Alpine MRV-F400

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Next on my to fix list.

What I know so far:

Gate resistors to all the FETs are all good measuring just under 98 ohms. All FETs removed as two were definitely dead. 110 ohm resistors from the 494 to the FETs also good.

Chassis ground trace was blown so I've replaced it with 16 gauge cable.

Outputs, I think two channels have shorted and the other two seem to measure OK. Will take them out and measure them again to make certain as it's been too long since I last worked on the amp.

This amp uses 2SD718, 2SB688 outputs. If there are shorted channels as I suspect, would sanken 2SA1186 & 2SC2837 be suitable replacements? Also, would it be wise to check the driver transistors for those channels too?
 
I've replaced all of the outputs with FJA4213/4313 parts. All tested OK before going in. I've also replaced all but the 6800uF 35V capacitors. I just need to fit the new IRFZ44's and put some fresh paste between the bits and I can test it out.
An externally hosted image should be here but it was not working when we last tested it.


One thing I have noticed is that mine has a daughter board with two 4558 type KEC opamps that is missing in a lot of other F400's I've seen pictures of online.
 
Seems my host for the images has gone down for the minute. The fets are now in place, micas in and fresh paste applied all round. It seems to work fine for a little bit but starts to distort getting steadily worse until it's virtually white noise.

DC voltage across the collectors of the outputs is 58V, which should mean 28V rails
 
Rail voltage remains the same at +/-29V at the collectors on the outputs. Regulated voltage also remains the same at +/15V measured at the supply pins of one of the KIA6259P opamps.

This behaviour seems to be consistent across all 4 channels.
 
Don't know what has changed between yesterday and today but it's no longer distorting. Only changes made was to remove the RCA inputs and refresh all the solder joints as I had both left and right output with only a right input with the switch set to stereo. I had taken one of the muting transistors out but apart from a big thump and a lot of noise at switch off it had no apparent effect.

One thing I have noticed is that channel 2 has 360mV DC offset which explains the pops when moving the filtering switch.

Ch1: -30mV
Ch2: 370mV
Ch3: 19.5mV
Ch4: -37mV

There is also evidence of noise from the inverters leaking through to the outputs at a very low level. (certainly not audible)
 
The two differential amp transistors must match as closely as possible for there to be minimum offset. These may have drifted some so they don't match. The simplest way to determine if they're the problem is to swap them with the two transistors from a different channel to see if the problem follows the two transistors.
 
Is there a way to test for this problem before fitting new outputs? I diode checked the lot prior to fitting and they were all between 0.55V and 0.57V drop. Is it possible that the problem exists further up the chain of parts? Channel 2 is where the original shorted output was.
 
Swapped the outputs between channels 1 & 2 and now the offset problem disappeared. (~30mV on both) Still had all sorts of intermittent issues so continued checking everything and finally found that E503 in channel 2's muting circuit was 47uF and should have been 4.7uF. Swapped it over and so far it's much better however there is still an intermittent buzz that comes and goes. Would like to get a look at the waveform given a chance.

The gain pot seems to be shot for ch1 & 2 as it measures 40k for ch1 and 45k for ch2 at minimum so I get this between L and R.
An externally hosted image should be here but it was not working when we last tested it.
 
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