Blackstar HT Dual

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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ryanuk
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Post by ryanuk »

Guys

I have an Blackstar HT Dual pedal Im trying to fix. I purchased from eBay. Shortly after I noticed a problem in the switching. Negotiated a 50% reduction on the basis I would fix. Im not a pro but have built fx and fixed amp problems before so am confident in tackling this.

Wanted some advice before I open the box though.

Symptoms are...
Unit always works in Bypass
Sometimes either green, red (or both) channels are silent.
Sometimes the volume of green or red channel will dip randomly.
I discovered a 'fix' is to unplug and reinsert the PSU.

This is a tempremental fault. It works fine for some periods but then one or both channels will die. Bypass always works.

Looking at the schematics (attached) Ive ruled out the PSU and regulator ICs as faulty as bypass is working.
The tube is not part of the circuit when the unit is bypassed.
The tube power supply ICs seem ok as usually one channel is working.

I think its something to do with the switching logic. Perhaps a faulty diode or FET.

My strategy will be to carefully open the box and eye ball the 3 PCBs for loose connections or dry joints. Examine and test cables linking the PCBs. I would then change the tube, and test the pedal.
If that doesn't highlight problems, I could lift and test diodes. If they were ok, I would then probably just replace all the J175s and opamps in the switching circuits. Test again.
If all that hasn't worked Id check the tube power supply.

Does anybody have any other ideas, comments??

RyanUK
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Blackstar HT Dual Schematic.docx
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alexradium
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Post by alexradium »

Take care of the high voltage in there

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bmxguitarsbmx
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Post by bmxguitarsbmx »

Sounds like a decent starting. When things work intermittently, it can be a pain in the ass. Have a look at troubleshooting flow chart from an electronics repair book. Those books are invaluable, as strategy is often the most important part of repair. Inevitably, once you fix it, you will look back on your strategy and wish you did it a more logical way. Those books are written by people who were very good at repair.

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ryanuk
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Post by ryanuk »

Hi

Didnt have many replies to my post but wanted to update that I managed to fix this pedal. Eventually the pedal stuck on channel 2 so deffo needed a repair.

My process...

- I checked the internal supply voltages (+22v, -/+15v and 270v) at Connector 11; this was pretty easy to do (being careful) as the connector pins (underside) are accessible as soon as you take off the base. As the voltages were ok, I eliminated all of the power supply section.
- I then checked everything in the signal path of channel 2. Channel 1 and bypass worked ok so I was able to eliminate a large part of the circuit this way.
- All ribbon cables were fine so these too were eliminated.
- The cab sim worked so I ruled out any problems here.

All the above left me with the switching circuit (FETs, diodes, IC8 and 9), and possibly IC2. However, half of IC2 is used for channel 1 so I also ruled that out.
I tested diodes/FETS using my multimeter; these seemed ok, although I wasnt 100% as they were tested on the board.
In the end I replaced IC8 (TL072) on a hunch and everything else seemed ok, and this part seemed to switch channel 2 acorrding to the schem. IT WORKED! IC8 was the culprit. So pleased!

Some notes on this pedal;

- The stock tube is a Sovtek 12AX7WX. I tried a different tube just to confirm this wasn't the problem - channel 1 worked so I knew the original tube was ok but I just wanted to confirm. I put the original back in when I fixed the pedal. Not sure a different tube would make a major difference in sound. Its a pain to swap out anyway.
- The build quality of the case is insane! That said, the clear material protecting the valve under that solid grill is a THIN piece of acetate! I was staggered. A very wink link given the rest of the build quality. I replaced this with a thick, better fitting piece of plastic. It wont offer any protection against spilled drinks but better than before!
- Also, the glow from the tube is actually prominently a amber LED just under the tube.
- Nuts for the pots and jacks are 7/16!

I studied the schems pretty hard so happy to help anyone debug theirs if they have an issue.

Thanks to bmxguitarsbmx and alexradium!

RyanUK

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Post by kossofos »

Hello Ryan. I have the same problem with my HT Dual. After sending the unit to the official service center, I live in Greece, they replaced the left footswitch with a latch type.The problem remains. I own the 1st rev, which works with 16 volts. I have a question for you. Can you tell me what footswitch I should use for the green channel? Is it momentary? Thank you and excuse me for my bad English...

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Post by ryanuk »

Kossofos
I think the switches are momentary as the switching is done by FETs and an IC.
A latching type would not in that situation.
Did the give you the old switch back? Can you test it???

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Post by kossofos »

Thank you for getting back. No, they gave me nothing back. So are BOTH switches momentary switches? I have a friend who Is a technician and he is looking into it.

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Post by ryanuk »

kossofos wrote: 03 Dec 2022, 13:58 Thank you for getting back. No, they gave me nothing back. So are BOTH switches momentary switches? I have a friend who Is a technician and he is looking into it.
Yes - both footswitches (marked SW2 and SW3) on the schematic are non-latching i.e. momentary DPDT. The only latching switch is SW2 which is the mini push-push DPDT that switches between "clean" and "crunch" for channel 1.

Hopefully your engineer can replace the switch.

I hope you get it sorted - its a lovely sounding pedal.

