Blackstar HT60 arcing after filter caps

[Rybow]

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

I show 34.5VDC coming out of R201 & R202 into the smoothing caps.

Thanks for the info on the AM radio. The schematic is still stuck inside the chassis, and it probably just needs a tube or 2. I’m just wondering how to plug my guitar...(half joking, half serious)

 

[abbey road d enfer]

I show 34.5VDC coming out of R201 & R202 into the smoothing caps.

Please listen. Check the heater voltage. That is what gives the best indication of how the transformer behaves.

 

[Rybow]

Please listen. Check the heater voltage. That is what gives the best indication of how the transformer behaves.

Apologies. I’ve been so focused on this R200 area.

Heater voltage is supplied to each power tube separately from the transformer. I measured 11VAC & 13.5VAC coming from the transformer. No caps in the path.

There is a centre tap joining pin 2 of one tube to pin 7 of the other that feeds heater voltage to the preamp tubes. I still have the tubes out, so there is no voltage on this tap. I did measure a small 1.1VDC on the preamp tube heaters

 

[abbey road d enfer]

Heater voltage is supplied to each power tube separately from the transformer. I measured 11VAC & 13.5VAC coming from the transformer.

OK, so the xfmr is correctly energized and the AC voltages are good enough. That means there is probably no excessive current draw. On the contrary. Something is either disconnected or gone.
You need to check hierarchically.
Check HT AC voltage between SP1/SP2 (on page 4) , then check DC voltage on both sides of R200.

 

[Rybow]

OK, so the xfmr is correctly energized and the AC voltages are good enough. That means there is probably no excessive current draw. On the contrary. Something is either disconnected or gone.
You need to check hierarchically.

This is very encouraging. I'm glad the transformer is in good shape, and the idea that something is missing is entirely possible.

I have 34.8VAC on SP1 & SP2. Between them I measure 2.8VAC

R200- 70.3VDC going in, 5.1VDC coming out

I only run the amp for about 10 seconds at most. You definitely feel the heat ramp up pretty quickly.

Check HT AC voltage between SP1/SP2 (on page 4) , then check DC voltage on both sides of R200.

Page 4 of the service manual? All I can find is the owners manual & quickstart guide

Edit: There is a 2 pin socket on the power tube board with no connection. I assumed because I couldn't see a corresponding connector that this was either a test point, or it is used if the amp is configured as an HT 100. Picture attached

 

[abbey road d enfer]

This is very encouraging. I'm glad the transformer is in good shape, and the idea that something is missing is entirely possible.

I have 34.8VAC on SP1 & SP2. Between them I measure 2.8VAC

That is very wrong! What happens if you disconnect them? I surmise they are Faston.

I only run the amp for about 10 seconds at most. You definitely feel the heat ramp up pretty quickly.

That was not clear from your previous posts. That suggests the HT winding is shorted, either internally (that would mean a replacement is necessary) or something along the line is drawing too much current. Since the 220R has fried, it suggests a short after it.

Page 4 of the service manual? All I can find is the owners manual & quickstart guide

It exists on the net. You must exert your googling talents.

Edit: There is a 2 pin socket on the power tube board with no connection. I assumed because I couldn't see a corresponding connector that this was either a test point, or it is used if the amp is configured as an HT 100. Picture attached

It is labelled "not fitted" for both the 60W and 100W versions. i have no idea what it is meant for...

 

[Rybow]

That is very wrong! What happens if you disconnect them? I surmise they are Faston.

They are Faston. I'll test that when I put the board back in. Taking another look at the bottom side & running some continuity

That was not clear from your previous posts. That suggests the HT winding is shorted, either internally (that would mean a replacement is necessary) or something along the line is drawing too much current. Since the 220R has fried, it suggests a short after it.

With the warming up issue, I am being overly cautious. After I run 4 or 5 cycles, you can feel warmth coming off the regulator heatsink and/or R200.
I may have found it. I decided to check continuity across some of the capacitors just in case. I have continuity across C142 as well as 2 of the main filter capacitors C138 & C140.

It exists on the net. You must exert your googling talents.

Will do. On a computer instead of a phone this time.

It is labelled "not fitted" for both the 60W and 100W versions. i have no idea what it is meant for...

Weird. It mirrors 2 pins on the 4 pin connector that connect to EL34 pins 1 & 7. That's why I thought maybe a test point

 

[Rybow]

That is very wrong! What happens if you disconnect them? I surmise they are Faston.

