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What's Z1 in a MA5002VZ?

Mystery Component

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#1 AC4AQ

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Posted 16 April 2014 - 04:54 PM

Hello, Crown folks!

Right now I'm trying to wrap up the repair on a MacroTech 5002VZ but the last little gremlin is stubborn. It has oscillation (actually vibration) in one output channel, causing a squarish/sineish waves of about 600 mV, 300 kHz to appear at the speaker terminals.

Anyway, while applying a signal to the unloaded amplifier and as I increased the amplitude, I heard something go Pop! Nothing burned on the output module, it appears Z1 on the speaker terminal ("Terminator") board ignited.

The trouble is there is nothing identified on the schematic as Z1, nor can I find a Z1 in any of the parts lists in that enchanted forest of a service manual.

If anyone has some thoughts on this mystery component, please share them.

Thanks,

Jeff / AC4AQ

#2 joust

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Posted 17 April 2014 - 08:05 AM

Hello Jeff,

I did find Z1 on a Terminator diagram (cpn 102396). Z1 is a 'ground lift' link, possibly installed for grounding issues.
If Z1 is installed, the main ground (also Common output) is connected to chassis ground (usually, the source AC ground).

If Z1 has flared, it is possible you have an issue with your grounds.

Alain

#3 AC4AQ

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Posted 17 April 2014 - 11:15 AM

Thanks, Alain,for your response.

   Ultimately the problem was C524: Among the original faults in this amplifier was leakage in C524, I substituted a 270 pF, 400 WVDC cap and this was a mistake. The extra 70 pF was enough to introduce an instability, though what mode I haven't and needn't determine.

   Now I need to figure why I now have a 185 mVDC offset in one channel, in Stereo mode only...

#4 joust

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Posted 21 April 2014 - 08:42 AM

You don't have this symptom in Bridge mode?
If not, then Ch2 preamp may be at fault, since the Ch2 preamp is kind of 'bypassed' in Bridge mode.
Note : 185mV is not extreme. I believe the amp will work fine even with this offset.
Note : the owner's manual states that if an [out of tolerance] output offset is present, to verify the ODEP circuit.

Hope this helps!

Alain