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Post by andreus »

Hi Ryan, I have a similar unit. It is Blackstar HT-Metal and maybe you can give me some advice on where on the circuit could be the
problem? It stopped responding to pressing switches and does not produce overdriven sound anymore. LED's also do not produce
light anymore. Only bypass clean sound goes through. This kind of situation has been happening in the past too, but somehow
the unit still continued operating after I put it aside for several days. But now it stopped responding to pressing switches for good.

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ryanuk
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Post by ryanuk »

andreus wrote: 18 Mar 2023, 20:36 Hi Ryan, I have a similar unit. It is Blackstar HT-Metal and maybe you can give me some advice on where on the circuit could be the
problem? It stopped responding to pressing switches and does not produce overdriven sound anymore. LED's also do not produce
light anymore. Only bypass clean sound goes through. This kind of situation has been happening in the past too, but somehow
the unit still continued operating after I put it aside for several days. But now it stopped responding to pressing switches for good.
Andreus

Sorry. I've just seen this. Let me have a think. The HT Metal is a different unit and I'm not familiar with the switching but let me see what I can find..

RyUK

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ryanuk
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Post by ryanuk »

andreus wrote: 18 Mar 2023, 20:36 Hi Ryan, I have a similar unit. It is Blackstar HT-Metal and maybe you can give me some advice on where on the circuit could be the
problem? It stopped responding to pressing switches and does not produce overdriven sound anymore. LED's also do not produce
light anymore. Only bypass clean sound goes through. This kind of situation has been happening in the past too, but somehow
the unit still continued operating after I put it aside for several days. But now it stopped responding to pressing switches for good.
Andreus

So, just had a think about this... the HT Metal is a similar typology to the HT dual so whilst the gain structure may be different, we can use the HT Dual schematics as a guide.

***Before I go on - warning as usual - you need to be careful poking in this pedal. The tube gets supplied with 270v DC at the plates. If in doubt, take it to a tech. I am not a tech but have an interest and have studied and but up a degree of knowledge. I'm not sure what your experience level is so please shout if anything unclear. ***

So, you switch the pedal on and you're stuck in bypass ie. bypass and no red/green lights? Can you confirm?

This tell us that some of the circuit is working (to an extent). Its getting power as its FET bypass logic. If you're getting bypass sound, the FETs are switched on to do so. Also there is a bypass buffer which appears to be powered and working in your pedal.

My pedal was temperamental also, and suddenly got stuck too! So I suspect a the same part at fault.

Some things to check...

There is an LED under the tube (yes, all that glow isn't just the tube - Blackstar put an LED under there for MOJO! :lol: ). If this yellow LED is on, then the +15v power supply rail is working. A good sign.

I assume the value heater is glowing? You may need to take the boards out for this as the yellow LED can make it hard to see. If the heaters are glowing, its a good sign that the -15v power supply rail is working.

These checks help diagnose the +/- 15v power rails. Important as these power the switching circuit.

If you have a multimeter, you could check the voltages throughout the internal power supply. The unit takes in 16vAC (or 22vAC depending on version) from an external PSU, which then splits to power (a) the 15v regulators for the opamps and LED power and (b) the circuit which steps the voltage up to 270v for the tube. You could also check for +/-15v at the op amps (pin 4 and 7), 12v at pins 4/5/9 of the tube for heaters, and a large voltage (>120v) at tube pins 1 and 6.

You could also do some audio tracing at FETs 4 and 5 to check if you get audio from the 2 channels.

My hunch is that you will find that the power rails are good and you'll get sound from probing. Which will confirm a switching issue. I would then use a meter to check diodes in the switching circuit (see page 1 of the schematics). And those diodes at the gates for the FETs. You can also eliminate the switches by checking with a multimeter. But I think these would be ok.

From a process of elimination I suspect your pedal is suffering from a problem with IC8 or IC9 (or both) which are TL072 op amps. These are cheap to replace, but go careful as there a difficult to remove without board damage. I would perhaps socket them (although I didn't do this in my pedal).

Follow the above tests and this will give you a better idea of whats going on. Be safe and best of luck!

RyanUK

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Post by rich_hill44 »

Hi Ryan, I hope you pick this up and can provide insight. I bought a HT Dual first version (16v) secondhand. I didn't have a 16v power supply so I tried an 18v. The amber LED is lit around the valve. However, none of the other lights are working (Channel 1 green, Channel 2 red) and it's not producing any sound output. Wonder if the info I've supplied suggests a idea of what part of the circuit is most likely causing the issue. Also, do you think it unwise to use an 18v power supply (my universal pedal power supply only has options for 9v, 12v and 18v)? Any thoughts much appreciated. Kind regards, Richard

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Post by rich_hill44 »

In a slight update, I'm convinced the amber glow is actually coming from the valve (I removed the protective cover above the valve to have a closer look). I can't see any light that I'd say was an LED.

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Post by leonardorafael »

Ryanuk thanks for your info and kindness. I dont know about electronics, but trying to figure out some info that is not available for owners of HT-Dual. If I figured out well your schematics, are there two valve tubes in the pre-gain section ? Is it true by-pass ? Does it has buffer in the output ?

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