Pilot light on - everything else dead. Blackstar HT-60

[glebert]

Looks like they use the PIC chip to turn the bias on when a speaker and input plug are inserted? Interesting...Can't say I like it...

[Steve Blackdog]

This is from the BS test spec. Any idea what a “Hum Balance Test Jig” might consist of?

BIAS AND BIAS BALANCE SET-UP:-

1. Using a DC Millivolt Meter, with a full scale range of 200mV, connected between TP1 & TP7 adjust PR2 for a reading of 50mV
2. Connect the Hum Balance Test Jig to the Speaker Output (do not disconnect the resistive load)
3. Connect a DC Millivolt Meter, with a full scale range of 200mV, across SP1 and SP2 of the Hum Balance Test Jig
4. Adjust PR1 (Mid PCB) for a minimum reading
5. Repeat steps 1 through 4

BIAS CONTROL TESTS:-

With the DC Millivolt meter still between TP1 & TP7:-

1. Change the STANDBY switch to ‘STANDBY’ and check reading drops below 5mV
2. Change STANDBY switch back to ‘ON’.
3. Remove the Input Shorting Jack and check reading drops below 5mV
4. Replace the Input Shorting Jack
5. Remove the Speaker Jack and check reading drops below 5mV
6. Replace the Speaker Jack

if anyone is interested the whole test spec is attached (courtesy of Blackstar Tech Department)

[Helmholtz]

This is from the BS test spec. Any idea what a “Hum Balance Test Jig” might consist of?

Actually no, but you don't need it.
Bias balance adjustment for minimum hum can be easily done by ear with speakers connected. Or use a DMM that displays mV AC.

But that would be the last step of amp maintenance.

What are your results (post #56)?

[Steve Blackdog]

What are your results (post #56)?

New fuses are due to arrive tomorrow!

[Delta362]

I haven't heard of a hum balance test jig. I just have the amp powered and adjust a hum balance pot for minimum audible noise thru the speaker without a signal being injected.

Thanks for sharing their test procedure!

[Steve Blackdog]

With the bias fuse in the voltages are as Helmholtz predicted. My DMM is not great, so voltages might be out.

TP13 = -64.5V
Pin 5 of V3 and V4 = -55V

what do you suggest next?

I have my new tubes, but don’t want to damage them if not safe to do so.

I am still concerned that the max voltage at pins 3&4 of V3 and V4 is 370V. It should be 570V.

all tests with 12AX7s in and EL34s out.

thank you!

[Helmholtz]

With the bias fuse in the voltages are as Helmholtz predicted. My DMM is not great, so voltages might be out.

TP13 = -64.5V
Pin 5 of V3 and V4 = -55V

If voltages are steady, they look good/safe and C122/123 should be good as well.

I am still concerned that the max voltage at pins 3&4 of V3 and V4 is 370V. It should be 570V.

What voltage do you measure at SP4 (OT CT)?
Do you use a bulb limiter?

[Steve Blackdog]

If voltages are steady, they look good/safe and C122/123 should be good as well.




What voltage do you measure at SP4 (OT CT)?
Do you use a bulb limiter?

tested with 12AX7s out as well and SP4 = 445V. Same on pin 3 &4 of the el34 sockets.

yes, I am using a light bulb limiter with a 40w bulb.

[Helmholtz]

yes, I am using a light bulb limiter with a 40w bulb.

On a bulb limiter all amp voltages are lower than normal, as the bulb "steals" a part of the mains voltage.

Do you have a 100W or at least a 60W bulb?
If so, use it in the limiter and insert all tubes and correct fuses.

[Steve Blackdog]

I had best check all of our lights for another incandescent bulb! They are increasingly hard to find in the UK.

[nosaj]

I had best check all of our lights for another incandescent bulb! They are increasingly hard to find in the UK.

What do you replace your refrigerator light bulb with or the one in your stove? These are usually 25 or 30 or 40 watt bulbs

nosaj

[nosaj]

[eschertron]

And remember, this isn't rocket science.. or so they tell me. If all you can find are 40W bulbs, wire two in parallel for an 80W 'steal'. It doesn't have to be pretty, just work.

[nosaj]

And remember, this isn't rocket science.. or so they tell me. If all you can find are 40W bulbs, wire two in parallel for an 80W 'steal'. It doesn't have to be pretty, just work.

You really don't ever need more than a 40 watt bulb. All the dim bulb is there for is to protect everything from a serious failure or miswire. After The bulb lights up and dims back down you are mostly safe for full AC power.

nosaj

[Steve Blackdog]

Okay, so I will stick with my 40w lbl for now.

As mentioned before, my LBL shines bright with two tubes, irrespective of tube or socket. I have narrowed it down to the heater circuit. It’s very simple. 12VAC from the PT one to each Pin 2 and both pin 7s to earth and each other.

this has me scratching my head? I am wondering if that is to be expected with a 40w LBL?

ps. I wish I could out a light bulb in parallel to my brain and double its output!!

pps. Just found a 100w light bulb in outside porch!!