That makes sense. At least in simple terms, looking at the board, I can see Q1 and Q2 are both connected to the track which goes directly to VR1 and VR2 via resistors R11-R14. Is there a particular transistor you can recommend?
I have noticed there is definitely some noise when I turn the volume up, it's particularly bad on the phono inputs, but it's present on all inputs, the tape inputs are the quietest. And sound is leaking between some of the inputs (I can't remember which). I wasn't going to worry about it right now though. I thought I'd finish changing the components, deoxit everything again, and then see if there are still any problems.
I have noticed there is definitely some noise when I turn the volume up, it's particularly bad on the phono inputs, but it's present on all inputs, the tape inputs are the quietest. And sound is leaking between some of the inputs (I can't remember which). I wasn't going to worry about it right now though. I thought I'd finish changing the components, deoxit everything again, and then see if there are still any problems.
Madders - Good job looking over the complete specification AND the application. There is usually no one-size-fits-all replacement for one existing component series without studying the application. Moreover, things change just as we get complacent. For example, the KSC1845E variant is no longer available, and it had higher gain ratings than the KSC1845F variant.
In cases where the higher gain is desired, a potential replacement is the BC550C. That is a "very low noise" device but has a voltage rating of only 50V Vce. In many circuits that is more than adequate, such as the equalizer amps and flat/tone amp sections we see frequently. In that regard, it is very much like the originally installed 2SA725s which were only rated for 35V. Installing a higher voltage rated replacement is unnecessary, and lower noise characteristics might be the better criteria for selection in such a case. The KSC1845F is a low noise device and has a higher voltage rating which makes it a safe choice, but an ever quieter device may be more suitable.
In cases where the higher gain is desired, a potential replacement is the BC550C. That is a "very low noise" device but has a voltage rating of only 50V Vce. In many circuits that is more than adequate, such as the equalizer amps and flat/tone amp sections we see frequently. In that regard, it is very much like the originally installed 2SA725s which were only rated for 35V. Installing a higher voltage rated replacement is unnecessary, and lower noise characteristics might be the better criteria for selection in such a case. The KSC1845F is a low noise device and has a higher voltage rating which makes it a safe choice, but an ever quieter device may be more suitable.
tsd71
RIP Tom Petty
View attachment 686005
That was a Mattsd list, I have always used KSC1845's there without any issue, maybe I will update my notes to a KSC2383
I usually use what MTF suggests, as in this post on the same receiver on the same flat amp board.
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Thanks. I like to at least try to understand why particular components have been recommended as suitable replacements, but in most cases, it's beyond my understanding. I notice there is also a BC546 available with a higher voltage rating, but it has a higher noise figure. Looking at the datasheets, I'm not sure how to make a comparison between the DC550 and KSC1845 noise level, since only the DC550 datasheet provides the figures with dBs.
I think the conclusion is that the KSC1845 is a better choice than the KSC2383 because it is low noise.
So I saw that Maplins stock the BC550C and there is a store right next to where I work, so that helped make the decision between the BC550C and KSC1845. I picked a couple up on the way home. Unfortunately, it's made no difference. The voltages are almost identical to the original 2SC1885s:
Q1 - E: 28.6V (28.5)
Q1 - C: 45.4V (47.5)
Q1 - B: 27.3V (29.1)
Q2 - E: 28.3V (28.5)
Q2 - C: 45.5V (47.5)
Q2 - B: 27.0V (29.1)
These are on house power (after first testing with DBT).
Watthour I think you previously said this could be caused by the low voltage on the collector. The voltage on pin 10 is only 49.0, and it should be 51.5. Should I be going back to the power supply board and rechecking things there?
Q1 - E: 28.6V (28.5)
Q1 - C: 45.4V (47.5)
Q1 - B: 27.3V (29.1)
Q2 - E: 28.3V (28.5)
Q2 - C: 45.5V (47.5)
Q2 - B: 27.0V (29.1)
These are on house power (after first testing with DBT).
Watthour I think you previously said this could be caused by the low voltage on the collector. The voltage on pin 10 is only 49.0, and it should be 51.5. Should I be going back to the power supply board and rechecking things there?
