BLACKSTAR HT5TH strange behavior

Forum to discuss amp circuits besides the 18 Watters and their ilk.

Moderators: JMPGuitars, CurtissRobin, Daviedawg, Graydon, zaphod_phil, rock_mumbles

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Wed 11/06/19 3:27 am

blackstar-amplification-ht-5th.jpg
I have got a Blackstar HT5TH 2x10 Combo with a very strange behavior.

I've been having some issues with my ht5TH combo (5th anniversary edition) recently. For some reason, the gain and volume controls on the od channel are not working as they should. With volume at 0, the amp can still reach very high volumes and distort by turning the gain knob alone. The volume knob still influences the loudness, but at a lesser level. The gain that used to distort already at 9 o'clock now stays clean up until about halfway. It is as if the lower volume threshold has been raised. Or rather the amp has turned into an old-school one-volume amp with an extra volume control that does slight adjustments. My guess is that the soldering has shorted at some place but i'd like to hear some expert opinions.
cheers,
Steve
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1422
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: BLACKSTAR HT5TH strange behavior

Postby JMPGuitars » Thu 11/07/19 7:40 am

First, try different tubes, just for the hell of it. Then test the pots. Check the outer terminals, then test the sweep range.

If that doesn't help, then test every component around that preamp.

This schematic is probably correct: http://schems.com/bmampscom/blackstar/b ... ematic.pdf
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
Amp Tools & Test Equipment
(anti)Social: Facebook · Instagram

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

Re: BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Fri 11/08/19 2:28 pm

JMPGuitars wrote:
Thu 11/07/19 7:40 am
First, try different tubes, just for the hell of it. Then test the pots. Check the outer terminals, then test the sweep range.

If that doesn't help, then test every component around that preamp.

This schematic is probably correct: http://schems.com/bmampscom/blackstar/b ... ematic.pdf
I have tried a variety of Tubes. When a new Tube is used. The amps works for two or three hours. And then goes back to the same behavior, as listed above.
I would assume that there is something wrong with some part in the preamp section that blows the tube.BTW,how do I test the IC's in there?

cheers,
Steve

P.S:
thx for the Schematic!
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1422
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: BLACKSTAR HT5TH strange behavior

Postby JMPGuitars » Fri 11/08/19 3:08 pm

gretsch61 wrote:
Fri 11/08/19 2:28 pm
I have tried a variety of Tubes. When a new Tube is used. The amps works for two or three hours. And then goes back to the same behavior, as listed above.
I would assume that there is something wrong with some part in the preamp section that blows the tube.BTW,how do I test the IC's in there?
Interesting. That makes me think that something is overheating. It could be a bad solder joint, or maybe an ill-fated component. Can you post a gut shot?

The easiest way to test ICs is with an IC tester. I have a super generic one that works pretty well... TC1 multi function tester. You can get them from $15 to $30, but check the feedback first. Besides that, some chips can be tested with a DMM if you look up the datasheet for the chip.

Googling it, it appears that people have had issues with some resistors becoming loose from overheating. A lot of posts reference R38 and R39. Check those out.

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
Amp Tools & Test Equipment
(anti)Social: Facebook · Instagram

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

Re: BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Sat 11/09/19 3:45 am

JMPGuitars wrote:
Fri 11/08/19 3:08 pm
gretsch61 wrote:
Fri 11/08/19 2:28 pm
I have tried a variety of Tubes. When a new Tube is used. The amps works for two or three hours. And then goes back to the same behavior, as listed above.
I would assume that there is something wrong with some part in the preamp section that blows the tube.BTW,how do I test the IC's in there?
Interesting. That makes me think that something is overheating. It could be a bad solder joint, or maybe an ill-fated component. Can you post a gut shot?

The easiest way to test ICs is with an IC tester. I have a super generic one that works pretty well... TC1 multi function tester. You can get them from $15 to $30, but check the feedback first. Besides that, some chips can be tested with a DMM if you look up the datasheet for the chip.

Googling it, it appears that people have had issues with some resistors becoming loose from overheating. A lot of posts reference R38 and R39. Check those out.

Thanks,
Josh
Hi Josh,

I will check these resistors and get back to you, after I have checked those.

cheers,
Steve
0 x

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

Re: BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Mon 11/11/19 6:16 am

gretsch61 wrote:
Sat 11/09/19 3:45 am
JMPGuitars wrote:
Fri 11/08/19 3:08 pm
gretsch61 wrote:
Fri 11/08/19 2:28 pm
I have tried a variety of Tubes. When a new Tube is used. The amps works for two or three hours. And then goes back to the same behavior, as listed above.
I would assume that there is something wrong with some part in the preamp section that blows the tube.BTW,how do I test the IC's in there?
Interesting. That makes me think that something is overheating. It could be a bad solder joint, or maybe an ill-fated component. Can you post a gut shot?

