Sansui 500a restoration questions

canuckaudiog

On a quest for high fidelity
Hey guys,

I recently acquired a Sansui 500a for the measly sum of $60.

Unfortunately I don't really know the history on it. But here is what I do know.

It appears to be all original, save for some tubes. I checked underneath and there was nothing obviously busted or overheated - all parts appear to be original and OK.

One 7189a tube is missing :(

I've put together a list of the capacitors needed, and I've been searching around AK and have read a few 500a restoration threads. They've been really helpful, except I am having trouble sourcing some parts.

For some of the capacitors, for example the 0.3uF/400V caps, I can't find any of the K40/K42 caps on eBay for sale with this value. The closest I can find is 0.33uF. Is it OK to substitute with this value or am I out of my tree? Another such value is 0.02uF.. these are tough to find and I'm not sure if I ever will.

Next question, based on my thread I started on the 250 I found, it seems the best advice is to get it all tubed up (so replace the missing 7189a), then run it on a DBT in series with a Variac. For the tube, what is my best option to replace it with? I don't want to have to do any modifications to make it work with another tube, I'd like to just slap one in if I can.. will the 6P14P russian tube work without any mods? Or am I stuck having to find another 7189a?

OK I think that's all for now. I nabbed the parts list from another thread so I believe I am OK there.. just want to be able to source all of the parts.

Thanks!
 
Member Bhagstrom did my 500a recently, maybe PM him with any questions, the power tubes are readily available but not cheap on ebay. I bought 2 "lots" of them, ended up with a pretty good matched set of GE's. Expect to pay about $40 each for NOS testing tubes....
Worth it however, great sounding receiver.
 
Hey guys,

Luck has shone on me! I found a variable autotransformer for a very very low price.

Having never used one of these before, it's my understanding that basically I start it on low low power, then continue to raise the power over the period of a few hours. Is this right? And what do you look for when raising the power? Just keep going up little by little every 15-20 minutes and if no smoke comes out, keep going?

Also, do I need to have a full complement of good, working tubes before doing this? I'm short a few but do have a few laying around that didn't test well.. but if this will risk anything then I will just wait until I get some good tubes.

Finally, once it is up to full power, how long do I run it for just to see it works and then shut it down? I understand the parts are faulty at their current age, but I just want to ensure the transformers are in good shape and that I can start restoring it. There's nothing worse than spending a bunch of money restoring something to find out there's something that is dead in the water and turns it into a heavy, expensive paperweight.

Thank you,

Jeffrey
 
Hook some cheap speakers to it when you bring it up on the variac. Put all the tubes in it, even the weak ones. By about 70V or so with it in FM or AM and tuned to a station it should make sound. By 90v it should be pretty clear (enough to prove out the transformers). If you get no smoke by then, AND the CAN CAPS aren't HOT, bring it up to 117 over a 5 minute period. leave it on just long enough to check the POWER SUPPLY VOLTAGES and the OUTPUT TUBE VOLTAGES. Then power down. Check the voltages against the manual. If ok, then go ahead and restore it.
 
Hook some cheap speakers to it when you bring it up on the variac. Put all the tubes in it, even the weak ones. By about 70V or so with it in FM or AM and tuned to a station it should make sound. By 90v it should be pretty clear (enough to prove out the transformers). If you get no smoke by then, AND the CAN CAPS aren't HOT, bring it up to 117 over a 5 minute period. leave it on just long enough to check the POWER SUPPLY VOLTAGES and the OUTPUT TUBE VOLTAGES. Then power down. Check the voltages against the manual. If ok, then go ahead and restore it.


Perfect thanks Larry!

Where do I probe to check those voltages?


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Check DC Voltages at the CAN CAPS (each one should have a voltage on the schematic), then the plate and screen voltages on the tubes. I don't have a copy of the schematic handy so not sure of the pin #'s.
 
OK so I fired it up on the variac. I got it up to 110V and was about to check voltages when I noticed two output tubes were redplating so I shut it down. However I did get clear sound out of it, the left having a very small hiss and the right crackling and popping a little bit with much more hiss and hum. I think just based on this I can verify that the receiver is worth restoring.

