Recapping a Toshiba C15/M15 Combo

It has been a while since I posted a DIY thread, so I thought I would chronical my latest AK commission: a nice little Toshiba Mini Combo -> SY-C15 Preamp/SC-M15 Power Amp.

These units turned out to perform better than original factory specifications, and are quite impressive little units.

I never do a recap on a unit that is not already performing fully, so, 1st any necessary repairs must be done. Then, the unit is tested lightly, as a baseline to compare against, after recapping. Then, the recap is performed, and extensive final performance and functional testing is performed.

Power amp repairs:

1. The power amp protection circuit cycled on and off, which I found to be some old, cold solder joints in the bias circuitry of the right output channel. A key to this repair was the fact that, the problem was vibration sensitive (it would come and go when the main circuit board was lightly tapped. Reducing the tap strength help to localize the area of the circuit board that was affected. Inspection with a jeweler's loupe in the area showed a few suspect solder joints, and probing on them showed which ones were the problem. Retouching the solder joints solved the problem.

2. The speaker output relays were then found to be just a bit marked, due to arcing. LIGHT burnishing of the relay contacts cleaned them up nicely.

Preamp repairs:

1. Everything worked, except the "Standby" function. The outputs would reduce in volume to various degress, as the "Standby" switch was cycled, but the signals would not fully mute. When the "Standby" switch is engaged, an internal relay shorts the output terminals to ground (after a decoupling resister, of course). The contacts on this relay were found to need a light burnishing. That got it.

I have attached a couple of pics of this little set.

Rich P

Recapping Boiler Plate

First a bit of boiler-plate recapping info:

Note: Most of the original capacitor series used in these old units are long obsolete. Selecting replacements, from current manufacturers and their series, is also a task requiring a qualified tech. There are many parameters to consider (operating temp, value, value tolerance, life expectancy, effective series resistance, internal losses, size, etc.).

I am a Pioneer specialist, so, all my work references the Pioneer parts series. This is because my master recapping database is based on Pioneer part numbers. These Toshibas offered no problem relating the electrolytic capacitors to the Pioneer equivalents...

The problem with most vintage gear (in excess of 25 years old), is the fact that electrolytic caps tend to dry out, since the electrolyte is moist, and must stay that way for proper function. Some caps drift severely, in the lower capacitance direction. Some caps fail open, and simply cease to perform whatever function they were designed in to do. Others that fail, do so by failing short, generally causing catastrophic damage to the unit. You may be enjoying a ticking time-bomb. Some will swell and leak, causing all kinds of corrosion damage to the circuit boards in the area, while others will literally blow up. Some caps measure OK on a capacitance bridge, but, in the circuit, leak DC, causing noise, bias shifting, instability, etc. I truly recommend re-capping gear you intend to keep and enjoy, or sell to someone you care about.

Re-capping generally requires a complete service manual, with all published supplements, addendums, errata, and modification sheets. Each board is gone through, electrolytic caps are each measured to get their physical dimensions (replacements must fit in the space provided. Not to fear normally, since modern caps tend to be 1/3 the size of the originals (sometimes causing other problems)). Then the caps are checked to see which Pioneer series they are in (indicated by Pioneer part number). Now you must attempt to determine what characteristics were inportant to the designers that established a specific Pioneer part number series. Hints come from what cap manufacturer series was used (usually, and hopefully, more than one), "if" you can find the very old data sheets you need (so far all the series I've encountered have been long obsolete), or mfg-to-mfg cross-reference lists. You must also track what kind of circuits the particular Pioneer part number series is used in (coupling, de-coupling, filtering, feedback, etc.). Without the original Pioneer part drawings, you must engage in significant guesswork, supplemented by informed/experienced reverse-engineering.

