Sunday, February 12, 2012

Service Review: The Blackstar HT40

This will be the first in a series of reviews of amps as they show up around here for repairs. I intend to look only at a limited number of parameters: ease of serviceability and availability of documentation. These opinions are my own


Recently a Blackstar HT40 like the one in the picture arrived. It had been arrested for attempted homicide, having tried to kill the owner with a jolt of electricity that actually melted a couple of guitar strings. It was bought used, so the warranty is not in existence.

The street price for these things is about $700 more or less at all the usual places. Cheap enough if you get good value but that's not what's happening here.

It looks like a nice stout production, but appearances can be deceiving.

During my preliminary information gathering process I learned that this amp has a processor on the circuit board that has some proprietary programming. That's the 32(?) pin number on the right with the sticker on it in the photo below. This processor with the special programming is made of unobtainium, kinda like the Maytag "Jazz" chip that caused my refrigerator to collapse and die. At least there I could get my hands on a schematic and a circuit board and it only cost eighty bucks.

You can read about that here.

In addition I found out that Blackstar service information is available only to authorized Korg service centers, which means it's not available to owners or to the independent service operators.

On opening up the amp for a look around I also noticed that the board is thin and flexible-and not in a good way, either, with circuit board mounted tube sockets already showing signs of distress and a wealth of surface mounted devices that are nearly impossible to remove and replace economically. For a 40w amp the output transformer is decidedly small.

I did find a burned trace on the circuit board and fixed it with a jumper and it now works intermittently when the jack is preloaded. Replace the jack? Didn't help.

One of the things we do when troubleshooting is to switch out suspect components as an information gathering and trouble localizing exercise-does this or that item change the parameters of the malfunction?

That's not going to happen with this amp because of the surface mount componentry. Between the lack of service information and the nonrepairability of this circuit board, most of these amps will end up in dumpsters or stuck in basements when they fail, which they all do inevitably.

How about the rest of it? Well, the cabinet is made out of MDF fiberboard-not the most damage tolerant product out there. Tonewise it is brittle and harsh sounding. In addition the solid state reverb is synthetic and cheesy sounding, like a cheap pedal from a junkshop.

I wouldn't recommend this amp to anyone. Amps always break sooner or later, and having one that can't be fixed economically is just a waste of hard earned money. Hell, even Crate will provide service information to owners and the trade.

Truth in advertising suggests that this should be called Crapstar.


11 comments:

  1. I purchased this combo on craigslist for a good price knowing it was toast. I did some searching after i got it and found you had worked on it. I have some surface mount soldering experiance, any advice on how to get it going again?

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  2. I hope you got a good deal on it. There are three problems here. One is getting technical information-i.e., a schematic, which Blackstar and/or Korg won't supply except to their service centers. A second problem is the programmed chip, which is made of unobtainium. Once you resolve those two issues, if you resolve them, you will then have to find out what was cooked in the preamp circuitry. Unless you have a pipeline into Korg, those problems will effectively prevent you from repairing the amp. If it was mine I'd throw the circuit board away and use the iron and the chassis to build something PTP like a single channel Marshall. There's plenty of room inside and the chassis looks strong enough. If you can fix it my hat's off to you-at which point you will have earned a guest slot here. I spent several weeks screwing around with it off and on and trying to get a schematic to figure out how the thing works. It was a money losing proposition all hte way around because I don't charge people if I can't fix what they bring me.

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    1. Thanks for the advice! I finally got some El34s in it and fired it up. I am amazed that it even makes a sound. It seems like the diode gain stage(s) were KIA, among other things. I will let you know if anything constructive hapens. It would be ironic to build a Marshall out of it...

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  3. I understand that the programmed chip is involved with the channel switching. I was able to get some sound out of it by completely bypassing all the surface mount solid state stuff, but there's olenty of room to build something interesting inside it. Best of luck.

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  4. This post just saved me $300 and a headache. Thank you!

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  5. I'm having trouble too, changed the STK830 resistances.
    I switched and audio still comes down.
    Without the schematic is hard to run after the fault

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  6. Great blog post, a lot of people don't realise how much of a gamble it is to buy a HT40 out of warranty. I really like your idea of using the chassis to build a Marshall clone. Would you mind sharing your thoughts on what kit would be suitable? Would any of the electronics from the HT40 be salvageable for such a new build?

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  7. I think all you could salvage might be the power transformer and output transformer. Probably best to gut it out and start from scratch. Maybe, say, a JTM45 or something.

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  8. can anyone identify r144 and r145 on the board for me? i have replaced a couple resistors that a shorted power tube took out but i cannot identify the value of r144 and r145 becuase they have been burned

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  9. can anyone identify r144 and r145 on the board for me? i have replaced a couple resistors that a shorted power tube took out but i cannot identify the value of r144 and r145 becuase they have been burned

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    1. Both r144 and r145 are 100 Ohms.
      Greg R.Cranky's

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