Crown Amp DC300A - One channel distirted, other good.

[petehall347]

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

you could see if adjusting output offset changes it .
then check q219 and q200 . d200 .
also wouldn't be a bad idea to check input offsets too

[budwich]

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

I am not sure I want to touch any of those as they were never touched to begin with. Why would there be such a large difference in this area when it wasn't disturbed.

I did some resistance measurement on the output pins to the "power bias" pins to see if that shows anything... this was with the chip demounted. Both outputs show the same resistance to the +10v pin. I also measure the appropriate circuit point to the 10v "rail" and both side show the same resistance. This does not leave much else at this point. I am leaning towards a bad op amp ic that "leaks" / breaks down under higher voltages that my meter can provide.

[petehall347]

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

you could try a new one . cant vouch for it .not used this supplier for years but never had anything fail from them . https://nikkoe.com/catalogsearch/result/?q=ua739

[budwich]

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

thanks... I will also try to find something locally or equivalent. I will also do some more measurements in and around the offset adjustments to see if any thing jumps out there... never know, I guess the pot could "oxide up" and result is a bad "controlled" result.

[budwich]

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

I measured and compared the offset adjustments across each side. They are basically similar and "optically" look in the same position. I am going to remove a few components the circuits associated with the "bad side" of the dual op amp so I can do some out of circuits on them (ie. capacitor and switching transistor).

[budwich]

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

I got a couple of ua739's for replacement of the op amp. Didn't help. The "bad side" still shows about a 10v dc offset at the output of the op amp for the channel. Some component of that side must be "leaking" the 10v dc "rail" thru into the output.

[budwich]

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

It would appear perhaps the Q219 (2n4125) is bad / leaking on the distorted side. Not sure what the function is as it is hooked to the point on the op amp for "output lag" along with the actual output. Measuring voltages at Q219 shows 9.08 v at base, 9.8 v at the emitter and 7.2 at the collector (which goes to the output of the op amp). This compares with Q119 (working side) which shows 9.26v on the base with 9.94v emitter and .01v on the collector. The "10v rail" that these are connected to (thru resistors and such) show 10.00v. Doing "dry resistance" measurement in circuit (no power), does not seem to show any differences between the two side but maybe that has to do with the subtending circuits (dc equivalent) being the same / OK and thus the transistor itself not really being measured without it being removed or possibly the meter not having enough voltage to breakdown the transistor junction.

[budwich]

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

I removed q219 (2n4125) and checked with a transistor checker... it checked out "ok". I replaced anyway. It didn't make any difference. Started check voltages at various point and compared across sides. Of course, the large positive DC voltage (8-9v) on the output of the opamp of the distorted side is an issue but I have not been able to locate where that it coming from.

I was checking voltages at the input / output offset pots. I have not touched / changed these. The "biases" for both sides are as expected... 5v and 0v for the input pot and 10v and -10v for the output pots. The resulting input offset "values" are 2.5 / 2.4 v (good side / bad side). However, the output offset value is somewhat different with -.6 / -1.08 v (good side to bad side).... almost a factor of double. Not really sure if that means much as I don't know how the op amp uses this input. Maybe some one can explain if the difference is normal?

[budwich]

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

Well, it been a year in pieces figuratively and literally. The amp is still apart. I thought I would give it some more investigation. To help things a bit better, I created a component label map to help find things easier. The boards don't have any labels on them and the crown manual shows the "component label" on the solder side but I find it easier to trace / test on the component while following the overall schematic. Attached is the "labelled" picture. Most components have been label but some have not but are readily identifiable from one side to the other (ie. 1XX, 2XX).

[budwich]

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

The new year bringing "rejoice". :-) With help of the labelled photo, I was able to more easily move around the board to locate components (test points) while navigating the schematic. I eventually found some voltages out of whack up near the +60v rail. The rail itself is fine but Q201 which I think provides some "bias" toward the output circuits (Q206) was not switching / tested faulty once removed. Its a MPSA93 and I replaced it with an MPSA92. After that "subtending" voltage around the -60v also moved towards normal as opposed to what was seen earlier so that "side" of the output "picked up" its operation.

I am probably not going to touch much else. Will let is run for a while and maybe check a few signal paths with a scope and frequency wave inputs. Then try and figure out how to put the thing back together... a year is a long time and I need to learn to take more pictures during the disassembly.

