Pilot light on - everything else dead. Blackstar HT-60

[Steve Blackdog]

Thanks for all the suggestions. I have eventually managed to get things so the bulb only shines when you put in V3 EL34. I found a connector to the FX loop seemed to have been a culprit in causing the light to shine when I pulled in the +/-12v leads from the PT.

so now, I have no heat in the tubes at all - no sign of any glow at all. When you add V3 LBL lights up.

what should I check next?

thanks again folks

steve

ps getting 490 Vdc on the plates on both V3 & v4.

[nosaj]

Switch v3 and v4 and see if problem follows the tube. If not could be socket related.
nosaj

[Delta362]

Did you measure resistance of pins 2 to 7 of each power tube out of the amp? Then, what voltage do you have across pins 2 and 7 of each socket with the tubes removed? Got another set of EL34 tubes that you can swap in for testing?

One other possibility is within the plug itself. I just fixed an HT Stage 100 that had a broken pin on the HT connector. When I pulled the connector off the board, a small part of one of the pins was broken, hanging partially out of the connector and shorting across the heater pins. I'm guessing the previous tech who worked on this amp broke the crimp when attempting to get a more snug fit. I replaced the crimp on the wire and reinstalled it. Now it's working good.

[Steve Blackdog]

Switch v3 and v4 and see if problem follows the tube. If not could be socket related.
nosaj

Swapping the tubes makes no difference- it is whatever is in V3 that causes an issue. I will check the socket again for a short - something that happens only when tube is in.

[nosaj]

Check for resistance between pins with tube out. There should be none. And a socket replacement would be in order.
nosaj

[Helmholtz]

Check for resistance between pins with tube out. There should be none.

You surely meant resistance should be infinite, as no resistance is zero resistance aka a dead short.

[Jazz P Bass]

The schematic is kind of vague as to what SP14 & SP15 voltages are but I do believe that each EL34 tube heater uses a separate and distinct 12Vdc polarity.

[Helmholtz]

The EL34 heaters are wired in series. I'm sure SP14/SP15 connect to a 12.6VAC heater winding.

[nosaj]

You surely meant resistance should be infinite, as no resistance is zero resistance aka a dead short.

Yes you are correct. My mistake.
nosaj

[g1]

Is the V3 socket (pin5) getting the same bias voltage as the V4 socket?

[Steve Blackdog]

Check for resistance between pins with tube out. There should be none. And a socket replacement would be in order.
nosaj

i have done as suggested and all seems ok.

[Steve Blackdog]

Is the V3 socket (pin5) getting the same bias voltage as the V4 socket?

I’m not sure what I should expect, but both pin 5 reading similar - but fluctuating continuously, between around 50 and 70v. Should I expect a constant? (Valves out).

I should mention that the 12ax7’s are actually getting warm. I can detect a faint glow.

also, the o/d channel led isn’t coming on - that was a problem when the ht fuse blew. Any thoughts on this?

[Delta362]

The preamp tube heaters are on a separate circuit from the power tube heaters so I am not surprised they are getting warm.

Pin 5 voltage should be solid. What's the reading on TP13 for V_BIAS (lower left of sheet 3) before it heads over the power tube bias adjust and balance circuit?

How are your +/- 12V6 looking? 3V3? Do any opamps show any signs of physical cracks?

I'm still uncertain about the OD LED issue just yet. Do you have a general idea which part of the circuit board components had the wisp of smoke and pop?

[g1]

I should have mentioned that these Blackstar's usually require something plugged in to the input jack to engage the bias, so you should have something plugged in there when checking power tube pin5 voltage.

edit: bias voltage always present. Input jack only engages the cathode circuit of power tubes. So you can measure bias voltage at pin5 with nothing connected. To measure idle current via cathode resistor test points, there must be something plugged in at the input jack.

[nosaj]

bias procedure
nosaj
http://dealers.korgusa.com/svcfiles/...0Procedure.pdf