I guess it was too early for me and must have not had my probe on right, or there was something loose. Now I get 68.5 VAC when measuring between them. Disconnect one and no voltage.

Edit: answered my own question

 

[abbey road d enfer]

I guess it was too early for me and must have not had my probe on right, or there was something loose. Now I get 68.5 VAC when measuring between them.

That seems wrong to me.  Check AC voltages on CN6 (5pin). Between 1 and 3, and between 4 and 5.

Disconnect one and no voltage.

Obviously. What is the AC voltage between the two wires (female Faston) when you disconnect them?

 

[Rybow]

That seems wrong to me.  Check AC voltages on CN6 (5pin). Between 1 and 3, and between 4 and 5.

After about 5 seconds I have 6.24VAC between 1 & 3 (blue wires), and 14.8VAC between 4 & 5 (brown wires). This is out of circuit, as well as back probing while in circuit

At the fuses I have slightly over 3VAC on both fuses that link 1 & 3, and 7.04VAC on the fuse for 4 & 5

Obviously. What is the AC voltage between the two wires (female Faston) when you disconnect them?

It is wrong. I found my notes from weeks ago when I first got the amp, and I was just collecting data. Prior to R200 being installed I had 191VAC on SP1 & 196VAC on SP2.

Disconnected I currently have 389VAC between them.

I bet if I disconnect R200 those voltages will read normal. The increased draw must be pulling the voltage down

 

[abbey road d enfer]

After about 5 seconds I have 6.24VAC between 1 & 3 (blue wires), and 14.8VAC between 4 & 5 (brown wires). This is out of circuit, as well as back probing while in circuit

That seems absolutely normal.

At the fuses I have slightly over 3VAC on both fuses that link 1 & 3, and 7.04VAC on the fuse for 4 & 5

Weird. You should read about 12Vac on fuses F1 & F2, and about 20-25Vas on F3. All measured referenced to ground.

I found my notes from weeks ago when I first got the amp, and I was just collecting data. Prior to R200 being installed I had 191VAC on SP1 & 196VAC on SP2.

Disconnected I currently have 389VAC between them.

That is consistent. Two windings in series, voltages add up.

I bet if I disconnect R200 those voltages will read normal.

But they actually do read normal.

 

[Rybow]

That seems absolutely normal.

A piece of good news!

Weird. You should read about 12Vac on fuses F1 & F2, and about 20-25Vas on F3. All measured referenced to ground.

The voltages are being divided somehow? Very strange.

That is consistent. Two windings in series, voltages add up.
But they actually do read normal.

My initial measurements were taken with SP 1 & 2 connected to the circuit referenced to ground. If I’m understanding correctly, a voltage in the area of 190VAC is what we should be seeing here in a healthy amplifier.

I was suspicious of C142 and removed it. Found a hole that has almost eaten away the + trace connection on one side. I’m starting to wonder if this was overheating from a soldering iron, or damage caused by a surge in the amplifier. R203 has similar burns, but not to this extent (pic attached)

 

[Rybow]

My apologies for the double post, but I have an update. I've been able to get the voltages restored, and have attached a spreadsheet. I believe they are looking good, but I would feel a lot better about taking my surge limiter out of the loop with some confirmation that we are in the right ball park. The bulb goes to full brightness when the amp is powered on, and dims to maybe half brightness after a few seconds.

I had assumed C142 was fine because it was a replacement. However, I found that the PCB had not been cleaned before the new cap was installed, so there was a quite a mess underneath. Ill post a pic of that  soon.

I just want to say thanks to everyone for helping me out with this. Special thanks to abbey road d enfer for sticking it out while I fumbled my way through this. You really helped me out, and I have a much better understanding then I did before. I truly appreciate your assistance!

 

[abbey road d enfer]

The bulb goes to full brightness when the amp is powered on, and dims to maybe half brightness after a few seconds.

that's definitely correct. When switching on, there is a current surgeddue to the transformer core being magnetized (very short but quite intense), the smoothing capacitors being charged (a tad longer and less intense), and the heaters going from low resistance when cold to higher resistance when hot, which takes about 10-15 seconds.
Have you done a test run? You should see that the bulb lights in accordance with the output level.
I wouldn't hesitate to plug it in without the surge limiter.

 

[Rybow]

that's definitely correct. When switching on, there is a current surgeddue to the transformer core being magnetized (very short but quite intense), the smoothing capacitors being charged (a tad longer and less intense), and the heaters going from low resistance when cold to higher resistance when hot, which takes about 10-15 seconds.
Have you done a test run? You should see that the bulb lights in accordance with the output level.
I wouldn't hesitate to plug it in without the surge limiter.

