Help fix PL700B with PL-14B board

[CASSETTE DECK]

Corrections:

caps will work, but

Post # 45: Series connecting caps will work, but the capacitance will be divided by the number of capacitors of equal capacitance value in series.

does not increase

Post # 49: Paralleling does not increase voltage handling, series connection does, but at half the capacitance for two capacitors of equal capacitance value in series.

The formula for calculating capacitors in series of different capacitance values is the same formula used to calculate the total value of resistors or inductors in parallel with different values.

Equalizing resistors must be connected in parallel with each capacitor to ensure that each capacitor in series is charged to the same voltage, and will completely discharge each capacitor when the power is turned off for safety and to avoid introducing faults when working on electronic gear.
Resistors should be wire wound or metal oxide or metal film for accuracy, stability and reliability.
High resistance value carbon film resistors have a tendency to fail open circuit.
Low resistance value carbon composition resistors can short circuit when they are burnt out or severely overheated.

 

[kevin]

Sorry Snoopy, WW----Wire Wound resistors. Ok for what they are made for but have no business within 10 yards of that amp....

Thanks for clearing that up Lee... I was looking all over that board for wooly worms !!!

 

[laatsch55]

Glad I cleared that up for ya. W W 's are in the fly box though.

 

[CASSETTE DECK]

I put in a new fuse and turned it on only to witness the a few resistors flame up and a blown test speaker.

These resistors look similar to 1 watt carbon composition resistors but they are actually 2 watt wire wound resistors with a bakelite body that turns into conductive carbon that can burst into flames and short circuit when severely overheated, which is the same thing that can happen with low resistance value carbon composition resistors.
www.youtube.com/watch?v=XowmjnizlIM http://en.wikipedia.org/wiki/Ayrton-Perry_winding
I have a 0.27 ohm 2W WW resistor like this at home that I have dissected ready to take photos.

Just eyeballin... I see some WW's on the back wall. I dunno if those should stay?

The WW have to go. They are inductive.

I totally agree. Resistors with a high parasitic inductance cause a lot of problems in circuits where high frequency AC or pulsed DC is involved, such as self oscillation, limited frequency response, distortion and phase distortion especially in an emitter resistor application.

Sorry Snoopy, WW----Wire Wound resistors. Ok for what they are made for but have no business within 10 yards of that amp....

I agree that inductive wire wound resistors should not be used in any critical part of an audio or radio frequency power amplifier circuit, but they are fine for capacitor discharge and equalization applications in DC and low frequency AC power supply circuits including the DC power supply section of this amplifier.

Resistors should be wire wound or metal oxide or metal film for accuracy, stability and reliability.

Wire wound resistors with a high parasitic inductance won't cause any problems in this particular DC
capacitor discharge and equalization application.