Saturday, May 30, 2015

Dual 510 Plattenspieler

Dual 510 Plattenspieler

Here we've got a German Dual 510 Plattenspieler. This came in with one channel out and for some repair from a previous "shop"

Whats nice about a lot of the Dual TT's is its fairly straight forward replacing interconnects. I prefer a end soldered connection. Custom length double shielded+RF RCA cables with strain reliefs were used, very similar to a co-axial type construction, I just love these cables, I use them almost exclusively besides the audio-quest interconnects now, ie long grain alloy ;)

you can read a lot of positives on the 510 series, the heavier mass platter and standard medium weight arm allows for fairly universal use, adapting cartridge bodies is also much easier requiring only some TK24 surmount vs than say a 506 series where a adapter is required. Like the 506 it uses a very nice, reliable 8-pole AC motor  I do want to note belts are readily available from many reputable resellers, both timing too. 

Its been a while since I played my "In the court of the Crimson King" album, but I have to say, love it on the 510! 

Friday, May 29, 2015

Sansui AU-717 Integrated Amplifier

Sansui AU-717 Integrated Amplifier

So here we've got a incredibly clean and pristine AU-717 integrated amp. Theres quite a few thing I like about the AU-717. The layout is very clean, they used proper shielding over all AF sections that we like to see, the shielding itself is attractive ,matching the clean black gloss finish elsewhere. The PCB locations and mounting make for some fairly quick work, especially when pulling the driver sections. I should note to properly pull the protection/PS one needs to pull the dual power transformers.  Also note, check your hex locations on the knobs first, on the 717 it really is a bitch.

Power Supply/Protection F-2663 PCB
Per my usual treatment I went with high temp, long life Panasonic FR/FCs strictly for this stage along with a pair of Nichicon high temp Bi-Polars (note some SM state 100/should be 220uf). The FR/FC series  delivers exactly what you want in a power supply, reliability.  Another addition is the installation of a new Omron relay, went with the 4PDT  MY-4 variant with the low noise action. 

Now as with most Sansui of this age and period we have some serious glue issues to attend to, the glue used is notoriously bad, it oxidizes to a degree and foreign particles create new tracing, as the glue sits on the component leads its causes oxidation and eats away at many resistors.  I recommend replacing any component that came into major contact with it and deep cleaning the PCB.  Another issue to note with the Sansui's are the fuse-able resistors. These absolutely need to go, when they fail they really fail and short. I recommend flame-proof MOX here. 



Along with the glue, you will encounter several coated diodes as well, the 1S2473 had a great modern drop in available the 1N4148 and the larger diodes consists of ultra fast UF4003/02. 

Driver Stages F-2721/2722
For the lyrics on the driver stages I used audio grade Nichicon KW and Elna Silmic II capacitors. Again this board has the notorious glue on it, make sure ALL of it is remove and address and questionable resistors that were coated.  This particular model did not, but some may have fuse-ables at R23/25/26/16 locations, again I recommend flameproof MOX. 

The Au717 uses an interesting 1st stage topography along with a field-effect transistor   FET (2SK97) differential package, and quasi complimentary pairs at TR01/02 & Tr03/04. It is recommend to update the TR01/02 and Tr03/04 as was performed in this unit with ZTX694B and 992's BJT's.  On a last note I like to replace  biasing/offset potentiometers, IF, their not the old big fat white ones (those are great) the Au717 uses a series of two pots (100/2.2K) to dial in the Offset and one 1K for the Bias, right angle bourns were used here along with the fore-thought too add test points, too bad we dont see insight like that often. 

Pre-Tone Control  PCB F-2720
One thing I really liked about working on the AU717, especially in this stage is the molex type connections. It took just a few minutes to pull this section along with the huge enclosed stepped attenuator and F2666/F2665 assemblies for cleaning.  All the lyrics here were replaced again with audio grade Elna Silmic II's. One important thing to note is the AU717 used quite a few VD1212 type varistor diodes. These should all be removed and replaced with something a little more reliable and new, two 1N4148 in series (cathode to anode) work excellent.  Also, note we have 4X 82.5 ohm fuse-ables that need to be replaced with MOX or equivalents. This is a great time as I mentioned to pull the F2666/2665 assemblies to clean the controls, and tap the large enclosed attenuator to clean and lubricate. 


EQ PCB F-2723
The EQ is fairly straightforward to work on, one can pull the extended shaft controls and simply pull the molex type connectors again ( :) ) All the lyrics were replaced with a low impedance , high reliability Nichicon PW and audio grade Elna Silmic II's here, the original C1/2 films were replaced with a better polypropylene WIMA film capacitor of the same value (250V). Again we see the VD1212 in X4 four position on this board D01-04 that need to be updated along with X4 four 82.5 ohm fuse-ables to replace. 

The rear panel F-2670 PCB has two lyrics I replaced with Nichicon PWs again, all the filter caps were replaced with X4 four 15Kuf 63Vdc Panasonic THA's, sadly these are nearing their run, it will be hard to find such a good filter cap replacement. 

Im very impressed with the AU717, I do wish the stepped attenuator had a vent, but that can be made, and as with most Kenwoods or Sansui's utilizing dual power supplies you usually end up pulling the Pformers out of the way. Still what a damn nice looking integrated, its is incredibly clean sounding I thought with just a touch of some lower end warmth. Quite impressed. 

