I am having an issue with both the 15ohm 7W resistors (R251 & R252) which is fed by the 14V output and 1.5K 5W (R301) which feeds the 75V and 18V outputs getting hot with the amp running in "measurement mode", ie nothing connected. I believe these might be two seperate problems, but I am not sure.

1> In one case, when I disconnect the 14V source from the Filter and Headphone printed circuit board (044-064), the problem with R251 & R252 goes away . Whatever it is, it must be a common to both Right and Left outputs. It must be that the output transistor is being overdriven and by the looks of the circuit it would seem to me that one of those transitors is being fully saturated. Any idea where the short is? Could it be in C223 a 100uF Cap which filters the base of a PNP?

2> I took out the caps on the 18V side, but still have a hot R301. Will continue to study.

Time to go to bed. :boring: Any help, of course, would be appreciated. :scratch2:

Thanks.

George

7 W and 5 W resistors will get hot. What are the measured voltages? What are they supposed to be?

I don't have a circuit diagram of a C-28, so can't be much help.

7 W and 5 W resistors will get hot. What are the measured voltages? What are they supposed to be?

I don't have a circuit diagram of a C-28, so can't be much help.


No resistor/component that is on a circuit board if designed correctly should get hot enough that it will burn your finger. If that were the case, you must heat sink the component. The life expectancy of any component that is under stress is reduced tremendously.

Well, those resistors are in the emitters of the output transistors that drive the headphones. Looking closer, these are driven as Class A amplifiers so there's plenty of current running through the emitter resistors all the time even at idle. This means that you are correct in that they're being 'turned on' all the time - but that's ok.

So, what exactly, is the problem that you're chasing with all this work?

BTW, do a search here as I, among others, have reported excellent results after rebuilding these preamps.

Cheers,

David

These 2 resistors get HOT in all C28, hot enough to blister your finger in every C28 I have worked on. The quick check is 14V on one side and 7V on the other, if your voltages are the same all is OK. The headphone amp is a class A amplifier and these load resistors dissipate 3.26 watts each, they are well within their power rating even if the get hot.

Just one other thing about that little built-in amp. I've used it to directly drive a set of Klipsch speakers in the past. It has the power of one of those little SET amps - Class A but Solid State. Pretty cool and it sounds great, too.

Cheers,

David

Do those emitter resistors get so hot that they will actually discolor the metal around them?

Another question I have is why are they getting hot with no loads connected? Is there a reason for that?

Also, on the schematic it shows the output from one of the taps after the bridge to be 150V on the schematic. I am read somewhere around 120V. Is that a problem? The 75V output is around 68V. Is that OK?

Is that also true for the 15ohm 7W resistor? It also gets hot enough to burn your finger.

Terry,

I will check those voltages to make sure that they are correct. They do not show any voltages at those points on the schematic.

Thanks guys for your help.

Q: Do those emitter resistors get so hot that they will actually discolor the metal around them?

Dshoaf: They _could_ discolor but only if the preamp was not ventilated well over the years.


Q: Another question I have is why are they getting hot with no loads connected? Is there a reason for that?

Dshoaf: They are, in fact, connected all the time. Since the output devices are biased as Class A, there is current flowing all the time. This is why the 2 output transistors mounted to the chassis also get hot.

Q: Also, on the schematic it shows the output from one of the taps after the bridge to be 150V on the schematic. I am read somewhere around 120V. Is that a problem? The 75V output is around 68V. Is that OK?

Dshoaf: That's right at 10% low. Which caps have you replaced in the power supply so far? And, again, what's the problem you're chasing?


Cheers,

David

Well, those resistors are in the emitters of the output transistors that drive the headphones. Looking closer, these are driven as Class A amplifiers so there's plenty of current running through the emitter resistors all the time even at idle. This means that you are correct in that they're being 'turned on' all the time - but that's ok.

So, what exactly, is the problem that you're chasing with all this work?

BTW, do a search here as I, among others, have reported excellent results after rebuilding these preamps.

Cheers,

David

The actual problem that has caused me concern was that when I had the unit playing I smelled what is burning electronics and I shut off the unit. I could smell that something was wrong. I opened it up and found that R305 that drivers the base of Q302 which is the pass transistor for the 14V output was FRIED! I replaced both it and the filter cap at the base. So, the question that needs to be asked is why did this occur? That is why I started checking the unit looking for areas where the excessive current issue was that caused R305 to fail. If you have any ideas I am all ears.

Thanks David and Terry.

Q: Do those emitter resistors get so hot that they will actually discolor the metal around them?

Dshoaf: They _could_ discolor but only if the preamp was not ventilated well over the years.


Q: Another question I have is why are they getting hot with no loads connected? Is there a reason for that?

Dshoaf: They are, in fact, connected all the time. Since the output devices are biased as Class A, there is current flowing all the time. This is why the 2 output transistors mounted to the chassis also get hot.

Q: Also, on the schematic it shows the output from one of the taps after the bridge to be 150V on the schematic. I am read somewhere around 120V. Is that a problem? The 75V output is around 68V. Is that OK?

Dshoaf: That's right at 10% low. Which caps have you replaced in the power supply so far? And, again, what's the problem you're chasing?


Cheers,

David

Thanks David. I have not replaced any of the input caps except the cap on the base of the transistor that outputs the 14V.

Another thread explains the problem that I am chasing.

The first filter after the bridge on the 150V supply is bad causing the voltage to be low, this is a common problem that is only noticed when the voltages are checked, the 2000uf x 2 14V supply cap is another common failure item, it commonly leaks electrolyte onto the bottom cover.

Terry,

I want to keep the "look" of those caps on my C-28 and as far as I know, those caps are not available anymore. I know that both C301 and C302 are multiple tap caps. I have heard that you can basically "clean out" the can and put together a series of capacitors to replace the current configuration. Do you have a recommendation of what combination of capacitors to buy (axial vs radial) to build this within the can? Would you put some underneath?
Can those cans come apart to be able to do this?

I know you have probably done this numerous times and therefore you configuration advice and cap types would helpful. Thanks.

Do a search on re-stuffing cans, several good write-ups on methodology. I tend to do it by what I have availible in stock.

OK, enough talk about his C28 and get to mine! LOL. BTW I forgot to mention that it looks like a previous repair tech may have gotten his soldering iron too close to those beige rectangular caps (?) replacing some transistors, and they have little melts into them. Is that a problem? I'm sure you noticed them, but I though I'd just mention them, just in case. I hate sloppy work like that.

I did notice the hits on the plastic cases, not a problem but cosmetically not nice, C28 is on it's way home with a full set of data, the biggest improvement is the noise floor is about 97dB down, just a bit better than spec.

Is there a way to improve the noise floor?

Thanks again Terry, You DA MAN!

Noise floor can be helped with several updates, extended grounding with copper braid between terminal strip grounds and the can filters seems to do the most.