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Post by mmartell on Mar 28, 2022 21:14:08 GMT -5
Have to have the volume turned all the way up to get minimum amount of sound for a subwoofer, even then it doesn't sound right. Hard to describe really.
Here is a link to the schematic I have. My board is Rev B but this schematic is Rev C. Judging by what was listed as changes I think these are mostly identical.
When it was first opened I noticed visible signs of heat and charring at the edge of the preamp board around the power resistors at locations R41, R22, R68 and R32. This was cleaned up and I don't believe this to be a problem. However if measured in circuit they measure closer to 2k ohm than the 3.6k ohm listed in the schematic. Would their function cause a problem at a lower resistance ? If so I can remove one leg and measure again to be sure...
Any other instruction is greatly appreciated of course.
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Post by mmartell on Mar 28, 2022 21:20:01 GMT -5
A few pics of the backplate, the preamp board and the amp board. |
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Post by mastertech on Mar 29, 2022 16:44:48 GMT -5
Have you checked your rail voltages?
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Post by mmartell on Mar 29, 2022 22:17:19 GMT -5
Yes -50.5v and +50.5v are good also have +/-14.5v for the opamps.
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Post by mastertech on Mar 30, 2022 10:44:47 GMT -5
I did a little checking and it looks like these units are plagued with bad electrolytic capacitors and cause low or no volume. So I would suggest you change all the electrolytic caps except for maybe the large filter caps. You could change them also but they may not need to be changed and they might be a little pricey. Your choice. But get a good 105c grade cap for all that you change.
I also read your thread on badcaps and you never indicated if you changed all of these already.
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Post by mmartell on Mar 30, 2022 12:24:52 GMT -5
IIRC I replaced the four small caps on the amp board. I have no non-polar electrolytics so I did not replace those on the preamp board.
Are you thinking the fault is probably on the amp board and not in the preamp section ? What would be a good way to confirm that ?
Was thinking of using the probe tool of yours I built for the marantz repair and inputting the preamp signal into another amp to drive the 4ohm woofer but not sure how much that would test would really tell me since the amps are already different obviously...
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Post by mastertech on Mar 30, 2022 15:26:02 GMT -5
At this point I am not thinking anything or any particular board. But I figured before we got into specific testing I would do some looking around and found out about the caps. Apparently on these units all the caps are junk, polarized and non polarized. And I personally have been down this cap road so many times myself for so many years that I would start there first if it was me.
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Post by mmartell on Mar 30, 2022 19:13:23 GMT -5
Fair enough I will look into getting some replacements.
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Post by mastertech on Mar 31, 2022 10:59:13 GMT -5
Did you at least test the speaker in something else or try another speaker in the unit?
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Post by mmartell on Apr 1, 2022 20:51:54 GMT -5
The amp won't drive a 12" 8 ohm speaker, sound is barely audible at max volume.
The amp associated with that driver plays the Polk 10" 4 ohm speaker no problem, even with the plate not mounted (open back sub box).
Yes I could sawzall the Polk box and jerry rig the other amp plate in but that's not very satisfying. I'd like to determine what's causing the preamp/amp to not be able to perform to spec.
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Post by mmartell on Apr 1, 2022 20:59:57 GMT -5
And there are four bipolar electrolytics on that preamp board (I changed the other two polar caps). The largest measures ok on my esr meter (22uf 35v, capacitance in spec and esr about 1.45 ohm). The other three are 2.2uf 50v. They read ok capacitance-wise but probably a little high on the esr range at about 8 ohms each. I notice they are decoupling each input so could be causing a problem ?
Any way to verify that with a scope or otherwise before trying to locate these ?
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Post by mastertech on Apr 1, 2022 21:39:43 GMT -5
In my opinion you need to ask yourself a few questions. Do you want to fix this just to get it working and then sell it off to someone without regard as to how long it might last? Or do you want to fix it so you can enjoy it for some time without worrying if it will fail at any time?
ALL, the caps in that unit are junk. If you find 1 or 2 or 3 bad ones and replace them only and leave the rest in there then the potential for a future failure at any time, even in a couple days is there.
Years ago I threw my ESR meter in a drawer and have not taken it out since. To many possible errors for testing even with the best meter. I could go on and on how they are basically worthless but that is another discussion.
Even if it still does not work properly after changing all the caps I am sure we can still get it working and you will have a nice fresh unit that will probably be better then when new. So it is your call on how you would like to proceed.
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Post by mmartell on Apr 1, 2022 22:39:11 GMT -5
Ok look humour me for a second.
This sub sounds terrible and must be turned up almost all the way to get some output. That's a fact. But I am only speculating that this is not normal for this sub - they are not all created equal.
So I ask myself, does it make sense to order and replace caps without taking a single measurement to confirm the amplifier is in fact in need of repair ?
Does that seem unreasonable ?
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Post by mastertech on Apr 2, 2022 10:14:03 GMT -5
Ok, I'll humor you for a second, LOL.
From here, I cannot hear your sub. I can only use your comments which were, Quote: "The amp won't drive a 12" 8 ohm speaker, sound is barely audible at max volume.
The amp associated with that driver plays the Polk 10" 4 ohm speaker no problem, even with the plate not mounted (open back sub box)."
So........ whether this is normal or not? Your words don't speak normal, LOL.
But if you feel like playing around then you could probably try sending a different preamp signal into the amp and if it is working, see how it sounds. Does that receiver you built for a signal tracer have preamp outs? You need to be able to control the preamp level signal so it would need a volume control. Looks like the best place to inject the signal is at R13 on its preamp side but use a 22uf, 35v cap in between.
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Post by mmartell on Jul 5, 2022 19:50:50 GMT -5
Managed to burn out the fets by taking measurements without having the heatsinks attached so got some parts from Digikey and changed out the caps and the mosfets. Caps didn't make a difference. When I measure the base of Q3 and Q6 I get 1.6v and -1.75v. I would expect them to be +/- 0.7v for proper biasing is that not correct ?
Those voltages are with respect to J2 btw.
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