Kenwood L-07M Major Restoration with BOM

[Will_Emanuel]

Hi all, it's been a while since I've finished this restoration project, I'm just now finally getting around to documenting all the work I've done on these wonderful amps. I will attach a comprehensive BOM to this first post that documents every part I used with component references to match the original service manual. I hope this info will be found helpful to those in the thread that need help finding suitable replacements for various components. This specific pair of amps were purchased in original condition, however the serial numbers were quite far apart between the two and shared minor assembly differences. These amps were eventually matched to have the same components by the end of the restoration process.

 

[Will_Emanuel]

The amps originally arrived to me exceptionally well packed and in good cosmetic condition. They quickly made their way down to the bench for further inspection.

First unit on the bench looked very original when I cracked it open, I couldn’t find any typical signs of previous work or previously swapped components. A brief examination of each unit immediately pointed out a major issue that is incredibly common on these units, that being the ground plane. The large plane is directly attached to the main connector PCB through a tiny pin connector that loves to break off and leave the entire amp without a ground reference, sending many of these units into protection and in quite a few cases, blow their output stage. You can see in the photo below that the small pin has broken off from the large blob of solder below and is no longer making a solid connection with the main connector PCB.

This whole scheme is very flawed by how it is connected to the main board. I will cover later in this thread the fix I use on these units that was passed on to me by @Connor G who was a major driving factor behind me pursuing these amps to begin with.

The output devices in each amp were all original and were individually pulled and tested to confirm they were still intact before proceeding. The bias tracking diode had lost one of its hooks for the mounting screw which is pretty common to see with these style diodes. This diode was also tested before moving forward.

The protection board was addressed first. The focus on this board was the electrolytic capacitors, transistors (more importantly the “outhouse” style TO-92s that are known for causing issues), diodes, and two resistors, one of which being a carbon comp, which are also known for drifting over time. All of the components that were replaced along with the replacement parts that were used can be found in the attached BOM at the top of this thread. One other thing I like to address is the flux that can be found all over these boards. I always like to deflux these boards after reworking them for a much cleaner end result.

Another typical gremlin when working with most of these 70s components is the glue used to hold components down to the boards. This glue should be removed in all cases, as it isn’t uncommon for the glue to eventually become corrosive over time and eat away at any components it comes in contact with.

Here’s the completed protection card after being reworked and defluxed. Along with the defluxing process, I also go through and reflow all joints on the board for good measure.

 

[Will_Emanuel]

Next up was the very dense amplifier card, which had quite a few components being replaced on it. The plan was to do all electrolytics here, transistors (Minus the uPA input package and accompanying FET pair, as well as the very high FT TO-220 driver transistors), Trimmer potentiometers, ceramic caps (In this unit, this isn’t crucial unlike it’s later replacement, the L-07MII, which uses the notoriously faulty “black flag” styrene capacitors), and most of the larger resistors. Those of you with a keen eye may catch the cracked trace in the last photo on the last connector pin. This pin was also a very important ground reference for the amplifier card itself, and caused a lot of troubleshooting headaches down the line until I discovered the cracked trace.

Here is the first amp card after its major refresh. Note in the final assembled photo that four of the original green high power resistors were still installed. I was waiting for replacements to arrive and those remained in place until I was able to swap them. Again, please reference the provided BOM at the top of the thread to see what components I used here. The board was reflowed and defluxed, with the aforementioned cracked trace for pin 1 on the connector was reflowed and the connector pins themselves were lightly polished with simichrome to remove the outer layer of oxidation.

To continue with the restoration, the first unit needed to be broken down to remove the power supply. Taking these units apart is exceptionally easy and straightforward. There are 11 wires that need to be carefully desoldered before separating the power supply from the rest of the chassis. Once the chassis is apart, the power supply board can be removed.

With the power supply out of the unit, there were a few cracks in the PCB around the mounting screw holes, which I have also seen in previous units. No joints were visibly cracked, though a few traces were bridged around the cracks anyways for good measure. The bleeder resistors in this unit run especially hot and are mounted right on the top of the PCB. When replacing these resistors, it’s best to mount the replacements lifted slightly off the board for sufficient airflow as well as to avoid further discoloration and damage to the PCB. The main components to address on this board were the bleeder resistors, as well as the three electrolytic capacitors, a few film capacitors (not really necessary), and the soft start relay. The two large original filter capacitors measured exceptionally well and I did not feel that these needed a replacement. For those who are looking to replace these, I highly recommend upgrading to a 22000uF 80V replacement from Nichicon’s Gold tune range, the LKG1K223MKZF in particular. This is a direct fit replacement to the originals size wise, though the PCB mounting holes will need to be slightly offset to account for the offset terminals on these capacitors. Many others I know have used these caps with that small modification to the power supply PCB with great success.

