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Sony TAN-5550 Restoration Thread

MartinAudio

MartinAudio

Young Collector
Some time ago, a friend of mine inquired about purchasing a VFET amplifier, and was veering towards a Sony TA-N7b. However due to both their expense and rarity, he decided to go with the more common and easily servicable TAN-5550 model. I've worked on a few of these at this point and know the ropes on what they need, some improvements here and there, and making them reliable. Replacing the "death diodes", noisy black leg TO92 transistors and obsolete package driver transistors, the often poorly contacting speaker relay, and more were all on the radar for this one.

After some time he pulled the trigger on one and had it sent straight to me for a full top down restoration. When it arrived I knew it was going to be interesting for sure! Somebody at some point must have smacked one of the volume pots hard, enough to shear off the shaft and scratch up the faceplate. Overall this unit was in good condition, especially the fake wood side panels that like to delaminate and fall apart over time.
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But on the side of good news, the internals were entirely original! No hack work, poor soldering, or modifications could be seen anywhere. A perfect restoration candidate once the cosmetics were figured out on replacing that potentiometer and knob assembly. More on that later.

After reviewing the ad, it appeared the seller powered the unit on and described it as working, and sure enough the amplifier did in fact work upon powering up. The left channel however was intermittently cutting out, and the sound was dull and murky, not something that should be the case for a TAN-5550, so this unit was definitely tired and needing a proper top down restoration to get it back into performing beautifully. Upon taking off the convienent mesh cover plate, we can see a full compliment of original Sony branded 2SK60 and 2SJ18's! It appears that someone poorly attempted to redo the thermal paste, but used about 3x the necessary amount, and smeared it everywhere. That will all have to be cleaned up and new silicon pads will be put in their place.
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Let's first tackle what Sony calls the Class A board in their service manual. This is a combination board containing the speaker protection relay, a +/- 60v regulated power supply, and the amplifier front end circuitry using two Sony 2SK58 dual JFETs in a differential pair. This is also where your DC offset adjustments are made via the top blue trimmers.
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As discussed before, we were experiencing channel dropoff and this right here was our culprit. This isn't the first 5550 I've seen with what appears to be a defective relay. If you look closely, you can see the contact plates which are meant to be straight are all crooked and misaligned causing poor contact.
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After replacing that with a brand new Omron relay, our dropoff problem went away! But that isn't the only thing we need to do, we are just getting started. In a short order, here is a list of things done to the Class A board.
  1. Replace all electrolytic capacitors with Nichicon low impedance 105C rated parts.
  2. Replace the two trimmers with Bourns 3386-P series parts.
  3. Replace the infamous VD-1221 "death diode" with two 1N4148's in series.
  4. Replace the speaker protection relay with an Omron MY4-02-DC24V part.
  5. Replace all the transistors with equivalent, appropritately chosen On-Semi transistors.
That last one on the list is quite important, as using new parts ensures no future problems can arise which could damage the highly coveted VFETs. This is especially important with the 2SC1124 and 2SA706 transistors as they are known to be noisy and develop leakage, so changing them with TTC004B/TTA004B is a must. After all that work, we have a board looking more akin to this.
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The next job to tackle is the power supply/filter capacitor assembly. This board simply attaches to the existing filter capacitor screw terminals via 4 copper plates attached to the PCB. This board provides your +/- 53vdc for your amplifier assembly, along with a dropping resistor to provide voltage to the green power indicator bulb.
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Besides being caked in dust, this is a bit of trickier issue to deal with, as the filter capacitors are no longer available in this size profile. The diameter of the capacitor actually expands where it meets the bracket, and there just isn't anything like this made by anyone. The .5" screw terminal capacitors on their own are already relatively uncommon, so these are NLA at this point. Because the PCB is lifted and supported by these capacitors, it is necessary to choose the right ones and I was not able to find anything from my suppliers. So a bit of development later, and I designed a new power supply PCB to replace the original, one that can accomodate brand new modern capacitors that are the right diameter.
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New board assembled.
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Vs the original.
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This new board allows multiple improvements to be made over the original, such as increasing the power rating of the power indicator dropping resistor to a 5 watt ceramic, as it can get very very hot and cook the resistor over time. The original part was only rated for 3 watts, so this should last much longer. The new board also incorporates provisions for modern fast diodes, along with more optimally placed WIMA capacitors to filter out any switching noise. Instead of using the cheap wire wrap terminals like they did originally, I am using machined brass terminals made by Mill-Max. The ground terminal originally was attached to one of the capacitor screws, but now it is attached to its own dedicated ground terminal on the PCB.