The next step is to attempt to find modern manufacturers for parts that conform to the design criteria determined in the previous step, followed by locating distributors that handle the replacement parts (in the onesy-twosy quantities you will need), and determine the per unit cost. (Note: I prefer to keep things original. I don't try to second think the original designers, unless the future owner pays significantly for an upgrade or modification. I do tend to use a bit better part than the original, but not to point of using the very expensive, esoteric, sometimes snake-oil parts). It gets to be a lot of fun when the part you need is not made by anybody in the size and/or voltage you need (like finding low voltage electrolytics in the sub-1uf values). Then you need to substitute a different kind of part altogether.

I then created an overall Pioneer database for electrolytic caps, which I can then draw on for other models down the line, and from which, I create a model-specific database, which shows each cap, on a per-circuit-board basis, with all the associated info (original part number, dimensions, value, working voltage, new mfg part number, distributor part number, price, etc. etc. etc.). The database also contains a table of consolidated data, where all like parts are grouped, used as a purchasing list for the model.

Here are some of my choices for the various types of caps that may need replacement:

1. For the low leakage or low noise lytics, I use Xicon LLRL series parts.

2. As to the standard 85degC, 20% lytics, I prefer to use the Panasonic FM series. When they are not available (possible, since this is a new series for Panasonic), I use the Panasonic FC or the Nichicon PWs. They are an incremental upgrade to the originals, in that they are 105degC, lower ESR parts, low tan-theta (relating to internal losses), plus, the Panasonics are pretty, with those gold labels and all.

3. For standard 85degC, non-polars, I use Panasonic SUs. No upgrade here, but there are fewer choices in this type.

4. For the parts requiring low ESR, of course I'm using the Panasonic FM, and FC Nichicon PWs. No upgrades in this case, except for the temperature tolerance.

Emphasis:

Recapping is not the panacea for modern man. Recapping is the last thing I do to a unit, other than cosmetic work and final/performance testing. I won't recap a unit, until I have determined that it is fully functional. Recapping is more for an attempt at adding longevity to the mix. The last thing you want is for a 30-year-old 'lytic to fail short (they also fail open, and drift out of spec). Recapping is not a good "repair" technique (falls in the realm of 'shotgun repair' which I hate).

Rich P

Supplemental Recapping Operation

I have done alot of recapping, and I like to test the old ones every now and then, to see what their condition was. In general, I have found that they are not far off, just at or below the capacitance spec for the parts. I have found a few bad ones. What I add, with a recap, is a retouching of every solder joint on the affected circuit boards. Here is where the real problem is, in my experience: old, oxidized, fractured, and cold, joints that have failed. The life expectance for a wave soldered joint (used in circuit board manufacturing) is right at 30 years. They tend to oxidize, and fracture, etc. I have repaired more problems caused by bad PCB solder joints, than by bad 'lytic caps, by at least an order of magnitude. So, for me, the solder joint retouch is the more important aspect of my recapping service.

Note: Styrol (polystyrene) capacitors are extremely sensistive to heat. They cannot tolerate temperatures above 85 degC, and will be damaged is subjected to higher temps. Soldering involves temperatures in excess of 200 degC. You must clip a heatsink clip between the capacitor and the solder joint when retouching joints associated with styrol caps.

Rich P

The SC-M15 Power Amp

Since this unit arrived with a problem, it was first to to be recapped, after it was brought up to full functionality.

The first pic is of the unit, before opening it up. OK, so I polished the cap and transformer covers first.

The manufacturer's specifications for this amp are as follows:

...Continuous Power Output
......20 Hz to 20 KHz (both channels driven):
.........45W x 2 (4 ohm)
......1 kHz (both channels driven):
.........50W x 2 (4 ohm)
.........45W x 2 (8 ohm)

...Total Harmonic Distortion (THD):
......0.02% (at rated power, 8 ohm)

...BTL/Bridged Mono (20 Hz to 20 kHz, 0.05%, 8 ohms):
......90W

...Frequency Response:
......DC ~ 70 kHz (+0db to -1db)

...S/N (IHF A Network):
......115db

Final, Measured, Performance Data (Post Recap):

