Quad 306 - left channel issues and failed components. Please send help!

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02-Sep-2018 01:00 PM

Gus

Gus

Posts: 4

Good morning

I have been the proud owner of a Quad 306/34/FM4 for 12 years since my father gifted them to me for my 16th birthday.  He has also had one from new.

I have just purchased the Dada upgrade kit for both the 306 and 34 but need to fix my issues before I install.

Problems are on the left channel.  R22 and R31 have both failed (pictures attached).  Is this failure likely to have been caused from issues elsewhere?  If so can someone more clever than I point me in the right direction?

As a minimum I need to replace the two resistors.  R22 (560 ohm 2% 2.5W TFP3) seems difficult to obtain.  Can Dada supply what I need?

Also, to my horror, I have discovered that the previous owner has been in here meddling.  There is a 1k resistor across R14 and R16 on the left channel and R14 and R15 on the right channel.  There is also a wire bridging C3 and the adjacent integrated circuit on both channels.  Please can somebody tell me exactly what this does and whether I should remove when I install the upgrade?  The Dada kit obviously replaced C3.

Sorry for the lengthy post.  Fingers crossed I get a response.  Life is slow without a working amp.

Gus

https://imgur.com/a/y66Abt5 

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04-Sep-2018 07:08 PM

Not Available

Posts: 8

Hi,

Please also check the resistors in the op amp circuitry.

For reference the topic of last year: "306 M12890-6 Upgrade and Fix"

R3 (3.3kOhm) was not 3.3kOhm anymore causing the DC voltages to shift.

Maybe this caused your amp having completely burnt components.




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05-Sep-2018 01:52 AM

EJP

EJP

Posts: 1530

These are factory modifications. Do not remove. The 1K resistor for example is R14A on the first schematic in the service manual, and the wire link is part of the setup of the op-amp.

I can't recommend that you try to diagnose and fix this yourself. Send it in for service.

EJP

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06-Sep-2018 11:20 AM

Gus

Gus

Posts: 4

R3 is within tolerance on both sides.  This weekend I will go round and compare values to see if I can see any discrepancies. 

Thanks EJP.  I'm interested and keen to learn hence not sending it in.

Any other suggestions?

Can somebody point me in the right direction to find a replacement for R22?

Thanks, Gus.

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06-Sep-2018 11:26 AM

Stefaan

Stefaan

Posts: 583

All components of the 306 are in the Webshop.

You can use the search-function to find them in a easy-way.

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06-Sep-2018 12:31 PM

EJP

EJP

Posts: 1530

At a wild guess you have a short c/e circuit in T9.
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11-Sep-2018 10:16 AM

Gus

Gus

Posts: 4

Good morning

I have done some extensive testing on all components over the weekend.  All appears to be in order with the exception of C10 on the bad side.  This shows OL on a capacitance test.  Would this result in the failure of R22 and R31?

I have also measured the resistances between the base emitter collector on the 4 transistors (17556).  These test consistently out of circuit. T9 on the right channel (good side) has a higher resistance (6.21 Mohms) than the other three (all between 3.5 and 4 Mohms) but I assume this isn't an issue?

I would be grateful for any further thoughts?

Thanks for the guidance so far.

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25-Sep-2018 02:47 AM

EJP

EJP

Posts: 1530

If C10 is hosed nothing will work, and it could certainly affect R31. Change both C10 and C11.

Testing transistor junction resistances isn't useful. You should use the diode position of the meter, with the red lead on the base for NPN transistors and the black for PNP transistors. You should get readings of 0.5-0.7V for small signal transistors, lower for drivers and power transistors.

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26-Sep-2018 01:32 PM

Gus

Gus

Posts: 4

Thanks for taking the time to respond EJP!

I have installed the Dada upgrade kit and have replaced R22 and R31 on both channels with Kiwame carbon film resistors.  I couldn't find any other suitable resistors for R22 (high power rating/low tolerance etc).

It all works and sounds fantastic however there is a slight hiss on both channels that I am adamant wasn't there before. It's a hiss and not a hum.  Could this be due to the resistor type used for R22 (5W Kiwame) or is this unlikely?  Any other suggestions?  The hiss is on both channels which makes it unlikely to be an install/soldering error.

Thanks

Gus

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27-Sep-2018 02:17 AM

EJP

EJP

Posts: 1530

The hiss will be coming from the input end.
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