Sansui 1000A redplating!

[luRaichu]

Someone left this amp on the curb… It had a blown fuse but other than that it started alright last night. It has trouble bringing both speakers up and for the most part I was only getting audio from either the left or right speaker. Pots are super flaky!!! They need DeOxit.
As you can see in the pics at least one of the largest tubes are redplating. That’s a scary dark spot.
I have never owned any vacuum tube equipment before, all my TVs and hi-fi amps are solid state with printed circuit boards. I successfully recapped a few computer motherboards (surface mount & through hole) so I do have soldering skills. Just would like to know how to proceed with restoration… Thanks

 

[BmWr75]

That is a great find and great tube receiver. DO NOT power it on again!! You run the risk of destroying the output transformers which would be a huge loss.

It needs a bunch of work to operate safely. All the capacitors probably need replacing, new power tubes needed most likely (two power tubes closest to front panel are shot based on the ashy looking getter flash) and the bias most likely needs to be adjusted. I picked up a 1000A last year and the bias pots had never been adjusted, were still sealed in place with red paint.

Search the Tube audio forum for "Sansui 1000". You will find threads on the 1000 and 1000A.

 

[TriodeLuvr]

That's a shame. I hope you didn't replace the fuse with something larger in order to power it up. These old amps must be checked internally for proper voltages around the output section, and certain capacitors must be replaced before the unit is powered up.

Jack

 

[luRaichu]

Here’s a better look at the tubes… now that they’re cleaned up I’m not sure whether they’re cooked or not. Notice how the redplating tube darkened the glass a bit.

 

[BmWr75]

Here’s a better look at the tubes… now that they’re cleaned up I’m not sure whether they’re cooked or not. Notice how the redplating tube darkened the glass a bit.

You are a long way from needing to worry about whether the power tubes are good or not. There is much other work to be done before power tubes become a concern.

As @TriodeLuvr said, if you installed a larger than recommended amperage fuse there is a good chance you cooked something in the amp. There is usually a good reason for a fuse blowing and protecting components in the amp from frying.

 

[gadget73]

The caps are almost certainly toast. Those grey caps are notoriously unreliable. It may or may not have destroyed the tubes.

 

[Catmanboo]

You lucked out, getting the later model 1000A with the 4 bias pot version. You likely have at least 2 toasted tubes, the 2 in the foreground of the 7591 quad. Note tannish- color of the internal flashing. They may still test good or marginal, but condider a new matched set-AFTER repairs are made. Examine closely the underside. Note all the gray Suzuki & Elna capacitors. They are oil-soaked paper types, most likely leaky as hell & often what cause red-plating of the output tubes & can kill output transformers if not remedied. Add to that the power & bias supply electrolytic capacitors, being better than a half-century old will have to go also. Even in it's current condition, quite the find! Hope it came with the steel case, but if not, no biggie - except that's where the serial number is, if I recall. I have the same unit, no serial number anywhere to be found.
Should you elect to tackle the restoration, lots of info available. To farm out, the cost may be prohibitive. Good luck with it!

 

[Tom Bavis]

These Japanese caps were trouble by the '70s and are much worse now. A friend of mine has a 1000A he got cheap; one of the 7591 tubes has a hole melted in the glass.

 

[TriodeLuvr]

These Japanese caps were trouble by the '70s and are much worse now. A friend of mine has a 1000A he got cheap; one of the 7591 tubes has a hole melted in the glass.

I was really glad they were all out of warranty by the time I had to deal with them on the bench back then.

 

[Mr Boochie]

Change all the main electrolytic power and bias supply caps.

Change all the coupling caps. Old Japanese ELNAs have a short life span. Buy four or five of the proper fuses.

You should have a dim-bulb current limiter and Variac for the first ramp-up. With the tubes removed, measure the voltages in the power supply to be sure you are in range (maybe 20% higher due to the higher outlet voltages and no tube load). Report your results.

Good luck!

 

[luRaichu]

Yeah, you’re right, it’s about time I get a variac. What wattage of lightbulb do you recommend?
How much do the caps cost? What type, brand should I use? I’m an extreme cheapskate and so far Jim McShane hasn’t replied to my email about his cap kit for this amp. Might have to buy caps from DigiKey.
While testing, I used an unknown amperage fuse that was laying around. I just wanted to see if the amp worked or not.