Next up I'll be tackling installation of the power supply board, rebuilding the power amplifier board, dealing with the cosmetics, and more.
 
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Once the power supply assembling is done, it is only a matter of installing it into the unit and firing it up. In this unit, I opted to replace the old connector as well with new connectors made by Hirose, which are a perfect drop in fit for the originals. Unlike the original connectors, these are keyed and can only be installed in one way, along with having a locking tab so it cannot be easily removed. Overall, these are far better than the original cracked and discolored parts.
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Moving on from that, it's time to get into the most important step, and that is overhauling the amplifier board. This is easily the most time consuming aspect of the job, as there are many components that will be changed out to ensure the amplifier both sounds fantastic and operates reliably and consistently for many years to come.

Here's the board in all of its glory before any work was performed, a completely original example.
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But of course, before I even get to component replacement, let's take care of the thermal goop mess the last tech so kindly left for me. I generally opt for thicker thermal pads for this particular amp as the heat sink is rather cheaply machined and has sharp edges around the TO3 pin holes, and if you use thin pads the sharp edges could cut into the material and short the body of the transistor to ground. That would be catastrophic and is not a chance I'd like to take, even if thinner higher performance pads may work slightly better. Here's half of them redone to show the contrast between new and old.
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But of course, here is the fully monty all cleaned up with new pads, with all that nasty goop cleaned off. Such a satisfying part of the job!
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Now that we got the thermal paste mess all taken care of so it doesn't get all over my hands while handling the assembly, let's get into working on this board. There's a lot to cover so I'll just once again put it in a list.

  1. Replace all the electrolytic capacitors with Nichicon low impedance 105C rated parts.
  2. Similarly to the Class A board, replace all the transistors with modern appropriate devices.
  3. Replace the bias trimmers with Bourns 3386-H parts.
  4. Replace the four VD1221 "death diodes" with two 1N4148's in series.
  5. Replace the carbon composition resistors with 5% carbon film resistors.
Due to the excellent design of this amplifier, it is very easy to remove the amp board and quickly reinstall to verify the work. Since I am replacing so many components, I like working on this board in stages, so if I make any kind of mistakes I can trace back the issue painlessly. All it takes is removing four plugs and a few screws and the amplifier is out. I wish everything was this easy! So I'll post the amplifier progress in the stages, beginning with the caps, trimmers, and death diodes.
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Once those devices are changed, its down to methodically replacing all the transistors. This board had far more black leg TO92 transistors than the Class A board, along with those nasty driver transistors that get noisy and leaky. Out they all go, and in with new modern devices.
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Once all this work has been completed, it is only a matter of installing the new Hirose connectors for the board and cables, and this is about as rock solid as it gets.
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This in my opinion is where the biggest improvement in sound can be heard. Rebuilding this board is essential not just for the reliability although that is a big factor, but it greatly improves the sound of the amplifier and removes a lot of the veil I hear on these before work is done. Now that we are at the end stages of the restoration, a lot of the positive traits these VFET amps have are in full swing and are clearly audible. That airy and detailed top end and well articulated midrange is all there, free of any grain or glare.

Resolving the knob situation was a bit more frustrating than I initially thought upon receiving it. These knobs are unique to this model and are rather small, but I was able to find a set of knobs somewhat close from a Sony STR-313. Not exactly the perfect 1-1 match that I'd like, but it is far more preferable to the sheared off knob and the bare hole left on the face.
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Beyond this point, it was simply a matter of setting amplifier bias and DC offset, running it for an extended period of time, and eventually installing it in the main system to evaluate it and make sure everything was sounding as it should prior to shipping to the client. Adjusting the amplifier is incredibly easy thanks to the easily accessible test points at the top of the amplifier pcb corners, and offset on these is very stable and with new trimmers setting it is a breeze. When these amplifiers are properly restored, they are very stable and effortless to adjust and get in motion.

Next post will be covering stability and sonic impressions in my main system, and some final comments about these amps.
 
