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2SK30A vs. 2SK68A and intermittent volume fluctuations in photo stage in Kenwood KR-9600

H

hatman

Active Member
Thanks to AK members my Kenwood KR9600 has come a long way with recap, repairs and mods. My current repair involves a JFET substitution as a possible upgrade.

My search for a sub for 2SK68A has this novice a little befuddled. Forgive me if I am asking technical questions that are not answerable in a “Cliff Notes” version, requires I have a real electrical background to benefit from and would probably hurt my little brain if offered.

AK members have noted that as a substitute, the 2SK30A is a lower noise JFET than the 2SK68A ( K & M versions are in the KR9600 Pre Amp), and an overall improvement over the stock 2SK68A. I have attached the specs for both JFETs and the PreAmp Schematic. Additionally I located a FET Database at : http://www.minor-audio.com/data/data_fet.html
Pretty sure EchoWars and other members were looking for more specs on these FETs.

I have the 2SK30A(GR) version that has the same IDSS (2.60 - 6.50mA) as the 2SK68A(M).
I have also the 2SK30A(Y) version that is close, with an IDSS with 1.20 - 3.00mA vs. 500u - 1.5mA found in the 2SK68A(K). Will that difference cause a problem?

The following also concerns this novice:
The Drain Power Dissipation of 2SK30A is 100mW and the Absolute Max. Power Diss for the 2SK68A is 250mW. Are these comparable PD's? If so, is the difference a problem?

Also, is it possible with the available specs, to determine comparable “low noise”claim? NF for 2SK30A =0.5 & 5.0db: 2SK68A = 0.6 & 1.5db, meaningful? Found these specs at DatabaseArchives; http://www.datasheetarchive.com/
Their cross References for 2SK68A are:
2SK68A: 2SK106- 2SK117- 2SK121- 2SK127- 2SK163- ECG458

Now if you are interested in why I am thinking of replacing the four JFETs with 2SK30a, a quick answer is I am about ready to shotgun the PreAmp board to solve static hiss and intermittent volume spike issues. The long answer follows. Hopefully the AK experts will read on and advise me if I am “off the mark”.

Assessment: (Tools; Fluke 279, freeze spray, lots of DeOxit used)
1. All noted problems also occur when playing the turntable through the KR9600's PreAmp and another AMP, which eliminates the KR9600's Power Amp boards as the problem. PHONO 1 has an intermittent habit of boosting the left channel volume for a few seconds a few times in an hour. Additionally, with no signal and at about half volume a weak hiss is intermittent in both channels. No problems with AM / FM/ AUX .
2. Phono 2 has the intermittent volume spike problem in the right channel along with a constant stronger static-like hiss in the right channel.
3. Freeze spray on Qd8 & 9 can cause volume spikes.
4. Probing and tapping around did not reveal any wire/connection problems, solder bridges or prior repairs. Turntable plays fine through other receivers.
Questions:
1.Replace the four FETs with 2SK30A . Are they a better replacement?
2 Are Qd19,20,21, 22 part of the Phono section and/or could they be responsible for the problems with the Phono modes? Include in shotgunning board?
3. Shotgun the Phono section transistors?
 

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1. FET's are much simpler devices than bipolar transistors (on an atomic scale). They do go bad from time to time, but it is a much rarer event than with a bipolar transistor...perhaps because of their simplicity. I have never seen a bad 2SK68A FET in any piece of gear, but readily admit this does not preclude it happening.

2. Methinks that you do not quite understand the functions of the circuits you are looking at. Let's clarify:

---The four 2SK68A FET's, and the six bipolar transistors immediately to the right of them, consist of the 'Phono 1' amplifier for both the L & R channels.

---Directly below these transistors are six more bipolar transistors. This is the 'Phono 2' amplifier for both the L & R channels.

---To the right of both of these on the schematic (looking separate, but in reality on the same PC board as the phono amps), is a Class AB buffer stage. ALL inputs are routed through this stage, regardless of source. If you are having problems with noise with phono 1 or 2, and any other souce (Aux, for instance), it is here that I would look.

3. Intermittent 'jumps' in volume level are not uncommon with phono stages. It usually indicates a bad polystyrene cap in the feedback loop of the phono stage. In the Phono 1 FET amp, these caps are (my description is for the L channel...you may interpolate the corresponding parts for the R channel) Cd15 and Cd19 (Cd15 is mislabeled as Rd15). For the Phono 2 amp, the feedback caps are Cd17 and Cd21. If you are getting intermittent jumps in volume level when listening to any other source besides the phono amps, we are probably also talking about a bad cap in the feedback loop, but it will not be so easy to find.
 
