Crown Amp DC300A - One channel distirted, other good.

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

it wouldn't harm to check all those cement emitter resistors are good

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I checked them all... they were within the value and "accuracy" of my meter... .3 ohms (value .33).

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

you should use a blocking capacitor on the positive probe to measure AC .also if you use an audio tracer such as headphones . 0.1uf or thereabouts if i remember right

I was thinking of trying a fm wireless headphone... using the input to the base as the "probe"... which is a phone plug / rca plug and then wireless out to the headphones.

I am thinking that the issue might be the high DC levels around in places... so the capacitor is probably a good idea.

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

I was thinking of trying a fm wireless headphone... using the input to the base as the "probe"... which is a phone plug / rca plug and then wireless out to the headphones.

I am thinking that issue might be the high DC levels around in places... so the capacitor is probably a good idea.

always a good idea to use low value blocking capacitor .

[petehall347]

Re: Crown Amp DC300A - One channel distirted, other good.

in my experience any amp left in a working state then doesn't work properly is either corrosion or dampness got into the board or resistors . corroded connections being the main thing .

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I agree. I have been measuring components at the component wires. I should probably do some more check at the board layer / junctions just in case it is a poor solder joint as opposed to a faulty component.

This unit is over 40 years old so capacitors of their time might have an issue (drying out or otherwise) which to me is a "distortion maker".

[Agent24]

Re: Crown Amp DC300A - One channel distirted, other good.

I have to investigate the meter but I am sure that I have used it to check the incoming AC frequency on a few house circuits...

Some probably can, I know mine doesn't.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

I think the problem with the meter use was what point I was probing. I don't really know where to look for signals.... at the base inputs to the transistors, at the emitters, other places? In the case of the "first attempt", I put the positive (red lead) on the base of the output transistor "group" that is associated with the positive rail. Nothing really got measured there but the meter did try to do some counting as but basically sat at 0. The next attempt was at the base lead of the transistor group of the negative rail. It is quite likely that this caused a current flow into the based from the chassis ground (black lead) towards the red lead. I suspect the meter doesn't expect to see "reverse flows" and has limited / no "blocks" to prevent it... maybe.

Anyway, I will do some more thinking / looking / checking before blindly going forward probing things.

[Agent24]

Re: Crown Amp DC300A - One channel distirted, other good.

First place I'd be checking is the output pin of that dual opamp, IC1.
Looks like pins 1 and 13 are the outputs. Not sure which channel is which. They seem labeled 1&2 not L&R.

Hard to read the text though. Can you take a better photo at all, or do a scan?

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

:-((

I guess I did some damage somewhere with my "unknowing probing". Replaced the speaker line fuse. This was on the good side of the amp. At the time, I was checking to see what I would see on the good side with the 1khz input signal.

Anyway, after replacing the fuse, turned on the amp at low volume, then tried to listen to the test cd again. The amp ran for about 30 seconds, noticed no sound coming from the good side. At this time, the amp shut down because it had blown the main line fuse (10 amp)... oh oh. So it appears I made my problem worse with some sort of short somewhere but not an immediate short as it did power up for a bit as opposed to an instant fuse blow.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

First place I'd be checking is the output pin of that dual opamp, IC1.
Looks like pins 1 and 13 are the outputs. Not sure which channel is which. They seem labeled 1&2 not L&R.

Hard to read the text though. Can you take a better photo at all, or do a scan?

not sure if this helps as my scanner is pretty old. The circuit diagram is a "fold out" so it takes two pages to get the whole schematic in. Note the page separation and even though it looks like it aligns, it is an overlapped view and not "thru flow" from one page to the next.

[andy1]

Re: Crown Amp DC300A - One channel distirted, other good.

If you say that you may have further damaged by blowing the working channel, do a no load check. By that I mean disconnect the speakers, replace the blown fuses with the correctly rated fuses, switch on the amp and see if you get any DC voltage on the speaker terminals of that channel. If there is DC present (I think it will be!) it means that you have blown output transistors. You could also have the driver transistors blown as well.
You could always check each transistor out of circuit. If you find even 1 of them blown, replace the whole lot. These direct coupled stages will damage everything if one gets blown.

[Agent24]

Re: Crown Amp DC300A - One channel distirted, other good.

not sure if this helps as my scanner is pretty old. The circuit diagram is a "fold out" so it takes two pages to get the whole schematic in. Note the page separation and even though it looks like it aligns, it is an overlapped view and not "thru flow" from one page to the next.

Thanks, that is a lot better than before, but can you try at a higher resolution? 300 dpi? Some of the component values are still hard to read, unfortunately.

