|
SA900C
|
|
03-12-2016, 01:15 PM
As I would expect component numbers to be allocated in a logical order it seems more likely that R24 existed in earlier versions of the design but was later eliminated.
If the PCB wasn't redrawn to suit, a wire link should be present if you've correctly determined its function ...
03-12-2016, 02:06 PM
I had that thought, but re-checked the videos before posting. Here's a frame-grab:
Who knows for sure, but from a design POV, I can think of more reasons to add it subsequently than to remove it after the fact. The schematic says "Valid from serial nr 84091B302", so if Andy's is earlier that that... Also, when you look closely at the schematic, it's clear it's been modified. "R24" and "R25" are hand-written, and you can see evidence of rubbing out (also present in other areas too - the "joys" of being a small company, I guess). Though generally pretty good, I have had problems with those Piher preset resistors in the past. I'm glad to see that the quiescent current preset is a decent Bourns type (and is in the "failsafe" position in the Vbe Multiplier).
03-12-2016, 02:37 PM
Re power tests, point taken.
The readings of Q3 could be wrong as I may have got mixed up what with different Q's having different pinout's. I've been trying to draw a schematic of the area around Q4/5 today inc Q3 where . It's a bit different to the schematic and as you say there are a few changes. For a start Q4/5 are BC547/557B's not MPSA43/93's and there's an extra cap in there too. There are some extra components "bodged" on the underside as well. Sorry I can't be more specific as my head is swimming from trying to trace where everything goes. My confusion isn't helped as there are no dark "fullstops" that you get on some schematics to tell you where two crossing lines are joined electrically. As far as I can establish the pots are ok and the resistance disparity is caused by something else. I'll give some better info tomorrow when I've rechecked pinouts etc. Andy.
03-12-2016, 02:51 PM
If you could post a couple of really clear close-up pictures of the PCB, that would really help. Both sides, please. And a good one of the main board that's got the power devices on (I appreciate that photographing the rear of that would be difficult, and I wouldn't ask you to disturb that, for fear of causing other faults).
Do you ever travel down this way? Just in case we can't get to the bottom of this remotely...
04-12-2016, 03:12 PM
Thanks for your support Mark. I'm puzzled now, the amp seems to be working ok??
Yesterday I got in a right mess trying to trace out the circuit around Q3. I had an stupid attack and got confused by mixing up R22 and R25 and tried to trace connections that wern't there. Spent hours chasing my tail. This morning after reading your posts last night I thought I'd try to nail down the readings for Q3. I rechecked the pinout for the BD235 ( no wonder I get confused what with some types being EBC others ECB and others BCE) and took readings which are good LH = E -1.8v C 1.8v B - 1.2v bad RH E -1.5v C 1.9v B - 1.3v . I then got a screw driver and turned R22 (VR1) fully CCW/CW on the iffy CH a few times to see what voltage range was present on Q3's base, I got 1.7v to 2.2v. I then set it to 1.2v, same as the LH CH, rechecked readings and got the same as the LH CH. I the swapped the pre-amp cards to see if the fault follwed the cards to see if I could narrow the fault down. Started the amp expecting one to shut down/overheat, was still waiting 30 mins later. So I put a a signal in cranked both up to 160w both remained ok,. Puzzling. I rechecked both were at 300mV, all seems well. I wonder if rotating the pot has jiggled something loose if see what I mean. I'll re test tommorow. Cant understand why Q3's voltages don't match those you mentioned yesterday. I'll take some good pics tommorow and post more data. Being Sunday bit pushed for time. Cheers, A.
04-12-2016, 05:15 PM
Those readings make more sense
 The bad channel is slightly confusing, as with -1.3V at the base, that only gives 0.2V across the B-E junction. Hmm... But there is 3.4V between collector and emitter, which is slightly less than the 3.6V of the good channel. So there might be something else going on. But, if all is behaving presently, let's not worry too much for now. Cheers, Mark 
05-12-2016, 02:32 PM
(This post was last modified: 05-12-2016, 02:37 PM by Diabolical Artificer.)
Played some music through it today and whilst it didn't overheat, the LH channel was a lot warmer, so the problem has followed the pre-amp board. Could it be I'm just no setting it right? I've been setting it with the input's shorted, volume controls to 0 after letting it warm up for a while.
AFAIK the amp is the same as schematic apart from Q4/5 have another cap (4N7) from base to emitter, oh and the diodes and resistors under the board. You'll see that one side isn't used, it looks like the other half of the amp, (can't remember whether it's the neg or positive side) has provision for another pot the same as VR1. See pics - I've looked everywhere I can think of and tested all R's/C's etc, what should I be looking/testing for Mark? Thanks for your help on this, Andy.
05-12-2016, 03:03 PM
(This post was last modified: 05-12-2016, 03:08 PM by Diabolical Artificer.)
Cocked that up, took too long editing, here's the other pics.
I did a better job of this but took too long, edit timed out, the BB connection went etc,etc. hope you get the idea, Andy.
09-12-2016, 12:03 AM
Hi Andy,
Just to say that I haven't forgotten about this. I've printed out the schematic and a few of the photos and am piecing together a few thoughts. Hopefully at the weekend - this week has been a hectic one. All the best, Mark
09-12-2016, 06:14 AM
I know your busy Mark, there's no rush, It's taken me 9 months to get this far.
Thanks for looking into this, Andy. |
| Users browsing this thread: |
| 1 Guest(s) |