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Thanks EchoWars for the invaluable lesson and I know that many AK members have also benefited from this thread. I had the Phono 1 & 2 somewhat figured but, was unsure about the "Buffer Stage" relationship and why Phono 2 had no FETs. Great lesson, everything goes through the buffer stage. FET-less Phono 2 stage is a design choice? I have a better idea what the feedback loop looks like. Also learned that the well regarded polystyrene cap can fail, and with the symptoms found in my phono amps, the transistors were not the usual suspects.

Locating radial polystyrene caps is a challenge and a lot of the physically small polypropylene caps are limited to 50V, which I believe is acceptable.

This place has extensive list of rectangular polystyrene caps with .4" lead spacing and 1" long leads. http://www.americancapacitor.com/S Series/sindex.htm. They may be a wholesale company or manufacturer.

Mouser has the red square WIMA Polypropylene FKP2 series (2.5% 50/100V) with the 5mm lead spacing that will fit but, ratings will have to bumped up from 2400 to 3300pf and 8200pf to 0.01uf. They also carry some 5% XICON Polystyrene radials.

Saved me on this one. So now I will shotgun the polystyrene. Bravo again, Mr. EchoWars!
 
The two separate phono stages, one FET and the other bipolar, was simply a selling point. The FET stage is rated for 2.5mV input, and I think the bipolar is rated for 5mV. Even accounting for the difference in level, I think the Phono 1 FET stage sounds better.

Your proposed changes in capacitor values will have a profoundly negative effect on the RIAA phono EQ curve. You need to stick closer to the original values. The Xicon axial polystyrene caps at Mouser will work fine. They have a 8200pf 50V cap, and a 2200pf 50V cap (or you can parallel two 1200pf 50V caps to get the exact value, perhaps putting one of them on the back of the board ala Sansui ;)). If you simply use a 2200pf cap to replace the original 2400pf cap, you will boost the highs slightly. In the same vein, they sell a 2700pf cap that would mute the highs a bit.

2200pf 50V radial - #23PW222
2200pf 50V axial - #23PS222
1200pf 50V axial - #23PS212
8200pf 50V axial - #23PS282

Digikey also has polypropylene caps that will work...I'll let you dig up the numbers. If I was going to do this, I'd buy the axial caps (the 8200pf and the 2200pf) from Mouser and call it a day.
 
Sounds like Mouser will get my order for polystyrene caps. I will stick with the two 1200pf in parallel as suggested and keep the factory 2400pf. Thank you and keep well.
 
I can personally attest to the fact that wild volume changes on a phono section can be caused by bad Polystyrene caps. Thanks once again Echowars, for setting me in the right directin on my C-2 pre.
 
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hatman said:
Sounds like Mouser will get my order for polystyrene caps. I will stick with the two 1200pf in parallel as suggested and keep the factory 2400pf. Thank you and keep well.
Click to expand...

Be careful and don't overheat the cap when soldering. Polystyrenes damage easily.
 
Good advice that I will certainly heed. It seems like these may be one of the most heat sensitive type of caps. Interestingly, this preamp board is mounted back to back with Power Supply ‘B’, although it is separated by the metal chassis. This PS ‘B’ board is hot running and I wonder if that heat may contribute to the Polystyrene‘s health. The original standoffs for that board just about crumbled when touched. Using an EW mod, I was able to get a couple of the resistors off PS 'B' to reduce the heat load.
 
The 2SK30A should work as a sub. You should match them first for IDSS. I would try the 2SK30A (GR) first. As long as the voltage on the drains is around 25V, you should be fine. If is much lower, say less than 22V, go to the 2SK30A(Y). If the 2SK30A(Y) has a drain voltage higher than 25V, I would go with the GR, to give you more head room and set the operating point further away from Cutoff.
To match the IDSS, make a little fixture with a 12V supply and a 1K resistor in series with the supply and drain. Gate shorted to Source. Measure the Drain voltage with a DMM and match the FETs for the closest Drain voltage.


hatman said:
Thanks to AK members my Kenwood KR9600 has come a long way with recap, repairs and mods. My current repair involves a JFET substitution as a possible upgrade.

My search for a sub for 2SK68A has this novice a little befuddled. Forgive me if I am asking technical questions that are not answerable in a “Cliff Notes” version, requires I have a real electrical background to benefit from and would probably hurt my little brain if offered.

