Repair help needed - Creek Destiny - diyAudio
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Old 28th March 2014, 12:38 PM   #1
jomor is offline jomor  Greece
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Default Repair help needed - Creek Destiny

Hello everyone,

I have a Creek Destiny integrated amplifier (version 1) which has burned twice the output transistors. Here 's a brief story:

- on the first week of purchase it oscillated and burned the mosfets, repaired by the local dealer (replaced transistors without investigating the cause of the oscillation). Note that my speakers are inductive up to 48KHz where I can measure.
- after a few hours of playing (but years later and out of warranty, i don't use it often) it went to protection mode. I replaced four blown electolytics (at the pre-amp stage) and the amp came back to normal operation, but minutes later it oscillated again and burned the mosfets of the left channel.
- I replaced the mosfets ( 76639P ) and I got sound from both channels. I tried to check/adjust the bias. On the board it is printed "adjust vr1 to 21.5mV accross jp4 at idle" so I checked the voltage across JP4 on the right channel (the one that did not had a problem) and it was 8mV (which seems suspicious, shouldn't it be at the suggested value?) so I adjusted it to 21.5mV. I tried to do the same with the left channel (the one which I replaced the mosfets) but the voltage drop was 0.2mV and does not change at all when i turn the related trimmer.

Unfortunately I can't find a schematic so this is a quite difficult task for me. Any ideas?
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Last edited by jomor; 28th March 2014 at 12:43 PM.
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Old 29th March 2014, 04:27 PM   #2
jomor is offline jomor  Greece
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I was comparing the 5350's schematic with destiny's pcb and as far as i can tell it has the same components so the circuits must have very slight differences. What else could I test except from comparing voltages between left and right channel on the corresponding components? Does anyone understand what could be wrong and the voltage drop across R28 is 0.2mV no matter what the trimmer's adjustment is?
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Old 29th March 2014, 05:39 PM   #3
Mooly is offline Mooly  United Kingdom
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That's one of the strangest circuits I've ever seen.

Can you just confirm the amp actually plays on both channels OK as it stands ?

If so, then I'm wondering if the non adjustable bias is more of a problem with the characteristics of the replaced transistors.

Anyhow, it looks as though the voltage between the drains of Q12 and Q14 are what is going to determine the current. I would compare those voltages, one channel to other first, in order to get some idea of what is going on. You must measure between those points and not to them individually.
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Old 29th March 2014, 09:53 PM   #4
jomor is offline jomor  Greece
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Thank you very much for the reply. Yes the amp plays on both channels just fine. I didn't dare to turn up the volume or let it play for much time though, I am afraid to stress-test it until I resolve the bias problem. I will follow your advice and focus on Q12 & Q14
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Old 30th March 2014, 06:43 AM   #5
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Yes, compare the voltage between these two points on both channels.
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Old 1st April 2014, 09:05 AM   #6
jomor is offline jomor  Greece
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Quote:
Originally Posted by Mooly View Post
Can you just confirm the amp actually plays on both channels OK as it stands ?

If so, then I'm wondering if the non adjustable bias is more of a problem with the characteristics of the replaced transistors.
Hi, your hunch must be right on target. The amp plays on both channels just fine. The spare 76639p I purchased were bought from chinese seller since they are obsolete and could not be found in a respected distributor. I measured them and compared to the original ones (the right channel fortunately was not burned). While in off state the original ones have source-->drain diode measuring of 525 while the "chinese" had 510. But while in ON state, the originals have nealry zero measuring 001 while the chinese give me a source to drain diode measuring of 070-100. If I understand well this shows the ability of the mosfet to conduct, and if this translates to a quite high Rds-on then this could explain the bias adjustment problem, could it ?

The only close equivalents I could find are STP40NF10L and I don't see another option but to replace them with these.

edit: According to this article, the circuit demands "logic level" mosfets which means low Vgs-th (correct me if I am wrong, I am just finding out what these mean) and STP40NF10L must be ok on this (min 1V max 2.5V while the 76639p has 1-3V ), are they?

I also spotted a flatulent smd capacitor, C20 near the TL074, I think its 100nF ceramic bypass capacitor. Do smd caps fail and get flatulent like electolytics? Could this cause an oscillation?
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Last edited by jomor; 1st April 2014 at 09:18 AM. Reason: add extra info
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Old 1st April 2014, 10:46 AM   #7
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Its pretty rare (very very ) for an SMD cap to fail in low voltage circuitry such as around an opamp. I'd bet its just fine.

Rds and Vgs are two different things really. A low Rds means that the FET appears as almost a short when turned fully on... and thats what is needed for power control using FET's as switches to minimise losses. But low or high Rds, that in itself isn't the issue here.

In an amplifier like this, the FET's are not used in that way (as switches) but are used in the "linear" part of the transfer characteristic.

Vgs is a measure of how the D-S current varies with a change in G-S voltage. And I think that is where your problem lies, the circuit has a very limited adjustment range and its outside of the Vgs of the replacement devices.

I'll have another look at the circuit later today (its on a different PC) and see if anything can altered.
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Old 1st April 2014, 10:48 AM   #8
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And did you measure and compare the voltages I mentioned in post #5
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Old 1st April 2014, 11:04 AM   #9
jomor is offline jomor  Greece
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yes I did measure the voltages:

I dont have my notes with me (I am at work now) but if I am not mistaken Q14 has 3.8V (Q14 source to ground) while Q114 (the other channel) has 4 volts.

Can't remember Q12, I will let you know as soon as I return home. Thanks again for being helpful
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Old 1st April 2014, 11:09 AM   #10
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Quote:
Originally Posted by Mooly View Post
Its pretty rare (very very ) for an SMD cap to fail in low voltage circuitry such as around an opamp. I'd bet its just fine.
pressing the cap with the probe pushes the inflated skin back to flat so it could also be just a failed protective thin film cover
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