Ruark Vita Audio R4 Mk3 will not power up / overheating- fixed - CD replaced - Page 1
Forum section: electronics / repair

author: rh100605
author_rank: Regular Contributor
timestamp: September 29, 2024, 11:09:34 am
content: My Ruark R4 MK3 suddenly refused to power on. The overheating is dealt with in the second post.When I removed the back panel I was able to measure the power supply outputs with the mains power applied.Be careful the heat sinks are live and even when disconnected from the mains the 450 volt dc capacitor retains charge for up to 5 minutesAll voltages were near zero. I used a mains isolation transformer and then probed the output voltages with an oscilloscope. The standby voltage was pulsing around 500 mv.The power supply board consists of one bridge rectifier supplying 300V dc to two switched mode power supplies both with dual outputs.The first supplies 8.3 volt standby and 12 volts on the 6 pin connector (Pin 1: Gnd Gnd Vsb Vsb 12 V PSon ) PSon input is 3.3volts when active and enables the +/-17 volt outputs.It uses the Power forest PF6000AG sot23-6 IC marked 6000 which I bought on Aliexpress. Scribd has the first page of the datasheet. It is included below. The 4 amp FET used by the PF6000 is ISA04N60A. The standby voltage is regulated with a TL431 and optocoupler feedback.The 12 volt uses a second winding that is rectified to 15 volts then passed though a to92 voltage regulator to give 12 volts.The second switched mode supply outputs +/- 17 volts for the final audio power amplifiers and uses the LD7580 AGS Leadtrend 8 pin switching regulator.The LD7580 18Volt supply is derived from the PF6000 circuit transformer primary winding that also supplies the PF6000. A schottky diode is followed by a series transistor, which is switched on by the PSon line via an optocoupler and is regulated by an 18Volt zener No High Voltage is supplied to the LD7580.The 10amp FET is ISA10N65A.The +/- supplies are rectified by double Schottky diodes (Sirect MBR10B200 FCTH 200v 10 amp) and smoothed by two 1200 mfd 25v capacitors in parallel on each rail. Then a choke and a further 470 mfd electrolytic capacitor on each rail.Both + and - supplies have a load of 180 ohms ( 820 820 330 in parallel) on the pcb which leads to overheating. The 4 pin connector is Pin 1: + Gnd Gnd -Feedback is generated by a TL431 (2.5v reference) driving a photocell through a 10.0v zener (ZD6) and 1K resistor.The TL431 reference resistors are 60.4k and 4.75k ( R42/44) across the V+ and V- supply giving +2.5v from the V-line ( 0.073*34 =2.5volts ) when the output is +/-17voltsI replaced the PF6000 chip to no avail and then checked all the diodes in circuit with a DVM for 0.2/0.3 forward voltage and large reverse resistance to eliminate shorted diodes .I found D10 a Panjit MBR1200 diode ( 1amp 200V Schottky) nearly shorted and replaced it with an MBR 2200 ( 2amp 200Volt Schottky)This diode rectified the 15 volts to the 12 volt regulator.The whole power supply sprung into life and so did the radio.The CD player is CDM M10 4.11/5 uxxx. It was unreliable. I bought another model CDM M10 4.xx on Aliexpress from Hong Kong and swapped the boards which worked fine. ( Take photos of the wiring beforehand, all wires are colour coded see attachment)To be clear the attached pcb pictures show the power supply board without the heatsinks attached.

author: rh100605
author_rank: Regular Contributor
timestamp: August 06, 2025, 09:15:55 pm
content: I fixed a second Ruark R4 Mk3 that refused to power up. The power unit showed significant overheating damage.The PF6000A chip had failed explosively. The ISA04N60A 600v 4A FET was source to drain to gate shorted, the 0.6 ohm current sense resistor was open and so was the 1K ohm 1206 resistor in parallel with the current sense resistor.The 100 ohm 1206 resistor in series with the FET gate was also open.I removed the heat sink and then used flux remover to soften the burnt sealant on the pcb, removed it carefully and replaced all the broken components.The power supply powered up and the secondary +/-17 supply powered up. I left the power supply open to the air while I played a CD.The Ruark R4 Mk3 gets warm during use, even at low volume. The two 330 ohm load resistors and the electrolytic capacitors adjacent to them on the +/- 17 volts line got so hot you could not touch them. This would be permanent what ever the volume level and there are no vent holes in the case or thermal connection to the back panel.So I did some sums.Each 17 volt line is dissipating 1.6 watts (17 squared /180) into the load resistors which are adjacent (330 ohm) and directly below (2 by 820ohm) which makes the two 470mfd electrolytics very warm.I read the LD7580a datasheet which infers that the chip will handle no load and be stable. The application circuit shows a load resistor which is specified as NC or not connected.Encouraged I removed the both the 330 ohm resistors and a pair of the 820 ohm 2512 resistors(R45,51 or R46,52), leaving a 820 ohm load on each rail.The power supply powered up and remained stable with out the power to the final audio amplifiers being connected. There was no increase in ripple voltage with or without the final amplifier connected and considerably less heat.I decided to divide each of the loads by 4 using a 1.2k 3w axial resistor and two 3.3k 2512 surface mount resistors. Each load was now 695 ohms. The total power dissipation was now 800 milliwatts instead of 3.2 watts