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SCA 35 troubleshooting
SCA 35 troubleshooting
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Old 28th July 2019, 02:22 PM   #1
rkoonce is offline rkoonce  United States
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Join Date: Aug 2006
Location: Texas
Default SCA 35 troubleshooting

I rescued a basket case Dynaco SCA 35 integrated amp with completely shot EL-84s. They had all four been very hot, one hot enough to crack the glass, and the other three with no getter left, only a rusty red residue in the top of the tube. I replaced the output tubes and capacitors, and the 12AX7s and 7199s were still good. The amp plays nicely after I cleaned all the pots and switches, but gain between channels varies with frequency. I can adjust the balance control at a single frequency to get equal output voltages, but that won't work with music. Any ideas what to check?

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Old 28th July 2019, 04:27 PM   #2
petertub is offline petertub  Sweden
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Join Date: Jul 2015
Location: gothenburg,sweden
SCA 35 troubleshooting
It could be the PEC's ( the think units containing the tone-control components. It could also be something broken in one or both power stages.
To isolate disconnect the volume pot wiper and inject signal in the wire disconnected.

Connect a tone-generator and measure the output at the speaker connections ( must have a load, speaker or resistive 8ohm). Inject 40hz, 1000hz 5khz 10khz and note the values.
Repeat with other channel. If the channels are equal you might build a pair of PEC
equivalants and replace the existing ones.
If the channels are unequal you have a problem in one or both power stages, but you have at least isolated the problem by reducing the area.
Go work and report back!
My home is at www.tubular-well.se
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