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Kenwood Basic C1 Preamplifier Restoration/Upgrade

Lee, Thank you for your wonderful thread it gave me inspiration to refurb the Kenwood C1 as well. It is mostly complete except for a few questions. I replaced the two small ceramics near the volume control and I replaced the single larger ceramic right before the two filter caps in the power supply. Waiting on some 1200pf replacement bypass caps in the phono section that were not addressed. I am not a tech but it seemed like a good idea and costs were very small. My question is would there be any sonic improvements if I replaced the mylar caps with mkp or polystyrene? Looks like there are about 16 lower capacitance caps and I have read that mylars tend to drift over time and have poor stability, what are your thoughts? Next question is when I ordered somehow I ordered 6 npn (2n5550) and 8 pnp (2n5401) transistors, are they worth putting in?
 
Now to my listening tests to this point. I have not tried the phono section yet so these comments are aimed at the general preamp section. I have several other preamps and my Yamaha CX-1000 stands at the top I also have a CX-800. These seem like fair vintage comparisons. The C1 beats the CX-800 in every area. Noise floor on the C1 is not audible in any way the CX800 it is present the CX-1000 just audible without a music source and turned all the way up. The C1 has tighter bass and the bass dynamics are faster, cleaner and seem way more controlled. The C1 bass is superb clearly the preferred preamp! Mid range the C1 bests the cx-800 but the CX-1000 has a midrange control and seems slightly more articulate in the midrange, however it takes a good deal of listening to reveal the small difference. The treble region the C1 again beats the CX-800 with an open clear transparent air the seems very musical. The CX-1000 seems as good and perhaps ever so slightly better than the C1 in this region, again it is so close that it might be a subjective call based on price or expectations. I am now moving the C1 to my premier position based on it's clearly superior bass as well as, a roundness, musicality and fullness that seems to be absent in the other two. The two Yamaha's have not been updated in any way.
I must say Lee you have certainly hit this out of the park I could not be more pleased.
 
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Dan Reardon said:
...My question is would there be any sonic improvements if I replaced the mylar caps with mkp or polystyrene? Looks like there are about 16 lower capacitance caps and I have read that mylars tend to drift over time and have poor stability, what are your thoughts? Next question is when I ordered somehow I ordered 6 npn (2n5550) and 8 pnp (2n5401) transistors, are they worth putting in?
Click to expand...

If the mylar (polyester) capacitors are in the signal path, polypropylene (MKP) or polystyrene would be better wrt to sound (lower distortion). Note that polystyrene capacitors are typically hard to come by these days and are sensitive to heat and solvent damage. I would not bother with replacing the regulator transistors unless they are defective and the 2N transistors have a different pin out compared to Japanese 2S types.
 
Ben, Thanks for the response, just learned you name was Ben sorry for the Lee. I will check to see if they are in the audio path, although I am sure you already did.Thanks for the update on the transistors. I like this modification so much I wanted to just keep it rolling. This is a day and night transformation I am truly elated.
 
Reading every word of the thread. Keeping up with the latest posts. Oh and BTW are you guys saying that Lee's name is Ben ?
 
Just wanted to leave an update and ask a couple more questions. First off, the phono section sounds bloody fantastic now! I tested it out on a few records I'm familiar with and also enjoyed a brand new album that just arrived the other day. Very good stuff! I'm also really impressed with the noise floor; it's pretty much absent even with the volume cranked up. Definitely a big plus for me as my desktop speakers are quite sensitive and tend to show off any excessive noise floor.

Second, Dan Reardon mentioned nice tight bass response, but I honestly think it could be improved a bit. Did you replace the caps for the bass control? I noticed they weren't included and I'm thinking I may wack some replacements on my next mouser order. It's not like they're expensive if you're already paying shipping!

This brings me to one final question. The Kenwood C2 has an adjustable turnover frequency for it's bass and treble controls. I usually used it on 200hz instead of 400hz; is there a way to adjust the turnover frequency of the C1's tone control section? I don't know enough about tone control circuits to try and modify it, and from what I can glean from the horrible C2 schematic scans, it looks like it's using a resistor to adjust the turnover? Either way, any advice on this particular question, or information or recommended reading suggestions on the topic in general would be much appreciated!

Oh, and before anyone asks, I'm referencing using a C2 a lot and the reason I'm not still using it is pretty simple: I like the look, function, and simplicity of the C1 better. Also, having toggleable tape loops is very important to me!
 
