![]() With the DBT, I've measured 6mV bias voltage at SourceHĪ mere touch to the multiturn trimpot brought it immediately to 4mV (was the wiper contact loose on the track?). The 80W bulb clearly limits the current as 80V rails are measured at 55V. ![]() I've brought back the unit on, this time with a DBT. Then, before I've had the chance to write that down, the voltage started to climb to 33mV, then decreased back to 16mV then went all over the place in a matter of seconds as far as 100mV The protection circuit triggered and the relay disengaged almost at the same time I was powering down the unit. So I've found R43 and R44 legs to probe "SourceH" and "SourceL" from "Out" (at relay out).Īt first power up, I measured 10mV climbing slowly and stabilizing to 16mV as the unit was warming up. Every THT component on this board is literally drown in silicone I had to remove again a lot of silicone to get access to them. I managed to identify accessible probing points for the bias voltage, as no such point has been anticipated in the design, attesting again that it has not been thought as serviceable. If you measure the DC voltage across C4, and the DC voltage across R38, and report those two numbers, along with the value of TRIM1, I think I can help you figure out a suitable value for this additional safety resistor. At the same time, this resistor needs to be small enough in value so that, if TRIM1 goes bad, the output MOSFETs are protected, as described in the previous paragraph. This additional resistor needs to be big enough in value to allow you to still use TRIM1 to adjust the proper output bias current. The value of this additional safety resistor depends on some component details I don't know, including the resistance value of TRIM1. There will be increased crossover distortion until TRIM1 is fixed, but nothing will burn out. This will reduce the bias on the output MOSFETs, making them go cold. If the trimpot (TRIM1) goes bad, this resistor will now turn T9 fully on. Wire a resistor between base and collector of T9. This can be fixed by a slight change in design. When this happens, T9 will turn off, and the output MOSFETs will start to flow too much current. Eventually the TRIM1 potentiometer will get old and scratchy, and the wiper will lose contact with the track. You're done!īy the way, the bias adjust circuit is very poorly designed. Adjust TRIM1 until the meter reads 5 mV or 0.005 volts. So now you know how to adust your bias current: Connect a DC voltmeter on a suitable millivolt range between "Source H" and "Out". Since R29 = 0.33 ohms, the voltage drop across it (when 15 mA flows through it) will be 5 millivolts. That means R29 will have 15 mA flowing through it. This current will be divided equally between the three matched FETs (we hope). Let's pick the middle value for bias current, 45 mA. ![]() If your amp's had a rough life there may be other contributing issues, and it may be a good idea to give it a full overhaul.Click to expand.I think you have all the information you need in those two sentences. Mine hasn't crackled since, although these days I use the plug to switch it on/off so as not to wear out the switch. ![]() It's a bit fiddly to do, and it involves bending some small metal tabs in order to be able to dismantle and reassemble the power switch itself, something that needs to be done very carefully. I cleaned it with isopropyl alcohol and a cotton swab, and then polished the contact points with metal polish (make sure to wash off any residue! I used isopropyl and a cotton swab for this, too). Not an uncommon problem on old hifi equipment (I've since encountered it on a tuner too).Īs I wasn't sure whether it would be possible to get hold of a replacement switch that would fit and work properly with the rotary selector used on these amps for power off/on as well as speaker selection, I decided to dismantle the old power switch to give it a deep clean. I finally narrowed it down to the power switch, more specifically the contact surfaces inside it being dirty and/or corroded. I have this amp's little brother, the AU-2900, and it suddently started crackling in a very similar manner to yours. Sounds like it could be a dirty power switch.
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