Hi,
Hoping someone with more experience can help, I have a Trio KA-6004 amplifier, which has a popping sound when the input selector switch is rotated from aux to Phono1 or Phono2, I have done the usual of cleaning all the switches with compressed air and switch cleaner, inspected all ground connections, replaced the electrolytic caps on the tone control, pre-amp and both phono boards. This also happens through the headphone out, so I presume its in the pre-amp stage, but could be wrong. I did check that none of the metal transistors pins were shorted to the metal can on the pre-amp section.
The sound from AUX1 and AUX2 is perfect.
Any help would be appreciated.
Hoping someone with more experience can help, I have a Trio KA-6004 amplifier, which has a popping sound when the input selector switch is rotated from aux to Phono1 or Phono2, I have done the usual of cleaning all the switches with compressed air and switch cleaner, inspected all ground connections, replaced the electrolytic caps on the tone control, pre-amp and both phono boards. This also happens through the headphone out, so I presume its in the pre-amp stage, but could be wrong. I did check that none of the metal transistors pins were shorted to the metal can on the pre-amp section.
The sound from AUX1 and AUX2 is perfect.
Any help would be appreciated.
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Forgot to mention that the phono not only sounds low in volume, but also has a lot of noise on it, hard to describe the noise, It kind of comes and goes, electrical interference from something. Also looks like ceramic caps C404 & C406 should filter noise to ground by the looks of it, could these be suspect?
Is the noise on both channels? If so then I would suggest you check and see the rail to the phono stage is correct and clean (use a scope).
Popping sounds when switching inputs suggest a shift in DC level and again, if both channels are affected then that points to power supply issues.
The voltage across those two 56k's at the output of each phono channel preamp should be zero 0.000 volts at all times due to the AC coupling.
Popping sounds when switching inputs suggest a shift in DC level and again, if both channels are affected then that points to power supply issues.
The voltage across those two 56k's at the output of each phono channel preamp should be zero 0.000 volts at all times due to the AC coupling.
Great thanks Mooly, unfortunately I don't have a scope, its on my list to buy at some stage, Yes its on both channels alright. I will check that the voltage across the two Rd25 and Rd26 is 0 Volts tonight, I have not recapped the power board yet, there are only a few smaller caps there so I might as well do them too. Any idea what voltage I should have on pin 14 on the input it says 8mA? I did check the voltage from the transformer output is 60Volts which goes into a voltage regulator and I'm getting exactly 45V on the output pins.
Just a question with regards to the Main 6800uF power supply filter caps (I don't have replacements for these at hand) , could these cause the issue I am seeing, (ripple) since the phono stage is probably more sensitive, or would the Aux inputs also sound bad.
Just a question with regards to the Main 6800uF power supply filter caps (I don't have replacements for these at hand) , could these cause the issue I am seeing, (ripple) since the phono stage is probably more sensitive, or would the Aux inputs also sound bad.
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I would doubt (99.9%) that the reservoir caps have anything to do with this. It's worth remembering that phono stages can be intrinsically noisy when the inputs are just floating and so eliminate this possibility you should fit shorting plugs while testing. With those in place the phono stage should be quiet and with just a faint hiss if the volume is turned up.
It looks like the 45v regulator supplies all the preamp stages (so pin 14), however remember the phono stage will be all the more critical on that rail being clean.
You should also see zero volts across the 220k (R11, R12) at the input to the tone stages at all times and also when switching between inputs.
It looks like the 45v regulator supplies all the preamp stages (so pin 14), however remember the phono stage will be all the more critical on that rail being clean.
You should also see zero volts across the 220k (R11, R12) at the input to the tone stages at all times and also when switching between inputs.
Great might get some time after work to take another look at it, otherwise tomorrow. Shorting plugs, I presume your on about the RCA input plugs? Signal input connected to ground?
I will test pin 14 at the phono input, The 45v was at the regulator and was spot on 45V, but I will have to confirm I am seeing the same at pin 14.
I will also check R11 & R12
Thanks again for your help, will reply as soon as I know.
I will test pin 14 at the phono input, The 45v was at the regulator and was spot on 45V, but I will have to confirm I am seeing the same at pin 14.
I will also check R11 & R12
Thanks again for your help, will reply as soon as I know.