Well I plugged in my guitar and fired up without the surge limiter. Let it warm up for about 15 minutes, and then let her rip

2 things:
My whole house stinks which my girlfriend is not happy about. I might need to set this up outside for an hour or 2 to burn off whatever chemical residue is there

It sounded like shit. Distorted playing quietly, hit the strings a little harder and it would go very loud. Doing that transistor gate not quite closed thing. I did a quick bias adjustment by ear. Got it smoothed out and sounding half decent, but that will need fine tuning. My meter doesn’t pick up voltage @ D27 where I should be adjusting to 50mV. Not sure why. I’m also near the end of available adjustment on the pot. Power tubes may not be matched well enough?

Getting close now!

 

[abbey road d enfer]

My meter doesn’t pick up voltage @ D27 where I should be adjusting to 50mV. Not sure why. I’m also near the end of available adjustment on the pot.

The voltage on the grids  and the absence of voltage at D27 suggest the output stage is running in class C, which definitely does not sound right.
Now you need to check the bias circuit. Check voltages at wiper of PR1 (Bias Balance) and both sides of PR2 (Bias). The Bias Balance control should be enough to balance less-than-perfect matched tubes.

 

[Rybow]

The voltage on the grids  and the absence of voltage at D27 suggest the output stage is running in class C, which definitely does not sound right.
Now you need to check the bias circuit. Check voltages at wiper of PR1 (Bias Balance) and both sides of PR2 (Bias). The Bias Balance control should be enough to balance less-than-perfect matched tubes.

Finally got back to this. Didn’t notice until after I finally got the smell out of my house, but R142 and R143 were starting to burn out.

Took a closer look and saw some scorching under C8. I’m thinking that originally R200 burned out, but a charge slipped through that blew out C142 and potentially damaged other capacitors along the rail.

At this point I’m going to pull every electrolytic cap in this part of the circuit. I’ve noticed that all of them are pushed up on one side, while the majority on the PCB are seated flush.

 

[abbey road d enfer]

I’ve noticed that all of them are pushed up on one side, while the majority on the PCB are seated flush.

It could indicate they have suffered overheating, either by being submitted to excessive voltage (not too likely since the power xfmr would also be damaged) or submitted to AC, which can happen if one of the rectifiers is shorted. I suggest you have a look at them. Maybe the person who worked on it earlier had connected  a diode in reverse...?

 

[Rybow]

It could indicate they have suffered overheating, either by being submitted to excessive voltage (not too likely since the power xfmr would also be damaged) or submitted to AC, which can happen if one of the rectifiers is shorted. I suggest you have a look at them. Maybe the person who worked on it earlier had connected  a diode in reverse...?

I was really hoping that it was just a diode, but alas, all were oriented correctly & tested fine out of the circuit.

The resistors only overheat when I adjust the bias & bias balance up high enough to get a passable sound out of the amp. They are almost maxed at that point. I did measure the bias trimmer & I think it's set up as a variable resistor. Pin 1 is shorted to the wiper. Pin 3 to wiper varies from 20k or so down the 250 ohm

I still can't get a read at D27. I measure -0.6 mVdc (on the correct side). I did find that pin 3 on the 4 pin connector gives me 48-50 mVdc initially, then drops quickly to about 34 mVdc. Might be my meter loading it down?

One piece of good news. The simulated output works perfectly

 

[abbey road d enfer]

The resistors only overheat when I adjust the bias & bias balance up high enough to get a passable sound out of the amp. They are almost maxed at that point.

What do you mean "maxed"? Do they burst in flames? The operating temperature for these resistors may be above 50°C, which is where it starts to burn skin.

  I did measure the bias trimmer & I think it's set up as a variable resistor. Pin 1 is shorted to the wiper. Pin 3 to wiper varies from 20k or so down the 250 ohm

That is correct.

I still can't get a read at D27. I measure -0.6 mVdc (on the correct side).

Are you sure? What's the "correct side"? The voltage should be measured across D27, that means one probe on each side, not between ground and one point. In particular, between ground and pin 4, you can only expect residuals. That's between pins 3 & 4 that you need to measure.

I did find that pin 3 on the 4 pin connector gives me 48-50 mVdc initially, then drops quickly to about 34 mVdc.

That's about what I would expect to find there

Might be my meter loading it down?

No. Your meter cannot load a 1 ohm resistor. It is normal to see a surge then a stabilized value.