Lunarfox, I replaced every cap in the entire C1. Like you say they were not expensive and while I was in there I did them. I followed Ben's advice and read every thing I could find by Cyril Bateman. Cyril has some very revealing capacitor tests that gave me an insight I never had. I also used Bi-Polar caps in the phono section that were physically larger than the ones that Ben recommended (he did mention they should be Bi-Polar but size was a factor). They do not look as good because of the shoehorning I needed to do to fit, but should sound better. I do not have any test equipment that I can run and show you or others objective testing but I can say subjectively this preamp sounds sublime. Just as you have commented the noise floor is completely gone. I agree with Ben, the op amps play a major role in that.
 
The C1 phono stage is very well designed and theoretically accurate within 0.2dB; the actual accuracy is dependent on the actual resistor (R19/20 & R21/22) and capacitor (C7/8 & C9/10) values in the RIAA feedback. The stock resistors can be upgraded to 1% metal film types (use exactly the same values as stock) and the polyester capacitors can be upgraded to polypropylene (or PPS types) of the same capacity (the challenge is sourcing capacitors that fit the limited available board space). The large 2200µF capacitor (C5/6) in the high pass filter for the RIAA can be replaced with a 470µF bipolar type if only MM cartridges will be used (F3=2.3Hz); with the MC cartridge setting, the F3 with the 470µF capacitor is 30.8Hz as opposed to 6.6Hz with the 2200µF capacitor.
 
Hello Leestereo & All,

I'm new to this website and having read this thread from start to finish, I have say to Leestereo this is an excellent thread and thank you for producing such a clear and concise set of instructions for the C1 upgrade. Most threads of this type are usually not this helpful.

I recently bought a C1 preamp because I was looking for a preamp to match with my Quad 405-2 power amp that does not have the brightness issues that so many modern preamps and dac/preamps do. With going back in time to vintage kit, I've also been able find some very detailed KEF speaker that have helped in reducing brightness and whilst on fleabay I came across the C1 for just £80, did some research and could not find a negative opinion about it.

Having listened to the C1, I like it! The mids sound slightly muffled, the bass could do with tightening up and there's lot of noise out of the speakers when nothing is playing and the volume is turned up, even with its problems it sounds good, after nearly 40 years it's drifted, but I can hear why so many like this pre.

The research I've done has led me to this thread and I'm willing to have a good go at this upgrade. I have some basic experience of soldering and recently replaced some caps in my plasma tv, but by no means can I declare myself an electronic engineer, I have a basic knowledge of what some components do and ohms law and that's about it.

I am about to finish compiling a list of the parts I need or are available from Mouser and I've encountered a few problems with certain components not being available, I would be very grateful to the folks on this thread for any help they could offer.


Best regards,
Rob
 
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Hello All,

I have a list of components ready to go off to Mouser and out of that list, what's below I have concerns (in brackets at the end of each line) about and would be grateful for any help before I sink my teeth into this project.

POSSIBLE PROBLEMS LIST:

POWER SUPPLIES:
I can't find low esr caps at 16v.
C48 / C49 2 NICHICON UPW - CAP ALUM 47UF 20% 16V RADIAL UPW1C470MDD1TD (NOT LOW ESR TYPES ?)
C50 1 NICHICON UPW – CAP ALUM 33UF 20% 16V RADIAL UPW1C330MDD (NOT LOW ESR TYPES ?)
THESE ARE LOW ESR BUT WITH HIGHER VOLTAGE:
C48/C49 PANASONIC FR - 47uf 25v EEU-FR1E470B
C50 PANASONIC FR - 33uf 35v EEU-FR1V330

FLAT AMP STAGE:
C9 / C10 ???? (DO THESE NEED REPLACING? / IF SO WHAT SPECS?)
C11 / C12 2 TDK - CAP CER 68PF 50V 5% C0G RADIAL FA28C0G1H680JNU06 (VOLTAGE TOO HIGH ?)

TONE AMP STAGE:
C27 / C28 2 TDK - CAP CER 10PF 50V 0.5pf% C0G RADIAL FG28C0G1H100DNT06 (VOLTAGE TOO HIGH ?)
C29 / C30 2 TDK – CAP CER 47PF 50V 5% C0G RADIAL FG28C0G1H470JNT06 (VOLTAGE TOO HIGH ?)

PHONO STAGE:
C1 / C2 2 TDK – CAP CER 100PF 50V 5% C0G RADIAL FG28C0G1H101JNT06 (VOLTAGE TOO HIGH ?)

SUBSONIC:
C16 / C17 2 KEMET - CAP FILM 0.33UF 10% 50VDC RADIAL 80-MMK5334K50J01L4 (TOLERANCE OK?)

UNDERSIDE OF PCB:

1 PANASONIC FR – CAP ALUM - Radial Leaded 25VDC, 100uF 20% LOW ESR EEU-FR1E101B (THIS OK?)

This (below) is on the underside of the PCB, I'm presuming this is factory work and not DIY? The C1 came with a white shaver mains cable attached so someone at some point has been inside.

kRG2cVl.jpg

Thanks,
Rob.
 
