Blackstar HT60 arcing after filter caps

[Rybow]

666, that would be the OP's number of posts for those not up to cryptic analysis at this juncture in time. 

Certainly someone named after one of the more well behaved Apostles would not be easily corrupted into Satanic beliefs. 

8

Haha! Also just to clarify, my response was an alternative way of saying “thank you”. I had Iron Maiden running through my head so it seemed fitting.

I do have a question about this resistor. Does it have to be flame proof? I did track down an exact replacement, but it was silly $$$ ($15 ea, 10 minimum). The one I ordered is wirewound and had a way better temp coefficient, but it is not flame proof.

 

[abbey road d enfer]

I do have a question about this resistor. Does it have to be flame proof? I did track down an exact replacement, but it was silly $$$ ($15 ea, 10 minimum). The one I ordered is wirewound and had a way better temp coefficient, but it is not flame proof.

Make sure you mount the resistor at a distance (1/2") from the PCB

 

[Rybow]

Make sure you mount the resistor at a distance (1/2") from the PCB

Thanks. Will do.

An attempt to repair this amp has definitely been made. R203 and C142 look to have been replaced. C143 and one leg of D26 have been desoldered for sure.

I’m not a fan of being the second guy in when I don’t know what guy one was up to.
I think I’ll be checking continuity in this area as well.

Edit: C142 positive leg no continuity. Everything else appears ok. Ugh

 

[abbey road d enfer]

An attempt to repair this amp has definitely been made. R203 and C142 look to have been replaced. C143 and one leg of D26 have been desoldered for sure. Looks like this resistor has already been replaced once, and that it has overheated the PCB.

Edit: C142 positive leg no continuity. Everything else appears ok. Ugh

What do you mean, "no continuity"? Continuity to ground? There shouldn't be, you should measure a capacitor discharge that resumes to about 200kohms in one direction, in the other direction you would measure the same capacitor discharge resuming to a diode conductivity (which does not return a usable value, because it depends on the meter).

 

[Rybow]

What do you mean, "no continuity"? Continuity to ground? There shouldn't be, you should measure a capacitor discharge that resumes to about 200kohms in one direction, in the other direction you would measure the same capacitor discharge resuming to a diode conductivity (which does not return a usable value, because it depends on the meter).

My apologies. I should have been more specific. I tested continuity between connected solder joints along the PCB trace. I tested all of the reworked joints in this manner, and that one failed in both directions. I fear the pad was lifted when the original capacitor was removed. I’ll try to reflow the joint first, but I might be looking at soldering to the trace or running a wire connection.

 

[abbey road d enfer]

My apologies. I should have been more specific. I tested continuity between connected solder joints along the PCB trace. I tested all of the reworked joints in this manner, and that one failed in both directions. I fear the pad was lifted when the original capacitor was removed. I’ll try to reflow the joint first, but I might be looking at soldering to the trace or running a wire connection.

OK. That happens often. You've been wise checking that.

 

[Rybow]

OK. That happens often. You've been wise checking that.

Wisdom from too many headaches over the years

 

[Rybow]

Got the new resistor in, but it went up in smoke the moment I turned on the power, so I’m guessing there is a short down the line.

I’ll pick up some more tomorrow. Tubes were in when I turned it on. 

My spare room doesn’t smell too good right now

 

[abbey road d enfer]

Got the new resistor in, but it went up in smoke the moment I turned on the power, so I’m guessing there is a short down the line.

It may not be a dead short. Many things can cause a large increase in current, particularly if something goes wrong with the solid-state "glue" that surrounds the EL34's.

  I’ll pick up some more tomorrow. Tubes were in when I turned it on. 

  Indeed, taking tubes off, particularly the power tubes, is part of troubleshooting 101. From the voltage list you posted earlier, it looks like there should be no issue with the power tubes, though.
If you don't detect a dead short, I would look for suspicious electrolytics first.

Do you have a surge limiter? You can make one easily, takes about 5 minutes to build and you won't regret it.
Install a bulb of about half the rated power draw of the unit it protects. I the bulb fully lits, there is something wrong. When the issue is fixed, you may progressively increase the bulb power.

 

[Rybow]

It may not be a dead short. Many things can cause a large increase in current, particularly if something goes wrong with the solid-state "glue" that surrounds the EL34's.
  Indeed, taking tubes off, particularly the power tubes, is part of troubleshooting 101. From the voltage list you posted earlier, it looks like there should be no issue with the power tubes, though.
If you don't detect a dead short, I would look for suspicious electrolytics first.

Do you have a surge limiter? You can make one easily, takes about 5 minutes to build and you won't regret it.
Install a bulb of about half the rated power draw of the unit it protects. I the bulb fully lits, there is something wrong. When the issue is fixed, you may progressively increase the bulb power.

I know I’ve seen the surge limiter mentioned a lot, but have never made one. Just got back from the hardware store.

I’ll pull the tubes and see what we get this time around with the surge limiter in play. Luckily I can get that resistor locally

 

[mjrippe]

Luckily I can get that resistor locally

Resistors are so cheap, buy ten this time 

 

[Rybow]

Resistors are so cheap, buy ten this time 

Yeah I’ve been kicking myself for not getting 10 on my digikey order. Locally I pay $2.20 a piece for these.

