New HT5-R - No Sound!

Discussion - HT-5 amplifiers.
Myles Loud
Posts: 5
Joined: Thu Feb 04, 2016 8:32 am

Thu Feb 04, 2016 11:52 pm

Is this the correct parts list? I want to order and get started; Also when installing the resistors do they only work soldered in one direction, hence the name , resistor?
1. ... UTF8&psc=1
2. ... B007Z7WGVC
3. ... UTF8&psc=1

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Posts: 1160
Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Fri Feb 05, 2016 3:47 am

You are thinking of a diode, not a resistor. A diode only lets current flow in one direction. A resistor will let current flow in either direction.

Assuming the MOSFET is bad, that should be all you need.

Based on that number, it looks like your was made in May 2011.

Myles Loud
Posts: 5
Joined: Thu Feb 04, 2016 8:32 am

Fri Feb 05, 2016 4:48 am

Thank you. I'm going to contact Sam Ash in the A.M. and see what they say about looking it over and if it might be covered. I will order the parts if not.

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Posts: 1160
Joined: Wed Jan 29, 2014 7:42 pm
Location: Virginia, United States

Fri Feb 05, 2016 3:29 pm

Good call. You are definitely out of warranty but, since it's a very well known defect, B* might just cover it. It's worth checking....

Posts: 1
Joined: Tue Mar 29, 2016 5:15 pm

Tue Mar 29, 2016 5:26 pm

God I can't believe this is still helping people.

I have had to change my user name slightly due to not signing into the new forum for so long but im back.

Just glad it is still helping people. My HT5 is still going strong nearly 4 years after the fix. Not even changed the valves. . . .Yet.

Just to clear up a few things as said the arc lines in the board were microscopic and will not be able to be seen even with an eye glass. I work for an electronic aerospace company so have access to very good equipment which helps and we only observed these lines under intense magnification.

If any one needs any help with this issue I am back and happy to help but thephantum seems to know his stuff more than me and seems a real helpful guy. As he has said this issue is isolated to a batch of boards from 2011/12 and please don't jump to the conclusion that this is issue with your amp if you experience similar problems.

All the best,


Posts: 1
Joined: Wed Sep 28, 2016 5:28 pm

Wed Sep 28, 2016 5:45 pm

I registered just to say thank you to everyone.

I had the same problem. Suddenly after 2+years my amp stopped working and i fixed it in 30minutes thanks to you :P

Just in case i can help someone else these steps might help you:
1) Open the amp's back case and locate TR2 and TR3. If they look burned then you need to replace them. Otherwise continue (in my case they were not).
2) switch the amp on and wait a couple of minutes (do not touch anything)
3) switch the amp off and UNPLUG the amp's power
4) quickly touch TR2 and TR3. If one of them is extremely hot then its probably the cause (otherwise this is not the same case).
5) remove the mainboard from its base.
6) locate the backside of the TR that was hot. Make sure there not problems with the PCB/solder.
7) drill two holes with a 0.8 drill between the legs of the hot TR. You don't have to desolder anything but make sure you do it slowly and do not drill through the TR: only the PCB.
8) plug the amp and check if it is working (don't touch anything while in power). It worked for me...

In my case it was the TR3. Here is a pic of my drilling :)
20160928_174011.jpg (146.32 KiB) Viewed 27206 times

Posts: 1
Joined: Sun Oct 02, 2016 6:05 pm

Sun Oct 02, 2016 6:35 pm

I have an HT5 with me for repair. I noticed it does have slots rather crudely cut through the PCB in-between the 3 pads for each STK830 MOSFET in the phase splitter. Whether this is a factory mod or not I couldn't say.

In any case the amp was quiet and distorted even on the clean channel. Inspection showed that only grid pin 7 of the output dual triode was getting any signal, almost no signal on grid pin 2. So I suspected TR2 and tried to change it.

Unfortunately the slots cut in the PCB had weakened it so much that the through hole pads just collapsed while I was trying to remove the MOSFET and clean out the holes. I had to cur the mess away and then rebuild some connections using fine copper wire. Not nice.

After all this the fault was still there. Then I remembered a problem I had with a Fender Pro Junior - the phase splitter just would not balance - one triode was almost fully conducting the other was shut off. Eventually I cleaned the PCB surface on both sides with isopropyl alcohol. Removed quite a lot of unseen dirt. The phase splitter on the Pro Junior then fell back into balance. I tried this on the HT5, didn't fix the problem although the PCB was quite grubby - covered in half removed flux resin.

Then I risked placing the probe from my digital scope (the scope manual contains dire warnings about the voltage limits of the inputs) onto the two 22K drain loads R38 and 39. A healthy signal on both of them. So it had to be some kind of open circuit between those outputs of the phase splitter and the grids of the output triode. I pulled pulled C45 (1uF 450V) and measured it. It was shot - 3nF and D of 4 ! just for laughs I pulled the other cap C46 which had been passing signal - that was in a pretty bad way too. Replaced both caps, checked the bias and balance and the amp was back to proper operation.

I have to wonder - electrolytic caps do not last well in hot environments and the Blackstar amps are notable for lack of ventilation. On the other hand maybe it was just a bad batch of caps.

Posts: 24
Joined: Thu Sep 24, 2015 6:38 pm

Mon Nov 07, 2016 3:57 pm

here's mine :(


Posts: 24
Joined: Thu Sep 24, 2015 6:38 pm

Mon Nov 07, 2016 10:20 pm

ok I'm confused about something.

I think I read here or in some other thread that R39 is supposed to be 22K Ohm, based on a schematic. However looking at the stripe pattern, only R38 looks like 22K Ohm (red-red-orange-gold). I'm having trouble even reading the colors on R39. I can imagine if resistance is lower than called for current would increase, leading to heat production and the burn pattern seen on the board.

So what's the deal with R39? What is the correct value and what is in the amp?

Posts: 1
Joined: Mon Dec 03, 2018 3:55 pm

Mon Dec 03, 2018 6:03 pm

Hi everyone!

I have a customers amp in, with the exact same problem. Even though this thread didn't see an update for quite some time I wanted to share an idea and hopefully get a response.
How about mounting the FET Transistors to the chassis to get the heat away from the PCB. They are fitted with a mounting hole for heatsinks anyway. The only problem I can imagine is, long leads picking up noise.

Any thoughts and comments are apreciated!

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