Blackstar HT-5 technical help please!

ecc83

Ok, Hearing nothing I can tell you this. If Emulated Out is working signal must be getting to FET 6 (j175*) which gates off the drive to the PI when headphones are plugged in thus stopping drive to the 12BH7.

The gate circuit is arranged around TR5 (BC182**) and so something is awry in this circuitry and stopping drive to the PI and hence PA. 

First check, link FET's drain to source. Sound? If so, 'cheat' the FET' s gate with a voltage. I am sure the tch can work it out from there.

*Almost and P chan FET will serve. ** Almost any ssig NPN transistor will do!

Dave

metalshredder

@ecc83 I need your help; I have an HT5 head with a very distorted clean channel.  I know it's not working properly because I have an HT5R combo that has a very clean clean channel, which barely begins to show grit when it nears full volume.  The HT5 head is just plain dirty at any volume.  It's not right.  I've changed the tubes, biased the power tube and set balance, and no real difference to speak of.  Any thoughts?  Point of reference, my combo amp, I bought brand new, the head I got used.  So there could be something the matter with it anyway.  I did send you a PM some time ago but I read above that you might not be getting your alerts.

ecc83

Had look at the map. Is the dirty channel "cleaner" than the clean channel when the former is at minimum gain?

If so I can only suggest FET1 or associated components. IC2a is the common input amplifier and IC2b is in the clean circuit. I have never had just one side of a dual chip fail but I guess it is possible. Check voltages on both output pins, should of course be close to 0V.

FET1 might not of course actually be faulty, the control voltage to its gate could be awry and biasing into some half on-half off-half arsed state.

This will be tricky to track down without a sine wave source and a scope I fear.

Dave. (PM sent)

metalshredder

@ecc83 was your May 10 post in response to me?  If so I apologize for the LONG delay, the site never notified me of your reply.  I'll go check my amp and answer your questions.  Also if that was to me, I didn't get a PM either.  maybe the site is messing up.

metalshredder

@ecc83 yeah, I'd say the dirty channel at min gain is a bit cleaner than the clean channel.  So you'd suggest Fet1?  Should I go ahead and replace the cap and diode too? Also do I need to discharge the caps to do the work?

ecc83

metalshredder said:

@ecc83 yeah, I'd say the dirty channel at min gain is a bit cleaner than the clean channel.  So you'd suggest Fet1?  Should I go ahead and replace the cap and diode too? Also do I need to discharge the caps to do the work?

As you want, have had FETs fail (not just in B's kit) but not a low V diode and cap in donkeys. I never bothered to dump caps but then I have been working on valve kit (and nastier!) for 50+ yrs.

Have to say, this sort of fault is very hard to trace without a scope.

Notification? NEVER got that from FBD. If anyone knows how to set it up, do tell.

Dave.