I just moved my pair of test speakers from ch1 & 2 to ch3 & 4 along with the inputs. (the outputs were all replaced at the same time) I've not yet tried removing the muting transistors for these channels, I believe the one that I removed previously happened to be channel 1. The lack of any pcb labelling makes figuring out which corresponds to the right channel a little difficult.
Took the muting transistors out this morning and tested it, still the same problem. (confirmed I've got the correct muting parts as it no longer mutes ch3 + 4) Distortion slowly gets worse once it starts. Starts off as clean 1k tone. This is just a few seconds after it starts to distort.
This is about 30 seconds later which is about as bad as it gets.
I've confirmed that after it's distorting I still have +/- 29V at the collectors on all of the outputs and I also have tested that I have +/- 15.5V on at least some of the easy to access opamps. I have also checked the continuity of the RCA shield to the centre tap is good too.
Presumably this points towards the opamps and the logical route will be to trace the signal from the input to find out at what point it distorts.
An externally hosted image should be here but it was not working when we last tested it.
This is about 30 seconds later which is about as bad as it gets.
An externally hosted image should be here but it was not working when we last tested it.
I've confirmed that after it's distorting I still have +/- 29V at the collectors on all of the outputs and I also have tested that I have +/- 15.5V on at least some of the easy to access opamps. I have also checked the continuity of the RCA shield to the centre tap is good too.
Presumably this points towards the opamps and the logical route will be to trace the signal from the input to find out at what point it distorts.
Confirm that the signal remains clean at the base of the input to the differential amplifiers. That's Q501-Q504 for the 4 channels.
Signal at the base of Q504 (or what I believe to be Q504) is virtually non existent slight negative DC offset. I've not been able to locate Q501 & 502 yet.
A quick check at what I believe to be IC101 and IC102 to find that pins 1, 2, 3, 5, 6 & 7 all have 15v DC on them.
Sadly I ran out of time to check anything else.
A quick check at what I believe to be IC101 and IC102 to find that pins 1, 2, 3, 5, 6 & 7 all have 15v DC on them.
Sadly I ran out of time to check anything else.
Blooming thing is not misbehaving so I can't check anything as it all tests good at present. 15V dc on the input to the opamps seems to be the state it sits in with no input connected.
I think I now understand why I've been confused by the schematic pin readings for the opamps IC101 and 102. In the schematic, they are SIP-9's whereas in my version of the amp they are DIP-8's.
I have correct voltages at the supply pins but +14V DC out of every other pin. Meaning I get 14V DC out of the RCA's for that pair of channels, both shield and signal path. What's a little odd is that there is a 10uF cap and 100 ohm resistor in the path between pin 5 of the opamp and the RCA positive
I have continuity between RCA ground and centre tap momentarily on Ch 1, 2, 3 & 4 so there must be a capacitor between the two as it goes OL after a second. The RCA loop out (1+3 & 2+4) have permanent continuity between shield and centre tap.
I have correct voltages at the supply pins but +14V DC out of every other pin. Meaning I get 14V DC out of the RCA's for that pair of channels, both shield and signal path. What's a little odd is that there is a 10uF cap and 100 ohm resistor in the path between pin 5 of the opamp and the RCA positive
I have continuity between RCA ground and centre tap momentarily on Ch 1, 2, 3 & 4 so there must be a capacitor between the two as it goes OL after a second. The RCA loop out (1+3 & 2+4) have permanent continuity between shield and centre tap.
With nothing connected to the amp, what is the resistance between the B+ terminal and the secondary ground?
Are you using a grounded signal source?
Are you using a grounded signal source?
The resistance between B+ and secondary ground rises fast from ~30ohms up to roughly 4.2Kohms.
My source is my laptop which must have a secondary ground from it's transformer. I have continuity between signal shield and the psu ground to the transformer, but not past the transformer to the wall socket.
My source is my laptop which must have a secondary ground from it's transformer. I have continuity between signal shield and the psu ground to the transformer, but not past the transformer to the wall socket.
Yes there is a link between the two from what I can tell. It measures as such too between non bridging speaker terminals, an primary ground. RCA shields on all of the inputs are open circuit to primary ground. The RCA output is short to primary ground.
I didn't remember having the following files. It's 3 files with the circuit board layout.
If you don't want them or after you download them, let me know so I can pull them from the server.
http://www.bcae1.com/temp/mrv-f400-board-2x.zip
If you don't want them or after you download them, let me know so I can pull them from the server.
http://www.bcae1.com/temp/mrv-f400-board-2x.zip
Downloaded. Massive thank you Perry, I've been struggling along without that bit. By any chance, do you have a higher resolution version of the schematic now? I've got the low res copy
I've rechecked the voltage across the inputs and it seems that across the + input to the secondary ground I get my 12-14V DC. If I measure it directly off the ground for that particular RCA, I get 0V.
When it misbehaves and I loose the positive half of the waveform, the gain control affects the level of the noise produced regardless on whether there is an input. With my 1k tone in, I see the same distorted waveform at the input to the opamps (IC101/IC102) on pins 1 & 5 (dip-8) as I do at the speaker output. Oddly though if I check the input from the RCA with the scope, it's clean.
I changed my input from my laptop direct via 3.5mm jack to RCA's to using a USB XLR interface via a sonifex redbox converter which definitely ties the rca shields to ground. I got plenty of mains hum along with the usual misbehaviour.
I've changed the input coupling caps as they were still original. Typically, it is behaving perfectly again now. Mine seems to use different parts to the one in the service manual so the caps were 10uF rather than 22uF.
When it misbehaves and I loose the positive half of the waveform, the gain control affects the level of the noise produced regardless on whether there is an input. With my 1k tone in, I see the same distorted waveform at the input to the opamps (IC101/IC102) on pins 1 & 5 (dip-8) as I do at the speaker output. Oddly though if I check the input from the RCA with the scope, it's clean.
I changed my input from my laptop direct via 3.5mm jack to RCA's to using a USB XLR interface via a sonifex redbox converter which definitely ties the rca shields to ground. I got plenty of mains hum along with the usual misbehaviour.
I've changed the input coupling caps as they were still original. Typically, it is behaving perfectly again now. Mine seems to use different parts to the one in the service manual so the caps were 10uF rather than 22uF.
Is this better than what you have?
http://www.bcae1.com/temp/MRV-F400 schematic diagram smaller.swf
Right-click to zoom, left mouse button to drag.
http://www.bcae1.com/temp/MRV-F400 schematic diagram smaller.swf
Right-click to zoom, left mouse button to drag.
Used freezer on the opamp when I had noise issues and it had quite a profound effect virtually stopping it dead. I've replaced the original KIA6259P with an LM833N and am testing it again, though no doubt it'll behave itself for ages.
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