AK members have noted that as a substitute, the 2SK30A is a lower noise JFET than the 2SK68A ( K & M versions are in the KR9600 Pre Amp), and an overall improvement over the stock 2SK68A. I have attached the specs for both JFETs and the PreAmp Schematic. Additionally I located a FET Database at : http://www.minor-audio.com/data/data_fet.html
Pretty sure EchoWars and other members were looking for more specs on these FETs.

I have the 2SK30A(GR) version that has the same IDSS (2.60 - 6.50mA) as the 2SK68A(M).
I have also the 2SK30A(Y) version that is close, with an IDSS with 1.20 - 3.00mA vs. 500u - 1.5mA found in the 2SK68A(K). Will that difference cause a problem?

The following also concerns this novice:
The Drain Power Dissipation of 2SK30A is 100mW and the Absolute Max. Power Diss for the 2SK68A is 250mW. Are these comparable PD's? If so, is the difference a problem?

Also, is it possible with the available specs, to determine comparable “low noise”claim? NF for 2SK30A =0.5 & 5.0db: 2SK68A = 0.6 & 1.5db, meaningful? Found these specs at DatabaseArchives; http://www.datasheetarchive.com/
Their cross References for 2SK68A are:
2SK68A: 2SK106- 2SK117- 2SK121- 2SK127- 2SK163- ECG458

Now if you are interested in why I am thinking of replacing the four JFETs with 2SK30a, a quick answer is I am about ready to shotgun the PreAmp board to solve static hiss and intermittent volume spike issues. The long answer follows. Hopefully the AK experts will read on and advise me if I am “off the mark”.

Assessment: (Tools; Fluke 279, freeze spray, lots of DeOxit used)
1. All noted problems also occur when playing the turntable through the KR9600's PreAmp and another AMP, which eliminates the KR9600's Power Amp boards as the problem. PHONO 1 has an intermittent habit of boosting the left channel volume for a few seconds a few times in an hour. Additionally, with no signal and at about half volume a weak hiss is intermittent in both channels. No problems with AM / FM/ AUX .
2. Phono 2 has the intermittent volume spike problem in the right channel along with a constant stronger static-like hiss in the right channel.
3. Freeze spray on Qd8 & 9 can cause volume spikes.
4. Probing and tapping around did not reveal any wire/connection problems, solder bridges or prior repairs. Turntable plays fine through other receivers.
Questions:
1.Replace the four FETs with 2SK30A . Are they a better replacement?
2 Are Qd19,20,21, 22 part of the Phono section and/or could they be responsible for the problems with the Phono modes? Include in shotgunning board?
3. Shotgun the Phono section transistors?
Click to expand...
 
Thanks Warren for the detailed information on the 2SK30A as a sub. This rounds out this thread and gives novices like me a valuable lesson. Regarding your Avatar, I am old enough to remember the band Genesis, but would appreciate a recommendation for a Steve Hackett CD.

EchoWars was on target as usual. His recommendation to replace the PreAmp's polystyrene capacitors solved the very intermittent volume spike issue and saved me from shotgunning all transistors and FETs on that board.

Without these patient experts, a novices ability to restore, repair and modify our vintage units would be nearly impossible. So, I encourage strong support($) to the AK concept and lots of admiration for those gurus who make it happen. As the band at the local Road House says "Sip and Tip".
 
hatman said:
Regarding your Avatar, I am old enough to remember the band Genesis, but would appreciate a recommendation for a Steve Hackett CD.
Click to expand...
Spectral Mornings
Out Of The Tunnel's Mouth (the newest) is killer!
Genesis Revisited, 1996
To Watch The Storms
Defector
Please Don't Touch

These are all standouts.

Please buy them here:
http://www.hackettsongs.com/
This is Steve's official website. The old site was taken over by his ex-wife.
 
Thanks Doc, anxious to hear him again and will be ordering from his store. Think I will start with the Spec Edition "Out of the Tunnel's Mouth" and "Spectral Mornings".
Great players are inspirational and after hearing them I might initially feel like throwing my guitar out the window. Then a reality check results in more wood shedding, and that's good for the dust on the soul.
 
This post help me solve problems with my 9600. I had intermittent volume fluctuations on phono 1. Replacing the polystyrene caps with Echowars recommended replacements did the trick.
I also didn't like the heat on the "B" power supply board so I took Glen's advice and installed the single 25 watt 75 ohm resistor on the chassis. Simple and effective.
Thank you for the great post and information.
:D
 
Fhamre, good to see the valuable info provided by the experts get used. Echowars has put out a lot of nice mods for the KR9600. I have detailed records on a few of the basic ones. PM me if you could use them.
 