If I may ask, on the flat amp (AWG-038) are C1 and C2 installed with the negative toward the balance pot? That would be critical.
What happens to the transistor voltages it the Audio MUTE is turned on?
Since the power supply voltage is well within 5% of the design point I'm thinking it is not a problem. It certainly seems that the emitter biasing is correct, even with the slightly low collector voltage. In theory, at the 45.5V collector supply and considering the resistances that you have measured for R43/R11 and assuming (dangerously) the internal junction resistances of the transistor, the voltage at Q1-B should be close to 29.3V
I'm almost afraid to ask, but are we certain of the lead orientation for the replacement Q1/Q2? I probably could look back through the past six pages of posts, but...
Also, I'm NOT advocating this just yet, but it is a question for the board in general - What happens if a 270K resistor is installed in place of either R11 or R12? It might appear that the base would be pulled up to somewhere near 30.05V.
What happens to the transistor voltages it the Audio MUTE is turned on?
Since the power supply voltage is well within 5% of the design point I'm thinking it is not a problem. It certainly seems that the emitter biasing is correct, even with the slightly low collector voltage. In theory, at the 45.5V collector supply and considering the resistances that you have measured for R43/R11 and assuming (dangerously) the internal junction resistances of the transistor, the voltage at Q1-B should be close to 29.3V
I'm almost afraid to ask, but are we certain of the lead orientation for the replacement Q1/Q2? I probably could look back through the past six pages of posts, but...
Also, I'm NOT advocating this just yet, but it is a question for the board in general - What happens if a 270K resistor is installed in place of either R11 or R12? It might appear that the base would be pulled up to somewhere near 30.05V.
Hi Watthour,
I don't think I'll get time to check these things today, I should be able to check tomorrow evening and I'll post a picture of the board then too.
Previously Q1 and Q2 had the 2SC1885s, which looked original. So I think they must have been orientated correctly? The voltages now I'm using the BC550Cs are almost identical. I remember the 2SC1885s were orientated so the flat part of the transistor was parallel to the top and the bottom of the board. So looking at the circuit diagram of the board, the pin out must have been ECB or BCE. The BC550C pinout is CBE according to the datasheet, and I definitely remember orientating them at an angle when I installed them, but I could have it wrong.
I did recheck the orientation of the all the capacitors I'd installed on the board, and they appeared correct. But I will check C1 and C2 again. I get easily confused comparing the top of the board to a diagram of the foil side!
I don't think I'll get time to check these things today, I should be able to check tomorrow evening and I'll post a picture of the board then too.
Previously Q1 and Q2 had the 2SC1885s, which looked original. So I think they must have been orientated correctly? The voltages now I'm using the BC550Cs are almost identical. I remember the 2SC1885s were orientated so the flat part of the transistor was parallel to the top and the bottom of the board. So looking at the circuit diagram of the board, the pin out must have been ECB or BCE. The BC550C pinout is CBE according to the datasheet, and I definitely remember orientating them at an angle when I installed them, but I could have it wrong.
I did recheck the orientation of the all the capacitors I'd installed on the board, and they appeared correct. But I will check C1 and C2 again. I get easily confused comparing the top of the board to a diagram of the foil side!
i think you are chasing your tail with a noise non-issue, if that is what you are doing.
so I am to take it that you do not hear any diff between a BC550C and a KSC1845, as the input buffer, which sounds about right.
these voltages are fine, supply is a bit low but I think it is within the allowable tolerances of around 5%
Q1 - E: 28.6V (28.5)
Q1 - C: 45.4V (47.5)
Q1 - B: 27.3V (29.1)
Q2 - E: 28.3V (28.5)
Q2 - C: 45.5V (47.5)
Q2 - B: 27.0V (29.1)
so I am to take it that you do not hear any diff between a BC550C and a KSC1845, as the input buffer, which sounds about right.