The noise may be an actual RF interference issue - they didn't have any cell phones in the early '70s, and the feedback network is run out to the input selector switch for phono / mic selection after all. Do make sure that all case parts are making good contact, and the switch shaft may need grounding as well (in a way that is effective to hundreds of MHz no less, so some sort of copper tape or something). Bad grounding in general can also cause high-frequency oscillation, of which noise may be one symptom.
The phono stage does have some rudimentary RF filtering but nothing up to modern standards. As Mooly suggests, try shorted inputs. Perhaps better yet, shorted Rd3/Rd4, which would bypass input wiring (phono/mic section).
Is the noise appearing on both channels equally? Could be a power supply issue then. What does the homemade audio probe have to say on the +45V, or on the emitter of regulator transistor Qk2 (Ck9/Rk9 junction)? (Checking voltage and ground connections on the phono PCB should come first, of course. Sometimes you have ground connections running over screws, which may appreciated being loosened and hand-tightened.)
This old Kenwood has some circuits of no more than moderate PSRR, and while the regulator circuit is basically sound and should have decently low noise as-is, it is not up to highest standards. I would suggest the following tweaks to the regulator:
1. Swap out 14V zener Dk6 for a low-noise type. Zeners have made a lot of progress since then.
2. Ck13 is a 33µ/16V - nice try, Kenwood, but I'd say more like 220µ/25V, and if you can find a 22µ/25-50V to parallel...
3. Ck8 is basically OK but has probably seen better days at this point.
4. If Qk2 is giving trouble, replace by 2SC945, 2SC/KSC1845 or similar. Should work well at ~1 mA with high beta.
5. Feedback network impedance is somewhat high.... I guess zener noise was swamping any voltage noise coming from there. I'd like to bring it down by a factor of 3-5 (10 seems a bit excessive with 1/4W resistors) after the zener replacement, but finding a good resistor ratio with standard values is not easy... I need at least one E48 series part. I'd say see where the new zener and possibly Qk2 gets you and choose new values from there. Rk11, 12 and 13 need to be kept in proportion, and Ck10 increased to match (accuracy of Rk13 / Ck10 is not as critical by far, however).
There is a bit of drop in Rk11 due to Qk2 bias current, so changing the transistor may result in a somewhat different optimum operating point. As Qk1 bias current somewhat affects Qk2 operating voltage, that one has some effect as well.
Hmm, they chose a part with 50 V of Vcbo (2SC1061) as a pass transistor for a regulator with a 55 V input voltage? That's quite brave, I must say. Guess they needed the highish beta and 5 MHz fT. And why a TO-220 case part when all they needed was a 10 V drop and 42 mA? (No heatsink?) Maybe TO-126 wasn't popular yet. As long as it works, fine, otherwise some creativity may be required. Apparently you can still get 2SD313s though, which are listed as a cross and will sustain 60 V (note that these are made in TO220, TO-220F full plastic and TO-264 packages).
The phono stage does have some rudimentary RF filtering but nothing up to modern standards. As Mooly suggests, try shorted inputs. Perhaps better yet, shorted Rd3/Rd4, which would bypass input wiring (phono/mic section).
Is the noise appearing on both channels equally? Could be a power supply issue then. What does the homemade audio probe have to say on the +45V, or on the emitter of regulator transistor Qk2 (Ck9/Rk9 junction)? (Checking voltage and ground connections on the phono PCB should come first, of course. Sometimes you have ground connections running over screws, which may appreciated being loosened and hand-tightened.)
This old Kenwood has some circuits of no more than moderate PSRR, and while the regulator circuit is basically sound and should have decently low noise as-is, it is not up to highest standards. I would suggest the following tweaks to the regulator:
1. Swap out 14V zener Dk6 for a low-noise type. Zeners have made a lot of progress since then.
2. Ck13 is a 33µ/16V - nice try, Kenwood, but I'd say more like 220µ/25V, and if you can find a 22µ/25-50V to parallel...
