UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Sharp SG-FR35 loud hum

Chris55000 · 22nd Sep 2018, 5:08 pm

Hi!

THe following fault-finding guide is based on the lower-power Sharp SG-FR35E with the LA4282 output stage.

1) No sound/complete silence

Total silence, apart from obviously blown fuses, open circuit loudspeakers or their wiring is usually down to open-circuit speaker-muting contacts or cruddy contacts on the bank of function switches, which contact-cleaner will help with.

Open-circuit contacts in headphone sockets/jacks usually need the whole socket/jack replacing, these can usually be found online from ebay or various sources of surplus sources, or even old units scrapped at your local household tip-yard!

The headphone socket muting contacts can be tested for continuity with an ordinary multimeter.

Obviously failure of the 31.5V main supply would produce a completely dead unit as tho' it simply hasn't been plugged in!

2) Faint hiss or plop when switching on, but no sound and most other functions inoperative

This fault can occur if the stabilised supply from the 12V regulator Q513 (2SC1762) fails, usually due to the fusible resistor R544 going open-circuit - this sometimes happens when no other faults are present, a less likely possible cause is the stand-by muting transistor Q511 e/c short-circuit or ZD501 short-circuit.

If you need to repair the 12V reg, use a TIP41C for Q513, a BC182L for Q511 and the zener can be BZX61C13V. Fusible resistors are available online, various sources!

3) Distortion and/or low volume either or both channels

This fault can be due to the LA4282 being internally defective or the "bootstrap" coupling capacitors going open-circuit, there are two in this design, C517 and C519. Note in particular that either of these going o/c can result in eventual failure of the LA4282 due to unbalanced drive to it's internal output transistors.

4) Low volume and very poor bass

LS Coupling capacitors C511 or C512 partially or completely open-circuit.

5) Low volume or almost total absence of sound

Negative-feedback return capacitors C505/C506 low in value or o/c.

6) Loud hum immediately on switch-on

Internal failure or partial s/c in LA4282 output amplifier chip - replace with a new one and renew ALL the electrolytics detailed in 3), 4) and 5) above

7) Distortion, hissing, crackling, etc.

Various strange faults can occur if faults occur in the Zobel Correction Cell across the amplifier output, C513 to C516 and the resistors R515 & R516 - renew these components as a precaution if you find the LA4282 defective.

The above seven paragraphs used in conjunction with the S.M. will take care of any of the most likely faults encountered in the power amplifier stages! If you have further faults after you've repaired the output stages please keep us posted!

Chris Williams

22nd Sep 2018, 5:08 pm	#21
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,870	Re: Sharp SG-FR35 loud hum Hi! THe following fault-finding guide is based on the lower-power Sharp SG-FR35E with the LA4282 output stage. 1) No sound/complete silence Total silence, apart from obviously blown fuses, open circuit loudspeakers or their wiring is usually down to open-circuit speaker-muting contacts or cruddy contacts on the bank of function switches, which contact-cleaner will help with. Open-circuit contacts in headphone sockets/jacks usually need the whole socket/jack replacing, these can usually be found online from ebay or various sources of surplus sources, or even old units scrapped at your local household tip-yard! The headphone socket muting contacts can be tested for continuity with an ordinary multimeter. Obviously failure of the 31.5V main supply would produce a completely dead unit as tho' it simply hasn't been plugged in! 2) Faint hiss or plop when switching on, but no sound and most other functions inoperative This fault can occur if the stabilised supply from the 12V regulator Q513 (2SC1762) fails, usually due to the fusible resistor R544 going open-circuit - this sometimes happens when no other faults are present, a less likely possible cause is the stand-by muting transistor Q511 e/c short-circuit or ZD501 short-circuit. If you need to repair the 12V reg, use a TIP41C for Q513, a BC182L for Q511 and the zener can be BZX61C13V. Fusible resistors are available online, various sources! 3) Distortion and/or low volume either or both channels This fault can be due to the LA4282 being internally defective or the "bootstrap" coupling capacitors going open-circuit, there are two in this design, C517 and C519. Note in particular that either of these going o/c can result in eventual failure of the LA4282 due to unbalanced drive to it's internal output transistors. 4) Low volume and very poor bass LS Coupling capacitors C511 or C512 partially or completely open-circuit. 5) Low volume or almost total absence of sound Negative-feedback return capacitors C505/C506 low in value or o/c. 6) Loud hum immediately on switch-on Internal failure or partial s/c in LA4282 output amplifier chip - replace with a new one and renew ALL the electrolytics detailed in 3), 4) and 5) above 7) Distortion, hissing, crackling, etc. Various strange faults can occur if faults occur in the Zobel Correction Cell across the amplifier output, C513 to C516 and the resistors R515 & R516 - renew these components as a precaution if you find the LA4282 defective. The above seven paragraphs used in conjunction with the S.M. will take care of any of the most likely faults encountered in the power amplifier stages! If you have further faults after you've repaired the output stages please keep us posted! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!