Agreed. Sounds like an offset issue.
Things like this can be difficult to pin down. If I had this in front of me I would look to just replacing Q755 and Q757 (use something generic like MJE340/350) because those will be hot running and also possibly that dual PNP input device. For a test I would just fit a pair of suitable small signal PNP's for those.
Things like this can be difficult to pin down. If I had this in front of me I would look to just replacing Q755 and Q757 (use something generic like MJE340/350) because those will be hot running and also possibly that dual PNP input device. For a test I would just fit a pair of suitable small signal PNP's for those.
I had to go away for a while but tonight I can try to start swapping transistors between L and R if I can't easily see voltage problems with my voltmeter, still waiting for the oscilloscope and also realized I only ordered one lead for it so I will be working single channel at first 🤦♂️
Replacing the transistors I mentioned is something I would try but swapping isn't generally a great idea simply because the heat from doing so can in some cases temporarily 'heal' defective devices or change the symptoms in ways you don't expect.
Its up to you 🙂 but swapping isn't something I would normally recommend.
Its up to you 🙂 but swapping isn't something I would normally recommend.
It's funny you say that because it seems like that's exactly what's been happening. I re-flow solder, everything is fine then after a considerable amount of time, the problems come back but slightly differently so that seems it's even more likely to be the transistors.
That's possible, especially in places, where pcb becomes darker, burnt. You may connect speaker through capacitor , something like 1 uf , film or electrolytic bipolar, 100v rated, and no damage occurs to speaker. To catch the problematic part you need to write down most voltages of bad channel in non-working state, when protection engaged and there's no relay click, but amplifier is powered and enabled.
You could try a bit of freezer spray on the suspect parts. Cheapo alternative are the cans of air duster in high street shops. Those are butane based (so safety precautions obviously apply). Invert the can and dispense literally a few drops at a time via the straw. Don't blast whole areas... it takes practice to dispense drop by drop but it can be done.
small but strange update, I turned on the receiver and before long it popped and went into protect mode. I started going around and testing voltages and at one point when I touched this one, I heard the transformer hum and it also looked like mabye there was a spark happening along the trace then suddenly the fuse blew. I pulled the transistor out to see if it was shorted and I didn't see anything wrong with it but that trace circled had peeled up from the board and when re-installing it I had to put a jumper in back to R763. I replaced the fuse and tried to power the unit up again and I heard the xformer hum again the fuse popped. I checked all around and found one of the main driver transistors was shorted. It's Q763 and it's a MP1620. Looks like it xrefs to a 2SB1647. Should I order a pair of them and replace them on each channel or do the NPN ones as well?
At first ,check output transistors . If this one driver transistor failed ,higher than normal current flow through output pair and probably they both are shorted too , causing fuse to blow .If outputs failed , then Q756 must be failed too ,but open ,not shorted .
If diode test passed, then maybe it survived. I would temporarily short it, untill all issues are fixed, or use a powerfull diode jn parallel to it. And dim bulb tester, a incandescent lamp in series with mains, maybe lamp just connected in place of mains fuse.
I just went ahead and got all 6 of the transistors that mount to the heatsink, they're not expensive. I'll drop them in and see how it goes.
First measure them . To prevent further damage , use dim bulb tester , even less expensive than transistors , and faster to build.
Pretty much what everyone has said really. The transformer humming shows a massive overload. Check also that the bridge rectifier has survived. The 0.22 ohm emitter resistors need checking too.
Well I've had to order fuses along with the transformers and am going to build the dim bulb tester. I'll measure the rectifier and resistors.
The rectifier and resistors seemed fine. I mentioned before that I saw what looked like some arcing on the q755 transistor's emitter trace when the short was happening and upon inspecing that part of the board, those traces didn't look great so I wound up running a wire from that emitter joint to the R763 resistor joint. Yesterday I built my dim bulb tester and received the new transistors. Before doing anything else, I installed a new fuse and plugged the amp into the dim bulb and everything seemed fine. I went ahead installed all six new transistors and again, all good and I left the unit playing for two hours without issue. I've just reverted the switched leads on the volume board connector and will run this receiver in my main system for some time until I am confident it's stable and will report back.
You should recheck the idle current and compare with good working channel . Just compare voltage drop on 0,22 ohms resistors with amplifier turned on ,but volume set to zero ,and speaker not connected .If voltages on resistors would be very similar , probably no adjustment needed .Refering to schematic posted earlier ,there are no adjustment potentiometers ,but a few fixed resistors near vbe multiplier .
then