D150- One Channel Down!D150 troubleshooting
Posted 04 November 2013 - 03:57 PM
I've had 2 lovely D150 amps since approximately the dawn of time. Try to take them away from me: suffer my wrath. Now then: I'm not an engineer, but I've burned my way through innumerable carpets with dropped soldering irons (stinky!), so I fancy myself an expert of sorts.
Here's the prob:
I lent an amp out to a touring band (how long will I keep making the same old mistakes?), and when I got it back, one channel was effed. So I've been poking around. While doing that, I kept a log of sorts, and here's a copy/paste:
Turned on, stereo signal (music), 8 Ohm speakers (load).
ch 1 OK
ch 2 Low output, very distorted.
Pulled AC plug, and as the power died out, ch. 2 suddenly un-distorted itself and sounded fine for the last half-second or so before fading out altogether (bad cap somewhere?)
I had a few transistors lying around that matched: swapped both ch2 output transistors, tried the amp, no change. Swapped one driver, no change. Swapped the new driver over to the location of the other (old) driver, replaced first driver with old second driver: no change. Replaced all transistors with originals.
Bought a ua739c IC, swapped it for the old one, oops! Now ch2 doesn't show any output at all.
previously: drivers, output transistors may have been changed at some point, but many many miles on them w/o problems. Something burned on the board, many miles ago, there is a hardwired bridge along the bottom of the pc board between q108 (?) and q208 (?) (it's hard to read the pc layout scan from Crown!!), but it's all ancient history and appears to be ok- no shorts, no burns…
visual inspection: yes I found a couple of bad solder joints, fixed 'em, hard to say whether they caused the problem, but fixing them didn't fix the problem...
Turned on, no signal, no load:
No DC voltage present at output...
Fuse doesn't blow. Can't find -7.5 to 9.5V at pin 7 or +7.5 to9.5V at pin 14 of IC, but where should I be taking the - pole measurement from?…
checked bias resistors:
across R127 shows .294VDC, across R227 shows .039VDC.
across R120 shows .258 (variable), across R220 shows .039VDC
checked R115, R215, without removing them from the circuit:
hmmm, not really sure… it's SO HARD to read the Crown scan of the PC layout… but it looks like 115, 215, 116 and 216 are all both neither shorted nor open. They all need a minute or two to settle down, but read 5.8- 6.8 KOhm after a while. Likewise R117 and R217, but it's so hard to tell! The scan! AAAGH!
I give up. It looks like no resistors are screwed, but…
Poked around checking diodes, too, nothing conclusive...
Oh man, what I'd give for a legible scan of that 7891A PC layout! Or maybe y'all got a better idea?
Any gentle pushes in the right direction will be most, most graciously appreciated.
Posted 05 November 2013 - 04:49 PM
You're saying that you don't have or can't determine the psu voltages for the IC. Well, the reference is thru the ground (sleeve of input jack). I believe Q1 (pos rail) and Q? (neg rail) are configured as 'zener' diodes (to be confirmed)! Let's say I doubt that the IC is problematic: You did mention that one of the channels was working fine. Did you check the signals (with oscilloscope) at the outputs of the IC?
Can you measure the output resistance (without power) between the output terminals? There is a possibility that you may have a direct short on the output, but not through one of the output devices (Mains fuse would blow if you had a shorted output device.) Distortion could be caused by a shorted load (I didn't mean the speaker!)
You confirm that you don't have any DC at the output terminals?
Let's start with this!
Posted 10 November 2013 - 11:20 AM
First let me just say that it brightens my day to know that there are people like you who are willing to share knowledge so freely, and thank you VERY much! Sorry it took me a few days to get back; that's life around here sometimes...
I did finally find those PSU voltages at the IC; turns out the IC was not seated properly (duh). When I got the new IC to sit right, I discovered the exact same symptoms as before, so I guess that's out. Now reading 8.75 on the plus side, and -9.76 on the minus, not quite exactly within the -9.5 parameter, but pretty close.