3. Ck8 is basically OK but has probably seen better days at this point.
4. If Qk2 is giving trouble, replace by 2SC945, 2SC/KSC1845 or similar. Should work well at ~1 mA with high beta.
5. Feedback network impedance is somewhat high.... I guess zener noise was swamping any voltage noise coming from there. I'd like to bring it down by a factor of 3-5 (10 seems a bit excessive with 1/4W resistors) after the zener replacement, but finding a good resistor ratio with standard values is not easy... I need at least one E48 series part. I'd say see where the new zener and possibly Qk2 gets you and choose new values from there. Rk11, 12 and 13 need to be kept in proportion, and Ck10 increased to match (accuracy of Rk13 / Ck10 is not as critical by far, however).
There is a bit of drop in Rk11 due to Qk2 bias current, so changing the transistor may result in a somewhat different optimum operating point. As Qk1 bias current somewhat affects Qk2 operating voltage, that one has some effect as well.
Hmm, they chose a part with 50 V of Vcbo (2SC1061) as a pass transistor for a regulator with a 55 V input voltage? That's quite brave, I must say. Guess they needed the highish beta and 5 MHz fT. And why a TO-220 case part when all they needed was a 10 V drop and 42 mA? (No heatsink?) Maybe TO-126 wasn't popular yet. As long as it works, fine, otherwise some creativity may be required. Apparently you can still get 2SD313s though, which are listed as a cross and will sustain 60 V (note that these are made in TO220, TO-220F full plastic and TO-264 packages).
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Sorry for not replying sooner, I replaced all the electrolytic caps on the power board and did what you suggested sgrossklass, I put a 220uf and 22uf in parallel in for ck13, but am waiting for some new 0.01uF film caps to do them too, and a zenor 14V (low noise, well better than what that is in there, heard leds are supposed to be good too) I have noticed a hug improvement already, But the pop is still there when switching from Aux to Phono 1 or 2 on both channels, I have gone over the ground connections a few times already and all seem solid, that was the first thing I checked. I know what your saying about the 2sc1061 in fact I am measuring 49.9V on the input which is even more of a test for the 1061 ! , I disconnected the 2A fuse and I am measuring 45V exactly coming from the transformer, along with the two 27Volts are spot on. As for Qk2 its measuring ok in circuit, but that does not say much, I have a few new 1845's which i can use, just to rule it out. Mooly I have not had time to measure the voltage at the resistors yet, but I will do this soon, and good tip for shorting the inputs. I also noticed that after cleaning the power switch contacts a second time with cleaning alcohol and some really fine sandpaper made the biggest improvement of all. Just the popping left to sort. Mind you turning down the volume when switching is also a way around it
"What does the homemade audio probe have to say on the +45V" Good idea I did not even think about this, the high voltage film capacitor will filter out the DC but I should be able to hear the AC noise. I was only using it to monitor the audio signal path. I can even change the gain on the circuit board, but am not sure what is acceptable levels, and yes I fully understand that the older gear is way more noise prone than more modern equipment in most cases (that is why i have an kenwood DA9010 which blows most vintage gear out of the water, a nad c370-some really bad engineering design flaws, with electrolytic's right against heat sinks, and diodes but now modded, some technic's, pioneer and JVC amps) , I will give it a try before and after replacing the zenor and see if I can hear the difference out of interest
Stupid question - when you are switching from Aux to Phono, is there anything connected on the Aux input? Both DC resistance to ground and AC impedance vary quite a bit between the two.
Phono: DC ~56 kOhm, AC ~20 kOhm and change.
Aux: DC 220 kOhm, AC 220k || (20 kOhm + source impedance)
That alone could be responsible for the pop.
Just to be sure that it's nothing to do with coupling cap leakage, measure the voltage across Ri1/Ri2 in both cases.
That said, I would not be too surprised if the emitter follower at the input of the tone amp were prone to oscillation. The two 10k resistors in series and Ci3/4 (470p) in parallel to the emitter resistors smell like bodging to make it work. That said, much the same circuit was also found in the older KA-7002. Anyway, I would have liked to see the odd kOhm directly in series with Qi1/2 base, and maybe 100-150 pF in parallel to Ri5/6.