The easiest way to test ICs is with an IC tester. I have a super generic one that works pretty well... TC1 multi function tester. You can get them from $15 to $30, but check the feedback first. Besides that, some chips can be tested with a DMM if you look up the datasheet for the chip.

Googling it, it appears that people have had issues with some resistors becoming loose from overheating. A lot of posts reference R38 and R39. Check those out.

Thanks,
Josh
Hi Josh,

I will check these resistors and get back to you, after I have checked those.

cheers,
Steve
Hi Josh,

If these resistors are blown up or soldered out, this would mean, that the circuit board has a short somewhere in the area under TR2 or TR3.
It's going to be interesting to find out. I will get the pics from inside the amp as soon as I get back from my trip on Thursday, and let you know.

sheers,
Steve
0 x

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

Re: BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Wed 11/13/19 9:34 am

Hi Josh,
I have checked the PCB. It is already a new one, that has cuts between the TR2 and TR3's pins to make sure that no short would take place. Never the less, the two resistors R38 and R39 were soldered out on one side.
I have soldered them back, but the problem has changed to the fact, that never the less when I change the 12AU7 tube the behavior stays the same, that the I hear sound that accellerates quickly without using the Volumepot of the Gainsection. I have measured both, gain and vol pot of the OD-Section and they are fine, returning 90k for the Gain and 22k for the Vol.
Somehow I feel lost now.
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 1422
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: BLACKSTAR HT5TH strange behavior

Postby JMPGuitars » Wed 11/13/19 10:54 am

Get ready for a goose chase? I'd look for anything that stands out. Bad solder joints, anything that looks swollen or overheated, etc... Use a chopstick to test any actual leads, or tap components.

If that doesn't shed any light, you can pick a spot somewhere on the schematic and start the highlighter game.

If all else fails, smash it with a hammer and build a lite 2b to throw in there. ;)
2 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright
Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
Amp Tools & Test Equipment
(anti)Social: Facebook · Instagram

User avatar
colossal
Occasional poster
Occasional poster
Posts: 200
Joined: Thu 05/10/07 2:00 am
Location: In and around the lake

Re: BLACKSTAR HT5TH strange behavior

Postby colossal » Wed 11/13/19 12:34 pm

JMPGuitars wrote:
Wed 11/13/19 10:54 am
If all else fails, smash it with a hammer and build a lite 2b to throw in there. ;)
I will strongly second this motion. I will go further to suggest saving yourself the hassle and NOT troubleshooting, gutting it, and putting a Lite IIb circuit in there right now :lol:
2 x

Daviedawg
Superior Amp Tech
Superior Amp Tech
Posts: 529
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: BLACKSTAR HT5TH strange behavior

Postby Daviedawg » Thu 11/14/19 4:00 am

PCB problems seem almost commonplace these days across the range of brands. Poorer manufacturing practices or poor boards?

Dd
0 x

gretsch61
Newbie
Newbie
Posts: 91
Joined: Wed 02/20/13 5:53 am

Re: BLACKSTAR HT5TH strange behavior

Postby gretsch61 » Thu 11/14/19 10:04 am

Ok guys,

I have found the problem and solved it!
I have taken the box apart and took the chop stick method to check wether there were any parts not beeing soldered wright or soldered out by heat.
I also changed R38, R39. When trying on R10 (since it is connected to R38 and R39 directly), I detected that one leg was gone. So I found the problem that caused it all.
As a precaution I have resoldered TR2 and TR3, because even that Blackstar has a new PCB in there, the area around those two Transistors looks awfully brown.
After these changes, the amp works as good as new!

cheers, and many thanx for all your help especially Josh, who has lead me to this solution!

Steve
2 x

Daviedawg
Superior Amp Tech
Superior Amp Tech
Posts: 529
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: BLACKSTAR HT5TH strange behavior

Postby Daviedawg » Fri 11/15/19 3:16 am

Well done. Enjoy your amp.

Dd
0 x

User avatar
colossal
Occasional poster
Occasional poster
Posts: 200
Joined: Thu 05/10/07 2:00 am
Location: In and around the lake

Re: BLACKSTAR HT5TH strange behavior

Postby colossal » Fri 11/15/19 3:27 pm

Well done.

But there is always time to gut it and put in a turretboard Lite IIb circuit :)
1 x


Return to “That's Not 18 Watts”

Who is online

Users browsing this forum: No registered users and 1 guest