Set to restore a Kenwood Model 500

[hopjohn]

Filter capacitor removal and Relay / Rectifier board access.

I first disconnected the wiring from the bottom of each of the large filter caps. Then, I placed the amp atop two large books, each about three inches tall, carefully spaced such that the books are at the far ends avoiding any jacks or terminals on the back of the amp. Next, I sprayed soapy water around where the cap and bracket meet serving as a lubricant. I then gently wiggled at the two filter caps until they had become loose enough to take out by hand. With these capacitors out, it opens up easy access to the top half of the clamp and its mounting screws (shown already removed in Photo 1 below). The bottom half of the clamp was still nestled behind the transformer. To access these screws, I needed to remove it from the chassis. The transformer was held in place by four 8mm hex nuts on the reverse side which I removed with a socket wrench. Before pulling the transformer free I placed a rag beneath it to avoid any damage and freed up the wires at the back to provide a bit of slack. With the transformer off the chassis it can be maneuvered around a bit to reach to the two remaining screws left holding the clamp.(see Photo 2 below)

The second transformer is removed in the same way as the first. Access to the top screws was straight forward through the gaps to the inside of each of the rectifier heat sinks. (See Photo 3) The screws behind this transformer were a bit more challenging to reach. The capacitors are left in the way, but with a little patience and maneuvering of the transformer an opening for the screwdriver shaft presented itself. Once the clamp was freed from the chassis, getting the caps/clamp out required me to take the two screws from the relay/regulator mount out. With the screws removed, I was able push the circuit board upward slightly which provided some much needed space for the clamp/caps to slide left and then out.

With the filter caps removed and the transformers re-attached to the chassis, the amp can now be placed bottom down. The relay/rectifier board is now accessible for solder work to remove and replace the old relay. (Photo 4)

Photo 1: The Kenwood 500 atop two books backside down.

Photo 2 : Accessing the mounting screws of the filter cap clamp shown behind the transformer.

Photo 3: Removing the second transformer with the caps still in place.

Photo 4: With the filter capacitors removed the solder side of the relay/rectifier board is accessible.

 

[hopjohn]

Filter Capacitor Installation

The capacitors I chose are a millimeter or two too small for the bracket to hold firmly. To correct this, I attached two strips of craft foam with double sided tape to the inside edge of the clamps. (Photo 1) Once the strips are in place I use a black permanent marker and color in the top and bottom edges of the foam so they don't show. The caps/clamps are then installed in reverse order of the removal described in my previous post.

Photo 1 - Two strips of craft foam being applied to the inner edge of the filter capacitor clamp.

The wiring for all the capacitor connections was stripped back about half an inch or so. Ring terminals are crimped and soldered to to the four red (+) wires and the four white (-) wires. Some heat shrink tubing was then used for insulation. The ground strap was constructed from a short piece of solid 12 gauge copper wire taken from Romex cable and two more ground terminals, crimped and soldered. (Photo 2) WARNING: Watch your polarities here!

Photo 2 - Ring terminals are crimped and soldered to the filter capacitor wiring. A ground strap is constructed from 12 gauge wire.

Photo 3 - 2.2uf 250V bypass capacitors are put across each of the filter capacitors.

Photo 4 - Bottom view of the completed filter capacitor installation.

Photo 5 - Top view of the completed filter capacitor installation. This photo also shows the new protection relay installed (middle left of photo) on board (X00-1700-11)

 

[EchoWars]

No real need to shrinkwrap the ground jumper.

I have a few rolls of black athletic tape...like the stuff used to wrap barbells and such. It's 2" wide, but I just tear it down the middle to get some 1" wide pieces, and wrap them around the bottom of the new caps so they fit.

 

[turboyam]

The bypass 2.2uf caps you added, do you have to find the polarities for those (outside layers) for this purpose? .
Keep-up the great restoration!

 

[hopjohn]

No real need to shrinkwrap the ground jumper.

Purely cosmetic.

I have a few rolls of black athletic tape...like the stuff used to wrap barbells and such. It's 2" wide, but I just tear it down the middle to get some 1" wide pieces, and wrap them around the bottom of the new caps so they fit.

That's a good idea EW. I'm sure there all sorts of fixes. It gets ugly fast though like when the clamp is twice the size of the cap it's holding, discretion is key.

The bypass 2.2uf caps you added, do you have to find the polarities for those (outside layers) for this purpose? .