Hello to anyone experiencing phono 1 issues on KR9600. I found that the 2% polystyrene capacitors on mine were all deteriorating, so I replaced them with surface mount 2% ceramic chip capacitors. They fit perfectly on the bottom of the PCB, and the annoying intermittent problem disappeared. It was not just a jump in gain, but the RIAA would change as well, making one channel harsh and bright. I also replaced the 2SK68A JFET’s with Linear Systems, ultra low noise LSK-389A dual JFET’s. These are the same as dual matched LSK-170A’s on a single substrate. I used them as singles because I had some on hand, but Linear Systems does a noise verification on all their FET’s to assure no popcorn noise, which was also an issue with the 2SK-68A units. I couldn’t install the dual unit without some modifications, as the pinout was such that it made it too difficult or me. I ultimately just cut the three pins from each dual FET and used them as singles, otherwise I would have had to order a minimum quantity of ten pieces of the LSK-170A. The A units have a guaranteed idss of 2.6 to 6.5. On my unit, the popcorn noise was so annoying that I had to do something, and the lower grade 68A FET’s just weren’t good enough. I recommend using the Linear Systems
LSK-170A, even if the specs are somewhat different. They seem to work very well so far. Next it’s on to the regulator board and the excessive heat radiating from it. This board can be easily redesigned with a better heat sink, transistors and resistors.
 
Glad to hear about you using linear systems parts to solve your 2sk?? issues. We use them too in our DH-220c design (lsk489, lsj689) dual complementary pairs. Great parts to use for front ends.
Toshiba is still making 2sk208,209 parts too.
 
Very interesting. I have exactly the same volume problems on my Kenwood 7100.
Question: Have I located the feedback capacitors correctly in green? Enclosed schematic.

EDIT: Adding the entire phono-section schematic as well...
 

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Hello, and yes. Kenwood used polystyrene capacitors for the RIAA feedback loop. They are easily identified because they look different. They are white cylindrical In shape. .0082uf and .0022uf. The 100pf is probably a round flat ceramic and does not need to be replaced. I used 2% chip multi layer capacitors and soldered them to the bottom of the board. It is important to use high tolerance components here. I wanted to use mica caps but they were too large and expensive, but the chip components fit perfectly. There is a Kenwood thread somewhere, and you might find more information there. Polystyrene caps go turn to poop after they get old and are subjected to heat. I originally thought it was the FET and that is why my question was asked in this thread, but replacing them had no effect on the crazy noise. Regards, J
 
Joseph G said:
Hello, and yes. Kenwood used polystyrene capacitors for the RIAA feedback loop. They are easily identified because they look different. They are white cylindrical In shape. .0082uf and .0022uf. The 100pf is probably a round flat ceramic and does not need to be replaced. I used 2% chip multi layer capacitors and soldered them to the bottom of the board. It is important to use high tolerance components here. I wanted to use mica caps but they were too large and expensive, but the chip components fit perfectly. There is a Kenwood thread somewhere, and you might find more information there. Polystyrene caps go turn to poop after they get old and are subjected to heat. I originally thought it was the FET and that is why my question was asked in this thread, but replacing them had no effect on the crazy noise. Regards, J
Click to expand...

Thank you Joseph G! I replaced 2x 8.2 nF (0.0082 uF) and 2x 2.2 nF (0.0022 uF) with as good parts as I could source locally. Almost one hour of listening now and so far no problems. Knock wood. It was quite an intermittent problem before.

Also, tried both heating and freeze spraying the removed caps but could not provoke them to display any off-readings outside of the amp. Measured OK. Yet, I know from previous experience that caps can be notorious for measuring OK but failing intermittently.

Also - I must say - it appears to have been worth it! This amp sounded very good on the Phono input. Sounds amazing now...
 

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swingwhale said:
Thank you Joseph G! I replaced 2x 8.2 nF (0.0082 uF) and 2x 2.2 nF (0.0022 uF) with as good parts as I good source locally. Almost one hour of listening now and so far no problems. Knock wood. It was quite an intermittent problem before.

Also, tried both heating and freeze spraying the removed caps but could not provoke them to display any off-readings outside of the amp. Measured OK. Yet, I know from previous experience that caps can be notorious for measuring OK but failing intermittently.

Also - I must say - it appears to have been worth it! This amp sounded very good on the Phono input. Sounds amazing now...
Click to expand...
Glad you got it sorted out. The KA-7100 is a very nice amplifier, I recently completed a restoration on one and loved the way the phono section sounded.
 
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