306 M12890-6 Upgrade and Fix

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26-Apr-2017 08:14 PM

M6NTL

M6NTL

Posts: 4

Hello All,

Thanks for this forum, the opportunity to ask questions - and to buy an upgrade kit!

I have just bought a faulty 306, the seller listed as non working Left Channel (just Hum), but working fine on the right channel.

I haven't yet powered it up - I've looked inside and there are a couple of burned out looking resistors which I will replace before upgrading with your kit. I really have no idea why they died at this point.

My circuit board is M12890-6, are the component and voltages on the schematic going to be the same (the schematic is for M12890-3).

Many thanks in anticipation. Regards, Mark.

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14-May-2017 10:07 PM

Not Available

Posts: 6

Hi,

Same issue here with the same board revision.

But. Here the RIGHT channel generates the hum.
DC is asymmetric:
+DC is about 3.7V.
-DC is about -65.3V.
R17 and R18 (560Ohm) resistors are getting quite hot.

Actions (one by one) taken but with no results:
- swapping secundary windings of transformer;
- swapping diode bridges;
- replaced main capacitors with known good one.
About to do: replacing transistors T11 and T12 with the four surrounding resistors.
If this does not work then maybe the DC feedback?
Hope somebody has a hint or tip?

Thanks!

Harry
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15-May-2017 10:13 PM

Not Available

Posts: 6

Found it!
R3 (3.3kOhm) was infinite but looked normal.
Replaced it with a new one... Tadaaa!!!!

Good luck!

Harry
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15-May-2017 11:44 PM

M6NTL

M6NTL

Posts: 4

Hi Harry,

Mine is also the right channel, and has the same symptoms as yours - 20-0-60 DC Input, and about 19V DC on the Speaker posts.

R31 was cremated looking, but still measured 2K2, and R32 was open circuit. Replaced these, but problem remained.

So, I removed T7, 8, 9, and 10 (same WW resistors getting hot as yours), and tested OK on the bench. Fault still present.

I then removed T11, and 12, and the 10K and 9.1 Resistors - these test good on the bench too...

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15-May-2017 11:46 PM

M6NTL

M6NTL

Posts: 4

Ooh, just seen your R3 post - tested mine (in circuit), left channel measure 3K3, right channel measures 6K4 - nice tip Sir, thank you!

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16-May-2017 10:16 AM

Not Available

Posts: 6

I replaced the T11 T12 circuit completely because of that burnt resistor. Just to be safe.
And then the ZTX650 by a ZTX651 and the ZTX750 by a ZTX751.
Main difference between the -0 and the -1 is the higher collector emitter version of the -1. The -0 versions are obsolete as far as I could see.
Also check the other resistors in the DC regulation circuit around opamp IC1 TLC271.
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16-May-2017 03:24 PM

M6NTL

M6NTL

Posts: 4

Cheers!

I think I'll just replace all resistors on both channels with 0.5W 1% Metal Film, as I have a full range of these in my box.

I've also been advised elsewhere to replace the 4W WW resistors with 5W, because they are close to the limit at 4W.

Just need some new caps, and component value changes to correct the current dumping bridge values now!!!

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