Counterpoint SA5 Preamp Problem
This is my first post. What a great site. I hope someone can help me?
Sorry for the lengthy preamble...
I have owned by SA5 since the mid-eighties. It has given me very little grief and most of the few repairs required over these 20+ years were simply replacing (DIY) something that had visually fried, started to bulge, or a bad tube. I am armed ownly with a DVM, a tube tester, and only enough knowledge to get me into trouble and stubborness for DIY before sometimes finally going to a service tech.
Over the last few months I had noticed that my 5651 tube was taking longer and longer to turn on. Finally it was such that it did not turn on at all, yet after power down and up it would then turn on. I checked all the tubes and found that the 6JC6A tube had shorted. Upon replacing the tube, things seemed to work for awhile, but symptoms started again shortly after.
While the unit was on and the cover off, I noticed I had not seated 6JC6A tube straight, so gave it a wiggle. Well, what I found out, putting pressure on the tube, the 5651 next to it came gratefully on... but then off again.
So drastic measures, out came the preamp into my torture chamber (my rec room carpeted floor). Covers off, better lighting, and through wiggling and tapping I discovered 4 cold solder joints between the main and the perpendicular board, a crumbling tube socket (6GC5) and heavily discolored board area around same socket, in fact a little like carmel.
Joy oh Joy, something I can fix. Ordered parts/tubes, resolderd joints, replaced tube socket, scrubbed board area clean. Put it all together, it worked fantastic... for a couple weeks... :(
Again, 5651 not lighting up, 6JC6A shorted, (luckily I bought more than one), will still play, in fact the 6JC6A appears uncharacteristically bright, but certainly I am not leaving it on.
I have a feeling all I did was clean up the symptom, but not the cause.
All I have done further is checked everything over visually, put the DVM on every resistor I could see for anything obvious, but it looks like I won't be getting any accurate readings unless I start pulling resistors out? I have no way of testing the capacitors, or maybe I just don't know how?
Oh, and I did check for continuity the tracings from the main board, across the new solder joints I made, to the perpendicular board, and they seem fine.
I'd really like to get it up and running myself, and I do have a good service tech should I screw things up.
If anyone has any ideas, they would be much appreciated. Sadly, if it is such that it could be one of many things, I may concede and release this creature from my torture chamber and give to my tech.
Many thanks in advance for any help!
Anatech should be here any second. He's pretty much the Counterpoint guru.
Awsome! Thanks for the heads-up. I read some of his other replies. Hope he can help. Regards
Those connections between the main PCB and right angled tube PCB are always a problem. If you have recently gone over them, that's one problem gone.
If memory serves, there were some bad connections in the power supply that would interrupt the heater current to the power supply tubes. This is one of those times when seeing the unit would really help. Early documentation was both poor and a bit scarce, so I go on what they look like for the older ones.
There is a constant current diode as well I think. They break down over 100 VDC, and there should be a 91 volt zener across it. Check for shorts there, and also make sure the operating voltage is around 45 volts across that diode. If the zener conducts, the current will be uncontrolled.
The last thing would be to check the tube sockets of the regulator circuit, and you have found a problem. Make sure the PCB under the socket isn't arcing or tracking. Lights off and have a good look for light where there shouldn't be.
The one place where Counterpoint is very poorly engineered are the power supplies. Almost every one has design or manufacturing issues. The more expensive and complicated the unit is, the less chance there is of fixing it once and for all. In fact, each one I look at in detail becomes a journey now. Under warranty we just fixed them. Now I get involved in redesigning these simply to keep them running. I haven't had to do that with this model - yet.
For the last hint, try using Electroharmonix tubes for the signal path. Any good quality tube will work well for the tubes in the power supply. Try to stay with name brands such as RCA, GE, Westinghouse ... you get the idea.
Good luck, let me know how this works out for you.
Hi SY! Thank you.
Thanks for this! I will check all those things and let you know how it goes.
Well this fellow looks and reads nasty on my DVM. Unfortunately can't quite make out all the band colors? Yellow White White Red with Tolerance Brown? Or Yellow Brown White Red with Tolerance Brown? Or...??? Help!
Thanks in Advance!
Things are looking up!
My best guess for the resistor was 49.9k 1%.
As a quick fix I put together 2 x 22k 5% (44K total). The 5651 tube didn't come on before the warm up LED completed it's cycle, but it did eventually come on (within a minute after). It wouldn't come on at all before.
The other things such as the zenor diode checked out okay. And no leakage between pcb traces detected in the dark. (Checking things in the dark were eerie and cool at the same time!)
Will keep my eye on things.
Thank you for input!
location of resistor
Hope this pic works! Thanks for your help.
They didn't give us any PCB layout diagrams for these units. So, the best I can do for you is to attach a picture of one I had repaired. I have cropped the photo down to the area you are working in.
When working with high voltage on circuit boards, clean is what you need. That's one reason I keep my work clean. The other reason is that I can see if anyone else has been in there. :devilr:
Can you do me a favor and trace the resistor to where it connects there? One end will go to one or more components and the other end will do something similar. For example: one end goes to pin 5 on V5, pin 1 on V6, positive end of C52 and the bypass capacitor across it and the other end connects to pin 1 of V7 and many other parts, like 3 499 ohm resistors, the positive end of C56, and more. This would be the cathode of the pass tube, in other words, the output of the regulator. There are some decoupling resistors (499 ohms) between the gain stages and the regulated output. I can try to figure it out exactly from there.
As for the voltage reference tube, it will only light up (strike) once the DC output from the regulator has exceeded the breakdown voltage. So there is a delay there. Due to the high amount of capacitance across that tube (bad design example), and it's series resistor (182 K), there is an additional time delay before it strikes and glows. You should see between 82 and 92 volts across the voltage reference tube if it's operating okay. The regulated output voltage should be around 260 VDC. That will "bite" you if you contact any part of that circuit.
If you look at the component that appears to be a small can in parallel with a larger axial leaded diode, you should be able to measure a voltage drop around 45 VDC. It must be below 90 VDC by a comfortable margin, and above 10 VDC. The metal can device is a constant current diode (breakdown around 100 VDC), and the diode beneath it is a 91 VDC zener diode (1N5378). This measured voltage will rise and fall with the AC mains level, so you need to get a sense of what the average is. You can change this voltage by replacing the resistor in series with it. The nominal value will be 75 K ohm, but it may likely be a different value also. Increasing the value of that resistor (Say ... 82 K ohms) will lower the voltage across the current diode. You can figure out the drop easily enough. The CCS diode should be close to 1.6 mA. Ohm's law, figure out the drops in that circuit.
So, trace away please and I'll try to get you some answers. The continuity function on some meters make tracing easier.
|All times are GMT. The time now is 01:55 AM.|
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio