Sansui 1000A Repair

[Agorax]

Hello there, I recently inherited a 1000A tube receiver, I have replaced all the Elna and Suzuki oil caps, and have extensively cleaned all the controls.

The recap / Deoxit restored proper function to a lot of the tone controls, inputs, balance, and other functions.

However I have a weak right channel, it sounds a bit staticy and seems to be about 20% sound strength, compared to the left channel.

I have not messed with any tubes yet, any info on tubes and possibly what certain tubes on the unit actually do would be greatly appreciated. I have never worked with tubes, I mainly have owned 70s Marantz 2xxx 4xxx units.

All functions and inputs seem to be working, however my right channel is consistently weak. The stereo reverse function seems to work, I feel like that may be telling us the sound is processing correctly, just that the receiver can't deliver proper sound output to whatever channel is assigned to the right speaker.

 

[blhagstrom]

Hopefully it’s a weak tube and not a bad output transformer.

You can swap tubes side to side to check to see if the problem follows a tube.

The best thing is to have a tube tester.
It’s near required to own and maintain and repair tube gear.

 

[Agorax]

The transformers look original so I hope they're okay too. Im currently looking for information about the different tubes in the 1000A and what jobs they do.

 

[larryderouin]

The Service Manual has both the Schematic and a top layout diagram. Fairly easy to discern what tube does what from them. The most likely suspects are the 6AN8's right in front of the OUTPUT Tubes. Swap them left to right. If the lower level follows the tube, get a pair of them. The other pairing are the 12ax7's in the Front of the unit. These are the pre-drivers. Again, swap them L & R.

 

[larryderouin]

V1,2,3 are your RF AMP, Mixer and Oscillator for FM.
V4,5,6,7,10 are the IF Tubes and Discriminator for AM/FM
V8, V9 are the AM RF Tubes.
V11,12, 13 are the FM Multiplex Tubes.
V14, V18 are Predrivers (Audio Amp).
V15, V19 are Driver/Phase inverter
V16, 17, 20, 21 are the output tubes.

 

[Agorax]

Wow Larryderouin, that is extremely helpful, thanks for taking the time to share that info. Next I am going to thoroughly clean and Deoxit all the tube contacts.

When I initially powered the unit on (fresh out of the attic without any checking or cleaning) after a few minutes V20 started red plating (I assume this is the term used when most of the tube glows bright red). After a cleaning and partial recap this has never happened since, and I've had the unit powered on for a few hours at a time.

Now that I know a bit more about the tubes I will look into things further and share what I find.

 

[slimecity]

Wow Larryderouin, that is extremely helpful, thanks for taking the time to share that info. Next I am going to thoroughly clean and Deoxit all the tube contacts.

I recently had a hum on one channel of a little tube amp and what fixed it was me going thru all the valve pins with mini bottle cleaner brush soaked in contact cleaner. After that, I then prised the sockets closed with a tiny screwdriver - effectively making them tighter.

 

[Agorax]

Swapping those two pairs L+R didn't change operation at all, the right channel is still consistently weak. Im hoping it's output tube related, and not a transformer.

Cosmetically, the output tubes look very cloudy and tired compared to the rest of the crystal clear smaller tubes. (And like I said above, one has red plated in the past) I'm sure they've seen a LOT of heat as well.

Removal looks like simply undoing the 7mm ish nuts for the clamps and pulling them out.

Should I swap the output tube pairs left to right?

 

[larryderouin]

DO NOT LOOSEN the Spring Clamps. Push down on the clamp with the TUBE COLD, It will release from the tube and you can use your free hand to lift tube out. You can try the output tube swap, If you haven't installed them, install 10 ohm cathode resistors, and 100 ohm screen grid resistors. You can measure ma draw for each tube across the cathode resistors to help determine bias settings, and to balance out the output's.

I'd bet on the toshiba's being Extremely tired.

 

[Agorax]

I see, glad I waited for input before trying. After pushing the spring clamps down they come right out, I'm cleaning the contacts and swapping them now. Thanks again for the information.

Edit : After switching output pairs L+R, my right channel is still weaker than the left, however the left has developed some static, especially before it warmed up, almost constant static sounds.

Also the bass response on my previously good left channel has degraded, not coming in as well and a bit scratchy.

The bass response on the right channel, despite the channel being weak, has changed to very tight and rocks the house now like my previous left channel once did.

Extra Edit : I have not replaced the 4 large can capacitors on top. All four are Elna, two of the can capacitors are easy to read, 200uf 350v, however the two metal finished caps read :

Cap.20-20-20MFD 500v, 550 surge Cap.20-20-20-20MFD 350v These styles of cap markings I have not come across before.

Last Edit : I just learned these are called multi section capacitors and may not be as simple as replacing standard two terminal caps.

Once I figure out how many capacitors I need for the multi section caps, I think I plan to install modern capacitors underneath the chassis leaving the originals in place up top.

 

[larryderouin]

No Room for separates. It's crowded like any store 4A.M on Black Friday in there now. www.tubesandmore.com (AES) has a 4 section CE BRANDED 22uf 450V drop in, also for the 3 section.(See links below.) The 2 Singles, bump up to 470uf @ 450V. You can use Nichicon Radials and stuff the cans for the singles. These take a lot of abuse.

4 Section https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-20202020-f-electrolytic

3 section https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-202020-f

Larry

 

[larryderouin]

Try swapping V15 and V19 (6AN8's). One usually goes weak before the other. Don't just replace the one. Get a pair of Matched tubes from a reputable supplier. Jim McShane is one of the best out there. And his prices are in line with others who don't test tubes as well or at all.

