Thule Spirit IA100 (AmpDrive) turn-on-pop


2017-12-01 9:49 pm
thanks to Douglas Blake and Osvaldo de Banfield.
turned out to be C14 which has dried up resulting in a too short delay in the protection circuitry.

Original post:

Hey folks,
i recently brought a Thule IA100 back to life after it severely burned up on both channels, charred pcb, missing tracks etc.

It is a old rev.6 pcb without the load sharing resistors on the transistors. and some bodges. nasty

how it looks after fixing it (i added heatsinks later on):

took quite a while and lots of parts, but its now working again...
sort of

i still have a big problem with it.. when i turn it on or enable it from standby (relay klicks) it does a loud pop on my speakers.

about 10Vp for a few milliseconds, exactly the same with and without load attached.
its exactly the same on both channels.
i allready disconnected the Preamp (i wonder why people like it so much its just few analog switches and a volume IC..) and the problem persisted. it must be in the power amp stage.

it runs fine after that, but the initial pop may hurt sensitive speakers.

i allready replace the main filter caps and all other capacitors.
i checked connections and measured for broken components (had a broken resistor in the initial repair)

i traced down the STBY signal from the front panel to the protection circuit which sends it via STBY 1 and 2 to the power amplifiers.
All of this did work correctly.

when in standby it draws 10W, when turned on its about 20W.

i just cant figure out whats going on.
its gotta be something both channels use.. theres the rectifier, filtering and protection relay / circuitry.

The problem looks like this

Signals look fine (note: no load attached)

schematics (of rev7 which is almost the same) found here
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I marked the Delay circuit for you in the thumbnail.

If you are getting a thump at turn on it will almost certainly be caused by instability in the power supply and amplifier biases during start up. The relay is there to isolate that from the speakers.

It's fairly obvious the relay is pulling in right away, rather than delaying itself by a few seconds. Possible culprits are C14, Q18, Q6 and Z5.

On start up, the output of U2A should switch low. This will reverse bias D2, allowing C14 to charge through R61. When the charge is low enough, it will exceed the threshold of Z5, which would switch on Q18 and in turn Q6 to pull in the relay.

Reasonable delay in most amps is 3 to 5 seconds.


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