Tweet
I've been tasked with a Ross Mega Amp 400. Protection circuit comes on at power up and the Power Supply rail voltage is present on the output of the B channel. Took out the preAmp board and did some resolders. Problem cleared but is back when cold. Generally, after a minute it will clear and outputs drop back to 0 Volts with no input applied. Sometimes more than a minute. I thought I could swap the identical preamp boards from A to b side to confirm it is a problem on the preAmp board. Also thought Id stop by the forum for some good advice (as always) as the item seems to be worth less than the labor to take the cover off. Anybody out there fixed one of these? When protection circuit not active Amp works fine so we are chasing an intermittent issue with no schematic. Makes for a long day on an Amp owned by a divorcee with a voice but no money. Thanks in Advance for your opinions and time. (No caps are swelled but I had considered recapping it)
-
-
I would suspect the power amp board before the preamp board.
Resolder the power amp. -
These units don't really have a power Amp board. Each channel has four transistors on mini boards wired back to the preAmp. I'm going to rework the solder on thos four boards this morning just to get some forward progress. I had a Classic 30 come in from a new friend that I'm anxious to get a look at. Thanks for the reply!Comment
-
I googled a little.
You have a 200+200W rackmount *Power Amp* there, so no preamp boards at all inside it.
What you call preamp boards are in fact Power amp ones, and I guess what you call miniboards are more mini than you hinted (post stamp size?), simply a neat way to solder connecting wires to power transistors.
So resolder the small ones if you wish, can't hurt, but main problem seems to live in the large ones.
Resolder *everything* in sight.
Use a Bic plastic pen or chopsticks to beat and slightly move every part, wire, connector, pot, etc., *slightly* press and flex different points in the PCB , maybe you can find an intermittent that way.Juan Manuel FaheyComment
-
Yes, resoldered the postage stamps, did little more than make it shiney inside. I'll pull the board and give it the once over, just hate to put labor into a blind resolder effort. However, the fact that it does work just fine at times certainly suggests that it is just cold solder. Thanks for the support!Comment
-
After a through resolder my symptoms are now the "B" channel peak light blinking and the protect flashing instead of a solid on. Makes me wonder if I didn't just reconnect the original problem? Looks like time to see if customer wants to invest in a service manual or just take it to metal salvage.Comment
-
I have had the same symptoms and just stumbled on to my solution.
When attempting to remove the B channel main amp board, I noticed the quick disconnect on the far rear right(the side where AC comes into the amp) green and brown wires were loose.
The connector on the main board was missing the key stud and the pins were spead where the green wire would not mate with pin. I pulled the connector and the protection flashes. I straightened the connector so the socket engages solidly and has firm retention. The amp is working great.
I was about to look for a thermal fracture on a solder joint like my convergance amplifier on my bigscreen television, but I found the problem and didn't see any fractures around the components that typically get hot.
Good luckComment
-
A buddy brought me one of these to look at with the only noted problem of crackling in the sound when rotating volume controls. No big deal there just clean the pots. During my visual inspection of the overall amp I go ahead to do the wiggle test on various components on the PC boards I find one of the large round power resistors loose on one lead. I remove this board and resolder the broken connection. After installing the A channel board and cleaning the pots I turn it on. Both channels playing fine at this point then I notice a small of amount noise when rotating the volume pots. I shut it down and clean them again.
When I power it back up now only B channel has audio then shortly after power up the protection light illuminates. Also when powering up with both volumes all the way down channel A emits a static noise and then a pop, no noise on the B channel.Comment
-
I did power it and put a program in it before touching anything. The volume controls did have a high amount of noise during rotation and both channels played and no protection light then. Its a steady illumination.
well one nice thing I discovered later with this unit is a cover in the center on the bottom can be removed giving easy access to the bottoms of the three boards so you dont have to remove them to solder anything.Comment
-
Check to see if there is a dc voltage on the volume controls. That will cause a lot of noise when rotated. If it's there, check for bad caps or opamps.Originally posted by monsteranus View PostComment
-
Would that cause a crackling and a sudden pop noise when powering it up?
I cut it off after the first power up from the initial pot cleaning to clean them a second time. I pulled the plug from the speaker out to not have the pop in the speaker. I turned it on, then inserted the speaker plug. Its when the protect began to illuminate.Comment
-
You have a problem with the A channel circuit. You can leave the speakers off until you find the problems. I do not have a schematic for this amp, but I assume that you can take readings from the working B channel and see where the differences are.
Is the input section all discrete devices or IC driven? If it has discrete transistors, test all of them in the A channel. If it has ICs, do any of them work in both channels or are they separate to each channel?Comment
-
-
Thanks Doctor. I think this schematic may be for a later model unit. Its showing more fuses than what this amp has in it. I compared a few obvious components that didnt match marking numbers either.Comment
Comment