dsl 401 no sound

[T-bone]

hi all, i have a problem with a dsl 401 not producing any sound,, very faint when turned up full,,,,replaced main cap as the previous one was beginning to bubble, played it for a couple of hrs, turned it on and nothing,,,br1 mesures to be ok, main cap is fine,, could someone give me an idea as to taking some plate and screen mesurements to compare,,, pins, etc,,, and also a quick metre check on ot's,,, ive looked on the net with these amps, they seem to have a prob overheating dont they, any help would be really appreciated,,,,

[Enzo]

Very faint sound is not the same thing as no sound. At least you know the speaker works, right?

Plug a cord into the FX send and run it to another amp. Is that signal OK? And plug a signal into the FX return, does sound come out the speaker now?

Are the heaters running in ALL the tubes?

[T-bone]

response enzo

sorry enzo, i should have stated that there was very small output signal running the amp @ full. thank you for your reply, a sig from the fx send to an additional amp sounded fine,, but there was no sound injecting a signal to fx in, the heaters in all power tubes read 380v,, ive taken br1 out and check seemed fine,,, reflowed he board, and checked all componants from standby switch all the way down ht 1, including zeners,,,,,,,,,,, any thoughts mate?

cheers

[Enzo]

If the FX send works, then the preamp tubes are running.

Not sure what you measured 380v on, the heaters are those little orange glowing things in each tube, and they run on 6v.

SO the power amp section is the problem.

First, try flipping the speaker impedance switch back and forth. Might as well check the solder to it as well. ANy help? ALso, just try plugging a different speaker into the amp.

Turn the amp on, set the controls midway, turn the reverb up halfway too. Now rock the whole amp some to crash the reverb springs. Does that come out the speaker loud? or is it also super quiet?

V1 and V2 seem to work. Have you substituted a differnt 12AX7 in place of V3 or V4?

I would check voltage on pins 1 and 6 of those two tubes, and 3 and 8.

Pin 2 of each of those is the "input" to the stage. Set your meter down and leave the black probe on the table, not grounded. Now withthe amp running, touch pin 2 of V3 and then V4 with the red probe. The meter probe wires will act like an antenna and inject some hum signal into the amp. If it comes out the sspeaker, then the stage may be working.

[T-bone]

reply with thanks

hi enzo, once again mate thanks for taking the time with me and my project,,

these are the volts, from from power tubes
pins 7 & 9,,, 380v
fil pins 4 & 5 -30ma
pin 2 -14v
and pin 3 1v

the volts from the preamp tubes ae as follows:

all pre amp valves are running from 204-215v on pin1 ecxept v4,,,,
its running350v

pin 6 reads 198-205v on all valves exept v 3,, its running 349v
pins 4&5 are reading 2.9v on all preamp valves
pins 2&7 on valves 1,2,& 3 are all reading -25v exept pin 7 on three,,, its reading 216v

and heres something ,, all pins 3&8 on pre valves 1.3.&3, are reading 1.5v exept valve 3's pin 8 reads 220v

valve 4: pin3 34v pin 8 34v

and pin 2 on good old valve 4 reads 5.8v and pin 7 reads 20v

i have replaced all valves with known working valves, tried a new speaker,, tried rocking the amp for reverb crash,, still very faint,, tried the impedance switch,, and no luck with the antannea trick enzo,

these mesurements seem way too inconsistant ,, do you think i should check for faulty componants on HT line perhaps?

[Enzo]

One side of V3 is a cathode follower, so the plate is connected direct to B+, pin 6, so the high voltage is normal there. That means the cathode, pin 8, will also chow B+ type voltages. V3 is normal.

You could have a bad master volume control. Turn the master up. Now use your meter probe or a small screwdriver and touch pin of any power tube - its grid. it won;t be loud, but it should introduce some hum.

[T-bone]

response with thanks 2

cheers enzo,
you are dead set right mate,

"One side of V3 is a cathode follower, so the plate is connected direct to B+, pin 6, so the high voltage is normal there. That means the cathode, pin 8, will also chow B+ type voltages."
Bloody blind i se it now thank you,,

placing probe on pin 2 grid induces hum through unit!! having said that,,,, checked all following 10k resisistors leading to master vol, they all seem ok!

am i reading this scematic right,, the master pot seems to be a duel/or split pot,, taking responsability for bias as well?

any thoughts
warm regards

[Enzo]

The bias voltage is fed THROUGH the master volume pot, but the pot doees NOT adjust the bias in any way.

If touching the grids of the power tubes makes hum, then at least you are with me that far. Not loud, but you can hear it. So move farther back. Can you touch the end of the 10k resistors away from the tube and still hear the hum? Then turn the master all the way up and touch the top ends of the pot. Easy place for that is the end of R115, R116. And being careful, can you inject hum at the plaates of the PI tube, pins 1 and 6 of V4?

And if that works we are left wondering why you can't inject a signal at the grid of V4.

[T-bone]

cheers and thank you again

hi enzo,,, thank you for your time,, and very educational comments,,i touched the grids of the power tubes and the 10k resistors and made a small hum? Then i turned the master up and then touched the end of R115, R116.....we have hum as well.....

than i injected hum at the plates of the PI tube, pins 1 and 6 ............................................
a slight tick, and an almost inaudable hum..

be buggard if i would have thought to inject a signal at the grid of V4...............
unfortunatly, same result mate,, 1khtz injection at grid 4 produced very small signal output..
the bias voltage reads 1 volt at con 5,,

mmmmm is it back to the ot now i wonder??
thanks heaps for your time,, this a very educational experiance

[T-bone]

ot maybe?

are there any general tests for ot's, ive several dmm, function gens,,,10mhz oscilloscope,, maybe you can give me an idea where to start?


should i inject a sig from the speaker end??

cheers mate,, look forward to your reply,, as im sure a lot do......

[Enzo]

But pin 2 of V4 makes no hum when touched? (And try pin7, the other grid)

Then it sounds like V4 isn;t working.

SImple substitution is my test for the OT, but since touching the power tube grids results in sound from the speakers, I think your OT is working.

[T-bone]

response with thanks

gday enzo,,

have tried pin 1 & 6 of v4, slight buzz issues from speaker,, voltage readings fom those pins are 178v from pin 6 & 336v from pin 1

pins 2 And pin7 of V4 makes no hum when touched
readngs from these pins are as follows enzo,,

pin 2,,, 5.8v and pin 7, 16volts

sooo,,, is it the signal path we need to look at?

cheers mate look forward to your reply''

[T-bone]

od mute

can you tell me what od mute on pin 7 of v1 is all about
has it got somthing to do with chanell 2,,, footswitch maybe?
thank you in advance

[oc disorder]

On page 2 of the schematic ref 3 C (left hand side) is T 11 a J174 fet which is an electronic switch in this instance that shorts to ground the grid (pin 7 V1) when the OD channel is not being used.
Yes it is activated by the foot switch or front panel switch.

[T-bone]

cheers

hi ,, thank you for replying,,, as your prob aware, my skills are all about asking questions at the moment,, i see the jfet your refering to,,, and it clearly indicates ODMUTE,,, cool thank you i really should look more closely,


any thoughts on why limited sound on output of this amp,,

im kinda stuck on what else to troubleshoot,,, great fun, but somtimes its frustraiting to keep backtracking checking componants voltages,,etc,, to keep everything in your head! i really dont want to buy a ot to see if this one is faulty,,, seems everything i do,, the same prob keeps popping up,,,, everywhere i inject a sig,,, the same vol keeps coming out of the speaker...

love talking to you guys,,,
awsome fun !! warm regards