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    Hello and Mod question

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    ejc

    Posts : 25
    Join date : 2010-10-16
    Age : 65
    Location : Connecticut

    Hello and Mod question

    Post by ejc on Sat Oct 16, 2010 10:15 am

    I just discovered the forum and decided to pop in.
    I recently decided to reenter the Stereo/2 channel fold with some vintage gear.

    I just picked up a Dynaco PAT-4, please forgive me it's SS but once I get it going I'll be looking to pick up a PAS-3 and ST-70 or the newer incantations of them.

    I have started going through the PAT-4. I replaced the power supply, replaced the volume control with a 24 step attenuator, removed the Tone and Loudness controls from the circuit.

    Next the I/O board will be replaced and the selector switch (the old one has cracked wafer). I am also replacing the old solid core wire as I go along.

    I have 2 questions.

    1- The Tone Controls. I disconnected the tone controls. The treble control on the schematic looks like the PC board lugs should be left open. The BASS CONTROL on the other hand looks like the 3 PC LUGS should be shorted together. Am I reading this correctly?

    2- The Phono section. I plan on removing the Ceramic and High inputs and just going with the Magnetic Input.
    A- do I need the PEC currently attached to that lug
    B- If removed do I need to connect a 150K resistor from the center contact to the RCA shell, which appears to be what the PEC does?
    C- The reason for my confusion, is the manual discusses changing the SPECIAL INPUT to a phono input by moving a wire from the PC Board to the selector switch, But I don't see it connecting through the PEC?

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Hello and Mod question

    Post by GP49 on Sat Oct 16, 2010 1:26 pm

    ejc wrote:

    1- The Tone Controls. I disconnected the tone controls. The treble control on the schematic looks like the PC board lugs should be left open. The BASS CONTROL on the other hand looks like the 3 PC LUGS should be shorted together. Am I reading this correctly?

    Yes. You are correct.

    ejc wrote:
    2- The Phono section. I plan on removing the Ceramic and High inputs and just going with the Magnetic Input.
    A- do I need the PEC currently attached to that lug
    B- If removed do I need to connect a 150K resistor from the center contact to the RCA shell, which appears to be what the PEC does?
    C- The reason for my confusion, is the manual discusses changing the SPECIAL INPUT to a phono input by moving a wire from the PC Board to the selector switch, But I don't see it connecting through the PEC?

    If you wish to remove the PEC entirely and use the Phono LOW input, take out the PEC and run a wire from the Phono LOW input jack to where the PEC's Pin 3 was previously connected. Install a 100KΩ resistor from the center pin of the Phono LOW input jack to its ground lug. This is because inside the PEC, the 150KΩ resistance is paralleled by the 270KΩ and 56KΩ, which results in an overall 100KΩ (approximately) to ground at the PEC's Pin 2. The "nominal" load into which standard moving magnet and moving iron cartridges are designed for is generally 47K; you may wish to use that value instead.

    You are correct about the instruction for rewiring the SPECIAL input for phono. Such rewiring does not go through the PEC.

    If you are replacing the selector switch with a different one, you will also have to rewire the circuit board for proper RIAA equalization because the original switch selects RIAA, NAB tape, or flat equalization for the preamp stage. You can consult the circuitry for the PAT-5, which had fixed RIAA equalization, to see how it is done. Such a change also eliminates a set of switch contacts in the phono stage feedback loop.

    ejc

    Posts : 25
    Join date : 2010-10-16
    Age : 65
    Location : Connecticut

    Re: Hello and Mod question

    Post by ejc on Sat Oct 16, 2010 1:37 pm

    Thanks for the reply.

    So the Special Input if converted to Phono does not require the same 100K resistor?

    So is there any advantage/Disadvantage to keping or removing the PEC?

    I'm trying to decide whether or not to set it up with 1 or 2 Phono Inputs, or go w/ 1 Phono and 3 High Level Inputs.

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Hello and Mod question

    Post by GP49 on Sat Oct 16, 2010 10:10 pm

    It's not that the Special input converted to phono doesn't REQUIRE it. They just didn't DO it. The phono load resistor should have been installed at the jack, if that conversion was done.

    The advantage to getting rid of the PEC is that its internal parts are not of the highest audio quality, although as minimal as its circuitry is, it doesn't really DO much other than signal attenuation.

    ejc

    Posts : 25
    Join date : 2010-10-16
    Age : 65
    Location : Connecticut

    Re: Hello and Mod question

    Post by ejc on Sat Oct 16, 2010 11:54 pm

    GP49 wrote:It's not that the Special input converted to phono doesn't REQUIRE it. They just didn't DO it. The phono load resistor should have been installed at the jack, if that conversion was done.

    The advantage to getting rid of the PEC is that its internal parts are not of the highest audio quality, although as minimal as its circuitry is, it doesn't really DO much other than signal attenuation.

    Thank you again.

    I guess the PEC goes and the resistor is in (for both if I go with 2 phono inputs).

    ejc

    Posts : 25
    Join date : 2010-10-16
    Age : 65
    Location : Connecticut

    Re: Hello and Mod question

    Post by ejc on Wed Nov 10, 2010 11:43 pm

    The PAT-4 modification is progressing and taking some interesting turns.

    To date I've replaced the power supply, removed the Tone Controls, Replaced the Vol Control with a 24 Step attenuator, disconnected the Loudness Comp Switch, removed the PEC, replaced the I/O board,replaced a bad selector switch, used shielded cable for the phono wires and replaced the solid core wire as I've gone along.

    The results so far have been better than expected. The PAT-4 is quiet and sounds clean. No noticeable hum or hiss. Very pleased with the results so far.

    I still have to recap and swap out the resistors. Once that is done I'm tempted to have a local tech spec it out.

    The wierd turns taken in the process: In reconnecting the preamp I thought I had some problems with my rewire, turns out my SWTPC 215/A became unstable and blew. My guess is the power supply became unstable after 40+ years and smoked a couple of transistors. Unfortunately it also took the drivers in my BA HD5s. Parts Express had aa reasonably priced drop in replacement and they're back in service, although I have to use headphones (Senheiser 414s from the late 60's, I believe they were the first to use foam ear pads)for the time being.

    The 215 is probably not worth fixing, so I may pick up one of Bob/Roy's ST70s sooner than expected.

    I also have not solved my switching problem. It seems if I try to connect a line level input to the Special or Tape amp positions on the switch it keeps getting routed back to the low level section no matter how I try jumping the switch. In the end, I will probably use the loudness switch as a source switch for the Aux connector.

    Anyone have any other ideas?

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