It’s setting there on your workbench, looking good. You don’t know anything about its history for sure, and it may not have been powered up for a long time. So… before you power it up, there are some things you ought to do: cleaning, preconditioning, adjusting things. I know, the temptation is great to fire it up – but if you do, there is a small but real possibility of seeing smoke.
This article, and any that may follow it, is not intended to educate experienced audio Do-It-Yourselfers. If you are one of those, feel free to read on and send critical letters to the editor. (Just be aware that for some editors, any feedback is good feedback.) The intent is to help make the use of vintage (i.e. affordable) equipment more practical for the ‘common man’.
- The service manual for this model.
Without this, you won’t know which pots do what, or what their settings should be.
- A variac – either a real one or a PMV (Poor Man’s Variac, using light bulbs).
You use this to wake up the amp gently after its (presumably) long sleep.
- A VOM (aka DMM). Used when checking/setting DC-offset and idle (bias) current.
- A small paintbrush, of the type used to apply varnish. You use this as a duster.
- A can of compressed air, the sort used to clean computer keyboards.
- Electrical contact cleaner – Deoxit or Permatex (item #24379 in case there are others) recommended. I can’t rec-ommend anything else, because some of them leave residue, and I don’t know which ones.
- A (probably Philips) screwdriver for removing the amp’s cover. Some amps make you remove the faceplate to get at the switches.
- A small common screwdriver for adjusting pots.
The Poor Man’s Variac (PMV)
This photo is not of my PMV, which is too ugly for public viewing. This PMV belongs to the gentleman known at Audio Karma.org as Morden 2004, who graciously granted permission for the use of the photo here.
Photo #1 – Morden2004’s PMV
How to make your own: Materials:
- A ceramic receptacle for standard base incandescent light bulbs and a junction box to mount it on.
- A power strip, preferably with switch. Surge suppressor not required.
- A length of 2×4 to use as a chassis. Needs to be long enough to mount both junction box and power strip.
- Light bulbs: 25, 40, 60, 75 and 100 watt incandescent.
- Mount the junction box on one end of the 2×4. Open holes at both ends.
- Cut the power cord in two about “from the power strip. Remove about 2” of the outer covering from each end of the cord, exposing the (insulated) conductors. Strip about ½” of insulation from each conductor.
- Bring the cord ends into the junction box. Reconnect the ground (green) and white conductors and tape the connections.
- Connect the black wires to the terminals of the receptacle and mount the receptacle to the junction box.
The light bulbs provide the ‘settings’ for your variac.
We need an amp to use as an example; here’s my Sansui A-7, in its unopened state:
Photo #2 – Sansui integrated amp, model A-7
1st operation – cleaning
The majority of integrated amps (and tuners) have top covers that are also side covers; they are fastened with two screws on each side of the amp. The Sansui A-7 is different, just two screws in the back. When it’s off, you can see into the Area of Operations.
Photo #3 – The Sansui A-7 – cover off
These innards are clean. Your amp may be clean too, depending on where it was used and stored. If things are dusty (or worse), start with the can of compressed air. You may wish to remove the amp’s bottom cover now, to give the dust an open ‘way to the egress’ (sorry, I blame this on P.T. Barnum). If there’s significant dirt left, see what the small paint brush can loosen up, then use the compressed air again. If there is still a layer of crud, maybe yellowish from tobacco smoke, you can resort to isopropyl alcohol and cotton swabs. Don’t use any more alcohol than necessary.
One reason for this seemingly compulsive fussiness is that as you are cleaning, you will (hopefully not) see any evidence that electrolytic caps have been leaking. If you find any leakers you need to replace them before power-up. Now you can clean switches, using the electrical contact cleaner. In this amp the switches are accessible after the top cover is removed; in some amps you have to take the faceplate off too. Give each switch, both rotary and pushbutton, a good dose of cleaner, operating each one several times as you proceed. The cleaner will evaporate quickly.
If your amp has DC-offset pots, it makes sense to clean them now, in the same fashion; carefully note the pot’s current position, so you can return to it. Just spraying the pot with cleaner won’t accomplish much; you have to turn it through its travel a few times. Some folks advocate cleaning the bias pots during this operation. If that’s your plan, make very sure to return the pot positions to where they were. Folks who are not the above “some folks” prefer to leave the pots as they are, in the expectation (hope) that the settings will be good when checked, or that they will function correctly if they have to be adjusted. This – marginally competent, remember – writer takes the second choice, modified. I don’t mess with those pots now, but if I find that they need adjustment when I get to that step, I go back a step and clean them before moving them with power on. A “jumpy” bias pot can make for fried components downstream from it.
This amp doesn’t have DC-offset pots; you can see the bias pots in photo #3 – about in the middle of the picture.
