Hello everyone,
Its been a while for me... I bought this Foreplay probably around 2004, I was fresh out of high school and passionate about music and in love with hi-fi audio; very inexperienced and quite ignorant. I've grown more patient and have a high regard for quality and craftsmanship.
That being said, the Foreplay was the first DIY electronic I had built. After some time, work getting in the way, life, growing family, I was forced to get away from things for some years. Now to today, I am still just as ignorant and inexperienced with electronics and circuitry, but still love hi-fi audio. It was always a dream of mine to buy one of Doc's Paramour or ParaBee(?) kits. I have come to terms that between financial means, and family demands, and the fact that Doc is not producing those kits any longer, I will not be able to spend that kind of money on my dream to have a pair of 2A3, or 300B tubes pumping my Fostex FE167's (also built two sets of enclosure with these, neither will ever leave my possession).
I am now starting to gather parts to build myself a 300B two channel amplifier. In preparation I pulled the Foreplay out of storage, blew the dust off and fired it up. Needless to day some things were not right, and decided to use my ever-growing patience and craftsmanship to clean things up under the hood. Well lets just say I am not quite sure how this thing was running before. How ever it was, it did have a faint hiss and was unbalanced L to R. I fixed the obvious issues and scrutinized every connection, transistor, & resistor. I do not know (or have to looked into yet) how to test a capacitor. Here is what is going on...
Left channel is perfect, My right channel is dead, completely silent, no hum, no hiss, nothing. The tube is very dimly lit, and depending on the tube I have in it the voltage readings change. I had the original pair of GE tubes that came with it and a pair of Amperex 12AU7A's that I can thank my kids for bending the pins on and me breaking when trying to straighten them back out. Here is the hard Data I can provide:
Terminal | Expected V | Amperex V | GE V
2 | 164 | 193 | 188.7
7 | 153 | 174.5 | 177.8
10/11/12 | 143 | 162.3 | 167.3
A1 | 47 | 62.2 | 61.7
A3 | 1.7 | 1.9 | 1.9
A4/A5 | 44 | 48.2 | 49.8
A6 | 143 | 161.6 | 166.7
A7 | 47 | 62.1 | 61.9
A8 | 51 | 67.4 | 67.5
A9 | 44 | 48 | 49.6
B1 | 47 | 122 | 59.9
B3 | 1.7 | 1.8 | 1.8
B4/B5 | 44 | 47.9 | 49.8
B6 | 143 | 161 | 166.9
B7 | 47 | 122.9 | 59.9
B8 | 51 | 131.4 | 2.7
B9 | 44 | 47.9 | 49.9
I did not check resistances with the tubes in place but can provide similar info as above if needed. Anything else that would be helpful I'l happily get any thing to get this figured out.
I used the following procedures to check the FET's on the C4S boards:
"
1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.
2) First, touch the meter positive lead onto the MosFet's 'Gate'.
3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.
4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.
Such a simple test is not 100% -- but is useful and usually adequate.
When MOSFETS fail they often go short-circuit drain-to-gate. This can put the drain voltage back onto the gate where of course it feeds (via the gate resistors) into the drive circuitry, possibly blowing that section. It will also get to any other paralleled MosFet gates, blowing them also.
So, if the MosFets are deceased, check the drivers as well! This fact is probably the best reason for adding a source-gate zener diode; zeners fail short circuit and a properly connected zener can limit the damage in a failure! You can also add subminiature gate resistors -- which tend to fail open-circuit (like a fuse) under this overload, disconnecting the dud MosFet's gate."
I used the following procedure to check the PNP & NPN transistors on the C4S boards:
"
Step 1: (Base to Emitter) Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see "OL" (Over Limit).
Step 2: (Base to Collector) Keep the postitive lead on the BASE (B) and place the negative lead to the COLLECTOR (C). For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see "OL" (Over Limit).
Step 3: (Emitter to Base) Hook the positive lead from the multimeter to the to the EMITTER (E) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor. For an good NPN transistor, you should see "OL" (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.
Step 4: (Collector to Base) Hook the positive lead from the multimeter to the to the COLLECTOR (C) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor. For an good NPN transistor, you should see "OL" (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.
Step 5: (Collector to Emitter) Hook the postitive meter lead to the COLLECTOR (C) and the negative meter lead to the EMITTER (E) - A good NPN or PNP transistor will read"OL"/Over Limit on the meter. Swap the leads (Positive to Emitter and Negative to Collector) - Once again, a good NPN or PNP transistor should read "OL". If your bipolar transistor measures contrary to these steps, consider it to be bad. You may also be able to use the voltage drop to determine which lead is the emitter on an unmarked transistor, as the emitter-base junction typically has a slightly higher voltage drop than the collector-base junction. Remember: This test only verifies that the transistor is not shorted or open, it does not guarantee that the transistor is operating within its designed parameters. It should only be used to help decide if you need "replace" or "move on to the next component". This test works on bipolar transistors only - you need to use a different method for testing FETs.
"
Considering I probably have not posted since the antiquated forum 12 years ago, I hope I have posted this in the correct section of the forum.
My unofficial signature:
Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel Performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate
