Blackstar HT50 won’t turn on

jdbwales

Hi, I bought a backup HT50 mark one during lockdown ready for gigs restarting, it worked at the time. I tried it again today, it crackled, made a loud and horrendous noise and shut itself off and won’t switch back on. 

 So, obviously no warranty and now can’t return it to the seller, so it’s sort it myself or take to a tech time.
 What do we think? I’m assuming a power valve or both of them have gone and taken the HT fuse with them. I have a spare matched pair of EL34s here but will need to order a fuse - any ideas on what may have happened, how I’d test my hypothesis and what fuse I’ll need? I know @ecc83 is the Blackstar guy but I’m open to ideas, thanks in advance!

darthed1981

@ICBM may be of help as well.

jdbwales

darthed1981 said:

@ICBM may be of help as well.

That’s very true, but I always tag and/or bother John so he could escape this one...

impmann

Word of advice - don’t just change the fuse and try again... the fuses on HT50s don’t just go pop.

It could be as simple as a power valve flash over. I’d bank on a pair of output valves and a bias.

Id suggest send it to a Blackstar Authorised Service Centre - they have access to the schematics etc details on the website.

If you get really stuck PM me. :-)

ecc83

I agree, probably an OP valve taken out the HT fuse but since a new set will need a re bias best take to a proper service tech. There were problems with the very early HT-60 PCBs but never with the HT-40/50s AFAIK.

Dave.

jdbwales

Thanks gents. Trying new valves and new fuse this evening

jdbwales

Right - mixed results so I’d appreciate any help @impmann @ecc83 @ICBM  - sorry to tag! 

I tried the amp with a new fuse and without any power valves in at all - worked, stayed on, seemed fine. 

Then I tried a set of Marshall EL34s - same problem, crackling sound, shut down, blown fuse. Now - these valves have been lying around for about 5 years and have done several house moves so I’m not 100% that they were working anyway. 

Any ideas? I’m leaning towards getting a fresh set of EL34s and trying again as I think the fact it stayed on with no valves in would suggest duff OP valves. 

I’ve biased my old HT50 before with no issues and this is the same MK1 model, so if it’s just a new set of valves biasing is fine!

ecc83

Try again with the valves in but no speaker plugged in and nothing in the input jack. In that condition the 34s are biased hard off and should not blow a fuse. If they do you most likely have a fault in the bias supply circuitry.

Dave.

jdbwales

ecc83 said:

Try again with the valves in but no speaker plugged in and nothing in the input jack. In that condition the 34s are biased hard off and should not blow a fuse. If they do you most likely have a fault in the bias supply circuitry.

Dave.

Thanks Dave - will need to get hold of a pair of decent working EL34s first, the Marshall ones were buggered annoyingly. Will let you know how I get on!

impmann

The o/p transformer in a mk1 Ht50 is fairly over engineered, tbh. And contrary to some mythology on forums etc, they don’t just fail unless there’s an underlying fault. Put it another way... I can tell you demand for replacements for that amp is very low.
If you can hear crackling noises with the valves in, I really would not suggest putting another set of valves in it and trying again. Crackling says a big short. It could be the load resistors, it could be tracking, it could be the driver FETs... 
Drop the chassis from the cabinet and take a good look at the board before you go any further.

jdbwales

impmann said:

The o/p transformer in a mk1 Ht50 is fairly over engineered, tbh. And contrary to some mythology on forums etc, they don’t just fail unless there’s an underlying fault. Put it another way... I can tell you demand for replacements for that amp is very low.
If you can hear crackling noises with the valves in, I really would not suggest putting another set of valves in it and trying again. Crackling says a big short. It could be the load resistors, it could be tracking, it could be the driver FETs... 
Drop the chassis from the cabinet and take a good look at the board before you go any further.

Ah, well that doesn’t sound good. It’s not so much a crackle I guess as a hideous noise then shut off, not sure if that makes any difference. 

 I’m a bit gutted as I’ve had my other HT50 and gigged it for years without a single issue, but get a backup just in case and the bloody thing goes straight down...

impmann

jdbwales said:

impmann said:

The o/p transformer in a mk1 Ht50 is fairly over engineered, tbh. And contrary to some mythology on forums etc, they don’t just fail unless there’s an underlying fault. Put it another way... I can tell you demand for replacements for that amp is very low.
If you can hear crackling noises with the valves in, I really would not suggest putting another set of valves in it and trying again. Crackling says a big short. It could be the load resistors, it could be tracking, it could be the driver FETs... 
Drop the chassis from the cabinet and take a good look at the board before you go any further.

Ah, well that doesn’t sound good. It’s not so much a crackle I guess as a hideous noise then shut off, not sure if that makes any difference. 

 I’m a bit gutted as I’ve had my other HT50 and gigged it for years without a single issue, but get a backup just in case and the bloody thing goes straight down...

TBH, they are usually pretty bulletproof - the ones I look after have done multiple tours and festivals for years without anything other than normal servicing/valves...

Have a look-see inside and let us know.

ecc83

I would agree with Impman. I was not involved with the amplifiers nearly as long as he but the 'loaners' got a good thrashing! I also agree that the transformers are very reliable. I only had one HT-5 OPT fail and a power traff in an Artisan 30 but that was because of an arcing GZ34 and the guy kept feeding the amp fuses all night until it eventually died!

I hate valve rectifiers!

Dave.

jdbwales

Thanks gents - tonight’s project will be to inspect the chassis! Will there be any visible damage to look for?

jdbwales

Can’t see anything visibly blown or damaged inside. I’ve taken a load of photos if @impmann can see anything I can’t? 

https://imgur.com/gallery/UmIfJrr

impmann

The good news is, the board is looking very clean. No sign of a big short and nothing obviously wrong...

jdbwales

impmann said:

The good news is, the board is looking very clean. No sign of a big short and nothing obviously wrong...

That’s fantastic news, thank you for looking @impmann ; - I couldn’t see anything broken or blown. I think what happened was the power valve went dodgy, then took out the fuse, then muggins here put a set of dodgy years-old, already knackered valves in and the same thing happened. I’ll put out an ad on here for a cheap but working set of EL34s and try again I reckon...

Kittyfrisk

^  Matched tested pair of Svetlana EL34's here  https://www.thefretboard.co.uk/discussion/200204/fs-various-thermionic-diodes-valves-more-added#latest

jdbwales

Kittyfrisk said:

^  Matched tested pair of Svetlana EL34's here  https://www.thefretboard.co.uk/discussion/200204/fs-various-thermionic-diodes-valves-more-added#latest

I’ve contacted Rod, will have those off him if indeed a fresh set of working 34s is the next step!

jdbwales

Thank you everyone, new EL34s and a bias (with a non working brand new multimeter hurdle on the way) and its working and sounding great! All help very much appreciated.

impmann

Great stuff!!!