Been on hiatus from DIY for too long. My procurement skills are a little rusty

 

[Rybow]

New resistor in and  did the test with my new fancy surge limiter in place. Cost me a whopping $3.00 in parts. 25W bulb lights up to full brightness instantly. No tubes are installed.

Next I’ll pull the connectors leading to the power tubes to try and isolate the problem. Just letting those filter caps drain

 

[abbey road d enfer]

New resistor in and  did the test with my new fancy surge limiter in place. Cost me a whopping $3.00 in parts. 25W bulb lights up to full brightness instantly. No tubes are installed.

Next I’ll pull the connectors leading to the power tubes to try and isolate the problem. Just letting those filter caps drain

25W is not enough. the rated power draw of the amp is probably about 150W. Try a 75W bulb. Make sure you use an incandescent or halogen bulb, not an LED or CFL.

 

[Rybow]

25W is not enough. the rated power draw of the amp is probably about 150W. Try a 75W bulb. Make sure you use an incandescent or halogen bulb, not an LED or CFL.

Right. Half the power draw. Not half the output power. Come on Ryan. Get it together.

That would be 180 watts, so I need to find a 90 watt bulbs. Is it safe to leave this on for awhile to check voltages? It would be nice to check the pins on the tubes where I was missing voltage, and/or trace the path to find where it stops or shows a major variance

 

[abbey road d enfer]

Right. Half the power draw. Not half the output power. Come on Ryan. Get it together.

That would be 180 watts, so I need to find a 90 watt bulbs. Is it safe to leave this on for awhile to check voltages? It would be nice to check the pins on the tubes where I was missing voltage, and/or trace the path to find where it stops or shows a major variance

Buy several bulbs. Increase rating progressively.
You have to watch out for signs of undue heating, noises, smoke, as usual, but you can leave it ON for several minutes .

 

[Rybow]

Went to 3 stores. Found 100w & 150w but that’s it. Incandescent bulbs are thin on the ground here

100w- pilot light flashed on and went out.

150w- pilot light stayed on. I measured about 5VDC on the EL34 pin 4, & 2.8VDC on the preamp tube plates.

Tomorrow I’ll pull the board and trace that rail. I’m going to identify a couple of points where I can break the circuit to isolate the drain while also hunting for a short

I can’t see any caps with obvious bulging or leakage, so I might have to start pulling & testing

Learning lots! Really enjoying my first foray into tube land. I have a Marconi type 319a AM radio that I inherited. I’ll make that a repair project after this. Much simpler

 

[abbey road d enfer]

Went to 3 stores. Found 100w & 150w but that’s it. Incandescent bulbs are thin on the ground here

Most of the times you don't need more. For more power hungry loads you need to change the lamp for outdoor lighting type with an R7s bulb that can go up to 1kW..

100w- pilot light flashed on and went out.

150w- pilot light stayed on. I measured about 5VDC on the EL34 pin 4, & 2.8VDC on the preamp tube plates.

This is a good omen. That means that the amp staus on, albeit a tad underpowered (-20%), without frying. Then you can trace the voltages, and in particular why HR disappears at some point.

Tomorrow I’ll pull the board and trace that rail. I’m going to identify a couple of points where I can break the circuit to isolate the drain while also hunting for a short

IMO it's not a short. If there was significant drain, the transformer could not keep on delivering 5V to the heaters. You must look for an open in the HT distribution.

Learning lots! Really enjoying my first foray into tube land. I have a Marconi type 319a AM radio that I inherited. I’ll make that a repair project after this. Much simpler

Don't be fooled by the restrained number of tubes. Several tubes in old radios are dual or even triple, or operate a dual function, and understanding how a superhet works is essential. Very often the mixer/oscillator tube is at fault. The rest works more or less even with drained out tubes.
And you must be very cautious, this is an AC/DC piece; it's not isolated from mains.

 

[Rybow]

Most of the times you don't need more. For more power hungry loads you need to change the lamp for outdoor lighting type with an R7s bulb that can go up to 1kW..
This is a good omen. That means that the amp staus on, albeit a tad underpowered (-20%), without frying. Then you can trace the voltages, and in particular why HR disappears at some point.
IMO it's not a short. If there was significant drain, the transformer could not keep on delivering 5V to the heaters. You must look for an open in the HT distribution.
Don't be fooled by the restrained number of tubes. Several tubes in old radios are dual or even triple, or operate a dual function, and understanding how a superhet works is essential. Very often the mixer/oscillator tube is at fault. The rest works more or less even with drained out tubes.
And you must be very cautious, this is an AC/DC piece; it's not isolated from mains.

Thanks for the info. That 5V was what I was measuring on the EL34 grid.

Now that the resistor is in place, and current protected, I’ve been able to get some numbers. Running @ 150w:

R200- when it was blown we had 530v just hanging out.  Now I have 69. 8VDC there, and 4.965VDC on the other side. With a voltage drop of 64.835 we have a current of 295mA being drawn. No wonder this resistor cooked as that comes out to 19 watts dissipation @ reduced power (if my math is right)

No change when I disconnect the power tube PCB from the main board.

R203- the next big resistor in-line gets the 4.965VDC from R200 and reduces it to 2.974VDC.

I’m thinking to replace the 22uF 450V caps just to be on the safe side

 

[abbey road d enfer]

Thanks for the info. That 5V was what I was measuring on the EL34 grid.

You should measure a negative voltage there, about -50-60.
What is the heater voltage? What is HT right at the 1st smoothing cap?