Blackstar HT60 arcing after filter caps

[Rybow]

*please go to page 4 for the latest development

looking for some advice on what may have caused the 630mA fuse to blow in an HT60

Long story short, my friend bought this amp used and cheap. It had no power tubes  and a 12at7 in the phase inverter which should be a 12au7. He put in new power tubes. Amp powered on, no sound. He dropped it at my house along with some homemade vodka. The HT fuse looked fine, but my DMM showed no continuity. I inspected the PCB, and saw no signs of damage

My digikey parts just arrived, but before I fired this thing up with a new fuse, is there anything I should be checking on?

The 3 PCB mounted fuses & mains fuse are good, and the 12ax7’s have been tested in a Tubemeister 5. I don’t have a 12au7, but I do have a couple 12 bh7’s which I read are essentially a 12au7 with higher power handling.

I can also say that the previous owner was not too bright. Both fuse holders have been mangled by screwdrivers. One is actually broken and will be getting replaced.

One last question. I’ll be checking bias (for the first time in my life), and I have the procedure, but this amp has a bias balance adjustment as well. Anyway to do that without a signal generator or oscilloscope?

Thanks in advance! Hope you all are keeping safe & busy!

 

[mjrippe]

Fuses do wear out and fail similar to a lightbulb.  Replace it once and if it holds, all is well.  If it blows, then you have to find the problem.

 

[Rybow]

Thanks mjrippe. Put in the new fuse and it did hold through multiple on/off cycles

Still no output from the amp. Nothing. It is dead quiet save for a quiet pop when the main power is flipped on.. I’ll flip out a couple of the preamp tubes, but I suspect there is a bigger problem here. The input jack feels very loose, so that might be done. I believe this jack is switched to mute the amp with no cable inserted, but I’m not sure.

Looks like I’ll be pulling the chassis to play a game of “find the short”. Any suggestions would be welcome!

 

[abbey road d enfer]

It had no power tubes  and a 12at7 in the phase inverter which should be a 12au7.

Very frequent substitution; this is not a cause for "no sound".

I’ll be checking bias (for the first time in my life), and I have the procedure, but this amp has a bias balance adjustment as well. Anyway to do that without a signal generator or oscilloscope?

Most of the times, bias balance is done by comparing bias currents and making them equal. Does this amp provide test points for each tube idle current?

 

[PRR]

> bias balance is done by comparing bias currents and making them equal.

Yes, as step 1.

Post-1970s tube-pairs were prone to buzz even with nominally equal current (inconsistent guts). Sometimes "bias balance" was to null buzz and make poor-match pairs usable. So IMHO: trim for equality, trim for least buzz, and either find a near-optimum both ways or find a better pair.

 

[Rybow]

Very frequent substitution; this is not a cause for "no sound".
Most of the times, bias balance is done by comparing bias currents and making them equal. Does this amp provide test points for each tube idle current?

Thanks for the info on the tubes. Can tick one thing off the list

The balance procedure I have is to max the channel and master volumes, send a 1khz tone into the input until the waveform clips, and then adjust until the clipping is symmetrical

Bias voltage is measured across D27 and adjusted to 50mV.

The replacement power tubes are a matched pair from the tube store. When power is applied, one does heat up faster than the other. After about 30 seconds they appear equal in brightness. This difference does follow the tube.

I’m going to dig into this thing more today, here are some more details:

- by no output I mean that the amp is muted. No static, hiss, pops, clicks. It is dead silent including the emulated out. This has me looking at the input as it is designed to mute the amp when no cable is plugged in.

- the second stage 12ax7 is significantly dimmer than the other 2 preamp tubes

- when I pulled the chassis, I found the main PCB connector to the output PCB was pulled out. No change after reconnecting, but now I know someone has been playing around in here

Today I’ll be testing continuity through all the connectors, and looking for shorts surrounding the input. There is an IC right near the input that I’ll get the data sheet on and test the pins.

Thanks for all the help! Not a bad way to kill some quarantine time

 

[abbey road d enfer]

The balance procedure I have is to max the channel and master volumes, send a 1khz tone into the input until the waveform clips, and then adjust until the clipping is symmetrical

Bias voltage is measured across D27 and adjusted to 50mV.

Proper balance setting should be done in three different steps:

• Balancing idle current without signal or alternatively for minimum hum
• Balancing the drive signals for minimum distortion
• Checking symmetrical clipping

Since it's a guitar amp, where some degree of 2nd harmonic is not only tolerated but often welcome, the 2nd and 3rd point are not as important as the 1st IMO.
However, since they have not provided means of measuring individually idle currents, they had to design a procedure for setting bias balance.
Since you don't have access to the drive balance, you have to accept a compromise; if it was for me, I would choose the minimum hum setting.

The replacement power tubes are a matched pair from the tube store. When power is applied, one does heat up faster than the other. After about 30 seconds they appear equal in brightness. This difference does follow the tube.

That is not a sign of malfunction.

- by no output I mean that the amp is muted. No static, hiss, pops, clicks. It is dead silent including the emulated out. This has me looking at the input as it is designed to mute the amp when no cable is plugged in.

If the amp was muted, you'd still hear some hiss and hum. I think you need to take the voltages on the output tubes; could be HT not reaching the anodes.

- the second stage 12ax7 is significantly dimmer than the other 2 preamp tubes

Check the heater voltage and swap tubes, but again, not significant usually.

