Blackstar HT5 bad sound

I got this nice little amp to fix. It fizzed. I checked the tube : OK … then I saw R39 resistor was burnt.

I spiced the FET PI, replaced the burnt resistor and compare voltage : good !

FET-PI

 

(I used IRF150 model instead of IRF830 …)

 

A few days later, my customer came back : his amp had the same problem, again !

R39 burnt !

Burnt_R39

I found this topic on the Blackstar forum (very helpful, thanks a lot, guyz !) : http://www.blackstaramps.com/forum/viewtopic.php?f=32&t=2135&sid=6267625e63d87d9553d419c17f908d83

BadTraceTrace broken, to rebuild …

Once TR3 is removed, I could check it (see : http://www.decibelcar.com/menuelectro/93-fettest.html , very helpful, thanks !) : TR3 OK

R39 and C46 removed, let’s check if there is a short somewhere :

ShortWe should get an infinite resistor between Source and Drain holes … and we read 50kΩ !

It seemed this amp had a bad PCB, so I drilled hole (0.7mm) to isolate TR3 pins, as suggested in the Blackstar forum :

Drill

 

and checked :

checkTR3pinsPerfect !

 

I removed R38, TR2 and C45 : no short there !

Soldered new 22k resistor, TR3, C46 … amp to the test !

Bias : 

Tips : Measure voltage across 4,7ohm resistor on cathode (=R33, common 12BH7 cathode resistor). Adjust to mV reading of approximately 46mV. Adjust balance pot for least amount of hum from speaker

12BH7 voltages :

  • Anode : 470V (!)
  • Grid 1 : -29V
  • Cathode : 45mV

Blackstar-ht5-Bias-R33

Hope this helps

 

 

 

Website Pin Facebook Twitter Myspace Friendfeed Technorati del.icio.us Digg Google StumbleUpon Premium Responsive

This entry was posted in Amps. Bookmark the permalink.

16 Responses to Blackstar HT5 bad sound

  1. Kostyan says:

    thank you so much! very helpful!

  2. bobatious says:

    Excellent instructions and photos — thanks.

    I just fixed an HT-5C with the exact same problem.

    R39 was very fried and so was the trace to R10.

    After removing R39/TR3/C46, leakage amount was even higher than you observed, but only between drain and gate.

    I remedied the PCB leakage via carefully digging “trenches” on either side of board using an exacto knife until measuring no connection. This seemed safer than drilling through, and was effective. As the board is double-sided as opposed to multi-layer, there is little chance of connection _within_ the substrate.

    Also I upgraded the R39/R38 pair to from 1W to 2W metal film resistors to get a little extra safety margin.

    Thanks and cheers.

  3. t-geek says:

    Thanks bobatious for the tip with exacto knife !

  4. mike says:

    i am confused r33 is this the real tiny resistor labeled r33 in little white ink or the big white box that says 7w33ohmj i saw a video of a guy biasing the ht 20 and it looked like he clipped his multimeter clips to the bottom of that box first but i looked again and he is clipping it to the white box at the bottom is this what you are talking about

  5. Marco says:

    I have a problem, I made the repair you say but I get that voltage at all points of the valve 220v only appear approximately ‘ve measured SP5 and I get at most 350V and 220V SP4 . I have problems with the transformer ????

  6. Dear T-geek.
    Had this one on the bench with the same problem. After reading your post – problem solved! Million thanks!!!

    • t-geek says:

      Glad to help, Laslo !

      Recently, I had to re-read my post as I got another Blackstar HT5 with the same exact bug to fix !

  7. Barry says:

    here is a video for the process to fix this common issue as per blackstar circular

    https://youtu.be/iSrQEPKTlZw

  8. I had a Blackstar HT5-5TH with no sound. After much searching, I found that the op amp on the back board, next to the Send/Return jacks, was bad. Signal in from the return jack would not pass through it. I should have known earlier when I found signal coming out the Send jack, but inserting signal into the Return jack did nothing.
    Thanks for lots of ideas and info on what goes wrong with these amps. I think they are overly-complicated.

  9. Igor Naymark says:

    I had absolutely same problem. But I lifted S pins of FETs, cut PCB traces and connected them to capacitors with wires.

Leave a Reply to Marco Cancel reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>