[budwich]

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

well, the joy was somewhat short lived... :-( The amp has been running / working well for while (10plus hours use). I was switching inputs on the ic150a that feeds it and moved to the phono input. Large pop and now I have a distorted channel again. I have switched between inputs before (tuner, aux, tape, etc) but never tried the phono at the time because there is nothing connected to it (phono is unplugged... it has a speed control problem on the unit). Anyway, it would appear perhaps that the initial problem might have been caused by the IC150a preamp possibly.... maybe.... or just coincidence that the failure happened again. Won't know more until I take the dc300a back apart and check out the same components to see if the same failure area is back. :-(

[Agent24]

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

Switching through inputs on the preamp should not cause that. Although the preamp has a separate phono preamp circuit and if that were faulty... maybe check it's not outputting DC or such when phono is selected? But it seems unlikely as there will be DC blocking caps somewhere along the audio path anyway. If the preamp was oscillating at high frequency, perhaps, or such, is maybe more plausible.

Possibly more likely is still a fault with the power amp. Maybe an intermittent joint or component somewhere that worked for 10hrs only. Or something was running out of spec and became damaged after 10hrs, due to a root fault elsewhere.

[budwich]

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

ya, I am thinking that it isn't anything to do with the phono but I will check out that board / output just to be sure. Just got the thing back up on the "bench". It is the other side that failed / is distorted. I have to go thru the similar checks to see what's up. Preliminary voltage checks on a few areas shows similar results to the previous failed side so I need to go thru and figure out what I did before... I guess I should have made better notes. I wasn't expecting to be visiting back so soon... :-)

I am not sure about any oscillation. Why would that happen after such a run period... to high an input?

[Agent24]

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

If there was oscillation, I was thinking more like if the phono preamp section had already gone faulty at some point, but you didn't know because you hadn't used it until now. But it's just a random guess.

DC on the preamp output could be a problem, even with a DC blocking capacitor, if the phono amp was say dead shorted and outputting rail voltage, it could still cause a voltage spike when switched in, maybe?

Since it was the other power amp channel that failed this time, I wonder if both sides have the same worn out components and it just happened that this channel lasted a tiny bit longer.

Hopefully someone else has some better ideas though, mine are just random guesses at things that could cause such a problem.

[budwich]

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

thanks again for the comments / ideas. I like your thoughts. One thing that I tried at the ic150a, was touching the wiring going from the phono preamp board into the main preamp. As expected (at least to me), there was a "hum" induced into the subtending pre-amp stage which was quiet before hand (ie. no sound from no input plugged in but the position selected). So that to me seems to indicate that the pre-amp and phone stages appear to be "functioning". From this, your comments follow that possibly the dc300a section possibly had some similar issues on both sides which possibly took a bit more to manifest. I am working thru some of the same checks as before. I also need to de-solder some components to complete some of the checks (ie. off board component level tests along with some resistance / impedance testing across portions of the circuit comparing from side to side). I know from reads along the way, that others have replaced a number of components as part of "shop effort". I have been reluctant to follow suit as I fine that with my limited experience / capability, the less that I disturb, the better chance of success. Hopefully, this 45 year old box won't need a full "overhaul".

[budwich]

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

I tracked back poor / high positive dc voltage on the output of the op amp, compared to less than .6v on the good side. This is similar to the previous trouble of the other side. Then traced back (resistance... power off) and found low resistance at the output from the mpsa92 (itself tested good) on the side that is distorted (compared to the good side). It traced down to q120 (2n4125) which appears to be in the feedback path from the output stage. It was blown. I replaced it and now things seem to be up and running. From my reads on the net, people seem to do an R&R on all 2n4125. I have not at this point but if another failure occurs, I may follow suit. I haven't done a "full operational test" yet in terms of frequency along with input switching. I will let the thing run for a bit to see if there is some temp issue possibly causing a problem with time.

[budwich]

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

Another short lived "joy" moment... :-( It was playing at medium volume for about 2 hours or so. I did some input switching and everything remained fine. I then decided to turn up the volume on the pre-amp and then "thud" as the external fuse (10 amp) on the amp blew. New fuse blows on power up so something has gotten significantly worse this time. I guess its back to the "bulb limiting power up" to see what's up.

[petehall347]

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

maybe its oscillating or bias tracking isnt right .

[Agent24]

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

I find it interesting it broke when you 1) changed inputs 2) changed volume

I can imagine each of these actions has possibility to create a noise burst from dirty switch and potentiometer contacts.

I wonder if some feedback etc network or such in the amp designed to handle transients is not working properly (e.g. Bad capacitor) and the amp becomes unstable due to sudden change in signal, be that due to high level, fast transition time, etc.

(Again, wild speculation with no real evidence or possibly even sanity)

[budwich]

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

I guess at my level, anything is possible... :-) I did check voltages as per schematic at various point shown once the channel was repaired. Things looked OK. The pots / switch seem ok as when it was running, audibly, I didn't hear anything like crackle , etc prior to the "thud" of the fuse going. I don't know anything about its "internal operation" or design... beyond my knowledge limits. I will see if I can isolate the power inputs from the boards and see if the thing stays on and go from the result.