VOX DEFIANT SS AMP. REPAIR - diyAudio
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Old 21st April 2011, 02:02 PM   #1
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Default VOX DEFIANT SS AMP. REPAIR

Hi all,while i received a non working a 1960's Vox defiant solid state amplifier and i decided to repair it.
after opening it i found 3 shorted BJT and some blown up resistors ant pots.
my only problem is that i cant adjust VR 12,13,14 and if i turn it one without adjusting them R103 & R106 goes too hot.
so if any one can help me i will appreciate it
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Old 21st April 2011, 02:26 PM   #2
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Measure your voltages without the output transistors (TR16 and 17) installed. Replace TR15 with a resistor (about 400 Ohms @5W) and measure. E
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Old 21st April 2011, 02:33 PM   #3
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you mean voltages shown in schematics
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Old 21st April 2011, 04:39 PM   #4
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Yes. This way you can adjust and measure away without destruction. E
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Old 21st April 2011, 04:45 PM   #5
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how can i
Quote:
Originally Posted by mickeymoose View Post
Replace TR15 with a resistor (about 400 Ohms @5W) and measure. E
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Old 21st April 2011, 05:38 PM   #6
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That is the exact same circuit as my sunn concert slave amplifiers they are in a BTL configuration.

They have a 910 ohm and 22 ohm resistor in the bias circuit and I think my sunn concert lead amp had 820's (im not sure as it was 30 years since I had that amp).

Or maybe I was going to change to/or from 910's to 820 or vice versa or something,Due to an over heating issue of the bias resistors when I changed the output devices from a 2N3055 to the more heavy duty 2N3772 and 2N3773 series transistors.

Anyhow I don't have the concert lead amp anymore but I do still have the slaves and they use 910 ohm on the top half of the bias.

The last time I fired it up I was driving it quite hard and it devolped similar issues and one or two of the resistors became hot enough to start smoking although the amp seems to be working okay every time I turn it on.

So,I just put them away and the have been sitting.
Sorry, I don't have any more info as I have not taken the time to find the cause of the problem yet.

I dont see the logic in replacing TR15 with a resistor as this is a transformer coupled stage and has nothing to do with the bias of the output transistors.
Just removing TR15 from the circuit will cause the desired effect. jer
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Old 21st April 2011, 05:44 PM   #7
Bone is offline Bone  United Kingdom
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R105 & 106 are going to run hot they will be dissipating about 1.5 watts each. I would replace them with 5 watt resistors, to improve reliability.
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Old 21st April 2011, 06:09 PM   #8
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Check the resistances of R103 and R106 as I suspect that these are old carbon type and the do change value quite a bit with age and I suspect that this is the same problem I have with my amp aswell.

Especialy after once they have over heated.

I have run my amp very very hard through the years and have blown it up lots (this used to be my favorite past time because they are so easy to fix) and I have used every thing from plastic tabs (TIP3055) to 2N3773's and it has been very durable and reliable to me.

The only issue I have had when was when I used 2N377x's in the output stage.
This made the amp idle quite hot and would trigger the thermal shutdown switch with the cover on due to the terrible heatsinking methods of the day.

The amp played very clean and much more powerful in this configuration.
Except for a very slight tinge of crossover distortion noise which indeed warranted a change in the bias that I never got around to doing.

It served me well with the cover off for many many years with out any other major issues and the crossover distortion didn't bother me as I used it for my guitar setup anyhow.

But as a P.A. amp all the those issues disappeared when I switched back to 2N3055's
only it was not as powerful and not as clean when it approached its power limit towards and in to clipping.

I love those amps,I lost one but I still have two of them and I will put them back into service shortley as I am on the hunt to find more of them.

I hope you get your VOX working and are able to enjoy such a simple, reliable and powerful design from the past as I have.

Good Luck! jer

Last edited by geraldfryjr; 21st April 2011 at 06:22 PM.
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Old 21st April 2011, 06:17 PM   #9
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Very good point,Bone, I forgot to mention that I did end up doing that on my first sunn concert lead amp back in the day when I still had it.

And I never had I problem after that.

Except one day at a practice session the filter cap went off with a POW like dynamite within the first 15 minutes of jamming and left me without any sound for the rest of the day.

Boy that really sucked and with me being the lead and most experienced guitar player we got nothing acomplished that day,ha,ha,ha,ha,ha! jer

Last edited by geraldfryjr; 21st April 2011 at 06:27 PM.
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Old 22nd April 2011, 01:45 PM   #10
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i already replaced old R103&R106 with new 820 ohm 5 watt resistors.
and it is said that the two transistors are biased at 20 milliampere which mean about
10 mV drop across each emitter resistor. is that method is valid to adjust the two pots ??
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