these voltages are fine, supply is a bit low but I think it is within the allowable tolerances of around 5%
Q1 - E: 28.6V (28.5)
Q1 - C: 45.4V (47.5)
Q1 - B: 27.3V (29.1)
Q2 - E: 28.3V (28.5)
Q2 - C: 45.5V (47.5)
Q2 - B: 27.0V (29.1)
Hi rcs16. Yes I don't think the BC550C made any difference to the sound, but I wasn't specifically listening for a change in the noise, I was looking at the voltages. They are within 5%, but I thought the fact that the base voltage is below the emitter voltage is a problem. According to the schematic the base should be 0.6V above the emitter, but I am seeing it is 1.3V below.
okay i see now, it as simple as it gets for a single stage emitter follower, isolated by capacitors.
q1,2 emitter followers on the flat amp awg-038. so if q1,2 are removed you should have the base bias set by the voltage dividers made up of R11/r43 and R12/r44, close to the printed +29.1V. If good q1,2 are installed correctly, their emitters should be 0.6V less than the bases or +28.5V, assuming +47.5V on the collectors. if that does not measure up check R13/14 and those bjts and the coupling ecaps c1,2,5,6. should be easy to fix.
Thanks rcs16. I think what you're saying lines up with exactly what Watthour was saying a few posts ago. So I already tested the resistors, and they seem okay:
R11: 215K (220)
R43: 118K (120)
R12: 220K (220)
R44: 119K (120)
R13: 12K (12)
R14: 12K (12)
I tested the old Q1/Q2 transistors and they were good, and I've since replaced them with new BC550C and the voltages are exactly the same.
So I guess that just leaves the capacitors? I don't think I can test the capacitors with my DMM - it's just a cheap one. They are all new though. I will recheck again that I have definitely put them all in the right place and the right way round.
R11: 215K (220)
R43: 118K (120)
R12: 220K (220)
R44: 119K (120)
R13: 12K (12)
R14: 12K (12)
I tested the old Q1/Q2 transistors and they were good, and I've since replaced them with new BC550C and the voltages are exactly the same.
So I guess that just leaves the capacitors? I don't think I can test the capacitors with my DMM - it's just a cheap one. They are all new though. I will recheck again that I have definitely put them all in the right place and the right way round.
Well, since we just lost all posts from Saturday I'll try to refresh this.
Since the unit is operating normally and the bias resistors checked good, the odd bias voltage may be the result of your power supply discoveries. Your new multimeter may reveal different bias voltage once the trannies are back in place.
Since the unit is operating normally and the bias resistors checked good, the odd bias voltage may be the result of your power supply discoveries. Your new multimeter may reveal different bias voltage once the trannies are back in place.
It's been a few weeks since I had chance to get back to this. I put the BC550Cs back in, and tested the voltages with my new multimeter:
Q1 - E: 28.7V (28.5)
Q1 - C: 45.6V (47.5)
Q1 - B: 29.1V (29.1)
Q2 - E: 28.4V (28.5)
Q2 - C: 45.6V (47.5)
Q2 - B: 28.9V (29.1)
I don't know what happened, but the voltages look good now. Maybe my old DMM was faulty.
So my last problem is the noisy phono. Should I expect some noise? I've only tried with nothing connected, and I have to turn the volume almost halfway before it comes really noticeable. But it's a lot lot nosier than the other inputs. Both phono 1 and phono 2 produce exactly the same noise. I've De-oxited everything twice. And to summarise from earlier posts, I've changed the 6 transistors and the electrolytic capacitors, and I've confirmed they are all in the right way.
Anyone have any ideas what else I can try? Thanks!
Q1 - E: 28.7V (28.5)
Q1 - C: 45.6V (47.5)
Q1 - B: 29.1V (29.1)
Q2 - E: 28.4V (28.5)
Q2 - C: 45.6V (47.5)
Q2 - B: 28.9V (29.1)
I don't know what happened, but the voltages look good now. Maybe my old DMM was faulty.
So my last problem is the noisy phono. Should I expect some noise? I've only tried with nothing connected, and I have to turn the volume almost halfway before it comes really noticeable. But it's a lot lot nosier than the other inputs. Both phono 1 and phono 2 produce exactly the same noise. I've De-oxited everything twice. And to summarise from earlier posts, I've changed the 6 transistors and the electrolytic capacitors, and I've confirmed they are all in the right way.
Anyone have any ideas what else I can try? Thanks!