Sansui SM-88 rehab

[mrwood200]

Hello all!
I am getting ready to rehab my Sansui SM-88 integrated amplifier & would appreciate any input!

I am planning on a full recap w/any appropriate upgrades (e.g., add safety cap & thermistor)
I would welcome any comments, but specifically have some questions on the power supply:
- Any suggestions on improvements? The filtering looks a little light, especially the 1st 2x 20u caps.
- Looks like the power transformer has a 120V tap; I will look to use this.
- I was planning on replacing the diodes, but these are in the bias circuit & sounds like it is OK to leave as is.

This should end up being one of my better amps, so I will try to do a good job on the rehab. Looks like I will end up use the AUX input & will focus on this signal path. Thanks for any input!

 

[Tim D]

I'd look at replacing the power supply caps and maybe the coupling caps. I see that the heater supply is using a selenium bridge rectifier. That might be a consideration for replacement, but you'd want to add a dropping resistor since silicon has lower forward resistance than selenium.

I'd check all the tubes and measure all the resistors in-circuit. Any that read high by more than say 10% should get replaced. I'd clean all the switches and pots.

 

[Mr Boochie]

Can you post a photo of the wiring side of the chassis? Sometimes, these older Japanese units have notoriously leaky (DC) coupling caps. You might want to check that and replace, as needed..

 

[mrwood200]

Can you post a photo of the wiring side of the chassis? Sometimes, these older Japanese units have notoriously leaky (DC) coupling caps. You might want to check that and replace, as needed..

I'd look at replacing the power supply caps and maybe the coupling caps. I see that the heater supply is using a selenium bridge rectifier. That might be a consideration for replacement, but you'd want to add a dropping resistor since silicon has lower forward resistance than selenium. I'd check all the tubes and measure all the resistors in-circuit. Any that read high by more than say 10% should get replaced. I'd clean all the switches and pots.

Thanks - all caps & selenium rectifier to be replaced as part of this project. I will check resistors as well. I plan to leave the cans & rectifier in place, and wire in replacements underneath (see photo):

Not too bad underneath! Got some room to work. Overall, the amp is in very good shape.
I feel fairly confident in recapping/replacing resistors per the schematic, but would benefit from any advice for 'common-sense upgrades' from schematic (e.g., power supply filtering, bypass caps).

 

[Mr Boochie]

Check for DC voltage leaks on those gray non-polar coupling cap. The look poly types --- so you maybe good to go.

Unit looks really clean!

 

[kvflyer]

I have done a few Japanese amplifiers/receivers over the years. Most of the time, the gray Elna, Suzuki etc. capacitors, which are in an aluminum can, resembling electrolytic capacitors (they are not) are leaky. Also, those low value electrolytic capacitors, often cathode bypass, are literally open. Slow and methodically, you should be rewarded with excellent results.

 

[Mr Boochie]

I have done a few Japanese amplifiers/receivers over the years. Most of the time, the gray Elna, Suzuki etc. capacitors, which are in an aluminum can, resembling electrolytic capacitors (they are not) are leaky. Also, those low value electrolytic capacitors, often cathode bypass, are literally open. Slow and methodically, you should be rewarded with excellent results.

Yeah. It maybe better to just replace them all with new quality poly caps. Especially, if the unit has been sitting for a long time. Caps guts maybe all dried out and brittle.

 

[kvflyer]

Yeah. It maybe better to just replace them all with new quality poly caps. Especially, if the unit has been sitting for a long time. Caps guts maybe all dried out and brittle.

To agree, low value electrolytic capacitors (which the Japanese used; most USA manufactures did not) are dried out and essentially "Open". Paper in oil coupling capacitors are usually so leaky that they may destroy the output tubes or even the output transformers. Caution is advised.

 

[Mr Boochie]

Japanese PIO caps seem to have the shortest half-life of any coupling cap, of that era. They are notorious in heated, tube amp conditions.

 

[TriodeLuvr]

I am planning ... appropriate upgrades (e.g., add safety cap & thermistor)

Safety cap? What's that?

