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GK1 Repair

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AKSA

GK1 Repair
« Reply #20 on: 22 Feb 2006, 11:02 pm »
Steven,

If you reduce R3 from 1K8 to 1K5 your offset will come down to sub 10mV.

You could operate it as is no problem, however.

I have MANY 1K5 resistors, Steven, so if you'd like one give me a shout.....!   :lol:

Cheers,

Hugh
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stvnharr

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GK1 Repair
« Reply #21 on: 23 Feb 2006, 04:40 am »
Thanks Hugh,
I'll have to go look thru my resistor supply cache.
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stvnharr

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GK1 Repair
« Reply #22 on: 25 Feb 2006, 10:38 pm »
Hugh,
Installed 1k5 at R3, but only lowered offset a couple mv's to 32-33.
Sound is okay.  Should I just leave things alone now, or reduce the R3 a bit more?

Steve
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AKSA

GK1 Repair
« Reply #23 on: 26 Feb 2006, 01:38 am »
No, Steve,

Leave it as is.  Next time I visit I will have another look at it.

Cheers,

Hugh
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stvnharr

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GK1 Repair
« Reply #24 on: 26 Feb 2006, 05:24 am »
Sounds good to me.
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stvnharr

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Deja vu all over again
« Reply #25 on: 17 Mar 2006, 01:39 am »
Hugh,
The channel with the highish offset, to which I wired up the blue On/Off LED has now gone out.  I knew I should have just left things alone!!!!!

This happened late last night while listening to music.  Left channel went KA-THUMP and then HMMMMMMMMMMMM.  So I shut things off, disconnected all connections, turned back on, saw LED off, and knew it was deja vu all over again.  Music had been playing for about 30 minutes when it all happened.

Anyway, here are a whole bunch of readings which may give some light on which transistors have let loose.

                                       Good Channel                    Bad Channel
Across R12                        1.26v                                315mv
Across R8                          .633v                                .48v
Across R4                          1.59v                                1.50v
Top R12                            14.8v                                14.2v
Bottom R8                        -14.8v                               -14.6v
Top R16                           -20.9v                               -22.3v
To R15                              21.23v                               22.5v
Top R6/7                           .637v                                 .66v
To R9/10                            24mv                                12.34v
To R10/11                          2.6mv                               14.34v
Top LED                             12.96v                              13.57v
Btm LED                            14.88v                               14.69v
Top R11                             37.9mv                              11.44v
Btm R11                              3.3mv                              14.36v
LED on/off                            ON                                     OFF

Do I correctly surmise that T4 (557) is gone, and maybe T3 (550)?



Steve
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AKSA

GK1 Repair
« Reply #26 on: 17 Mar 2006, 05:34 am »
Yes, Steve,

Somehow or other you have stressed T3 and T4 and both are gone south.

All other readings look OK.

I will send a couple in the post.  This is unfortunate, but it could be you overloaded something with the new LED.

Cheers,

Hugh
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stvnharr

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GK1 Repair
« Reply #27 on: 17 Mar 2006, 06:47 am »
Hugh,
Thanks.
I will replace the transistors and then test everything.  If all okay then I will put the blue Led back on, then test again.  I just had it on the +17v and 0v posts from the transformers.  I also have to make the dropper resistor a fair bit stronger as the light was extremely bright.
Thanks again.

Steve
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AKSA

GK1 Repair
« Reply #28 on: 17 Mar 2006, 09:47 am »
Steve,

You shouldn't put an LED on AC!!  It quickly destroys them;  you need to put a diode in series with the dropper resistor.

Better to run the LED from the raw DC across C15.

Cheers,

Hugh
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stvnharr

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GK1 Repair
« Reply #29 on: 17 Mar 2006, 11:00 pm »
Hugh,
Hm, I should have followed last weekend's advise to "not change a thing"!
I found that with the new case that the open tubes didn't have much glow and that I wanted a little "on" light.  Now I'm committed with a hole.

I think I can find a suitable place to tap in to DC.

It just goes to show that "it's always something".

Steve
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Felipe

GK1 Repair
« Reply #30 on: 18 Mar 2006, 12:26 am »
Quote from: AKSA
Steve,

You shouldn't put an LED on AC!!  It quickly destroys them;  you need to put a diode in series with the dropper resistor.

Better to run the LED from the raw DC across C15.



I have my LEDs connected to AC on 17V in the GK1 and to 35V in the AKSAs.
No problems whatsoever . Been running for months in the aksas with no stress.
Do my LEDs have a diode incorporated maybe ?? Didnt find much difference that connecting them to DC....and its a lot easier.

Regards,
Filipe
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AKSA

GK1 Repair
« Reply #31 on: 18 Mar 2006, 03:03 am »
Felipe,

With a large dropper resistor, current is adequately limited both ways.

However, as a general engineering rule it is verbotten and it is not an elegant solution.

There really should be a diode in series with the dropper.

Cheers,

Hugh
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Geoff-AU

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GK1 Repair
« Reply #32 on: 18 Mar 2006, 06:42 am »
LEDs are a diode (light emitting diode is their name), however their reverse bias breakdown is only a few volts. like Hugh says it is bad practice to power an LED off AC, because even if it seems to work, current is flowing in the wrong direction during the negative portion of the cycle and LEDs are not designed to tolerate this.
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stvnharr

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GK1 Repair
« Reply #33 on: 18 Mar 2006, 07:02 am »
Hmmmm, the AC issue seems likely to have been my troubling issue, since I tapped into the 17v AC post.  It just took 30-35 minutes of music to kick in, rahter that the last time, which was a millisecond.  At least I got off a whole lot easier this time!!!

I will definitely be tapping in to DC!!!!
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rabbitz

GK1 Repair
« Reply #34 on: 18 Mar 2006, 02:32 pm »
I had to tap in the 6VAC filament wiring as I needed a higher current than the 17VAC could provide for a remote volume control.  As advised by Hugh, I came off one transformer 6VAC to a full wave rectifier > 1000uF cap > 7805 reg. Works a treat and use this supply for the LED as well.
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stvnharr

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It's not over yet!
« Reply #35 on: 23 Mar 2006, 06:10 am »
Hugh,
I've replaced T3 and T4, but still not working.

Here's some relevant readings:
Across R12    7.6v
Across R4      2.2v
Top R6/7       5.2v
Offset            5.9v

Most other readings look close to the "good" channel.

Should I just replace the whole lot?  I have enough to do T1-6, although T1 and T2 are not likely to be a matched pair.
Are the newly replaced T3 and T4 now automatically bad?
Capacitors all seem okay.

Steve
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AKSA

GK1 Repair
« Reply #36 on: 23 Mar 2006, 07:57 am »
Steve,

It's tricky and time-consuming for me to do this in a public forum, particularly one called 'GK1 Disaster'!

7.6V across R12 looks really bad;  it should be 1.25 volts, and indicates the LED is faulty.  Even 2.2V across R4 is not good, as this indicates a lot of current is flowing - it should be 1.5V - and this could indicate the diff pair is blown too.

I think I'd better see this up close, I'm sorry.

I posted off the transistors a couple days back.

I'm sorry for delays.

Cheers,

Hugh
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stvnharr

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GK1 Repair
« Reply #37 on: 23 Mar 2006, 10:32 am »
Hugh,
Thanks for the timely reminder!

Steve
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AKSA

GK1 Repair
« Reply #38 on: 23 Mar 2006, 11:55 pm »
Guys,

I've just been told by Steven that it's all fixed;  it was the LED and diff pair, and all is sweet.

I will now close this thread!

Cheers,

Hugh
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