One Channel dead on HD3S

[tomh]

Hi,

I've just finished my World Designs HD3S headphone amplifier, took me about 8-10 hours in total I reckon. I was excited to hear music after turning it on, only to be dissapointed that it was only coming from channel. One channel (Left) seems to be working just fine, but the other (Right) is completely dead (no hum no nothing).

Measuring the test voltages theres is alittle variation between the two channels, but both are in the right ballpark. I'm not sure if the variations are within tolerances, or indicative of a problem:

Test Point Voltage Channel
A 198 (205)
B 195 (200)
C 186 (190)
D 0.892(0.9) Left
E 96.8 (102) Left
F 187 (192) Left
G 14 (13) Left
D 0.975(0.9) Right
E 89 (102) Right
F 187.8 (192) Right
G 13.7 (13) Right


Other than measuring the voltages, so far I've:

a) swapped the valves. Right channel stays dead, left stays working.
b) doubled checked all the components and connections. Everything seems correct
c) Looked for solder bridges/checked joints on both sides of the board. Can't see any.

Two things I've done that deviated from the letter of the instructions are:

1) The Left (ie working) channel dip switch is back to front, I believe I have correctly compensated for this by reserving the position of the switches (ie "on" as marked is "off" and vice versa). I don't think this will have any ill effect, but correct me if I am wrong. (I'd be a bugger on de-solder so I'd rather not if there are no ill effects)

2) For the heater links on the board. The instructions said to use "1/0.6 solid core wire", whereas the parts list indicated the "wire 14 0.0076 brown/grey" was heater wire, so I used that. Again unsure if this will have any long term ill effect?

If anyone has any suggestions for what I might have done wrong, or what I should check I would be really grateful. I had promosed my better half that she could have the dining room table back by the end of the weekend

thanks in advance
--
tom

[John T]

Hi Tom,
Without details of what the voltages should be and/or a circuit diagram, your voltages don't mean much. However, if as you say they are all within spec (on both channels), this should indicate the actual amp is working correctly and therefore the fault must lie somewhere in the input/output wiring to the PCB. It's best to go over this with a meter and check for continuity.

This should locate the problem.

[tomh]

Originally Posted by John T

Hi Tom,
Without details of what the voltages should be and/or a circuit diagram, your voltages don't mean much. However, if as you say they are all within spec (on both channels), this should indicate the actual amp is working correctly and therefore the fault must lie somewhere in the input/output wiring to the PCB. It's best to go over this with a meter and check for continuity.

This should locate the problem.

Ok, thanks for the advice. If it helps the values in brackets in my original post are the correct values (though I guess you'd need to know what was being measured too). I don't know if they are within spec, as a correct voltage is given but not a +/- acceptable deviation. They are "fairly" close though.

The figure with the largest deviation for the non-working channel is the "anode of triode section" where is the correct value is 102v, the working channel is 96.8v, and the non-working channel is 89v. Any idea if that difference is significant?

In the meantime I'll re-check the wiring in the output section.

cheers
--
tom

[JerryT]

Originally Posted by tomh

The figure with the largest deviation for the non-working channel is the "anode of triode section" where is the correct value is 102v, the working channel is 96.8v, and the non-working channel is 89v. Any idea if that difference is significant?


tom

I don't think the figures are likely to be significant. 10% (OK 13%) is quite typical in many valve circuits.

I'd look closely at coupling capacitors and at wiring from input to output. Sounds like a dry joint or possibly a couping cap that has gone open circuit.

Not serious but a pain to track down sometimes.

HTH

Jerry

[tomh]

Well, after spending 3-4 hours double checking all the conenctions, and resoldering any joint that didn't look perfect, and still no joy. So I put it all on one side and left it for a week or so.

Came back to it today, still not working. jumpered the output straight from the pcb to the headphones and it all works fine! Turns out that the 1/4inch to 3.5mm headphone adaptor that I was using was faultly and only passing one channel. Doh!

Sigh, why didn't I check the obvious things first

Right, I'm off to listen to some music. Not going to make any snap judgements this early, but straight off the bat theres definately some quite "right" about the sound the HD3S makes.

Thanks for the tips and advice.
--
tom