[ticket]

Hi All.  I eventually completed my Cornet 2 a couple of weeks ago after a very slow build – I actually built most of it over a year ago, but only recently completed the chassis.  Just to give you a little background: I’m no expert, but this is my fifth amplifier build and all the previous ones worked perfectly first time, including the Bugle/power supply I built 4-5 years ago.

I tentatively powered up the Cornet 2 using my variac – no pops or bangs, but also no red/green light and initially seemingly no lit-up vacuum tubes – I then noticed that the rectifier tube appeared to be working, but not the other 3.  I switched off and started checking things and reading posts on here to determine what might be wrong. 

A week later I powered up again – same result, but this time I checked all the voltages on the circuit board – all very pretty much smack on.  Rectifier tube lit, but apparently nothing from the other 3.  I did notice that the heater voltage at R223 appeared a little low at 5.98.  Sorry, this is turning into a long post!

Another week of reading….. then yesterday I powered up – same result, though heater voltage a little lower at 5.71 (might be accounted for by the level at which I set my variac).  I then checked voltages at centre pin (?) of each (non-working) tube – 5.96 and then noticed that the tubes were warm!  OK I thought, I’m going to put it in my system. I set the volume very low and there was a bump and then …. music!  The bump was the stylus hitting the record! 

So, I have music and pretty good too.  I’m wondering if it’s a little thin, having read various comments on heater voltages, but it sounds pretty good to me.  I’m certainly hearing things I’ve never heard before even with my aged and deteriorating hearing! 

But I still have no LED!  Any thoughts?  If I’d put it in the wrong way round, would it simply light up green/red rather than red/green? I did do a continuity test suggested by Poty on the LED and it seemed to work.  However, I’m wondering whether to replace all the parts in the ‘LED circuit’ and, though I’m a little reluctant to take everything apart, I would like to get it working. 

Any help is much appreciated.  Sorry for the length of this post.

[poty]

If you have sound from the device - basic voltages are OK. Reversing LED is out of scope as you have already mentioned. Cold joints are possible, but it seems (taking into consideration that all other joints are working first time) not the source of the problem. You maid have soldered Q100, Q101 incorrectly (I mean wrong base-collector-emitter placement)? Could you check this?

[ticket]

Hi Poty - thanks for your response. 

I've checked positioning of Q100 and Q101 - seems fine.  I've also resoldered.  Still no working LED. 

Is it possible to check the transistors or do another check on the LED?

Thanks

John

[poty]

Yes, it is possible to check the transistors. Before that - could you post the photo of the LED's area including the transistors?
If you have multimeter - could you measure the voltages across R106, R108, R104 and from both anodes of LED (outer pins) to the ground (inner pin or grounding post)?

[ticket]

Hi Poty

Measurements are as follows - 'outside' first:

R104 - 7.29v/3.76v
R106 - 0.00v/3.08v
R108 - 0.00v/7.25v

LED outside pins are both 0.00v.

I'll try to post a picture in a minute.

John

[ticket]

[poty]

John,

Measurements are as follows - 'outside' first:

R104 - 7.29v/3.76v
R106 - 0.00v/3.08v
R108 - 0.00v/7.25v

LED outside pins are both 0.00v.

Could you say how did you measure the voltages?

[ticket]

I'm not certain I fully understand you!  Red lead of multimeter to resistor, black to ground on chassis.  I measure the outside lead of the resistor first, then the inside.  Is that what you wanted?

[poty]

OK! All voltages seem OK for me.
I can think of bad LED or shorted pins (by solder bridge or something). Could you switch off the Cornet 2, wait a minute and then in Ohmmeter mode check the resistance from the outer pins (there are 2 outer pins!) of the LED to the ground. Try to check both ways (I mean one pair of measurement - black (minus) probe to the ground, red - to the LED pins; second pair of measurement - red (plus) probe to the ground, black - to the LED pins).

[ticket]

Hi Poty

Thanks again!  Checking the outer pins with the red lead to the LED and black to ground I get a reading of 0.2ohm on both.  With the leads reversed I get 0.3 on both.

John

[poty]

Well... It means that you have either bad LED or shorted pins. Check if the solder joints are not touching each other. If there is no obvious signs - you'll have to unsolder the LED and try it out of the circuit.

[ticket]

Great, thank you.  Probably a job for tomorrow, though I don't have any spare LEDs (ordered some earlier - 25 in fact!).  At least I can check the soldering.  Out of interest, do you know what the reading should be?

John

[poty]

It depends on many things, including the type of your multimeter. When the minus probe is at the center there should be around 500Ohm - 1kOhm. Reverse polarity should be in the range of >100k.
If your multimeter has a check for diodes you can test the LED in this mode. It can be even lit with the right polarity.

[ticket]

Thanks. Yes my multimeter can test diodes, so I'll look at this.

[ticket]

Solder bridge on the underside of the board!  Now fixed, lots of lessons learned - thanks Poty

[poty]

Not at all! Congratulations!

[ticket]

Cheers!