2230 recapped but right channel struggles when cold

[dan001]

Hi All, I recently recapped my 2230, all boards, and I thought it went well. Two problems that I previously had appear to have been fixed. The stereo light now stays on when it should, and the FM tuner now stays tuned to the selected station instead of drifting and needing re-tuning. However, I've since encountered a new problem. The right channel doesn't play properly when I first turn the unit on. It sounds like a poor connection where sound is distorted and not flowing freely. After the amp plays for 10min or so the problem goes away and it can play fine for hours. I know it's beyond the pre-out and main-in jacks because the preamp is fine when connected to my Bryston amp.

I'm no engineer and so at a loss as to what components on the amp board might be failing. Is the problem I described consistent with an output transistor problem? Should I consider changing other components? Any ideas?

Initially I was worried about what I put in for C751, but I would expect both channels to act up if that were the case. I pulled 0.47uf out of here, but thought I read somewhere it should be using 1uf film type. Not sure I got the right part. Is that orange film cap 1 or 0.1uf?

Thanks

 

[Oerets]

Welcome to AK!


Recheck you solder joints for a cold one.


Barney

 

[dan001]

Welcome to AK!


Recheck you solder joints for a cold one.


Barney

Thanks Barney, yes, I should have checked that first. It didn't occur to me as the problem started about a week after the recap was completed, and I'm still busy patting myself on the back for not blowing the unit up.

 

[Oerets]

Do yourself a big favor and build a dim bulb tester first.



Barney

 

[rBuckner]

The cap in the picture is 1.0uF. The 105 translates to 1:0:00000 pF or 1,000,000pF which equates to 1000nF or just 1uF. (Nerd moment complete!)

I've had these be intermittently noisy even as new parts so try changing it out if you have a spare or swap it with the other channel to see if it's at fault.

 

[Mike Sweeney]

I've had these be intermittently noisy even as new parts so try changing it out if you have a spare or swap it with the other channel to see if it's at fault.

The only noisy ones I've had were the cheap Chinese imports. I sent them back.

 

[Scotteboy]

I agree with all the input above and would also ask if you cleaned all the switches and potentiometers well, I have had simple controls give these type of problems like a stereo/mono switch that is just a little dirty. Other possibilities could be a leaky or failing pre driver or driver transistor. Output transistors are usually pretty brutal in announcing problems ! Almost forgot to mention solder traces or broken traces on the PCB. All the details....... Good luck and please let us know what you find, its almost always new information !

 

[mbz]

I assume bias was checked (part of all good rebuilds...)

 

[dan001]

Thanks for the feedback all. I did clean the pots and switches with dioxit, and I did the bias adjustments on the power supply and amp boards. I could not do the clipping adjustments on the amp boards and phono board as I don't have an oscilloscope.

I built the dim bulb tester today, but without the switch as I couldn't find a dual gang box that wouldn't scratch the heck out of anything it touched. This single box I found at Lowes is really smooth on the back, sides and corners, but it only came in single size.

Over the next few days I'm going to double check and reflow all the solder joints I previously touched. If that doesn't work, I guess I should try putting the old caps back, one at a time? One more question: Should the dim bulb tester glow bright for a split second before becoming dim? I have a 50w bulb in there and that's what happened when I tested it.

Thanks again.

 

[audiojones]

Sounds like the power amp's bias (idle current) is too low. You should have 5mv across emitter resistor R774 on both channels, but I'd recommend doing an initial adjustment with the dim bulb tester connected just to see how the bias pots react. See if you can get them both set to about 3mv with the DBT connected, if they react smoothly then take it off the DBT and set to 5mv.

A general rule of thumb for adjusting the drive symmetry (clipping adjustment) if no scope is available is to adjust the collector voltage of the second output transistor to 1/2 of the collector voltage of the first (non-adjustable) one referenced to chassis ground. In other words if you have 70vdc between the collector (outer case) of the first output transistors and chassis ground then set the other one to 35vdc between its collector and chassis ground. It's not perfect but it'll get you very close. If you can't reach that adjustment point or the bias current adjustment point then there is still some work to be done.

 

[Steven Tate]

It’s normal for the bulb on a DBT to go bright for a few seconds after power up. It’s an indication of the large current flow as the caps charge up initially. Once charged and unit is idling, the bulb goes dim if there are no large, faulty current draws.

 

[dan001]

Well, no luck yet. I redid all the cap joints on the right amp board that I had previously touched. I let the unit play for ah hour and biased it, 35.6v on the power supply board an 5.1mv on each amp board. The trim pots appear to be okay, although jumpy when trying to dial in the correct voltage. After letting the receiver cool for an hour and turning it back on, the same problem persists, and as before the problem goes away after the receiver warms up.

Which amp is left vs right? Is the amp board close to the speaker terminals the right or left channel amplifier? I assumed it is the right channel amp and may have been trying to fix the wrong board.

 

[catrafter]

The right amp is on the right side as you stand in front of the receiver. Pretty clever, huh?

Tom

 

[dan001]

Decided to measure the bias over a period of time. Very interesting results. While the power supply board was fairly constant at 35.6 - 35.5 warm or cold. The amp boards were another story. I could understand a bit of change as the amp warms up, but the right channel is downright nasty as it dropped to 0.2mv for a few minutes without me touching anything.

Should I be replacing R765? If so, does anybody know the mouser/digikey part number for this? Or could it be something else further upstream? Again, any feedback is most welcome.

Also, what kind of tolerance should be allowed here? The left channel climbed to 6.1mv in minutes 11 through 12, is that too high? I didn't go that far on the right channel. Finally, I did let the receiver cool for 1 hour between tests.

Thanks

 

[Golanr]

Not sure about the 2230 but the 2275 has two pre drivers on each board (H701/H702) which must be kept at the same temp for ideal performance. Not sure if the 2230 have the same thing? Also try to deoxit your bias pots as well.

 

[dan001]

I tried to deoxit the bias pots and still no luck. I was actually able to confirm that the pot is not the issue. While getting the 0.2mv rating when the unit was cold I turned the pot quite a bit and no change of the reading. After the unit warmed up I was able to dial in 5mv again. So power is not making its way to the pot.

Wonder if maybe the diode thing wedged into the heatsink could have something to do with this?
Any idea if J754 can be used to measure anything here?

 

[fernarias]

Get some freeze spray. Let it warm up until it plays normally then freeze each transistor. Do the output transistors last.

If you're going to do more of these then even a cheap transistor tester (they sell at 20) would help. I have been noticing that NPN output transistors are usually below Hfe spec and are getting ready to fail (Hfe of 14 or less). Basically, every amp has needed new outputs except three (my personal experience).

 

[dan001]

Resolved. Thanks to all for the suggestions along the way. In the end, all it needed was a tightening of the screws that hold the output transistors in place. Not sure what possessed me to pick up a screwdriver and tighten those screws, but it worked.

Someday I'll get to Audiojones' suggestion for adjusting the drive symmetry without an oscilloscope.

Cheers