[PSP]

Hi Hugh,
Well, actually there was a thunderstorm.  My brother-in-law was watching TV during a thunderstorm and suddenly he saw a flash of light from the vicinity of the TV... he didn't see a lightning strike or hear anything, but lightning must have hit a power line some distance away and followed a path into his home.

A few days later, he turns on the TLP and AKSA 100 that I built for his 50th birthday and which are mounted just above the TV, got a gawd-awful squawk from the speakers, and shut down the power.

Looking at the dead 100w AKSA amp, R2=10R is toasted and open, R1 is OK, and C1 and C2 both read OK... this is true for both channels.  Can you suggest other parts I should check before applying power to the amp modules?  Both power supplies are apparently OK.

The TLP is also toasted, but I'd like to get the amp fixed first.  Obviously I'll build a surge protector before I give him the amps back...

Many thanks,
Peter

[AKSA]

Hi Peter,

This is bad news indeed.....  I'm very sorry about this.

Try replacing R2, 10R, then with fuses removed, switch on and see what happens.

It's most likely that the diff pair devices are fused, and possibly the VAS.  There is also a chance that the outputs are kaput, but it's 50:50.

If you monitor the output stage idle current with a DMM across the 100R fuse resistors at switch on, if they are blown there will be a very large voltage, more than 18V.  I'd expect around 12V across them momentarily with standard biasing;  it can only withstand this for a few seconds, so try to make it a snappy measurement!!

If it's 12V or thereabouts, switch off, put in good rail fuses (7.5A each), and switch on.  Then measure bias across the emitters, and output offset voltage, which should be less than 30mV.

If these checks come through, the amp is otherwise undamaged.

Good luck, Peter, a bolt out of the blue can be quite damaging!

Cheers,

Hugh

[Geoff-AU]

Surge protectors will help block small zaps, but the only thing that will stop a lightning hit is physically unplugging your equipment during a storm.

That said, I don't bother unless it's really close...  

[PSP]

Hi Hugh,
I'm lucky this time... all that was wrong with the 100W AKSA was R2.  I replaced those, checked voltages, bias, and offset and all is well.  The final test = music, and that works too!  

The TLP also seems OK, except for one strange voltage:

transformer and diode bridge are OK
Earth to R7 and R7' = 134vdc
Tube pins 4-5 = 6.13vac
Across R8 = 31.5 vdc, should be 12-13.

On switch-on, I hear the normal hum for 5-10 seconds, then quiet.  It plays music and sounds fine (although I haven't listened carefully in my main system).  Thoughts?

Many thanks!
Peter

[AKSA]

Hi Peter,

This is good news!!  Your brother has been very lucky......

31V across 1K is 31mA, which is about double what it should be.  Is this a 1K resistor, in fact?

The voltages are otherwise OK, however, and I'm not surprised it's playing OK, but again, the voltage across R8 is perplexing.  Could you measure it again?

Cheers,

Hugh

[rimljan99]

If you got separate earth line you could buy some surge protectors from Belkin.


They really work

[PSP]

Hi Hugh,
Following your PM, all is well.  I'm listening to the TLP and amp as I write.

Thanks for your help,
Peter

[PSP]

Hi Hugh,
While my brother-in-law's 100W AKSA has been making music for a week now, my GK-1R took a shot in a storm while we were away for the weekend.

Because a stormy weekend was predicted, I turned off the GK-1R.  Since  it was protected by a Jon Risch line filter/surge protector, then  a simplified Felicia line conditioner, I thought it would be OK.  Not enough protection, I guess.  The symptoms and evidence:

1.  The GK-1R was fine on Friday before we left (I've been running it 10 days straight to break in a cap at C1, listening carefully all the while).    

2.  Sunday night on our return, the GK-1R plays music from only the left channel.   The right channel sounds dead, unless the AUX source is selected and the HT bypass is enabled... under those conditions, the GK-1R plays from both channels, and all sounds good.

3.  With the HT bypass de-selected, the left channel sounds normal, the right channel is utterly silent, except when the volume pot is moved... then you hear a "scratchy pot" noise from the right channel while the pot is being moved, otherwise the right channel is silent... until you select the AUX source and enable HT bypass, then you hear nice music from both channels.

Thoughts??

Needless to say, next time I'll pull the plug on everything in the house when a storm approaches.  My mother used to do that all the time, she used to unplug the toaster!  I thought she was a bit nutty at the time but I guess not.

Many thanks,
Peter

[AKSA]

Peter,

Chances are you've toasted EITHER the power supply transistors on the SS section of the right channel, OR the input diff pair on the right channel.

Either/or, and possibly both.

There is absolutely no chance you've hurt the tube.  It should be fine.

Measure power supply voltages on the right channel SS section;  they should be +/-14 to 15 volts.

Measure them at the rails, after the series pass regulator transistors.  There's a very good chance that the regulator transistors are toast.

Cheers,

Hugh

[DSK]

Needless to say, next time I'll pull the plug on everything in the house  when a storm approaches. My mother used to do that all the time, she used to unplug the toaster! I thought she was a bit nutty at the time  but I guess not. ...

I remember years ago, parts of Melbourne were hit with a massive surge and many electrical appliances that were plugged in but switched off at the wall outlet were fried. Ever since then, I tend to unplug things not in use.

[PSP]

Hi Hugh,
I finally got the GK-1R opened up and made some measurements.

Dead Channel:
LED is not lit
LED cathode (K) = 13.5v (wrt 0v on the PCB)
LED anode = 14.5
T5 emitter = 14.5v
T6 emitter = -13.6v


Good Channel:
LED is lit
LED cathode (K) = 12.3v (wrt 0v on the PCB)
LED anode = 14.2
T5 emitter = 14.5v
T6 emitter = -14.6v

Thoughts?

Many thanks,
Peter

[AKSA]

Hi Peter,

First up, replace the LED with a standard, green 5mm item, inexpensive at Rat Shaque.....  The voltage has dropped to such a low value the output stage cannot work to spec.

This will fix the obvious problem from your measurements, but there may be other damage.  The power supplies would appear OK.

Hope this helps!

Cheers,

Hugh

[PSP]

Hi Hugh,
I replaced the LED on the dark side and the new one was dark too!  The original LED tests good, and lit up as I tested it with my multimeter.  Voltages were similar to the last readings:
  anode = 14.6v
  cathode = 13.5v

Since AC will be down for a day or two, please e-mail your next suggestions to me.

Many thanks,
Peter

[Grumpy_Git]

Hi all

I just had my own little scare with my GK1-R and 55N, it was sitting in the bedroom turned on with nothing playing when i heard a crackling from it, I went through and saw the speakers were pumping away, turned everything off and after a minute or two back on. Everything seems fine and both channels are playing music.

Any suggestions as to what happened?

Nick

[AKSA]

Nick,

I can't work this one out at all, unless it was a slow mains voltage spike!!

Hugh

[Grumpy_Git]

Cheers, thought I'd ask anyway as i was catching up posts on the circle.

Just bought a pile of 14 valves off ebay, 3 mullards, what looks like 4 military grade and the rest cheapies, not bad for £28, its a bit of piece of mind for if any die, I'll have instant replacements.

Think I'll be waiting a few months at least before I consider a lifeforce as well, sorry Hugh.

Nick.

[PSP]

Just to document the "final answer" on this problem, after a number of e-mails with many voltages measured, Hugh suggested that I replace T3 and T4 on the bad channel.  Instant music, problem solved.

Peter

[AKSA]

Thanks Peter!

Nick,

I wouldn't bother replacing everything if it all works, and works well with no nasties in the sound.

Generally, when passives fail traumatically (overvoltage, overcurrent, reverse polarity or heat damage) you can see it;  semi damage is often invisible, but the acid test is this:

THE BASE EMITTER VOLTAGE OF A FAILED SEMICONDUCTOR, NPN OR PNP, IS ALWAYS EITHER ZERO VOLTS (SHORT) OR WAY OVER THE CORRECT 0.6V (USUALLY 5V MORE)

Cheers,

Hugh