A coworker got this beautiful receiver for me. It is in mint condition with no cosmetic restoration necessary.
So far I have recapped the entire unit, replaced about 8 light bulbs, 5 transistors, all diodes and zeners, and added two rectification bridges instead of the stock bank of diodes. Also added some capacitor bypasses and increased capacitance where advisable (like at the tone control power filtering cap, from 220 uF to 2,200 uF)
I am trying to get this receiver perfect and it is almost there, the FM works flawlessly and so does everything else.
The problem, there is some very faint background transformer hum that I thought would be fixed with new caps and zeners. It will produce this hum on all inputs and without anything but speakers connected and with no volume at all. Only clue is that increasing the bass will increase the amount of hum.
I may be asking too much from a 1970 unit couple with very sensitive large speakers that reveal just about any issue. Any ideas on how to eliminate this very faint hum?
So far I have recapped the entire unit, replaced about 8 light bulbs, 5 transistors, all diodes and zeners, and added two rectification bridges instead of the stock bank of diodes. Also added some capacitor bypasses and increased capacitance where advisable (like at the tone control power filtering cap, from 220 uF to 2,200 uF)
I am trying to get this receiver perfect and it is almost there, the FM works flawlessly and so does everything else.
The problem, there is some very faint background transformer hum that I thought would be fixed with new caps and zeners. It will produce this hum on all inputs and without anything but speakers connected and with no volume at all. Only clue is that increasing the bass will increase the amount of hum.
I may be asking too much from a 1970 unit couple with very sensitive large speakers that reveal just about any issue. Any ideas on how to eliminate this very faint hum?
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Does it hum if you apply shorting links to the selected input ? Had to ask 
I was going to say turning the bass up could be a clue that it is entering the signal path at the front end, but that may not apply if this oldie uses tone control stages configured within the feedback loop of the power amp. Many old designs did just that.
I guess carefully shorting the signal at various strategic places may help pin it down, for example shorting the power amp inputs, then the tone stage inputs etc.
If the hum is pure and deep and at line frequency then its probably stray pickup. If it has any kind of harshness or rasping quality (and at twice line freq) then it could well be PSU derived. Scope checks ?
Oh yes, very nice by the way
I was going to say turning the bass up could be a clue that it is entering the signal path at the front end, but that may not apply if this oldie uses tone control stages configured within the feedback loop of the power amp. Many old designs did just that.
I guess carefully shorting the signal at various strategic places may help pin it down, for example shorting the power amp inputs, then the tone stage inputs etc.
If the hum is pure and deep and at line frequency then its probably stray pickup. If it has any kind of harshness or rasping quality (and at twice line freq) then it could well be PSU derived. Scope checks ?
Oh yes, very nice by the way
Very nice receiver! I have a few questions:
Without source connected if you increase volume...does increase noise too?
What is the S.P.L of your speakers?
Was the back noise there before upgrade?
it can be a ground loop problem...try to move some of those wires and see if noise increase or decrease.
Try to move those wires that have AC voltages away from inputs.
Without source connected if you increase volume...does increase noise too?
What is the S.P.L of your speakers?
Was the back noise there before upgrade?
it can be a ground loop problem...try to move some of those wires and see if noise increase or decrease.
Try to move those wires that have AC voltages away from inputs.
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Does it hum if you apply shorting links to the selected input ? Had to ask
...If the hum is pure and deep and at line frequency then its probably stray pickup.
Oh yes, very nice by the way
Thanks and yes, I think this is the way to go next.
Thanks and no, increasing the volume does not increase the noise. And it will do it with the volume all the way down and with nothing but the speakers connected.
The Cathedral speakers I use are at least 100 dB, maybe even more. Nothing gets past them as far as noise.
The noise was there before and after the recap.
I think I will try to isolate the AC wires from the rest, it could be stray pickup since it seems to originate from inside the unit. Since these receivers where glitchy from the factory (it does have the updates), I wonder if it is just an attribute that you have to put up with.
In the following photo, you can see the AC wires wrapped up with what appears to be mainly speaker output wires running to and from the speaker switch.
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It is quite evident in the headphones as well, the cheap Koss kind...
I upgraded the power cord to a think ground type but let the ground off the chassis in order to prevent exactly what you reference. You can see the green ground wire tucked out of the way in the picture below.
Even took the unit to another part of the house with a free AC outlet and nothing around, still hum.
I will open it up later and start moving wires around. For the Sansui line up, this is one of their finer sounding units along with the 9090DB. I have however come across two 881 units that had hum like this that could no be cured.
Thanks for the suggestions.
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with amp on...try to move some of those wires next to the transformer away from it! and pay attention to noise if increases or decreases. check for wires that have AC voltages and put them as far as possible also start playing with the ground wires. I know those speakers with that high S.P.L will be very sensitive to "ANY" noise and they will "amplify" the noise.
it can be a "ground loop" problem!
it can be a "ground loop" problem!