I don't own an oscilloscope, but will pick one up as soon as I get paid. Haven't used one in many years, so I'm looking forward to it.
I checked the resistances across the outputs- no shorts. Across the plus and minus terminals of both outputs, as well as across the two plus terminals, I see a slowly growing resistance ending at around 15K ohms. I guess that's my meter charging caps in the circuit.
And yes, I confirm that there is no DC at the output terminals.
I'll keep poking around, comparing voltages, until I can get an oscilloscope on it. Till then, any more suggestions are very much appreciated!
All the best,
Posted 11 November 2013 - 10:15 AM
So guys, what do you say?
Posted 13 November 2013 - 07:14 AM
Somewhere, way back in a corner of my mind, I had a feeling I'd seen a D150 schematic in my old junk. So I opened the box of papers I'd been avoiding for aeons, and lo and behold, stuck in with the schematic for my Ampex SA-10 was a D150 schematic, not perfect but much better than the one on the Crown site. I have no idea where or when I got it, but it's on US- sized paper, and I haven't lived in the US for over 20 years, so it's at least that old. I'll take it to the copy shop today and get a hi-res scan, and post that here. I hope it helps someone out there!
All the best,
Posted 18 November 2013 - 01:05 PM
Soo... I'm getting around now to figuring this amp out. First I decided to make a legible diagram of the PC layout, and here is what I've got so far:
the linked diagram has not been checked by a real engineer, so please don't use it as a reference yet! I haven't checked it for accuracy, nor for proper PNP/ NPN orientations and the like.
For the moment, here's my question:
I'm having trouble reconciling the schematic (see above posts) with both Crown's (nearly illegible) layout diagram and the PC board in the actual amp, which I have in front of me. To be precise, it's those transistors Q102 and Q202. I'm aware that they are in a TO-92-18R package, which would make their pinout C-B-E as opposed to the usual E-B-C, right?... but still, the schematic has their bases going directly to pins 1 and 13 of the IC, which doesn't seem to be the case in reality. Also, there are two diodes on the board which seem not to appear on the schematic, as far as I can tell, in fact bridging those very two pins 1 and 13 with the collectors of those very two transistors. What gives?
THANKS AGAIN! (he screamed... sorry...)
Posted 06 December 2013 - 10:17 AM
My old D150 broke. I started a thread here in the hope of getting advice on fixing it, where I lamented that the schematics and PC layout diagrams on the Crown site are illegible. After pulling out a lot of hair, trying to decipher the scans, I got a tickling sensation telling me that I may actually have those diagrams somewhere. Now listen, I am chaos personified, but I swear, I found an ancient copy of the amp's schematic in an old carton full of miscellaneous mixed papers within seconds, as though some mystical force were guiding my hand. Wit that and a lot of patience, I was able to more or less re-draw the PC layout, so that I could begin troubleshooting. But I wasn't so sure that my drawing was accurate, so I began with some trepidation.
Now I'm getting to the punchline:
Yesterday, I had occasion to look through that same box of old papers, for something entirely unrelated. This time I got all the way down to the bottom of the box, and what did I find there? Ha! The rest of the Crown document, containing parts lists, blowup-drawings, and two lovely, pristine drawings of the PC oard with all its components. Here's links:
The link to the drawing that I made myself is now dead, because I'd hate to have anyone burn up an amp as the result of some mistake I made...
Now then: I STILL haven't found the problem in this amp. I have done a lot of checking, and found the following: No DC at output. I've checked output x-sistors, drivers, the IC, diodes 210 and 211, none of them can be causing the problem. I have checked voltages at several points and definitely found discrepancies between the two channels. But for one thing, I haven't got an oscilloscope (working on getting one but I'm a Mac user and all the ones I see and might be able to afford only work with PC), and for another, basically none of the voltages I read match up with the schematic. I'm thinking that's down to leaky elkos, so I think I'll recap the beast and see what I get. Meanwhile, your tips are very welcome!