If you are refering to this, I've never bothered with it as I've not seen or heard people here making a fuss about it. Maybe someone can shed light on what turboyam is referring to and whether it's truly beneficial in a bypass application such as this.

 

[turboyam]

It is what i was referring to. Thanks for the link by the way! awesome info there!.

 

[EchoWars]

For a bypass cap, orientation doesn't matter WRT the foil. It barely matters when used as a signal cap, especially when the unit has a metal cover.

 

[turboyam]

For a bypass cap, orientation doesn't matter WRT the foil. It barely matters when used as a signal cap, especially when the unit has a metal cover.

Thanks EW

 

[hopjohn]

Disassembly of the front end.

The goal in disassembling the front section of the amplifier is eliminating the front housing. Doing so frees up all three boards for solder work. Elevating the amplifier allows your work table to act as a resting place for the boards. I've again used some nice thick books to do the job. There is probably two dozen or more screws holding all of this together, so be sure and document and/or photograph this step in detail to assist in reassembly.

It's helpful to know that the service manual mistakenly labels the tone amp foil pattern as the preamp. I find it useful to print off these PCB images and highlight each of the electrolytic capacitor joints. Using this as a guide will greatly ease in identifying the old parts as you desolder and eliminating the need to flip the board back and forth repeatedly potentially breaking wire wrap connections. There are 20 electrolytic caps on this tightly packed board.

Tone Amp (X11-1310-00)

Tone Amp Foil Diagram

A view of the tone amp in stock form reveals quite a number of the orange Elna low leak caps. There is also a load of wire wrap connections. Photos are invaluable should one snap off.

The old capacitors are removed and new Wima caps (red) and Kemet (white) replace anything 3.3uf or smaller. There's a couple of 10uf 25v (not room for larger film options here) replaced with Elna Silmic II 10uf 35V and four 47uf (two 10V & two 16V) caps all replaced with Elna Silmic II 47uf 16V.

 

[hopjohn]

Filter (X12-1130-10)

Filter Foil Diagram

Filter board in stock form

The filter board has just four electrolytic capacitors to be changed. All of which are the orange low leakage types. The two stock 10uf 35V are swapped with 10uf 50V Elna Silmic II and two 1uf 50V are exchanged for Wima MKS2 1uf 50V. There are ten 10 Megaohm carbon composition resistors on the board that I just left alone. Their proximity to the heat sensitive polystyrene caps greatly dissuaded me from bothering to change them.

Filter board with capacitors added

 

[turboyam]

. Great job!. Can't wait to do my 8100!.

 

[gort69]

Nice work John - great project.

A few questions -

- What is the benefit of replacing the small electrolytics with film caps?

- What is the downside to replacing an orange low leakage cap with a standard cap?

- I've noticed in some Kenwoods that there are dark orange and light orange caps on the same board? Is there a difference?

Thanks

 

[hopjohn]

- What is the benefit of replacing the small electrolytics with film caps?

- What is the downside to replacing an orange low leakage cap with a standard cap?

- I've noticed in some Kenwoods that there are dark orange and light orange caps on the same board? Is there a difference?

The benefits of film over electrolytic caps has been discussed and debated at length. If you want the long answer do some searching and you'll find plenty of heated discussions. The short answer is that film caps do a better job of blocking DC and that they sound better. When a circuit can benefit and whenever they fit in a reasonable way you can replace an electrolytic (and know I include tantalum in this) with a stacked film.

Modern standard caps, or general purpose caps, have higher CV specifications than caps specified as low leakage like today's Nichicon KL and the orange low leak Elnas and Chemi-Cons used in the seventies . This CV spec indicates a caps leakage properties. The orange caps are known to have had a CV value, under similar test conditions, hundreds if not thousands time lower than modern general purpose caps. (This value is provided in the Pioneer tuning fork guide on capacitors should you want to verify this ) It's known that lower leakage caps were used in key circuits of solid state 70s gear to provide a lower noise floor. Caps of this era called low leakage were often referred to as "low noise" interchangeably. So using a standard cap in place of a low leakage one can actually increase the CV spec of the original gear in these key circuits and potentially increase the noise floor.

The orange and dark orange caps are similar in design. As I spoke on previously, the darker orange caps were made by Nippon Chemi-Con (today known as United Chemi-Con) and were also designed to be low leakage or "low noise" caps. These darker orange caps were Nippon Chemi-Con's LR series and will be designated as such on their jacket.