Obviously any time a person talks about sound quality of any component, it hinges a lot on the rest of the system's ability to resolve what said component is capable of. So this is just the short list of gear in the system:

Soundsmith the Voice mounted on a Technics SL1200 -> Darlington Labs MP-7 Phono Stage -> Krell KRC-2 Linestage -> TAN-5550 -> Bozak Concert Grands.
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My Bozaks are around 90dB efficient, and are an easy to drive 8 ohm load. In my opinion this is where the TAN-5550 likes to be, these amplifiers are not high current power houses and using one to drive demanding 4 ohm inefficient speakers would not be an optimal match. While these amps are rated for 50wpc, on the test bench I was easily hitting 80wpc numbers before clipping, and at 50wpc the amplifier had plenty of power headroom to support it. These amps are definitely underrated by Sony in their spec sheets.

Where the 5550 excels in is clarity. This amplifier presents a very airy, open, and transparent top end. Typically amplifiers I have heard that have these qualities usually come at the price of being grainy, etched, or just overall unpleasant to listen to on less than stellar source material. This isn't the case with this amplifier, it remains clear, composed, and smooth. Terms like "sterile" or "cold" need not apply here.

On top of that, the midrange performance is nothing to dismiss either. Talk about an engaging, liquid, and present quality. Playing something like I've Got The Music In Me by Thelma Houston really exemplifies the amazing rendering of vocal content this amp can do. It is such a pleasure to listen to and just kept me engaged every time I was listening to anything where midrange shines.

Soundstage width and height is very good, and depth is quite good although it is left a little short compared to some of the higher end tube amplifiers I have had here. But overall, it is very very good at rendering real images when fed a quality chain.

The only complaint I have with this amp is the bass response. Compared to my mighty Aragon 4004, it doesn't have the absolute grip, extension, and slam that the Aragon is capable of putting out. It sounds a bit less dynamic and punchy but what it does produce is tight and clearly articulated. It just doesn't have the last word in bass slam that I've heard from other designs, but those designs don't compare to the 5550 in other qualities, so it is really a "pick your poison" sort of situation.

Overall I love these amplifiers, and I know I am in for a fun listening ride when one shows up at my door and gets the full restoration treatment. Hope you all enjoyed reading my thread!
 
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Very nice work Martin. Glad to see another vfet amp saved.

Some observations:
MartinAudio said:
This new board allows multiple improvements to be made over the original, such as increasing the power rating of the power indicator dropping resistor to a 5 watt ceramic, as it can get very very hot and cook the resistor over time. The original part was only rated for 3 watts, so this should last much longer.
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What is the current in that power button/ lamp circuit?
MartinAudio said:
The new board also incorporates provisions for modern fast diodes,
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The new board is a very nice upgrade indeed. What is the benefit of the fast diodes?
MartinAudio said:
The ground terminal originally was attached to one of the capacitor screws, but now it is attached to its own dedicated ground terminal on the PCB.
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Yes, very nice.
MartinAudio said:
opted to replace the old connector as well with new connectors made by Hirose
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Good upgrade for sure.
MartinAudio said:
I generally opt for thicker thermal pads for this particular amp as the heat sink is rather cheaply machined and has sharp edges around the TO3 pin holes, and if you use thin pads the sharp edges could cut into the material and short the body of the transistor to ground. That would be catastrophic and is not a chance I'd like to take, even if thinner higher performance pads may work slightly better. Here's half of them redone to show the contrast between new and old.
Click to expand...
Yes, better looks and easier installation but as you alluded to already, the thicker the pad, the less heat transfer to heatsinks. I would say that the uglier / messier mica install would trump silicone pads, for both, heat transfer and mechanical integrity.
MartinAudio said:
Resolving the knob situation was a bit more frustrating than I initially thought upon receiving it. These knobs are unique to this model and are rather small,
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let me know if you are still looking for one. I will check my pats bin to see if there is an extra one laying around.
 
rottalpha said:
Very nice work Martin. Glad to see another vfet amp saved.

Some observations:

What is the current in that power button/ lamp circuit?

The new board is a very nice upgrade indeed. What is the benefit of the fast diodes?

Yes, very nice.

Good upgrade for sure.

Yes, better looks and easier installation but as you alluded to already, the thicker the pad, the less heat transfer to heatsinks. I would say that the uglier / messier mica install would trump silicone pads, for both, heat transfer and mechanical integrity.

let me know if you are still looking for one. I will check my pats bin to see if there is an extra one laying around.
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Hey Peter! Love your work, thank you for dropping by. To answer some of the questions:

For the current of the actual bulb circuit, I don't exactly remember what type of bulb the 5550 uses to get a current draw rating for you. I just upped it as the last few 5550s I had here used a 3 watt rated part and both the resistor and the PCB underneath it was discolored from the heat generated, even though the resistor was stood off the board. I figured upping the resistor wattage while still standing it off the board since I was making a PCB anyway was advisable.

