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 Post subject: Sunn Concert Bass Amp - Troubleshoot and Repair
PostPosted: Jul Mon 29, 2019 1:42 pm 

Joined: Aug Sat 01, 2015 8:54 pm
Posts: 163
It's not often I work on transistorized amplifiers, however a couple of times a year I take on such a project. This Sunn concert, built around 1973 was owned by the same person since around that time and was looking for electronic repairs with no cosmetic work. I decided to take on the task.

The complaint was noise in the system, though, i did not attempt to turn it on without first a visual inspection. With that, seeing the state of the capacitors and the fact that the main board was floating, having lost all of the mounts over time, id decided that some repairs would first be required before a smoke test. The broken mounts were further enough to pull all of the resistors from their heat sinks as well over time.

The some capacitors were strange sizes, and by strange i mean costly, a few were checked for leak-down and ESR, the rest were replaced wholesale. all sized needed to be reverified because someone had been in here before. Special consideration was given to the main filter caps, given their cost, though, they tested good enough for now.

Swapping the caps from the main board also meant cleaning the board with spirits to remove all of the old electrolyte. The pads are delicate on this amp and care should be taken to remove and add components. Once the main board was finished, new nylon stand-offs were added, the board holes slightly reamed to M4 and the board remounted. Subsequently, the resistors were placed back into the heat-sink clips. A smoke-test confirmed the amp works but artifacts are in the system, Im reserving judgment until the pre-amp is completed.

The next task, the pre-amp board, was removed also to swap capacitors and apply Deoxit the pots. After reassembly, the artifacts persisted. The first hint as to the problem was the fact that the artifact noise became attenuated with the volume; this put the issue behind Q103. Localizing the issue to two components with the o-scope I broke out the signal tracer to isolate the noise generation to the .01uf coupling capacitor off of Q102. Confirming this in noise mode, it was swapped out and retested. The problem was solved and final testing of the amp conducted.

Having been proven out the amp is reassembled.

 Post subject: Re: Sunn Concert Bass Amp - Troubleshoot and Repair
PostPosted: Aug Sun 04, 2019 10:06 pm 
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Joined: Apr Sun 23, 2017 11:22 pm
Posts: 1180
Location: 44035 (Near Cleveland Ohio)
Geez, Rubin. Tightening pot nuts with duck bill pliers??? Hopefully your customer didn't see that video. . . . :oops:
And this from the man who carefully aligns slotted screw heads. Worlds are colliding . . 8)


 Post subject: Sunn Concert Bass Amp Troubleshoot and Repair
PostPosted: Sep Mon 09, 2019 1:48 pm 
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Joined: Jan Tue 15, 2019 2:43 am
Posts: 2
Since I installed a sub I wanted to keep bass frequencies out of my door speakers, so I turned the 100 Hz slider down several notches and turned up the gain on my amp until I was happy with the bass level. Not the approved method of setting amp gain, but it works well enough for my purposes.

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