SOLVED - SANYO DCX-2000L No sound after shorting the 6.3V line - Page 1
Forum section: electronics / repair

author: michalism
author_rank: Regular Contributor
timestamp: June 28, 2019, 09:07:07 pm
content: HelloI'm restoring this Sanyo DCX2000L Stereo receiver and one of the final tasks was to replace a burnt 6.3v bulb which is illuminating the radio stations window.After I replaced the bulb, I turned on the unit to check, and somehow I ended up shorting the 6.3v line which is giving power to the board that hosts all these bulbs at the front. There's a 2A fuse on that line and it got burnt so I replaced it, and all seemed well....the lights came up, the tuning meter works, as well as the stereo indicator lamp.BUT, there's no sound!!! Switching to different sources and/ or changing the speakers switch makes no differenceAll voltages seem correct.I am suspecting that the amplifier is in protect mode, because I measure 20VDC at the speaker terminals. As far as I know, this is not normal, is it?My question is where to start my investigation from. I do have the service manual, but there's no description about the speaker protection circuit explicitly.I have started checking on the internet for resources, but I'm also checking here to save some time.Also, another thing I noticed and I find weird, is that the fuse holder of the fuse at the 6.3V line, shows continuity even without the fuse in it! Is this normal?Any help would be much appreciatedThank you!Michalis

author: Chris56000
author_rank: Frequent Contributor
timestamp: June 29, 2019, 06:00:34 pm
content: Hi!You will read continuity across the fuse you removed as the lamps, the mains-transformer 6.3V secondary and the five scale-illumination lamps in parallel form a continuous circuit to d.c., you'd get no lighting-up if they didn't, the five scale-lamps would measure about 30-50 ohms cold each typically!Now, regarding your sound, (or lack of!), there is no speaker protection on this simple unit as the electrolytics C615 & C616 block the mid-point d.c. (about 21V) from pin 2 of the STK016 output amplifiers IC601/IC602.There is a 1.6AT fuse in series with the 42V h.t. line to each of the output amplifier ICs, shown separately from the O/P board on the circuit-diagram, altho' the PCB layout diagram shows them as being tracked on the PCB itself.If either or both fuses are blown, you will need to replace either (or both!) of the STK016s, and I recommend also replacing C615/C616 (use 1000u 50V) and C607/C608 (use 47u 50V) whenever these devices are renewed.https://www.ebay.co.uk/itm/1pcs-STK016-STK-016-Advanced-Power-MOSFET-SIP-ZIP/253898917506?hash=item3b1d8e0682:g:NewAAOSwGs1brFWIf neither of the fuses on the output board or connected in the 42V line have failed, test for continuity from the 'OUT' pins of the output stage board to the rear loudspeaker terminals, 'SYSTEM A' and 'SYSTEM B', the '+' terminals should show continuity to the board in either position of the 'A/B' selector switch on the front panel and the '-' terminals on the loudspeaker terminal boards go direct to chassis earth/0V line.Lack of continuity should lead to a physical examination of the wiring to and from the system selector switch itself and the condition of the wafer contacts.If you get continuity from the 'OUT' terminals of the O/P board to the speaker terminals, connect a 1k safety resistor across the + and - terminals of each speaker terminal-board and switch your unit on - there should be NO d.c. measured across these resistors. If you get a zero or less than 0.1V d.c. reading across 1k safety resistors across the speaker terminals, it should be safe to reconnect the loudspeaker units themselves.If you're happy with these tests so far, turn the unit to 'AUX' or 'TAPE' or detune it off a station and turn the volume control to maximum, you should hear a faint hiss/hum in each speaker if the STK016s are operational.If no sound can be heard, place the blade of a small screwdriver on the 'IN' pins of the output amplifier board, (see layout diagram) you should get a click or louder buzz when each 'IN' terminal is touched. If this produces no sound and the fuses are not blown, the STK016s are internally open-circuit and again, it will be necessary to fit replacements.Try this so far and if all tests well from the 'IN' terminals of your output board, you have a failure further back, which I'll go thro' in another post if necessary!Chris Williams

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 29, 2019, 07:44:34 pm
content: There is no protection circuit as such; none is needed since this unit uses output coupling capacitors.The 21 volts you measure tells me the output IC's are OK (see below). You may have one or more of the 2.5 amp fuses bad, or a problem in the voltage regulator circuit involving transistor Q701, which feeds the tone circuit.note:You would measure the 21 volts on the speaker side ONLY until your multimeter bleeds off the initial charge (how long depends on the meter's input impedance), and none at all really if speakers are connected. In normal operation you would hear a "bloop" sound at power up and that would be that.

author: michalism
author_rank: Regular Contributor
timestamp: July 01, 2019, 06:59:17 am
content: Thank you so much for taking the time to respond!!The continuity on the 6.3V line still is not clear to me, but I will leave it on the side for now...Initially I thought that the short I caused in turn resulted in the muting of the system, but after I had a better llok at the schemetics I realised this is not possible...But it is strange that last time I checked everything worked fine, and some time later it does not!I forgot to mention that I have replaced all electrolytics on this system. A lot of them where bad, but I replaced all anyhow.In my first post I say that the system seems to be in protection mode; that's because I saw a relay on a board and I just jumped to the conclusion that it is there for speaker protection, but as you also point out this is not the case. the relay is on the AM conversion board, so totaly irrelevant for my case.Tonight I will check your suggestions and make some more measurments to try and figure out what is wrong. I have ruled out a bad since both ICs have the exact same behaviour, and all voltages ar identical.Thanks again!Michalis

author: michalism
author_rank: Regular Contributor
timestamp: July 03, 2019, 07:46:31 am
content: Hello again!I'm happy to report that the system is working fine! However, I have to embarrassingly admit that it never failed...I must have had some kind of connection wrong or I messed up somehow and totally mis-directed myself and you.Really sorry about this.Anyway, just to complete the picture, I have done the checks that Chris suggested and all measurements were correct (continuity was ok, and no DC on the speaker terminals with the resistor connected).Then I proceeded by connecting a small 8 Ohm speaker to the output, turned-on the radio and I got sound! that was a surprise. I have touched nothing!so to close this thread, I'd like to apologise for wasting your time.For me it was a great learning experience, most notably learning about the output coupling capacitor and the troubleshooting methods!!A big big thank you!!Michalis