2nd operation – the slow wake-up
For this step, I recommend that you set the top cover back on the amp. No need to install the screws; the thing is that there is a small chance that one of those electrolytic caps will ‘pop’.
Connect your variac to power, with its power switch off. Connect the amp to the variac, power switch off, volume knob at 0 (for luck). If you are using a PMV, screw in the 25 watt bulb; if a real variac, set it at 45 volts.
Turn on the amp’s power switch, then the variac – watch for smoke, ready to turn off the variac. The amp’s speaker protection, if it has such, will likely stay active, but the caps will charge. The bulb in the PMV will dim as the caps charge. If no smoke is rising, leave the setup going for about 20 minutes.
If you are using a PMV and nothing bad has happened, turn its power off, replace the 25 watt bulb with the 40 watt bulb, and turn on power again. With a real variac, turn the voltage up to 60. Repeat the 20 minute wait.
Repeat the procedure, 60 watts, 75 watts, 100 watts (or 75 volts, 90 volts, 105 volts). If the amp passes, the procedure is finished. The amp should handle full power OK. Turn the amp off.
Somewhere during the process the protection circuit should deactivate a few seconds after power-up. If it hasn’t done that by the last step, cross your fingers and plug the amp into a regular receptacle (no variac), and turn the amp on. If the protection circuit stays in indefinitely, there is a problem that is probably beyond the expertise of the marginally competent. You’ll need to go to someone who’s better than marginal.
3rd operation – checking/setting DC-offset
This amp has no DC-offset pots, and yours may not, but you still ought to check the offset. It should be within a few millivolts of zero. Sensitive ears will detect sonic degradation at 15 or 20 mV offset (my ears aren’t sensitive, I’m going by expert opinion here). Anyway, even if your ears aren’t sensitive you could consider corrective action desirable if the offset is more than 20 mV, as a matter of pride. The deteriorated component is probably a regulating resistor or cap; figuring out which one is the problem, and I can’t help you there.
If your amp has DC-offset pots, locate them on the circuit board by referring to the service manual.
With the volume pot set at zero, turn on the amp and let it warm up for at least 20 minutes. Then turn on your VOM, selector set at the 300 mV DC position and the red probe lead in the Volts-Ohms jack.
Connect the probes to the speaker terminals of the left channel, positive to positive. You should see a reading on the meter, either negative or positive (no sign).
The service manual will probably tell you to set the offset to zero mV +/- 1 mV. If the reading is negative, turn the left channel pot clockwise – in small increments – to bring the reading to zero. You may find zero hard to get to, but don’t give up until you are at least within 2 mV. If the pot acts squirrelly, i.e. the meter reading jumps around when you move the pot setting, you may need to power down the amp and clean the pots again (clean both of them while you’re at it). I would tell you to check/set the right channel when you’re done with the left one, but you knew that already.
4th operation – checking setting bias current
Here’s what the Sansui service manual has to offer:
From the manual, a photocopy and a table
Here’s the real thing:
Photo #4 – Sansui bias pots
The pots are pretty easy to find, the aluminum-looking things with screwdriver slots about in the middle of Photo #4; the test points aren’t so easy. The white rectangles below the pots are the resistors referred to; points “A” and “B” are the outer legs on the left resistor, points “C” and “D” are the outer legs of the right resistor.The probes on your VOM leads won’t do for this, and alligator clips aren’t much better; you need clip leads like this one:
Photo #5 – IC clip leads
Some amps bring the test points to the top of the circuit board; clip leads work best there too. Some amps don’t provide test points, but have you remove a fuse and read milliamps across the fuse-holder; small insulated alligator clips are handier for connecting to a fuseholder.
With the amp turned off and volume control set to zero, attach the leads to the test points for the left channel. Don’t turn on the VOM yet, or connect the leads to it. Make sure that the leads are where they should be, and nothing is getting shorted.
Turn on the amp and let it warm up for at least 10 minutes. Then turn on the VOM, set it up to Volts-Ohms and 300 mV (DC amps and 300 mA if your are checking across a fuse-holder) and connect the leads to it. Let the meter reading settle. It can be 10% or so higher than what the manual calls for (they tend to be conservative), but shouldn’t be lower. If the reading is unacceptable, and you cleaned these pots while doing the other cleaning, Use a small insulated screwdriver (both handle and shaft insulated) to move the adjustment a small amount. If you see the meter reading go to zero and then return to a different number as you turn, the pot is ‘skipping’ and still dirty or corroded. You may not be able to get to the setting you want, because it’s in a ‘skip’ area. You should power down and try cleaning it again. Sometimes, especially with older pots, cleaning isn’t enough and the pots will have to be replaced.
If things go well, turn power off and move the leads to the test points for the right channel. When you have that one set, power down, remove the leads, and replace the amp cover.