[R_J]

Re: Crown Amp DC300A - One channel distirted, other good.

see if this is any better

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

thanks for the assist, RJ

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

If you say that you may have further damaged by blowing the working channel, do a no load check. By that I mean disconnect the speakers, replace the blown fuses with the correctly rated fuses, switch on the amp and see if you get any DC voltage on the speaker terminals of that channel. If there is DC present (I think it will be!) it means that you have blown output transistors. You could also have the driver transistors blown as well.
You could always check each transistor out of circuit. If you find even 1 of them blown, replace the whole lot. These direct coupled stages will damage everything if one gets blown.

While after a "deep sigh" and more reading and "thinking", I started back on this. I was able to "isolate" the output transistors (at least the collector to emitter section) without demounting them from the board.... demounting all transistors at this stage is probably not happening as this is significant effort and an unknowing experience (meaning I don't know how to go about the desoldering of such devices). Using a "gross resistance" test of the CE, I found two 2n5631's on the channel that I "took out with my probing"... these were shorted. I found two 2n5631's on the distorted channel... these had measureable resistance in both directions as opposed to "infinite" in one. I also found one .33 emitter resistor on the "blown channel" (all others on both channels are OK still).

Baby step forward... maybe... :-)

[circuitmonkey]

Re: Crown Amp DC300A - One channel distirted, other good.

Just a little friendly advice. Maybe consider wiring up a dim bulb to share the current when an "oops" happens. While it won't completely eliminate damage to you or the components, it will greatly reduce the risk and in many cases mitigate damage. There are plenty of good diagrams on the Google machine. It also works pretty well as a fault tester. If you apply power and the bulb(s) stay lit, you have a significant short and no further distortion testing can really be performed until the shorts are gone. Also, without having an auto transformer, this will help limit inrush current when you first power it up. It's a cheap process if you're creative. Around $10 at Home Depot.

Channel distortion is usually (not always) caused by bad caps or leaky transistors in the audio signal chain. It's really difficult (by no means impossible, but a lot more guess work) to narrow down the noisy component(s) without using a scope or signal tracer. One mismatched transistor in the pre-amp section could cause audible distortion. While replacing one or two might fix the issue electrically, not matching the transistors could result in distortion. Might get lucky, though. Cold/cracked solder joints can also cause cracks and pops.

Also, film caps (which are not generally marked for polarity anymore) can cause distortion if they are not oriented correctly. While they are said to be bi-polar, they have a "foil end" and that needs to point to the path of least impedance. I don't think that is your issue as it developed over time, but something to consider if/when you need to change them. Also, hard to test polarity without a scope, although I'm sure someone out there has figured out a way.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

thanks... good idea with the "limiter bulb". The rest of the "hints" are a good guide also in my "learning curve". At this point, I am truly hoping to at least bring back the "blown channel" with some luck and carry that luck (and "learning") over to the distorted channel... maybe.

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

continuing to do "on board checks", I can only further do E-B junctions on the transistors (no way to easily isolate the C-B due to multiple connection points). The diode voltage test of the junctions is good on transistors that had good resistance measurements across C-E. On the short transistors on the "blown side", the voltage was low / 0. On the two transistors that "failed" the resistance C-E test on the distorted side, they had good B-E voltages.... that would appear to confirm / indicated may be why the channel works but is distorted... maybe.... leakage across the C-E junction but functioning "control".

[budwich]

Re: Crown Amp DC300A - One channel distirted, other good.

Progress has been measured in "baby steps" over weeks / months.... some due to my slowness along with very slow "off shore sail boat times".... ultimately I hope I can remember how to put things back together if I ever get that far. :-)

I got some MJ15024 replacements for the originals output transistors. At this point, I only replaced two on each channel that I found which "appeared" to fail C-E resistance testing when compared to others (this is with the E isolated from the board... can't isolate B and C without totally desoldering / demounting).... either shorts or leakages. I did this at this point to limit the amount of "hands / soldering damage" that my "skills" will do to this thing.... :-)

Also replaced one .33 ohm resistor which was open.

Built my "bulb limiter" supply to hopefully reduce the "shock" of the first "plug in"... of course, I did a bunch of checks for shorts and comparing channel 1 to 2.
I plugged the amp into the bulb limiter and hit the switch. The bulb lit brightly for a second or so then basically dimmed to almost dark. The switch light remained lit so the limiter worked at limiting the current going to the amp and it would appear that I have no "immediate shorts". At this point, I am taking a deep breath, thinking on things and will go back to check for voltage at a few points to see if maybe there is enough juice going thru the limiter to get the amp "operational" at the basic electric level at this point. Hopefully, some baby steps will happen sooner as opposed to later.