Restoration Tips

Tricks and Tips

Here’s a few things we have learned, mostly the hard way, about working on these monsters.

The dial cord–When you take the front panel apart, you will have to deal with the dial cord.  It’s really a bitch to just let it get loose and then have to figure it out again.  Take some of that blue masking tape, the kind that doesn’t leave a residue, and wrap it around the wheel on the big tuning capacitor.  Wrap it all the way around and squish it up tight so that it will hold the string on the wheel.  Then remove the tension by lifting the string off the far right pulley on the front panel.  Then take the front panel apart.

When you go to put it back together, all you have to do is get the string around all the parts on the front panel, relatively easy.  Once you do that, then remove the tape, and voila, it all works again.

Heat Sinks–Practically all the units we work on have loose power transistors.  Remove the back half of the two heat sinks on the back.  They are held on with 5 or 6 screws.  Each transistor has two screws holding it to the heat sink.  These screws are often loose.  When loose, the heat is not transferred near as well.  Tighten all the transistor screws, keeping in mind that they are small machine screws….don’t over tighten.

Meter Bulbs–These are usually dim or vary in brightness.  It took us a long time to figure it out, but, these bulbs are practically always good.  They run at reduced voltage, and as a result, can last forever.  What does go wrong, and causes the problem of uneven brightness (even dimness), is the resistor on each of the little PC boards on the back of each meter.  This resistor is of the same type as used on the output driver boards, the fusible type.  It is light blue in color.  They get hot in use and drift in value, always up wards in resistance, making the bulbs dimmer, since the resistor is in series with the two bulbs on each meter.  We find them to measure anywhere between 22 and 50 ohms.  Replace these resistors with a new 22 ohm 1/2 watt metal film resistor, and the meters will be of uniform brightness, usually getting brighter.

The Headphone Jacks–The really cool thing about the 9001 is that the headphone jacks are hooked up to the outputs, before the protection relay.  Now what’s cool about that is that when something is wrong with one of the 4 outputs, the quick and easy way to figure out what’s wrong is right there on the front panel, those two headphone jacks.  You can measure offset right there, without having to hook your meter up to the speaker outputs on the back.  You can listen and see if audio is getting through, even if a channel is messed up.

There’s a 330 ohm resistor between the outputs and the jack, so you can plug your phones in with relative safety.  Use cheap headphones, not good ones.  When I evaluate a unit for the first time, I do it right there with the headphone jacks.  First I check offset, and any channel that’s within + or – 200mv. is probably going to be ok.  Then I listen to audio and that tells me the relative condition of everything in the unit.

Quad Boards–The quad boards, 8087 and 8088 are the ones that decode/synthesize the 4 channel information.  Sad to say, but after having worked on about 20 sets of these, I can tell you that almost half of the boards are bad.  The chips (IC’s) on these boards are designed to work with 25v.  They don’t have much tolerance for over or under voltage.  Anything over 30v or under 20v is likely to blow these chips.  How does this happen?

First of all there is no voltage regulator in the 25v. supply.  If the line voltage (house AC) goes up or down, the 25v. goes up and down with it in lockstep.  One of the things we do with our restoration is to add a 25v. regulator.  Because these QRX’s are approaching 35 years old, there is a good chance that in your unit’s past was a lightning hit, a voltage surge, or a brownout.  If it was bad enough, some or all of the Quad Board chips might be blown.

You’ll be able to tell easily when you try to do the Holy Grail alignment.  The adjustments do not respond as they are suppose to.  The bad chips need to be found and changed for new.  Here’s the important thing to make your quad boards last.  Don’t leave your 9001 on when you’re not around.  Get a good surge protector to condition your AC.  If you live in a really bad area for AC problems, get a quality UPS (un-interruptible power supply) like for a computer and put it in line.  It’s worth it.

Intermittent Audio that is not the 4 ch board–Sometimes there is intermittent audio, a channel will drop in and out, even after the 4 channel board is repinned.   It might seem to be the Dolby selector switch. Cleaning and lubing the switch might seem to make no difference.  It is possible to mechanically stress the switches when you reinstall the 4 ch board and the dolby board, causing an intermittent.  These boards mount to the chassis in the rear, and to the front panel.  If you disassemble the front panel, at re-assembly, mount the dolby board and the 4 ch board to the chassis, but do not tighten the screws.  Get the front panel on and tighten all the mounting nuts and screws on the front panel for these two boards, and then go back and tighten up the 5 screws that hold them on the chassis.