Looks like the power transformer has a 120V tap; I will look to use this.

If this is an import model, it will already be set to 120V. I just enlarged the underside photo - something looks odd about the primary wiring. The 120V tap is connected to the fuse, but the 100V tap appears to be connected to one side of the accessory outlets. Maybe it's a trick of the light. Be sure that fuse isn't exposed once you start using the unit. It should have had a cover from the factory.

Jack

 

[Mr Boochie]

Might be nice to put 20mfd/500VDC rated electrolytic caps (3) in the power supply rail off the 5AR4.

 

[TriodeLuvr]

I think B+ in this will never exceed about 300V. 350V caps with a 400V surge rating should last a long time.

Jack

 

[Mr Boochie]

I think B+ in this will never exceed about 300V. 350V caps with a 400V surge rating should last a long time.

Jack

I suppose. But, with PSU cap, I like a little overkill. As long they fit. It's too bad that schematic doesn't have nominal voltages.

Usually with 5AR4/GZ34 as the rectifier, I go 500VDC rating.

 

[TriodeLuvr]

Most units with a 5AR4 need its higher voltage/current capability. This amp could easily have used a 6CA4 or other less robust rectifier tube. The Stromberg ASR-120 is a good example of this (6CA4 powering two push-pull 6V6 channels). I've seen a few other examples of smaller amps like this one with a 5AR4, but it always seems like a mismatch to me.

Jack

 

[mrwood200]

Safety cap?....Be sure that fuse isn't exposed once you start using the unit. It should have had a cover from the factory.

Oh yes, gonna need a fuse cover!
Anyone know where I can get one for this style? I have looked and no luck so far.

Safety cap = X1/Y2 Line filter cap per http://www.justradios.com/safetytips.html

Thanks for the comments on the power supply -
I will check on the PS transformer wiring, but pleased to see that I have a 120V tap.
I do want to keep the general OEM design, so I plan to stay with the 5AR4, but interested in any common upgrades. When comparing this PS to other 5AR4 power supplies, I have seen others use 47u caps for the 1st 2x filters. However, I am hesitant to make any changes, especially without in-circuit voltages on schematic. I may play with PSUD or build per schematic, take some notes, then try any upgrades.

 

[TriodeLuvr]

Safety cap = X1/Y2 Line filter cap per http://www.justradios.com/safetytips.html

I have many years experience with both audio and RF, and I strongly disagree with that article regarding capacitors connected to the AC line. First, neither a cap across the line nor a cap to ground is a "safety cap." In fact, a capacitor from the line to ground is exactly the opposite, often referred to as a "death cap." Don't do it! A single capacitor across the AC line might be beneficial in suppressing RFI, but if you have a nearby source of RF strong enough to create a response in a tube-type stereo amplifier, what you really need is a RF line filter.

As for safety upgrades, all these older units should have a three-wire 18 ga line cord installed. In the case of your unit, wire it according to the schematic below. Note that the switch is moved so that's it's after the auxiliary outlets. You don't want equipment plugged into those outlets to stress this hard-to-get component. Also, the capacitor across the line is optional. I would also replace the fuse holder with one that can be mounted and wired inside the chassis. there's plenty of space for the right type.



Jack

 

[kvflyer]

Oh yes, gonna need a fuse cover!
Anyone know where I can get one for this style? I have looked and no luck so far.

Safety cap = X1/Y2 Line filter cap per http://www.justradios.com/safetytips.html

Thanks for the comments on the power supply -
I will check on the PS transformer wiring, but pleased to see that I have a 120V tap.
I do want to keep the general OEM design, so I plan to stay with the 5AR4, but interested in any common upgrades. When comparing this PS to other 5AR4 power supplies, I have seen others use 47u caps for the 1st 2x filters. However, I am hesitant to make any changes, especially without in-circuit voltages on schematic. I may play with PSUD or build per schematic, take some notes, then try any upgrades.

Instead of a fuse cover, you might just move the fuse holder inside of the chassis. It appears to have enough room.

 

[Lavane]

Be careful of those gray resistors also. The end caps can break off easily.