PK1048

Electronics / Sound

Weird Tape Deck Problem

June 16, 2020

I have been working on a long term project to convert my (and my Father’s) old reel-to-reel tapes to digital. As part of that I have been working on a Teac X3mkii deck. Skipping all the mechanical fun, at this point it is pulling tape very nicely (for a consumer deck) and I was impressed with the 8 KHz stability coming off of the MRL alignment tape. The purpose of this post to search for an answer to an electrical problem with the deck. The problem is ion both channels, but much more pronounced in the right. There is an about 18KHz tone present. See the following spectrum.

This the right channel, the left also has the problem, but much less so. Here the peak is about 40dB above the noise floor, in the left channel it is only about 15dB above the noise floor. Ignore the spikes of hum at the low frequencies, those are easy to fix in the computer, the spike at about 18KHz is not. It is not a pure tone and it is not constant. See the spectrograph below.

Note that the 18KHz pulses on and off.

I have traced it down to the playback head amplifier, see the schematic below. Thanks to HiFi Engine for the X300 Service Manual (close enough to the X3Mkii).

U101 is a 4558 dual op-amp and I tried replacing it with a 5532 with no change to the problem tone. I also looked at the power supply and replaced C306 the decoupling cap and added a 0.47uF right at pin 8 of the DIP to ground, the power supply noise did go down, but the tone is still there. I tried replacing C105 and C107 thinking that they may have gotten old and tired, no change in the 18KHz tone. I even replaced C102 thinking that perhaps the fake DC reference for pin 5 was getting contaminated, but no change in the 18KHz. I am out of ideas. The tone is not visible at pin 3 of U101 but it is there at pin 7. The FETs (Q101 – Q104) are for changing playback EQ based on the settings of two switches (LH1/LH2 and LH/EE), changing these switches does not change the 18KHz tone, but I am starting to think the FET switches are somehow involved.

Why am I obsessing over this 18KHz, not terribly audible tone? Well, even though this is a consumer deck (1/4 track running at 7.5ips), there is material at this frequency coming off of the tapes. See the following spectrograph of one of the tapes. There is clearly energy above 20KHz and I don’t want to lose it by trying to filter out the 18KHz tone (I have tried). The tone at 36KHz (twice the 18KHz) doesn’t worry me as there is no program material there and I can effectively filter it out.

If you have any ideas what I am running into here, please post a followup comment below.

Full size copies of the images above are located here (much easier to see than the small WordPress images above).

P.S. Thanks to iZotope’s RX for showing me the problem and helping me fix many others.

Sound

When is Ground not Ground?

November 5, 2019

A while back I was working entertainment tech for a small convention, I was the Sound Designer and had inherited the Power Design (usually done by the Lighting department, but we did not find a Lighting Designer until very late). The event was held at a hotel with a 4-section ballroom, call the sections 1, 2, 3, and 4. The stage was set in section 4 and we had a social event in sections 1 and 2 on Friday night with the airwall between 2 and 3 in place. After the social event the airwall between 2 and 3 was struck and the airwall between 1 and 2 was installed. For the remainder of the weekend events were on the stage in section 4 with audience in sections 2 and 3 and section 1 was the performer Green Room.

Power

We were told that we had one 100A 3-phase feed to use (as per the contract with the hotel). We were also told that there were feeders in the airwall pockets between sections 1 and 2 and sections 3 and 4. Call the pocket between 1 and 2 feeder A and the feeder between 3 and 4 we’ll call C and we’ll leave the label feeder B for the airwall pocket between sections 2 and 3 (which we were told did not have a feeder). We decided to use feeder C (airwall pocket between section 3 and 4, nearest to the stage).

100A of 3-phase 120v/208v power gives us 300A of single phase 120v power, but for safety I derated to 80% or 240A. I built the power budget and came out to just about 230A. I assembled my equipment (PDUs) and cable (cam-lock feeders, as well as NEMA 5-15 extension cords) to handle all our needs assuming we were getting all our power from feeder C near the stage.