...Total Harmonic Distortion (THD) at rated Power Out (both channels driven):
......Left Channel:
.........0.014% at 1kHz
.........0.07% max, from 20 Hz to 20kHz
......Right Channel:
.........0.006% at 1kHz
.........0.024% max, from 20 Hz to 20kHz

...Power Out at 0.02% THD (both channels driven):
......Left: 50.35W
......Right: 50.25W

...Power Out at 1.00% THD (both channels driven):
......Left: 58.16W
......Right: 55.28W

...Frequency Response (at rated power, both channels driven, ref'd to power at 1kHz level)
......Left: -0.16db to +0.01db
......Right: -0.16db to +0.01db

...Signal to Noise Ratio (both channels driven at rated power):
......Left: greater than 80db (measurement limit)
......Right: greater than 80db (measurement limit)

...Bridged Mono Data:
......Total Harmonic Distortion (at 90W output level):
.........0.011%
......Power Output at 0.05% THD:
.........110W

This unit has 2 circuit boards, with a total of 16 caps to replace...

Rich P

Initial Access to the SC-M15 Power Amp

The first pic is of the bottom of the unit. the screws marked 'A' (x4) are removed, to gain access to the solder side of the main board.

The next pic shows the bottom plate removed. The screws marked 'A' (x2), hold the temperature sensing diode arrays to the heatsink/cabinette. The leads are very fragile, so remove those screws 1st. The triple solder pads, marked 'B' (4 places) connect the main output transistors to the main board. Desolder these next, and stand the leads straight up (minimize the bending of these leads, to keep from breaking them). The screws marked 'C' (x4) hold the main board into the unit. Remove those next.

Rich P

SC-M15 Main Board Accessed

Once the screws and solder joints, retaining the main board, have been dealt with, the power switch must be switched to 'ON' and the 3 push switches must be switched to 'off'. then, the rear of the main board must be tilted SLIGHTLY up, and the board must be carefully slid backward, till the switches clear the front of the unit. The main board can then be pivoted backward, pivoting on the wiring to the output connectors.

The attached picture shows the main board, fully accessed.

Next, the power transformer must be released, by removing the screws marked 'A' (x4). Next, the power supply board is released, by removing the board mounting screws, marked 'B' (x6).

Rich P

SC-M15 Power Supply Accessed

Once the screws, retaining the power transformer and power supply board have been removed, the power supply board can be accessed. This is where experience and a sensitive hand pays off. You have to gently work the board free, tilting and shifting the board around to it is free, while also lifting the power transformer out of the way. It is tricky, but it will come free. The power supply board can then be pivoted up, on its transformer wiring.

Rich P

SC-M15 Recapped

The attached pic shows both, the power supply and the main board recapped.

First, the main supply caps are interesting, triple leaded, press-fit retained, short caps. While I could have made newer caps work, I would have had to drill the ciruit board and grind off some foil. Since the caps were not swollen or leaking, we opted to not replace these.

Next, you will notice that the caps that were replaced are mostly 1/3 the physical size of the originals, even though the capacitance and voltage values are identical to the originals (wonders of modern science).

The unit was then reassembled, reversing the aforementioned procedure.

Rich P

The SY-C15 Preamp

This unit was next to be recapped, after it was brought up to full functionality.

The first pic is of the unit, before opening it up.

The manufacturer's specifications for this amp are as follows:

...Total Harmonic Distortion (THD):
......0.01%

...S/N (IHF A Network):
......106db

...Frequency Response:
......10 Hz to 100 kHz (+0db to -2db)

Final, Measured, Performance Data (Post Recap):

...Total Harmonic Distortion (THD), 20 - 20kHz, both channels driven at 0dbV:
......Left Channel: 0.0068%
......Right Channel: 0.0067%

...Signal to Noise Ratio (both channels driven at 0dbV):
......Left: greater than 80db (measurement limit)
......Right: greater than 80db (measurement limit)

This unit has 3 circuit boards, with a total of 17 caps to replace...