 

[Rybow]

Proper balance setting should be done in three different steps:

• Balancing idle current without signal or alternatively for minimum hum
• Balancing the drive signals for minimum distortion
• Checking symmetrical clipping

Since it's a guitar amp, where some degree of 2nd harmonic is not only tolerated but often welcome, the 2nd and 3rd point are not as important as the 1st IMO.
However, since they have not provided means of measuring individually idle currents, they had to design a procedure for setting bias balance.
Since you don't have access to the drive balance, you have to accept a compromise; if it was for me, I would choose the minimum hum setting.
That is not a sign of malfunction.
If the amp was muted, you'd still hear some hiss and hum. I think you need to take the voltages on the output tubes; could be HT not reaching the anodes.
Check the heater voltage and swap tubes, but again, not significant usually.

Thanks for the info! I really appreciate the help.

Just going to edit this post as I go

Fortune favours the bold, so I got in there today. All voltages were referenced to chassis ground

Power Tubes
EL34-1 Heater Pin 7 6.68VDC,  Plate Pin 3: 528.5 VAC
EL34-2 Heater pin 7: 6.31VDC, Plate Pin 3: 530 VAC

Preamp tubes
12ax7-1 Heater pin 4: 12.61 VDC, Heater Tap pin 9: 5.97 VDC
12ax7-2 heater pin 4: -12.45 VDC, Heater Tap pin 9: -6.19 VDC
12bh7 heater pin 4: -12.46 VDC,Heater Tap pin 9: -6.16 VDC

For all preamp tubes I could not get a reading from Pin 1 or 6 to ground.

The first thing I tried to do was check the power tube Bias. This calls for 50mV "across D27" according to the procedure I saw online. Watched a video of it being done with the black probe on chassis ground, red probe to the anode. I get nothing. Turning the bias pot yields nothing. Running the diode test on my meter shows continuity both ways. This is with the diode in circuit though.

Maybe it's just a bad bias diode?

 

[Rybow]

Finally got some time to poke around a bit more. I have quite a few voltage readings from around the board. I'll try to get some pictures up showing where I am seeing what voltages.

Still no preamp plate voltage & no bias voltage at D27. I did find 44mVDC in the surrounding area

One thing I did discover today is that I have no voltage across the standby switch, but I do have continuity.

Any specific places I should be looking for this preamp plate voltage? Thanks!

 

[CJ]

check screen voltages  pin 4 on eL 34 tubes

report back immediately.

 

[Rybow]

check screen voltages  pin 4 on eL 34 tubes

report back immediately.

Hi CJ! Reporting back

Measurements taken at 15 min intervals after power on. One EL34 measured -0.040VDC resting at -0.038VDC

The other EL34 measured 0.089VDC steadily rising to 0.111VDC after 30 mins

 

[abbey road d enfer]

Hi CJ! Reporting back

Measurements taken at 15 min intervals after power on. One EL34 measured -0.040VDC resting at -0.038VDC

The other EL34 measured 0.089VDC steadily rising to 0.111VDC after 30 mins

What? You should have about 500Vdc there!

 

[Rybow]

What? You should have about 500Vdc there!

Yikes! That can't be good. I took all voltages with the black probe on chassis ground. Not sure I need to move it to another point to get a proper read?

I decided to go through and measure voltages on all the tubes. Attached is a pdf spreadsheet of everything I measured. It's almost as if I'm missing a power rail.

 

[abbey road d enfer]

It's almost as if I'm missing a power rail.

Not almost! It's exactly it. Check voltages at R200 and R203. Also check the 4-pin connectors.

 

[Rybow]

Not almost! It's exactly it. Check voltages at R200 and R203. Also check the 4-pin connectors.

Checked the resistors & connectors that you listed

R200- 533 VDC on one side (power supply side, nothing on the other side.
R203- -0.045VDC on both sides. I've been seeing this small voltage all over the amp

Looking at the back of the amp, left to right

CON 5 Blk: -0.044VDC, wht: 0.426VAC, wht: 0V, wht: 0V
I measured this both at the connector & at the pins on the power tube PCB

CON 1 wht: -60.56VDC, blk: 0V, wht: 60.5VDC, blk: 0V

Transformer Connector blue 1: 15.16VAC, Green: 0V, Blue2: 15.25VAC, Brown1: 35.25VAC, Brown2: 36VAC

The green wire has continuity to the standby switch black wire. With the connector out, just that pin is connected. With the connector in, the blue1 wire is shorted to the green. Not sure why.

Here is a link to my google drive with Hi res pics of the connectors in question.

https://drive.google.com/open?id=1Hh4Q4Whv-3ZoqHodHB2N9dDGVHGHjK1R

 

[abbey road d enfer]

Checked the resistors & connectors that you listed

R200- 533 VDC on one side (power supply side, nothing on the other side.
R203- -0.045VDC on both sides. I've been seeing this small voltage all over the amp

Don't look further, R200 is shot. Replace it.

 

[Rybow]

Don't look further, R200 is shot. Replace it.

Great!! Thank you very much for all your help. I’ll hopefully get a digikey order in a couple of days and report back with the results

 

[PRR]

Happy 666 !

 

[Rybow]

Happy 666 !

Yeah!! Rock on man!!

 

[CJ]

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. 

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