As far as I understand the benefit of the fast recovery diodes (MUR460 is what I used here) is to reduce any ringing or switching noise in the power supply.

I currently don't have any sort of thermal sensing devices here, but it would be interesting to compare thermal peformance between generic silicone pads, high performance silicone pads, and mica as these do run quite hot. Mica would definitely be more mechanically stout.

If you have any knobs that would be so helpful! Thank you for chiming in.
 
The bulb specs is in the schematic, IIRC 4.5v @40mA. The voltage drop is about 40v so the power dissipation is about 1.6W. The original resistor is a 2W IIRC. A 3W drop resistor is more then adequate. The 5W is even better but I never saw a 2W fried and I have seen a lot of these amps. What is important is distance from the board. I think the original resistor was mounted further from the board than the new one. Something to think about.

A soft recovery diode would have brought the less transformer ringing you seek. At 60hz I think the slowest diode should be more than fast enough.

There are very few types of synthetic pads that match or succeed the thermal conductivity of Mica. However, they need to be used with caution as one cannot apply the same torque the TO-3 can screws on the silicone pad as it can be done with the mica pads. I dread the goop mess too, but when the mating surfaces are uneven I prefer the mica and goop.....but YMMV. I am not saying you did anything wrong, just sharing my experience and logic behind it.

I am looking for the knobs stash this weekend.
 
rottalpha said:
The bulb specs is in the schematic, IIRC 4.5v @40mA. The voltage drop is about 40v so the power dissipation is about 1.6W. The original resistor is a 2W IIRC. A 3W drop resistor is more then adequate. The 5W is even better but I never saw a 2W fried and I have seen a lot of these amps. What is important is distance from the board. I think the original resistor was mounted further from the board than the new one. Something to think about.

A soft recovery diode would have brought the less transformer ringing you seek. At 60hz I think the slowest diode should be more than fast enough.

There are very few types of synthetic pads that match or succeed the thermal conductivity of Mica. However, they need to be used with caution as one cannot apply the same torque the TO-3 can screws on the silicone pad as it can be done with the mica pads. I dread the goop mess too, but when the mating surfaces are uneven I prefer the mica and goop.....but YMMV. I am not saying you did anything wrong, just sharing my experience and logic behind it.

I am looking for the knobs stash this weekend.
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I appreciate your input greatly, no offense taken. I'm still young and learning the ins and outs of this stuff every day. Electronics is a bottomless pit.
 
MartinAudio said:
I appreciate your input greatly, no offense taken. I'm still young and learning the ins and outs of this stuff every day. Electronics is a bottomless pit.
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Good job with the overall restoration! No pun was intended but meant to give you a different perspective and prep you for future rebuilds.

Consider using something like this:

At some in the future I will share more on that PSU board. There are some additional opportunities there. The last one I did in about 30 minutes and sent to the print house because I needed ASAP, but it can be further enhanced.

Another tip is that these amps are highly sensitive to the power supply caps. CDEs are likely better than the aged originals in the unit but there are other choices out there that would work better. than the CDEs. Kemet ALS series would be a good cap to consider.
 
drumbum said:
Did the sleazebay amp get reported yet?
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It is futile. It almost feels like fleabay clowns are in cahoots with these crooks.
I reported fraudulent listings before and got a denial response along the lines: our automated systems check the listings and there’s nothing wrong with the listing.
 
rottalpha said:
It is futile. It almost feels like fleabay clowns are in cahoots with these crooks.
I reported fraudulent listings before and got a denial response along the lines: our automated systems check the listings and there’s nothing wrong with the listing.
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ain't the beast system grand?
 
MartinAudio said:
This is for Mike, you may know him as Shxxl.
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Oh he finally pulled the trigger. That’s awesome. I’m sure he’ll love it.
MartinAudio said:
The unit got shipped back to my friend so I don't have it on me, but this is the ad he bought it off of.

Looks like that other seller just stole his pictures.
edit: reported the listing. Hopefully Ebay does something about it but I am not optimistic.
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Not only is the listing photo stolen, he’s also using an AI generated description :)
 
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