two coax. cables (input - pin 1 and wiper -pin 2)?

Ratio - 100k linear - are there two coax. cables (input - pin 1 and wiper - pin 2)?

Master - 1M logarithmic - are there coax. cables on Master (wiper - pin 2?)?

Presence – 2k linear – no coax. Cables (?)

OD trim - 100k linear (or logarithmic) - no coax. cables (also, is it better to use 100k or 345k pot?)

Kindly correct and answer where applicable. Thanks.

[erwin_ve]

Sven: the pre 1984 schematic answered all your questions.
Pots: when ending with a "A" it's audio taper. When ending with a "L" it's linear taper.
In the schematic clearly shows which pots have shielded wires and which not. (Cirkles round the wire.)
If in doubt you can check: pics http://thesubjectmatter.com/dumblearchive.html