 

[BmWr75]

Yeah, you’re right, it’s about time I get a variac. What wattage of lightbulb do you recommend?
How much do the caps cost? What type, brand should I use? I’m an extreme cheapskate and so far Jim McShane hasn’t replied to my email about his cap kit for this amp. Might have to buy caps from DigiKey.
While testing, I used an unknown amperage fuse that was laying around. I just wanted to see if the amp worked or not.

Jim McShane is in the process of retiring and have never seen him mention carrying kits for Sansui tube amps.

Tube Depot and Antique Electronic Supply are two web vendors that carry lots of capacitors for these amps. Suggest you read the values off the existing caps in the amp and work up an order.

Sticking just any old fuse you had on hand in the amp for test purposes was not a good idea. Hopefully no other damage was done.

 

[gadget73]

I use a lot of very basic yellow polypropylene caps in things. Nothing fancy, Vishay 716p or Illinois MPW are both fine and not expensive. Th he electrolytic usually end up any name brand from Mouser that fits the bill. Can caps are a clamp in, JJ and F&T both make that style for reasonable cost.

 

[Mr Boochie]

Yeah, you’re right, it’s about time I get a variac. What wattage of lightbulb do you recommend?
How much do the caps cost? What type, brand should I use? I’m an extreme cheapskate and so far Jim McShane hasn’t replied to my email about his cap kit for this amp. Might have to buy caps from DigiKey.
While testing, I used an unknown amperage fuse that was laying around. I just wanted to see if the amp worked or not.

100 to 150-watt incandescent bulb.

https://robrobinette.com/5e3_Modifications.htm#Light_Bulb_Current_Limiter

 

[kvflyer]

I have one of those. The redplating of one output tube is most likely caused by those gray Suzuki coupling capacitors to the 7591 output tubes. In my amplifier, I was fortunate enough to have replaced those capacitors before first power up. I have had it for decades now. Here is a picture of the output tube grid coupling capacitors. You can see wax has dripped out of one the output transformers from prior use. The partial schematic is showing the suspect capacitors.

 

[Mr Boochie]

I have one of those. The redplating of one output tube is most likely caused by those gray Suzuki coupling capacitors to the 7591 output tubes. In my amplifier, I was fortunate enough to have replaced those capacitors before first power up. I have had it for decades now. Here is a picture of the output tube grid coupling capacitors. You can see wax has dripped out of one the output transformers from prior use. The partial schematic is showing the suspect capacitors. View attachment 3451404

View attachment 3451413

Were all those caps only rated at 250VDC? I'd replaced them with 450 or, better yet 600VDC-rated poly film caps

 

[kvflyer]

I also replaced those early "M&M" silicon rectifiers because the construction was in its early stage and they may not be as reliable as modern diode rectifiers such as the 1N4007 (1A, 1000VDC) or 1N5408 (3A, 1000VDC). The replacement diodes are very inexpensive today. Note the exploded capacitor just right of the diodes!

 

[kvflyer]

Did you decide to keep the carbon comp resistors in the power rail, or was that the owner's decision?

Just a query. Thanks!

Were all those caps only rated at 250VDC? I'd replaced them with 450 or, better yet 600VDC-rated poly film caps

This picture above ^^^ is a Scott amplifier.

But in the Sansui 1000A, I just looked at the Service Manual and those 0.3µFD capacitors are 250VDC. I don't even own any 250VDC capacitors because they are just about the same price and size as 630VDC capacitors. Therefore, I agree with you, of course.

 

[gadget73]

Probably one high volt and one low volt. On the cathode side f the inverter it sees a lot less voltage, and it will never see the turn-on voltage spike. Plate side needs to deal with more. I've seen this in a lot of other gear too. The lower volt caps probably saved a penny or two per unit. I don't usually use different voltages when I rebuild things but thats partly for cost on my end. I just buy quantities of 630v caps rather than smaller amounts of multiple voltages so i get a price break. A 630v cap will happily sub for a 150, and often they aren't much different in size anyway.

 

[TriodeLuvr]

Probably one high volt and one low volt.

They are all 250V in the parts list. Maybe that contributes to the short life span of these parts.

Jack