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Long time ago, I had a problem like this one too. it was very tough to find it, but I fixed it. I got a piece of cooper wire and put one end attached to chassis and with the other end I started touching all Ground connections....at the end I found a "lift ground". it wast a tough dog to find it.
I did that and was able to reduce the hum just a tad by moving the ground of the rectifier board closer to it. I also separated all the power wires from the signal wires. This meant unsoldering some to route them differently (note diagonal run of the tone output to driver board cables) and wrapping up some together after separating them. I tried to separate DC from AC wires as well. All in all, I think 80% of the hum is gone and I really need to listen for it to find it whereas before it kind of stared at you. You can compare before and after in the first two photos.
Thanks for all the suggestions, the 5000A is back in operation.
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Blown Driver Transistor!
The fun was short lived! The unit was sounding great then moved it upstairs and once installed the left channel fuse blew. Tried again and then the main fuse went. Put it on the variac and once powered up, the left channel was in total short mode.
Checked the components and the 2SA566 driver transistor is completely shorted in one section. I am thinking of throwing in some Toshiba 2SA1930 and some 2SC5171. I know these are totally different case, but heck, they might be an upgrade to these obscure tired trannies.
The fun was short lived! The unit was sounding great then moved it upstairs and once installed the left channel fuse blew. Tried again and then the main fuse went. Put it on the variac and once powered up, the left channel was in total short mode.
Checked the components and the 2SA566 driver transistor is completely shorted in one section. I am thinking of throwing in some Toshiba 2SA1930 and some 2SC5171. I know these are totally different case, but heck, they might be an upgrade to these obscure tired trannies.
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That's tough luck although there should be no real problem fitting alternatives, although the characteristics may be different enough that the bias may not be "in range" with the pot. That was a problem that cropped up here on a similar vintage amp after replacing with modern devices,
http://www.diyaudio.com/forums/solid-state/233880-help-repairing-pioneer-m3-9.html#post3472090
http://www.diyaudio.com/forums/solid-state/233880-help-repairing-pioneer-m3-9.html#post3472090
Thanks Mooly, I like your illustrations in that thread. I have a very similar task ahead. If I can Bias these to a .9 - 1 -VDC, BE voltage, I think they will be happy. I have heat sinks for them as well. I do have one hole to drill to accommodate from TO-66 case to TO-220. Below the electrical characteristics for comparison and I will be replacing the complimentary pair, not just the PNP.
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Agreed, it is worth a shot, might get lucky and mitigate future failures of these drivers that are so common in these old receivers. I did a Sansui 8 for a friend, worked for about a year and then one channel gave up the ghost. It was minty with all the original OM and SM, and schematic in original paper! Still does not help the aging process and ticking time bomb factor.
Long story short, the transistors worked wonderfully. They are probably running on the cool side but the left channel now sounds better with more detail, so I can't wait to replace the transistors on the right side. It is a bit of an involved process, but the following pictures will make the job easier for the next guy. These drivers are much more detailed and are still available from Mouser. They are the Toshiba 2SA1930 and 2SC5171 and need to have the leads bent to accommodate the holes for the other transistors.
Complication came when I did not realize that the driver had not only taken out one of the outputs, but both. Good for me I had some vintage Sylvania ECG162 output transistors from some past rebuild, so these got placed one on each channel for some kind of balance along with the stock RCA SK3936 pair that remained working. The whole ordeal took about 4 hours and I have yet to do the other channel.
The sound is fantastic.
Thanks Mooly for reminding me to bring it back up on a light bulb, it probably saved the amp as there were two more shorted output transistors to deal with after replacing the drivers. The light bulb in the pic acting as a Variac was full bright.
Check out the pics.
Complication came when I did not realize that the driver had not only taken out one of the outputs, but both. Good for me I had some vintage Sylvania ECG162 output transistors from some past rebuild, so these got placed one on each channel for some kind of balance along with the stock RCA SK3936 pair that remained working. The whole ordeal took about 4 hours and I have yet to do the other channel.
The sound is fantastic.
Thanks Mooly for reminding me to bring it back up on a light bulb, it probably saved the amp as there were two more shorted output transistors to deal with after replacing the drivers. The light bulb in the pic acting as a Variac was full bright.
Check out the pics.
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Excellent stuff. I like your bulb tester set up, its a lot more sophisticated than mine
Using odd/different/unmatched devices is sometimes no bad thing because the result is more second harmonic distortion which is pleasing to the ear. Many new designs obsess over ultimate minimum distortion and what seems to me to be visual symmetry in the circuit diagram, and then along comes something like this that actually can sound better. A concept many designers can't seem to grasp.
Using odd/different/unmatched devices is sometimes no bad thing because the result is more second harmonic distortion which is pleasing to the ear. Many new designs obsess over ultimate minimum distortion and what seems to me to be visual symmetry in the circuit diagram, and then along comes something like this that actually can sound better. A concept many designers can't seem to grasp.
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