Here is the power supply board all refitted with new components, as well as a reflow and deflux. Note the top two photos are actually from the power supply PCB from the second unit, as I did not capture a photo of the completed first power supply PCB with the correct replacement bleeder resistors. Another thing to note is the film capacitor C6, which was still original. This can easily be replaced with the same caps as the other film capacitors that were changed on this board, but has a slightly wider lead spacing than the other 4 film caps. I have included a variant of the same Panasonic film caps I used for this project in the BOM that supposedly has longer leads to allow it to fit in place of C6. The finished board was then mounted back into the front half of the amplifier chassis.

 

[Will_Emanuel]

The next step was to remove the main connector PCB from the chassis. To do this, a lot of work needed to be done, with the majority being in relation to the rear panel. In the case of this restoration, I was planning on replacing the RCA jacks as well as the binding posts, so I removed the originals at the same time to make things easier. Along with the RCA and binding posts, the remote power connectors and power switch assembly needed to be removed from the rear panel, as well as the small Zobel network board. I also removed the ground plane from the rear panel at this time, as I noticed the joints holding it to the chassis were also looking rather weak and I wanted to completely reattach it properly before putting the unit back together. Once the rear panel was freed from the rest of the unit, I took the time to install the new RCA jack and binding posts.

The original RCA connector is mounted to this plate and has three separate ground connections. Two of which go to the main connector PCB and the third goes directly to the rear panel of the chassis. The original RCA connector was disassembled and removed from this plastic plate. The hole for the jack was then resized to accommodate the newer jack and the new jack was directly mounted in place of the original.

The new connector was only going to use two ground connections, one to the board and one to the rear panel. The main connector PCB was easily modified to accommodate these changes, I’ll mention this in more detail once the connector board itself is covered. The AECO jacks in particular make this wiring easy to solder into place, especially when using multiple wires to the ground connection on the jack. The rear panel itself was then modified to accommodate the larger cardas binding posts. With the delrin block being used on the new binding posts, the posts line up nearly identically to the original binding posts.

With the new RCA jack and binding posts attached to the rear panel, the zobel network board and power switch and remote assembly was reattached to the rear panel. The wiring between the binding posts and the zobel network were also replaced at this time.

The ground plane was then reattached at this time. In place of the small pin connector that was originally used to connect the ground plane to the main connector PCB, a small run of solid core copper wire was added to connect the plane to the rest of the amplifier, providing a far more reliable ground connection. With the rear panel reworked and out of the way, the focus shifted to the main connector PCB.

The main connector PCB was in for a lot of work. The focus on this board were the electrolytic caps, trimmer resistor, regulated supply transistors, diodes, various high power resistors, and the speaker protection relay. Along with this work, the aforementioned slight modification for the RCA connector needed to be made.

Pins 1,2, and 12 here are all for the RCA input connector, with pins 2 and 12 both being ground. To simplify the wiring from the RCA and only require one ground connection from the main board to the RCA connector, pins 2 and 12 were bridged as shown above.

Here is the finished reworked main board, with new components installed as well as a reflow and deflux. The next step at this point was to get the main board reinstalled in the unit. Before this however, the heatsinks needed a good bath.

With the heatsinks now as clean as they could be, the reassembly process can begin.

 

[Will_Emanuel]

The rear panel wiring for the RCA connector, Zobel network and binding posts, as well as the new ground plane implementation was done at this time, before bolting the rear panel of the chassis back into place. With the rear panel back on, I can really appreciate how these new jacks have completely transformed the rear of the unit.

With the main board being back in place, the output transistors can now be remounted on a fresh set of silpads and the protection and amplifier driver cards can be reinstalled.

With the driver cards reinstalled, it was time for a smoke test and a calibration. So far so good, steady bias and offset, and the regulated supply rail is right where it should be. Once a solid baseline calibration was done and operation was confirmed, I continued with the reassembly process.

One other modification that I felt was necessary on these was the addition of some solid chassis feet to replace the plastic originals. The new feet had to be slightly offset from the originals to avoid interference with the bottom panel screws.