Problems with the output driver boards–You cannot begin to repair these boards, if they have a problem, until you have changed out the fusible resistors and the caps.  Once you have done this you can re-install them and see if they will come in range for offset.  If they don’t there are two things that can be wrong (or both), either there is something wrong on the driver board or an output transistor is blown.  Usually it is the board.  If you check the output transistors and they are ok, check TR5 or TR6 as these are the ones most often bad.  In fact these are the only ones I have found bad so far in our shop.   I have helped people on line who have had others blown, but TR5 and TR6, depending on which channel, are the most likely suspects, especially if you have a high voltage on the output, almost rail.  These transistors get blown out by the 180 ohm fusible resistor (R33,34) drifting out of range.  I have measured these resistors as high as 400+ ohms.  When they go high, TR5/6 tries to correct and gets hotter and hotter until it blows.

Crackling when tuning FM–If the receiver has sat for a long time, when first brought up again, there may be a crackling sound, or cutting in and out as you tune up and down on a station.  This is caused by corrosion in the big tuning capacitor.  The ground side of the capacitor is made by the shaft making contact with the frame.  There are little spring spacers that keep the shaft under tension.  Apply whatever contact cleaner you use to these spacers and let it penetrate.  Then rapidly tune the dial up and down over the whole range from 88 to 108.  Do this a bunch of times and the contact between the shaft and the frame will clean up and the tuner will function normally.  This takes a while.  You may have to do it several times.

2X power stereo mode

I discovered something recently about the 9001. And believe me, I thought I had figured out everything there was to figure out about this receiver. I’ve worked on so many of these things now, that you could tell me a symptom, and I could most likely tell you the exact part or problem and where it was.

Anyway, this is most likely wrong with every single 9001 ever shipped. It has to do with the 2X power mode. The 9001 has a function where you can get all 4 channels to drive just the two front stereo channels, in bridging mode, which gives you 2X as much power or 120 watts per channel instead of just 60. This is achieved by using a channel to drive each side of the speaker, with the negative side driving out of phase with the + side.

Bridging mode normally works really well, and actually gives you the peak power of 4X what a single channel does. It also doubles the slew rate, and spreads the heat load over the heat sinks for all 4 channels. So, in bridge mode you usually get the sensation of much more power, and effortless drive, and great sound, better than the single ch version.

Well, the 9001 bridge mode never sounded as good as I thought it should. But since the 9001 is about quad, and I, and most of my customers, don’t usually run it in that mode, I didn’t think much about it.

I had a customer who’s unit developed a giant pop when switching to 2X. And then a unit on the bench that we had just recapped, did the same thing. I had to use the scope, pour over the schematics and actually think, to figure it out. Turns out that a cap that feeds the relay that switches to 2X mode has the cap marked backwards on the board. This cap only comes into play in 2X mode, but passes audio to the right channel, and is reverse biased by 8v (installed backwards). It’s a 50v cap, and I guess most caps could handle the reverse bias. We just got a new bag of 2.2uf @50v caps in, and I guess they couldn’t handle the reverse bias.

We later found that there were several versions of this board some with the cap in right, some not, and the labeling changed later on to be correct.  It’s hard to know which version you have without looking at the schematic.  As you look down on the cap from above, the + side should be toward the rear, and the – toward the front.

Well, a reversed biased cap doesn’t sound very good, even if it doesn’t fail. Now with that cap put in the right way, the 2X mode really shines. For a stereo receiver, it now sounds as good as the 9090, which to my ears always sounded a little better than the 9001 playing just stereo. Of course the 9001 always beat out the 9090 in quad mode, so I never thought much about it (that’s a joke).

Anyway, if you have a 9001, and you care, it’s C30 on the f2636 board. That is the board right behind the front panel with the bal and vol controls on it. You can get this board out without removing the front panel, if you remove the faceplate, and the bottom cover. It’s tight, but it will come out.  You can also change it by just taking off the bottom cover and fishing the cap out from the board with a pair of forceps or something.

I’d use a new cap. If you ever listen in 2X mode, it’s worth the trouble.  If you want a new 2.2uf/50v cap, audio grade, just email me and we’ll send you one gratis, no charge.  It’s a 15 cent part in quatity, but will cost you $5-10 by the time you place a minimum order somewhere and pay shipping.  I can put it in an envelope.

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