Has anyone any knowledge pertaining to the BSS MSR 604? It's a four channel, four way split active mic/line distribution system, two active balanced outputs and two transformer isolated outputs, separate power supply driving multiple racks. I was asked to repair a forty-eight channel system, three power supplies, with the user manual but no circuit diagrams.
The base problem was easy to find, easy to correct - in all the PS racks the main smoothing capacitors were loose in the circuit, the solder joints having given up with transport. just the ordinary problem of finding lead based solder to be compatible with older devices. But then I checked it, and the noise (some 200 Hz, some 50 and 100, but mostly white thermal resistor noise was at minus 92 dB, not bad, but this is mic level stuff. With 10dBs input gain (set on internal jumpers) this puts the equivalent input noise at -102 dB, while the manual insists it should be -115dB - fine for rock and roll, but somwwhat high for orchestral recording.
So, there's theoretically an external gain remote control, and I didn't test the through gain - oversight, I'll do it today - but the owners insist that a few months ago they did a rigorous check and it was in spec. This is checking half the racks, and all the power supplies, so it's unlikely it's a localised fault.
The base problem was easy to find, easy to correct - in all the PS racks the main smoothing capacitors were loose in the circuit, the solder joints having given up with transport. just the ordinary problem of finding lead based solder to be compatible with older devices. But then I checked it, and the noise (some 200 Hz, some 50 and 100, but mostly white thermal resistor noise was at minus 92 dB, not bad, but this is mic level stuff. With 10dBs input gain (set on internal jumpers) this puts the equivalent input noise at -102 dB, while the manual insists it should be -115dB - fine for rock and roll, but somwwhat high for orchestral recording.
So, there's theoretically an external gain remote control, and I didn't test the through gain - oversight, I'll do it today - but the owners insist that a few months ago they did a rigorous check and it was in spec. This is checking half the racks, and all the power supplies, so it's unlikely it's a localised fault.
with the possibility of phantom power being miss applied i'd be suspicious of the decoupling caps at the inputs and given the potential age of these unit it may be well worth simply replacing them all. chasing noise floor issues can be like chasing ghosts sometimes.
how many splitters per supply?
how many splitters per supply?
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i timed out on the edit for that last post.
so there's 16 units per supply?
the few times in the past that i had occasion to work with these they where configured 6 to a supply maybe 16 is asking a little much of the supply?
so there's 16 units per supply?
the few times in the past that i had occasion to work with these they where configured 6 to a supply maybe 16 is asking a little much of the supply?
Thanks. Four splitters per 1U rack unit, Power supplies rated to ten rack units each - and twelve of the rack units, so there's slight overkill. Unstabilised DC comes from the PS rack at +28 Volts or so and onboard stabilisers bring it down to ±15, and the scope tells me this is relatively clean apart for some slight HF switching noise. But I'm beginning to distrust the testing set up - when I switch 20dBs of gain into the preamp at the head of the splitter circuitry there is no noticeable change of noise level on the output - for all units. And putting tone through it shows the gain is actually there (and that the unit clips much earlier than the instruction manual would suggest - bad thing for a unit intended to be at the head of everybody's signal chain). Which suggests that the noise is coming in at the end of the signal chain, not the beginning. Scratches head. If it were just one of the rack units I'd be hunting a high resistance ground, but all three that I've checked intensively? And most of the time, to be able to access the boards and circuitry, I've got just one rack unit and one power supply up and running.
This is going to seem very silly and obvious when I find the problem, isn't it?
This is going to seem very silly and obvious when I find the problem, isn't it?
i guess i was not counting with a factor of 4, so you have 4 1u (rack unit) per supply then yeah supply loading would not be the issue.
are any of them stacked or linked for distribution purposes or are are they all independent?
re the remote attenuation: when phantom is applied to the main output(of an individual section) it applies a 10db pad
are any of them stacked or linked for distribution purposes or are are they all independent?
re the remote attenuation: when phantom is applied to the main output(of an individual section) it applies a 10db pad
The pad (or, as far as I can tell so far, the relay switching between high and low gain, both selectable between 0, 10, 20, 30 or 40dB of gain, working on a front panel switch when a rear switch is not enabled) is operated by phantom power applied from the house console, for remote operation. And that switch, combined with internal jumpers, is supposed to change the gain of the input amp, which it does on an external oscillator, without changing the output noise. I'll be back on it soon.
not sure if there's any functional differences between the 602 and 602 II but the block diagram on page 27 of the MSR 602 II user's manual seems to suggest otherwise but this doesn't help in the quest to find the noise issue.
if the supply units had cap troubles would it be worth looking at local line bypass caps at the opamps driving the output as a potential noise source?(simply suggesting places to look)
if the supply units had cap troubles would it be worth looking at local line bypass caps at the opamps driving the output as a potential noise source?(simply suggesting places to look)
Have now got six racks up to spec on the active balanced outputs (big, proud grin), three channels where the gain switching relay isn't making good enough contact, one electrolytic capacitor (25 volt working, input at that point about 28-29 volts - I should change all those on all the units) had blown its little can off and spread its guts over the power supply bit of the board on one unit. Since all those bits are parallel, cleaning that up helped a lot on all the other units, too.
I've got several more to do, and I'd like to know why the transformered outputs aren't as good as the active balanced, but I feel I'm getting somewhere.
Thanks for the encouragement.
I've got several more to do, and I'd like to know why the transformered outputs aren't as good as the active balanced, but I feel I'm getting somewhere.
Thanks for the encouragement.
Not if you drive it with its own amp.
So, problem was simple and easy to correct (when found) though why it is more evident on transformers than actives I'm not certain. No grounding on outputs of splitter (correct) and when the MTI level tester is run off batteries, figures are basically right - if using an external wart on the wall power supply, hum is picked up. Simple as that. I've worked out how to take the relays apart and spray them with switch cleaner, which has helped qite a few more channels - just four or five still with problems…
So, problem was simple and easy to correct (when found) though why it is more evident on transformers than actives I'm not certain. No grounding on outputs of splitter (correct) and when the MTI level tester is run off batteries, figures are basically right - if using an external wart on the wall power supply, hum is picked up. Simple as that. I've worked out how to take the relays apart and spray them with switch cleaner, which has helped qite a few more channels - just four or five still with problems…
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