After the unit was completely reassembled, I let it run on the bench for some time to keep an eye out for any issues that might arise. Everything behaved nicely and I went in again after some time to double check calibration and everything was still right where it needed to be. At this point I would typically be celebrating all my hard work, but as these are monoblocks, I was only halfway there. At this point, I turned my attention to the second unit and proceeded down the same path as before. I won’t include too much commentary to go with the pictures for this unit as the process was identical.

 

[Will_Emanuel]

Second unit was in similar condition to the first unit and also did not have previous signs of work.

Here is the final amp card, with the new high power resistors present in this photo. These resistors made their way into the previous unit as well.

Finished protection card, same treatment as the first unit of course.

Main board cleaned up nicely in the second unit

Rear chassis panel rework for new connectors in progress

Bridging grounds for RCA connection as was mentioned for the first amplifier.

More heatsink baths!

 

[Will_Emanuel]

Always clean your output transistors! It's the little details that really make these restorations pop.

Reassembly in progress..

Second power supply board now, this one also suffered cracking around the mounting screws.

Power supply going back together.

All back together, time for testing and calibration.

 

[Will_Emanuel]

And Finally, here we are. Both amps are restored and are functioning and sounding absolutely wonderful. Both units were reworked with the same components throughout and are now both calibrated to their factory specifications. This was the first set of monoblocks I had worked on, as most of the time I am occupied with mainly stereo amps. It definitely required a little more patience than I normally would need as there was twice as much work with these being mono amps, but the end result absolutely made it worth it. Thanks all for reading along, I hope this thread is helpful with those who also have these amps and others who may be interested in pursuing them. These amps are truly special and are some of the most impressive amps to have come across my bench. Once again, a huge thank you to @Connor G for giving me the L-07 bug when I first heard of a set of L-07MIIs that he had restored last year. He was also a huge help when it came to the specifics of restoring one of these and important things that should be addressed such as the ground plane. Ok, that’s enough talking from me. Enjoy some beauty shots of the finished pair!

 

[Connor G]

Superb work as always, really happy to see another pair of L-07’s properly restored for the next 50 years of enjoyment. Your attention to detail and workmanship is on another level and is what every technician should aspire to accomplish. The new owner of these is going to be in love!

 

[lokerola]

Wow man, awesome attention to detail. These look fantastic and I bet they sound great.

 

[Will_Emanuel]

Thanks for the kind words! I definitely don't like to skip any of the fine details in these restorations if I can help it.

 

[claneg]

Hi all,
it's just me that I can't see any of the posted images?

 

[Will_Emanuel]

Hi all,
it's just me that I can't see any of the posted images?

Images are loading just fine on my end. Might be something on your end.

 

[claneg]

Looks like there is a sort of country blocking: if I turn on the my VPN, setted USA, pictures are visible.

 

[Will_Emanuel]

Looks like there is a sort of country blocking: if I turn on the my VPN, setted USA, pictures are visible.

Interesting, All of the images are embedded from imgur. Maybe that is what is blocked.

 

[ghamilton]

Beautiful !

 

[rick85p]

Kenwood Laboratory Series components are refreshed for another lifetime. The best of Kenwood in the late 70s to the early 1980s. Awesome job!

 

[Sam Cogley]

Beautiful work!

Two questions: do you know why Kenwood/Trio original separated the ground connection at the 2 and 12 pins? It appears from the traces that the design originally had them connected, and was later modified to split the connection while for some reason they didn't remove the traces from the board.
Second: why did you use brown power cords instead of ordering some black ones to keep with the totally black theme of the design?

 

[Bman64]

Hi all, it's been a while since I've finished this restoration project, I'm just now finally getting around to documenting all the work I've done on these wonderful amps. I will attach a comprehensive BOM to this first post that documents every part I used with component references to match the original service manual. I hope this info will be found helpful to those in the thread that need help finding suitable replacements for various components. This specific pair of amps were purchased in original condition, however the serial numbers were quite far apart between the two and shared minor assembly differences. These amps were eventually matched to have the same components by the end of the restoration process.

Awesome work, thanks for sharing! Are you PrimeHiFi on YouTube?
When I open this link, it's just a bunch of undecipherable characters. Any suggestions?

 

[analogwisdom]

Awesome work, thanks for sharing! Are you PrimeHiFi on YouTube?
When I open this link, it's just a bunch of undecipherable characters. Any suggestions?

@Connor G is Prime HiFi.

The file attached to the OP is an Excel spreadsheet file, use Microsoft Excel or Google Sheets to view it.