Do any of these symptoms indicate something much more wrong besides old caps?

Thanks


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Redplating is usually bad output coupling caps with the Sansui's. But it could also be DIRTY contacts in the sockets, out of spec resistors, worn out tubes, or bias'd to high for the tube (possibly due to the caps and resistors).

Go ahead and recap it. Pay particular attention to the POWER SUPPLY. If it ain't workng right everything else will be off.

Make sure all of the signal path tubes are in good shape. These are the 6aq8's, 6BL8's and the 7189's. 6AQ8's can be subbed with 6dt8 for about 1/4 of the cost. But be aware that sansui's are kind of finicky about the 6DT8. They don't like them in the MPX section. (Luckily the MPX uses a 6bl8 and 2 12au7's. RCA Blackplate 12au7 are good here.) Your V3(6aq8) is used in the FM Front End and V12-V13(6aq8) are the preamp tubes. You should be ok with 6DT8's in any of these positions if any of the originals are bad.

And yes the 6p14p is acceptable as a drop in.
 
That's great information, thanks Larry.

It'll be some time before this project finishes but I hope to get it done soon. Just based on what I heard through it running on the variac this is a very sweet sounding receiver. I can't wait to hear what it sounds like with new caps and proper bias.

Thanks again for all of your help!


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Well I think I can rule out the tubes redplating (which I didn't think they were the problem anyway). I swapped the tubes around and tried it again - sure enough the same tube sockets redplated those tubes.

Anyway, it's now put away until I get the parts in to restore it. I'll be re-reading the other 500a restoration thread for more pertinent details - it should have all the info I need. If not I'll post a question here - but hopefully the next post I make will be of the restoration itself. I'm in the process of selling off some extras I don't need and will be putting that cash towards the Sansui so it shouldn't be too long I hope.
 
On of my tubes was red plating when I bought mine. From the research I did, it seems the output tube coupling caps were the likely suspect.

I changed them all out as part of my restoration and never had another problem with red plating.
 
Thanks Dan, I appreciate the info!

Phase 1 of the restoration has begun. I have placed my order with Mouser which is the majority of the capacitors.

Once the parts come in I'll get you guys some pictures. Do you think it would be wise to power it up on the variac to see if it is still working after I put these parts in? I always double check my soldering work but I like to power it on to make sure that everything is still A-OK. But I don't want to run the risk of blowing anything up, either. The caps from mouser are basically everything except the coupling caps, and the multi-capacitors (the 20uF x4 cap and the 40uF x2 cap).
 
I wouldn't risk it. Do all the work before plugging it back in.

Btw, what brand of 7189A tubes did yours have?
 
Hey, Cool :)

this was my very first self-paid gear at all, bought on a garage sale in Berlin for $100 in 1976!
And guess, it is still working!

Helge
 
OK so.. I am just finalizing up the rest of the parts that I need to order and I have a question.

With regard to the coupling capacitors, I am wondering if it is OK to use a 0.047uF in place of the 0.05uF capacitors?

Also, should I only go with K40/K42 capacitors? I found some 0.33uF capacitors that were cheaper than the K40 version the seller had (trying to buy it all from one vendor to save on shipping) but it is a K75. Any thoughts on this? I assume it's OK but in case this number jumps out at someone and raises a red flag I thought I would ask.

Otherwise my parts list is pretty well complete. I'll share my revised spreadsheet in case anyone would like it once it's all finalized.

Many thanks!
 
Yes, a .047 is the equivalent to the .05. No issue there as long as the voltage ratings are the same.

Haven't used a k75 so I'll let somebody else chime in there.
 
One kink I found in nosirrah's unit was the bias trim pots.
One of them fell apart when I touched it.
You may want to source some new ones.
I snagged a bag of 10 off ebay that fit pretty well.

Given the number of caps in that thing, I worked in sections.
I re-capped the amp and power supply (main caps) first and worked into the tuner area later.
 
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