If you have an 8 ohm load connected, that would mean the right channel has a DC output offset of roughly 106.6666... mV. Is that correct?
If so, that could be perfectly normal or it could mean that one of C802, C806 and C810 got shorted. I would check if they became short circuits (you don't even need to desolder them for that) and if not, try to trim the right channel without load.
If so, that could be perfectly normal or it could mean that one of C802, C806 and C810 got shorted. I would check if they became short circuits (you don't even need to desolder them for that) and if not, try to trim the right channel without load.
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All four emitter resistors measure .3 ohms, I even pulled the right channel resistors out of circuit to double check.
I took some readings of some of the transistors on the protection board per the schematic and some of the voltages were way off. I'll pull them and test with my Peak Atlas tester
I took some readings of some of the transistors on the protection board per the schematic and some of the voltages were way off. I'll pull them and test with my Peak Atlas tester
Dead output transistors very often take out the VI limiters. When the voltage across the emitter resistors shoots WAY up the base bias resistors for the protection circuits usually see way more than a quarter watt. And base current rating exceeded by a factor of 100 or more…..
I removed the protection board and found one faulty transistor but it was for the left channel. I pulled an exact replacement from the donor amp (same model) but had to use the right channel transistor, the donor amp left channel transistor was also faulty.
I'm still getting 20mV across one emitter resistor and around 12 mV across the other resistor of the Right channel. If I increase from 12 mV to 20 mV, the other resistor will shoot up to 27 or so.
I let the amp idle for about an hour before taking measurements. The Left channel DC Offset was at 25 mV, Right channel, 29 mV.
Any ideas where to look next?
If there was any thing that caused the problem then maybe. Was the speaker accidentally shorted, was there a mains surge or did this just happen suddenly, to both channels simultaneously. It is difficult to ask the blind man what he saw. Something caused it, I read through the thread but see no indications. Act of god? Unlikely, poking fingers yes but why both channels?
The current flowing through the emiitter resistors should be equal, assuming negligible currents flowing along the speaker path. If I recall, you confirmed equal resistances in the emitter resistors. Assuming this is the case, the disparity in the voltages across the emitter resistors suggests some undiscovered current flowing along the amp output.
If you disconnect the output devices, can you get the voltages on each channel the same? Just a quick look at the schema, there are four maybe more feedback point to the op-amp, then there is this resistor R726 that to me is a little weird meaning that adjusting the left channel bias would throw the other channel off. Have you tried biasing both channels at the same time, what happens?
Regardless the brand, to me it looks like the designer tweaked/hacked the thing to work, just an opinion.
Regardless the brand, to me it looks like the designer tweaked/hacked the thing to work, just an opinion.
@Nico Ras,
I am repairing this for myself, I am in no way qualified to work on some else's gear. I am trying to learn how to repair and restore audio amplifiers for myself as a hobby. I have done complete restorations of several tube and solid state amps successfully but this Sherwood SEL-400 is a bit more complicated than the others.
brief history....I purchased the amp used, while bench testing it, I shorted something in the Left channel and smoked some resistors and one transistor in the driver board of that same channel. I replaced the fried resistors and all of the old capacitors and transistors on the driver board but now have an issue with the Right channel as described.......but.......after replacing the output transistors and reinstalling the heat-sinks, I inadvertently pinched the wire connected to the collector of Q811 between the heatsink and chassis damaging the casing and causing a short which blew the relay contact. I know. I replaced the relay and repaired the lead from the transistor. The relay seemed to have done it's job because I can't find any other components damaged in the right channel.
I am repairing this for myself, I am in no way qualified to work on some else's gear. I am trying to learn how to repair and restore audio amplifiers for myself as a hobby. I have done complete restorations of several tube and solid state amps successfully but this Sherwood SEL-400 is a bit more complicated than the others.
brief history....I purchased the amp used, while bench testing it, I shorted something in the Left channel and smoked some resistors and one transistor in the driver board of that same channel. I replaced the fried resistors and all of the old capacitors and transistors on the driver board but now have an issue with the Right channel as described.......but.......after replacing the output transistors and reinstalling the heat-sinks, I inadvertently pinched the wire connected to the collector of Q811 between the heatsink and chassis damaging the casing and causing a short which blew the relay contact. I know. I replaced the relay and repaired the lead from the transistor. The relay seemed to have done it's job because I can't find any other components damaged in the right channel.
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I will pull the outputs this evening and take measurement across the emitter resistors and report back.
I use one meter and check both channels, I have not noticed any changes in one channel after adjusting the other if that's what you mean.
Do some comparisons without the outputs connected. Like I said weird common resistor in the negative line of the driver board, has its value changed. I cannot recall seeing anything like that before. Now you have experienced ouch! something that is not fun. That thing is direct connected throughout, one thing goes it sets of an instantaneous catastrophic chain of evens. Then there is the op-amp, does it work - that thing with all the summiong feedback to its input is another pain in the ***. Do you have a signal tracer, a little amplifiers that you can probe around with. A scope is a nice tool but you have to take your eyes of the work while probing around, another cause to make a short or stuff something up. I would try and trace the signal through the circuit and see where or if it goes wonky, then look for a fault in that area. As I say that resistor in the negative line makes me wonder. If one driver draws a little more current than the other channel it will cause off-set that you nor I will never be able to explain.