BTW, this amp employs the same kind of funny grounding scheme that I've seen in older Sansuis - a bus running from the back panel all the way through the phono amp, tone amp and preamp boards. You can run into some nasty surprises when trying to improve circuit PSRR there - it's easy to couple power supply noise into the ground. I would contemplate getting Ci23 on the Tone Amp board its own dedicated power ground return there (after lifting it from regular local signal ground), making its way alongside the supply line (pin 4) to power board pin 11. Then install just a little 100-220 nF ceramic or something in series with ~1 ohm where Ci23 was connected before. (Plan B, include a bit of a resistor in line with the power coming in at pin 4. 47-100 ohms maybe?) In general though this construction seems a lot more thoughtful than e.g. the Sansui AU-5900 which seemed slapped together rather haphazardly (older preamp circuit with low PSRR that requires clean supply + newfangled noisy regulator = hisssss).
Phono: DC ~56 kOhm, AC ~20 kOhm and change.
Aux: DC 220 kOhm, AC 220k || (20 kOhm + source impedance)
That alone could be responsible for the pop.
Just to be sure that it's nothing to do with coupling cap leakage, measure the voltage across Ri1/Ri2 in both cases.
That said, I would not be too surprised if the emitter follower at the input of the tone amp were prone to oscillation. The two 10k resistors in series and Ci3/4 (470p) in parallel to the emitter resistors smell like bodging to make it work. That said, much the same circuit was also found in the older KA-7002. Anyway, I would have liked to see the odd kOhm directly in series with Qi1/2 base, and maybe 100-150 pF in parallel to Ri5/6.
BTW, this amp employs the same kind of funny grounding scheme that I've seen in older Sansuis - a bus running from the back panel all the way through the phono amp, tone amp and preamp boards. You can run into some nasty surprises when trying to improve circuit PSRR there - it's easy to couple power supply noise into the ground. I would contemplate getting Ci23 on the Tone Amp board its own dedicated power ground return there (after lifting it from regular local signal ground), making its way alongside the supply line (pin 4) to power board pin 11. Then install just a little 100-220 nF ceramic or something in series with ~1 ohm where Ci23 was connected before. (Plan B, include a bit of a resistor in line with the power coming in at pin 4. 47-100 ohms maybe?) In general though this construction seems a lot more thoughtful than e.g. the Sansui AU-5900 which seemed slapped together rather haphazardly (older preamp circuit with low PSRR that requires clean supply + newfangled noisy regulator = hisssss).
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Oops, just noticed that this would probably degrade channel separation unless Ci23 were to be enlarged a fair bit as well.
One reason why I like circuitry with some inherent PSRR, it just makes such shenanigans a lot more unlikely.
Ideally both channels on this board should get their very own RC filtered supply.
If other sections of the amplifier are working correctly the problem has to be in the phono section. There is a capacitor which decouples the supply for both channels to earth near connection 7 value 10n. Is this doing its' job correctly.
Solder joins for earth connections can deteriorate with age. The feedback loop of the phono stage is routed through a switch. Remaking solder joins can resolve a lot of faults.
Solder joins for earth connections can deteriorate with age. The feedback loop of the phono stage is routed through a switch. Remaking solder joins can resolve a lot of faults.
Thanks so much for everyone's help, sgrossklass I have replaced all the coupling and de-coupling capacitors on every board, in fact every electrolytic cap has been replaced, and double checked all values and orientation was right, the working channels sound a hell of a lot better after the recap much cleaner, And yes the pop issue when switching was there before and after the recap, But I will measure Ri1/Ri2 when I get the chance (at my real job at the moment :-( so nothing to do with my work I done. As far as I know I had a source connected to Aux and phono when switching, as that was my thought as well, the input impedance might have something to do with it. I will confirm it though and connect something to all inputs tonight. "Ci23 on the Tone Amp board its own dedicated power ground" might be an option, but I will wait with this for a while, as the thing should work the way it is, unless it had this issue from new. mjona thanks for the tip, I will check the 10n decoupling cap (as only the electrolytic's were done) , and also reheat the ground connections, I reheated the ground connections on the phono boards already and they have good continuity to the chassis, but not the one going to the switch. I did check the continuity and it seems ok on the switch, but will reheat / re-flow it tonight
Sorry for delay in getting back to this, I finally sorted it, re-soldered every single earth connection on the Record, input selector switch, replaced the new low noise zenor and replaced all the 0.01uf caps on the power board with high voltage film caps. There is still a tiny pop when switching but it's a lot less, and can live with it (yes with inputs on all channels) . Its sounds much better after the full recap as well. Thanks everyone for your wealth of information and patients I learnt a lot.
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