Rich P

SY-C15 Top & Bottom Removal

The attached pic shows the bottom of the unit.

The top of the unit is removed, by first removing the knobs, labelled 'A' (x3). Then, the screws, marked 'B' (x4) are removed, and the top is slid off of the unit, toward the front.

The bottom of the unit is removed, by removing the screws labelled 'C' (x6). Then the bottom is slid off of the unit, toward the front.

Rich P

SY-C15 Top Removed

The first pic shows the unit with the top removed.

To fully access the solder side of the main board, the shield, marked 'A'. must be removed. The 2 pinned, quick release fasteners, marked 'B' (x2) are removed, and the shield can be removed.

The next pic shows the fully-accessed, component side of the main board. Pleae notice the 2 'lytic caps that are butt-up-against 2 hot heatsinks. This is bad news for cap longevity.

The next pic shows the fully-accessed bottom side of the main board.

Rich P

SY-C15 Tone Board Accessed

Unfortunately, there is a single 'lytic cap on there, to replace.

The first pic shows the top of the tone board. First, note where the jumper wires, marked 'A', are connected, then unsolder them. Then, un-solder the connections to the 3 slide switches, marked 'B' (x3). The tone board can then be lifted and pivoted on its wires.

The next pic shows the fully-accessed tone board.

Rich P

SY-C15 Recapped

The attached pic shows the main board recapped.

Again, you will notice that the caps that were replaced are mostly 1/3 the physical size of the originals, even though the capacitance and voltage values are identical to the originals (wonders of modern science). This nicely served to get some space clearance between those 2 caps and the hot heatsinks (noticed before recap). Also, the new caps will tolerate 105 degC, instead of 85 degC, so we are better off here as well.

The unit was then reassembled, reversing the aforementioned procedure.

Rich P

Thanks!

I really appreciate your time and effort restoring these obscure beauties, as well as making that writeup! I am looking forward to getting them back and running both my amps in bridged mode. I will post some pics when I get them back and they are in action :thmbsp: Thanks Rich!!!

Very informative. Most of it over my head, but still a great resource and fun read.

Can't wait to hear the difference from a recap and not recapped amp at Aarons.

Gary

A great read, Rich - thanks for the excellent series of posts!

Rich, You are a Wild Man!

Great Job Rich!
Both in the work done and the documentation. Im with Gary in the fact that it's over my head but reading and seeing a step by step is quite helpful. The concept is starting to sink in.

RC

Thanks to Rich's great documentation, I have been able to complete my 3-ring binders on these little guys. I have a 3-ring binder dedicated to each unit, comprised of all documentation including the original owners manuals, the service manuals, quotes from Rich, all work done as well as his writeup and photographs. Its neat to look at, is good to reference and should I ever decide to sell it, it should greatly help the selling value.

Rich, could please I have your comments in regards to replacing elecrolytics:

I have noted that you replace caps with new ones having the same voltage rating and capacitance value. Generally this must mean bending leads to meet the original holes and pads. As a result the base of the cap will not fit flush to the board. So I am wondering what you think of using caps that are of higher voltage rating than the original thereby coming closer to, or even being exactly the size of the original? And IF this is OK, then just how much more of a voltage rating would be OK?

This is an important question for me. In previous times, I have used the same or as close to the same voltage rating as the original — and always the same capacitance value. But for my current project, I have already purchased caps and selected them for — (a) the original capacitance, (b) higher temperature rating (105 as opposed to 85) and (c) physical size. The latter (c) factor means a higher voltage rating in virtually every case. But, the caps do fit directly into the original holes beautifully.

Not having your experience and so on, I did not choose for more detailed characteristics. But I trust that in most cases the ESR specs for the new caps will be either roughly the same or better. And BTW, the originals were nearly all some 3 flavours of 30 year old Elnas.

I am eager to read your answer as I have pondered this for some time — thanks in advance. ... Lorne