Contact with the Enemy

When we arrived on Friday morning we discovered that feeder A and C were not 100A, but only 80A. Feeder B was 100A, but if we used it, then we would end up with a PDU in a bad location due to cable lengths. Feeder A and C both came from a power panel in the kitchen behind the ballroom. Feeder B came from a different panel further back in the kitchen, and that panel also fed the panel that fed A and C. Like this:

Panel 1 -> Feeder B (100A)

Panel 1 -> Panel 2 -> Feeder A and C (80A)

We contacted Hotel Engineering and they apologized for getting us bad information. They offered to let us use as many of the different power feeds that we needed to, at no extra cost, to get the job done.

We were not comfortable using just the 80A feeder C as we were very close to that total draw and rather than use feeder B and have a PDU badly located, and against my better judgement, we decided to split the power between feeder C (for the stage) and feeder B (sound and lighting control points and video production control all at the back of the room). We knew where each feeder came from, we knew how the panels tied together, what could go wrong?

Friday night, after the social event, we started setting up the sound and lighting control riser; the video crew started building video production control on the other side of the room. I was the first to tie the two power feeds together when I plugged the mixer outputs into the snake feeding the FOH amp rack. As soon as the connectors mated I heard a hum out of the mains. I immediately knew what was going on. I called the crew together and told them we had a serious potential difference between safety grounds between feeder B and C. One of the crew grabbed a 100′ 12/3 extension cord and ran it diagonally across the room from feeder C to the sound board. We measured over 100mV between the safety ground pins. We assumed there was a wiring fault somewhere between the panels and feeders.

So what do we do now? The first thought was to use a bonding conductor to tie the two safety grounds together. We tried connecting the ground pin of that extension cord to the ground pin of the power from feeder B. That reduced the hum by about 20 dB, but it was still clearly audible. It was clear that we could not get a low enough impedance connection with the 12 AWG power cords we had with us.

We ended up, at midnight, redesigning the power distribution based on the parts we had on hand. We used feeder B since it could carry our full load. One of the PDUs that we added to the order at the last minute combined with multiple sets of camlock T’s of various genders, enabled us to get power where we needed it, but it was not pretty.

Splitting power at the first PDU
Audi / Cars

1998 Audi A4 quattro Avant – 250,000 miles and still counting

August 25, 2019

My 1998 Audi A4 quattro Avant (station wagon) crossed 250,000 miles few weeks ago and while it is still going strong, it does need some attention. This car is the 2.8L 30V 5-speed configuration (2.8 litres displacement, 30 valves, yes, that means 5 valves per cylinder, and 5-speed manual transmission). I started working on the suspension last year and got the rear springs and shocks replaced, I have the new front springs and shocks, but have not had the time to fit them. Oh, yeah, the springs are the Vogtland 35mm lowered sport spring set and the shocks are Bilstein Sport. So the car has been riding lower in the rear than the front since last December. It has needed a new clutch for the past 20,000 miles, I just keep nursing it along. The subwoofer in the cargo area in the back needs to have it’s foam surround replaced. I have a set of steel brake lines to replace the six short rubber hoses. Nothing major …

After meeting with my mechanic a few weeks ago we identified a couple other things the car needed. The first order of business was to address a very slight mis-fire at low speeds and under heavy load. My guess was spark plug, wire, or ignition coil pack as low speed and heavy load put the highest demands on the ignition system. The car was not showing any ignition related fault codes, and my mechanic said that any ignition coil fault would set a fault code. He also flagged that the valve cover gaskets were leaking and letting oil seep into the spark plug well and contaminate the plugs.

So this weekend I tackled the valve cover gaskets / spake plugs / plug wires and general cleanup. I also replaced the cracked brake fluid reservoir.