 

[gort69]

Thanks John

 

[hopjohn]

Preamp (X08-1560-10)

The Service Manual for the Kenwood Model 500 shows a totally different preamp than the one actually used. The schematic is different, the foil pattern is different, and the parts list is different. Everything is different. The SM is completely useless with regard to the preamp. The only way to know what capacitors are used is to pop the hood and do a visual check. Well until now that is. I've done the work for you. I'm providing a list here as well as a photo with all the values given for the 14 electrolytic capacitors used on on this board.

Preamp (X08-1560-10) electrolytic capacitor list :
C13/14 - 1uf 50V
C3/4 - 10uf 25V
C27/28 - 10uf 50V
C5/6 - 47uf 10v
C17/18/19/20 - 47uf 50V
C15/16 - 100uf 10v

Stock preamp board with capacitor values given.

The preamp in the 500 is one of those wonderful type of boards with wire wraps both top and bottom and just for giggles three pots dangling from it. This will limit your ability to maneuver the board to some degree. The two 20K resistors (one is just visible behind the red wires) on the board each have a lead tied to a 47uf 10V capacitor. The one on the left will test your soldering skills. I was damned glad to be left handed working on that one. Have fun righties!

Hello, Margaret. The backside of the preamp. The 20K resistor tied to the 47uf 10v capacitor on the left middle (foil side) of the board is playing peek-a-boo while mocking your soldering talents.

The preamp board complete with new electrolytic capacitors.

 

[turboyam]

You gotta looooooooove board connectors! when high quality of course. It can be great but also bad. Love the ones on a SAE power amp!.
Just got a KR-6600. Removing the amp board is a question of 3 connectors and 4 screws!.

 

[hopjohn]

Kenwood 500 Switch replacement

Parts used:
WARNING : ONLY USE SAFETY CAPS RATED FOR MAINS WIRING!!!!
Here is the safety cap I used here : Digikey: 490-9487-ND

This is the switch : E-Switch P227EE2B20A Digikey # EG1018-ND

It's a good idea to replace the power switch on the 500 since there is a nice replacement available for it. It's the E-Switch P227EE2B20A available from Digikey EG1018-ND. The plunger on my stock power switch had broken off right at the end where the cap goes on so it was going to have to go regardless. The original switch is a bit unconventional so the wiring will be a little different for the new switch. You can get full details about the differences from an old thread by EW, but here are a few photos showing how I wired it with the same protections as the original using two new X1 Y2 safety caps. The reverse side of the new switch has post terminals that will have to be trimmed off and then covered with electrical tape. Doing so will provide a proper fit.

The old switch with the 'NC' (normally closed) contacts in the middle. Notice the broken tip on the plunger.

E-Switch P227EE2B20A Digikey # EG1018-ND (20mm Mounting Spacing)

The new switch all wired up with two X1 Y2 .01uf 300V safety caps.

The switch installed. Notice the electrical tape on the reverse side of the switch to protect from any potential shorts where the terminals were trimmed.

 

[39cross]

This is a really wonderful documentation job, thanks for all the work you've put into sharing your restoration.
How have the sound tests gone in between stages? Is the switch the last step?

 

[hopjohn]

This is a really wonderful documentation job, thanks for all the work you've put into sharing your restoration.

I appreciate it. I hope everyone following along had fun reading and seeing the progress. The restoration has been going on for just over a month now, but is now pretty well complete. I expected this build to be a challenge and in some ways it was, but overall I felt it went pretty smoothly. This was due in large part to researching every thing on the web I could find and making note of everything of interest.

I intend to flesh out more details by editing some of the previous posts so anyone hoping to use this thread as a guide can look forward to more updates. I hope also to create a resources guide.

I'm not big on using superlatives to describe the sound of things as it is so subjective and often meaningless complicated by infinite variables from one listening space to the next. That said, I believe this to be a very special amplifier and after several listening sessions can understand why so many people rave about it (too bad for those that don't ignore that erroneous hifi review in whatever British magazine so many years ago). The 500 has a sound reminiscent of tube amplifiers where the high frequencies are reproduced in such a way that it just sounds right and doesn't fatigue you the way other amplifiers can. The low frequency response, particularly at lower volumes, sounds more musical and controlled to my ears. This 500, despite its minor cosmetic flaws, will be in my stable until they dig a special place for me.

 

[Alobar]

Really enjoyed following along with this thread even though most of it was well over my head. Good on you for saving and returning a great old Kenwood to its former glory!