 

[Agorax]

Unfortunately after replacing both multi section capacitors and both the large 200uf cans on top, I am now blowing the 4a main fuse immediately when pressing the power button.

I will regretfully have to look back over my work however I took my swell time doing this.

By the way - I did not purchase direct replacement multi sections, I purchased 22uf replacement Nichicons and installed them using a common negative.

 

[larryderouin]

the 200uf cans are wired in series, and IIRC are isolated from chassis ground. If it's blowing the 4A fuse, it's probably due to the cans being shorted to chassis. Take a look at them 1st.

 

[Agorax]

It's on. Second cap in the "series" was NOT in series, flipped it around.

Sound was good, (still weak right channel) however after about 5 minutes of light use a Vishay Sprague 25uf near the two big cans went Chernobyl.

Replaced the Vishay with a Bipolar Nichicon I had, I also installed 1/4 10 ohm resistors on pin 5 of the output tubes to ground.

Receiver is stable as of right now.

 

[Agorax]

Larryderouin, my right most transformer does not get warm like the other two. Pin 3 on all four of my output tubes reads 485V.

What supplies this high voltage? Could it possibly be a sign that the transformer could be okay, just not working in conjunction with something else?

I've also learned this is a four pot unit, and I've replaced the three diodes around the power supply area with UF4007's.

Any further advice you could share would be greatly appreciated.

 

[larryderouin]

Plate voltage on pin 3 is supplied by the Voltage Doubler CAN CAPS (the 2 in series.) Pin 3 on each pair of tubes feeds to a Output transformer, which is fed off point "W". Point "W" comes off the junction of th 2 200uf 350v can caps and The SD-1A diode. This is the Voltage Doubler.

You need to measure the bias voltage on pin 5 on each output tube and adjust the 4 pots so that they are all reading approx 32-34ma (voltage across a 10ohm 1/4w resistor off pin 5 to ground (you install them)). 320-340mv is starting point on the 1000a. Across the resistor means one probe (mini grabbers)on each end of the same resistor. The controls are interactive somewhat so you need to do this a few times to get them all lined up. Sounds like the Right side might be running "COLD". 32ma @ 485V is just over 80% of max dissipation. 31.5ma (315mv) would put you under 80%. That's not accounting for 3ma for the screen dissipation. Subtract that and you get an actual dissipation of 29 to 28.5ma or 14.06W(29ma), to 13.825W per channel. Bump it up to 34ma plate and get 31ma(after subtracting 3ma for the screens) for 15.035W per channel.

 

[Agorax]

Thanks for the always helpful information, however I'm having trouble getting a reading on my multimeter. I have been trying the 200mv, 2V, and 200mA settings with no luck.

I was measuring with the volume all the way down and no signal to the receiver.

Edit : Using the 200mV setting I believe I am getting erratic readings ranging from 0 to 5. It's dancing all over.

Placing the probes directly on the ground connections and tube socket pins is giving me the readings. Placing my probes directly on each side of my resistors gives me no reading.

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Recommended settings for setting all amps bias/offset. All SOURCES OFF or DISCONNECTED! NO FM or AM!

So;
Volume Down all the way.
Tone controls Centered.
Loudness /presence OFF
FUNCTION SELECTOR TO AUX. NO SOURCE CONNECTED!

On the other thread in Tube forum I posted the only thing I could find on the 6P14P family, data sheet wise. IIRC they are 14W tubes? Someone else correct me here if wrong. But 14w stick in my craw for some reason.

I normally set my amps for 75% of max rated dissipation. Haven't done 7189's yet. So we'll use 14W as an example. If it turns out different substitute the new number for 14.

with anode voltage of 450V
52ma two tubes are making 23.4w WAY TOO MUCH!!!!
39ma one tube is making 17.55w Still too much
36ma one tube is making 16.2w. Still too much

STILL USING 14W as an EXAMPLE 75% of 14 = 10.5
23ma will get you 10.35w per tube.
31.5ma will get you 14.175
31.ma will get you 13.95 (which if the tube is 14w tube is max diss)

23-26 is probably the safest range for the tube based on 14W MAX for each tube. The Voltage will probably climb to about -21v. That's ok. You don't want it to go DOWN toward ZERO any more than about -15v. And it'll be different for each of the tubes. Right now you have basically the same voltage and your dissipation is all over the place.

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Found this helpful post by you, I'll keep working on this.

 

[larryderouin]

Change the batteries in your meter as a start.

The 7591's (not 7189/6p14p-EV) have a max dissipation of 19W. 75% of that is approx 14.25W. 80% is 15.2W.
At 75% with 450v on the plates, you'll probably be right around 31.75ma (right off the top of my head.). Subtract 3ma for screen dissipation and your actual wattage would be around 12.93W. You CAN bump the ma up initially and subtract out the screen dissipation and get it back up to 14.25W or so. Mine runs at 480V on the plates and 34ma for 16.32W BEFORE subtracting the screen dissipation. 31ma gives me 14.88W per channel which is well within 75%-80% range.

 

[Agorax]

My replacement 50w heatsink resistors are in. Boy does the rear 6k get hot even using the separator plate as a heatsink, more than enough to give you a burn. These resistors operate up to 250 C (480 F) so I guess I shouldn't worry about the temperature, they are clearly a better setup than the original resistors between the transformer and output tube pair. I'm just hoping I rewired them right, the 1000a currently sounds excellent.