Here is how it looked after I had removed the air intake and air filter. I had not bothered to take a picture before I started any of the work, this was to help me put it back together if I needed it. You can see that spark plug #3’s wire is disconnected from the ignition coil.
Once the rest of the airbox and cables and hoses were removed,
I was ready to pull the right side valve cover.
Looking down at cylinder #3 you can see:
1. the pool of oil at the bottom of the spark plug shaft in the middle
2. the 3 intake valve cam lobes at the top
3. the 2 exhaust valve cam lobes at the bottom
4. the chain drive that connects the two camshafts for this bank on the left
Here is the entire right hand side head as viewed from the top.
Note the two fuel lines disconnected at the top left, one is the high pressure supply and the other the return from the fuel pressure regulator. I could not get the valve cover past them without disconnecting them.
And here is the left hand side.
Both sides open to daylight. Note that the designation Left side and Right side are from the driver’s seat, so the RHS is on the left in this photo and the LHS is on the right.
To get the LHS off I had to move the coolant reservoir, there is enough play in the 20 year old hoses to just move it aside once the three screws and one electrical connector are un-done.
Where did the brake fluid reservoir go ?
The white blob to the right is the new one.
There are four connections to the reservoir:
1./2. two connections to the brake master cylinder, front/rear
3. the blue lined hose just right of center is the connection to the clutch master cylinder
4. an electrical connection for the brake fluid level float
(yes, the car tells you if you’r brake fluid level is low)
This reservoir of brake fluid is also the source of the
clutch fluid necessary for the hydraulic clutch.
That is the new brake fluid reservoir installed. Since it is a “universal” part, good for manual or automatic transmission cars, I had to cut an opening at the end of the fitting for the clutch feed. Not a big deal, if you noticed before you installed it.
The other part of this project, the ignition coil module.
The black connector at the right is where power comes in and each of the three coils are signaled to fire. The silver brick with the fins is the electronics module that fires the coils. Each high voltage ignition coil, there are three of them, has two outputs. Unlike old school coil plus distributor, none of these coils have a ground connection on the output. The two outputs go to opposing spark plugs.
Getting technical here, each spark plug will fire twice for each pass through the 4-cycles of a modern engine. Both times at about top dead center, once when going from intake to compression stroke, and again at the “other end”, when going from exhaust stroke to intake stroke. A clever bit of efficiency if you do not need to tune each spark individually. Current ignition systems usually have one coil per spark plug to permit just such monitoring and tuning.
This is the bottom of the module. This is as close as you can get to the guts. Each coil is epoxy potted and the electronics module is encapsulated in what feels like rubber. Given that this example is still performing flawlessly after more than 20 years and 250,000 miles I’d say the assembly technique works well.
Having said that, I did not like the look of the rust on the iron laminations on the ignition soil cores, so I took to cleaning, priming, and painting it.
In this case, assembly really was the reverse of disassembly.
Everything went together very smoothly.
LHS valve cover replaced.
But I had to then remove the coolant reservoir again (thankfully it is only 3 screws and 1 electrical connector) to get at the spark plugs to replace them.
RHS valve cover back in place.
A side note about electrical connectors and connections.
Many mechanics and cars use grease to keep water away from electrical connections.
Keeping water away is good, but the grease does nothing to improve the electrical connectivity. Most, if not all, electrical connectors on Audi’s (and probably VW’s and Porsche’s as well as other German manufacturers) are designed to keep the water out. So I worry less about water and I do worry about the electrical contacts themselves.
DeoxIT is a chemical that cleans and lubricates electrical contacts, leaving a film behind that improves conductivity. I use it on every electrical connector as I reassemble things.
I also use it on mic packs in theatre, but that is a different category of discussion.
There was one casualty of the job. A small capacitor installed adjacent to the Ignition Coil Module was damaged while removing the Ignition Coil Module. A small blob of epoxy and it is back in service (note the shop towel to catch any drips as the epoxy sets).
Here is the freshly cleaned, primed, and painted Ignition Coil Module reinstalled at the front of the engine with a brand new set of spark plug wires. Note the repaired capacitor at the lower right hand mounting bolt.
The whole thing put back together.
I do not generally use the plastic cosmetic trim covers, while they make the engine bay look cleaner, they serve no really useful purpose and just get in the way when you need to do something.

Replacing the rear springs and shocks was the first major automotive project I have done in years. This is the first major project under the hood of this car, even though I have owned it for well over 10 years and 100,000 miles. Up until now I did not have the luxury of working leisurely on this car, we needed it for our daily commutes, so any work went to the shop and we got a loaner. Having said that, there has been very little that needed to be done to the engine.

The next items are a mix of general maintenance and projects.

  • The air filter needs replacing
  • It is due for an oil change
  • I need to find a rubber surround for the subwoofer
  • The front suspension is calling me
  • Those brake lines are not going to replace themselves
  • But I will pay to have a professional replace the clutch, anybody know where I can get an affordable lightweight flywheel / clutch kit for a 1998 A4 (B5 chassis) 2.8L 30V 5-speed quattro Avant ?

September 2023
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