Thursday, May 14, 2015

Kenwood C2 Pre-Amplifier Upgrade

Kenwood C2 Pre-Amplifier 

Here we've got a nice Kenwood C2 pre-amplifier. The unit came in for an overall recap and OPAMP upgrade. Leestereo of Audiokarma was very helpful with additional OPAMP information from his Kenwood C1 rebuild, big thanks to him and his work, truly top notch. 

open-booked main PCB assembly

Each stage has its own power supply, where as the electrolytic were replaced with high temp long Panasonic FR/FC capacitors, I prefer this application vs a audio grade. In a power supply were looking for an extremely reliable capacitor, the Panasonic FR/FC series deliveries just that.

 Note that the phono/tone/flat amp stages feed a 18.6-18.5Vdc rail to each OPAMP quadrant. the rated parameters of the NJM4580 and NJM2068 OPAMPs are 2-18Vdc.  For continued longevity of the OPAMPS D13/14/11/12 should be replaced with a 16-17Vdc zener to drop that operating 18.5-6vdc rail voltage. 

The following OPAMPS were upgraded along with new gold contact 8 pin DIP sockets for future rolling;
Phono - NJM2068DD
EQ stage - NJM4580DD
Tone Control - NJM4580DD
Filter stage - NJM4580DD

For all other electrolytic Elna Silmic or Bi-Polar MUSE ES were utilized, all low value lyrics were replaced with a polypropylene ECW Panasonic film capacitor.

I have to say, pairing this C2 with a Nikko 440 Alpha amplifier is quite impressive. There was a notable difference between the the original 2041DD JRC OPAMP and the NJM2068DD OPAMP. Impressed…. 

Wednesday, May 13, 2015

Bedini 25/25 Class A Power Amplifier

Bedini 25/25 Class A Power Amplifier

I wanted to share a little bit about this Bedini 25/25 Class A power amplifier. Rated at a mere 25W/CH it still has an impressive THD rating albeit fairly low dampening rating. I do want to note, apparently this is a very similar design to the 100/100 without updated 93/94 output devices.

This particular came in with some serious distortion issues, you could literally watch the wave form table-top with my Tektronix 2215 scope. Some previous work had been performed on the unit so unfortunately the quality fiberglass PCB had ignorant tracing issues. As well a axial general electrolytic was replaced with a trimmed audio grade Nichicon FG. I've noted across most major brands of the last few decades that axial tend to perform poorly compared to a radial and usually exhibit some form of excessive wear. In most cases the foot print only allows for a axial as you don't want much lead spacing exposed, but in this case it work perfectly.  Another update was to the complimentary differential pairs, vs a standard a/ab operation where we see a set of differential pairs, complimentary pairs are typically utilized in Class A operation due to the ON-ON type operation of the circuit. Standard KSC1845/KSA992s are damn near a perfect replacement, watch the pinout VS the original EBC.

Their is very little information available online, and the customer was incredibly useful in tracking done technical data, one leg that had to be worked out was a biasing issue.The threshold for the driver stage  is an impressive 550mV+ then you can achieve draw across the emitter resistor sets. Some models may not have a 2K potentiometer installed apparently. 

After figuring the Biasing issue and new components the floor was dropped considerably, audible distortion eliminated and a nice little amp was left.

Tuesday, May 12, 2015

AMC CVT1030MKII Pre-Amplifier


It was quite nice to get some haute equipment in the last few weeks, Id like to highlight a AMC CVT1030 Pre-Amplifier with the MKII addition. 

The AMC 1030 is a hybrid topography, utilizing class A arranged FE-transistors and high/lo gain 12au/AX7 valve variants. The FET's are incorporated into the solid state regulated power supply/filter stages, which is ideal considering we can clean up AC taps not required in  valve rectification with a little more reliability.Their are some neat features including  a direct/normal phase that bypasses tonal controls and includes a upgraded balanced output module (MKII.)

Power Supply PCB 90-1261-1
Because we are dealing with a valve topography and filter were looking at high voltage 250Vdc electrolytic here. I preferred the Panasonic EE series, these are phenomenal in the power supply, the EE is known for extreme reliability especially under duress; rated at over 10,000hours with very little capacitance change (20=-%) at +105C under load. For lower voltage lytics Panasonic FC/FR high temp long life capacitors were utilized.

Phono PCB MD90-1261
The phono stage is a modular PCB designed for only moving magnet cartridge bodies. All the lytics here were replaced with Nichicon low impedance and audio grade Elnas.

Headphone PCB MD90-1262
The headphone has a nice switching mechanism so you can cut out the outputs, it also is incorporating into the "phones/normal/direct" feature allowing tone controls still. Again the lytics like the phono were replaced with a low impedance Nichicon capacitors which are perfect for audio applications along with the audio grade Elna's. 

Pre-Amp Block (main) PCB MD90-1261
The original design had some decent proprietary films, we opted to go with the WIMA polypropylene 2.2uf 250V films, talk about BIG and expensive but the results are worth every penny. Some re-routing of grid resistors had to be performed, its important to note when rerouting valve resistors were talking some pretty decent radiant heating escaping, make sure you give plenty of room for them to vent so as not to heat up the foil side of the PCB or soldering mask. My favorite Panasonic ECW polypropylene were utilized where the WIMA's would not fit. All the electroytic capacitors on the main block were replaced with Elna and low impedance PWs.
I should note that the tracing on the socket side had been utterly destroyed by a prior "repair" centre, all new tracing had to be laid and new gold contact 9-oin PCB sockets installed.  The original ECC valves were replaced with all matched Electro harmonix 12AU/12AX7 lo/hi valves. Im a huge fan of EH, I use them in my push-pull amplifier and highly recommend them as a modern replacement source. 

All in all, I am very impressed with this pre-amplifier, it has some killer looks, very solid seated controls/knobs. Very simple straight forward audio signal topography with no bloated frills other than the IR receiver .