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It is generally recommended, whenever possible (depending on size and lead spacing), to use capacitors rated at 25V or higher (lower ESR and longer life). Voltage ratings for the C0G capacitors can be 50V or 100V. The Nichicon PW series is a low ESR type. The underside of your C1 looks stock.

In this thread,
C48/C49: https://www.mouser.com/ProductDetai...-250ELL470ME11D?qs=4%2boambldWwbpHx7aUKt3aw==
C50: https://www.mouser.com/ProductDetai...=sGAEpiMZZMtZ1n0r9vR22YeHFy5ijTdkGqGk6M0OXO4=
C9/C10 were not replaced.
C17/C18: since Panasonic stacked films are no longer available, WIMA stacked films are a good substitute, https://www.mouser.com/ProductDetai...=sGAEpiMZZMv1cc3ydrPrF7l45uRd9dVhGCTDaOJ/aBA=
 
Hello Leestereo & rjsalvi,

Many thanks for your help and prompt replies, I'm ready to buy fom Mouser now.

I've had a look at the lead spacings on the power and tone boards, the spacings where I can see the pins all look to be 5mm with only the very small caps being 2mm, can I assume this for all the caps?

This 100µF 25V capacitor that might be on the underside of the pcb, is this an electrolytic low esr type?

Thanks,
Rob.
 
I'll answer my own questions!

I've been a bit lazy here asking folks on this site to check the lead spacings for me, the thing was right there in front of me and so I had a look myself. I'm not quite sure what got in the way of my cognitive processes this time around.

The underside caps? I just bought them anyway.

My order has gone off to Mouser, I'm looking forward to this.
 
The beer fest I went to the night before probably didn't help things either, I wasn't the sharpest knife in the rack that day.

Let's hope there's plenty of plates full of fish!
 
I've received all the components last week from Mouser, but the kids have been off school with mid term holiday/vacation and I've have had no peace or quiet until today, they've gone back to school.

However, when looking at the C1 before starting I've noticed a replacement resistor has been soldered in, it looks to have the correct specs, this is resistor R62 330K ohm 5% 1W, I've indicated it in the photo.

My concerns are basically that this is a large resistor (big surface area), I presume to dissipate heat, they are meant to last the life of the unit itself, should I be concerned about any underlying problems that made this resistor fail? I get the impression my unit hasn't had an easy life.

xNn0Bbr.jpg
 
D8: 0.74v & OL
D3: 0.62 & OL

Transistor behaved normally, but when reverse biased, with the base/collector it gave a voltage.

All of the components were still soldered to the pcb, how will this affect readings?

Also the soldering on D3,D4 and D5 looks very poor quality, the all have readings of around 0.6v and are working, but I'm suspicious.


I'm in this to win it and so I'd like to put it beyond any doubt and just buy new components.

I've ordered the transistor and I'd be grateful if you would help me out with the specs for D8 and D3. The service manual doesn't help any with these two.

Should I replace D3,D4 and D5?

Thanks,
Rob
 
C1bob said:
...However, when looking at the C1 before starting I've noticed a replacement resistor has been soldered in, it looks to have the correct specs, this is resistor R62 330K ohm 5% 1W...
Click to expand...

Why do you think R62 isn't OEM (resistors with the stand-off type lead is typically stock)? Is the soldering on this component suspect? I do recommend the replacement of any component that is covered in glue (becomes corrosive as it ages), e.g., D3 (behind the glued capacitor).
 
C1bob said:
...Transistor behaved normally, but when reverse biased, with the base/collector it gave a voltage.

All of the components were still soldered to the pcb, how will this affect readings?...
Click to expand...

Component readings while in circuit are often misleading; best to remove to test. If the voltage readings match the schematic, there is no reason to think that the transistor is defective.

C1bob said:
...the specs for D8 and D3. The service manual doesn't help any with these two.
Click to expand...

D3 is a W06B type; general purpose diode, 100V, 0.75A
D8 is a 12.5V zener diode, 500mA
 
rjsalvi & leestereo,

Many thanks for your help in pointing me to to these components and the specs.

Leestereo, I've checked the soldering on R62 and you were correct, it's very definitely stock. It was simply the colour difference that aroused my suspicions, all those big resistors seemed to be in green in all the photos I've seen (including yours).

The soldering on D8 and Q1 also look stock.

And if it is stock, then there might not be any underlying problem (panic over?).

What has now concerned me is the soldering on D3,D4 & D5. It doesn't look very professional, there's no little conicals, they're nearly flat compared to what's around them, I've included some photos. Would this have got past factory QC?

BFpEEac.jpg


0YhqCBi.jpg
 
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