The Story

In 1986 I was in my first year of a two year T and I (Trade and Industry) Electronics vocational program at my high school. The T and I Electronics program was offered to Junior and Senior high school students and took a sizable chunk of the school day. At the end of the program you received a certificate indicting you completed X number of hours of vocation electronic training. The first year of T and I Electronics was centered around making you a competent electronic technician. As with many electronic vocational programs, the building of an Graymark Model 536 AM Radio kit was one of the tools used to teach you soldering, troubleshooting, and alignment skills.  As there was a lot of material to cover the first year of T and I Electronics program. We were only required to get the Graymark AM Radio working in breadboard format pictured above. You will notice initials on the breadboard, these are of the instructors as he had to qualify the build of each section of the AM radio, including measurements and conclusions before allowing the student to continue with the project.

Printed Circuit Board and Cabinet Assembly

My Graymark AM Radio kit has been in breadboard form for a little over 30 years!   I have chosen to complete the build by installing the components on the provided printed circuit board then mount it in the cabinet. ​

Graymark Manual

The Graymark Manual was very well written. It is more than just an assembly manual and provides students with information about proper soldering techniques, information about radio waves and the evolution from crystal set to that of that of a Superheterodyne radio that is to be built in this kit. In addition, it covers proper testing and troubleshooting techniques. The manual is broken down into sections corresponding to a block diagram of the AM radio. You start with the build of the Audio Amplifier, then build the Audio Preamp, Detector, IF Amplifiers, and Mixer/Local Oscillator sections. Each section starts with soldering the required components to the breadboard. The breadboard indicates proper electronic component placement and interconnectivity with other components. Next the manual provides a procedure on how to test the section just built including fields to record important data. Finally there is a quiz section to reenforce what was learned with the answers in the back of the manual. 

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Printed Circuit Board Prep

After 30 years, a tarnish had built up on the copper pads of the printed circuit board making it impossible to get solder to flow on the pads. The solder would just ball up and a thin layer of flux would separate the solder from the copper pad. I used a pencil eraser to remove all of the tarnish then I removed the pencil eraser residue with Windex glass cleaner. Below is a picture of the printed circuit board ready to accept solder.

Electrolytic Capacitor Replacement

As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace electrolytic capacitors that are over 30 years old!

Electrolytic capacitors should be replaced with one of the same or slightly greater capacitance and working voltage rating. 

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Audio Amplifier Section

One of the first steps in the Audio Amplifier section is to mount the speaker inside the front cabinet. I had to improvise and use a modified solder lug as a speaker holder due to some small parts of this kit had been lost over the years.

Picture below are the components of the Audio Amplifier section mounted to the printed circuit board and soldered on the copper foil side. I had to dig in my junk box to find the screws and nuts used to mount the volume control potentiometer. 

Testing of the Audio Amplifier section was pretty straight forward, the first thing that needed to be done is to solder the speaker wires and 9 Volt battery clip to the proper copper pads on the printed circuit board. You then connect a 9 Volt battery and power the Audio Amplifier section on by rotating the volume control to the ON position. The manual had you introduce AM hum into the input of the Audio Amplifier circuit using a screwdriver and your body as a natural antenna then listen for the sound from the speaker. I chose to use an my home brew Audio Frequency Generator to provide the input signal. Upon a successful test of the Audio Amplifier, you unsolder the speaker leads and move on the the Audio Preamplifier Section.

Audio Preamplifier Section

Pictured below is the printed circuit board with the addition of the components for the Audio Preamplifier section. Each component was soldered to pads on the copper foil side.

Testing of the Audio Preamplifier section was similar to that of the Audio Amplifier section.

Once again I used my home brew Audio Frequency Generator to provide the input signal but this time to the Audio Preamplifier section.  

 This time the audio was significantly louder at the same volume control setting as a new amplifier section was added to the circuit. 

Local Oscillator/Mixer/Detector Sections

The epoxy that held one of the windings on the loopstick antenna became brittle and fell off causing a winding to come off of the cardboard form. I rewound the winding and held it in place with a dab of hot glue.

In order to do any more testing, components for the Mixer, Oscillator, and Detector sections need to be installed and soldered to the printed circuit board. We can now test as a full AM Radio with these sections in place.

Testing of the Mixer, Oscillator, and Detector sections of the AM Radio is accomplished by adding a .02 mfd coupling capacitor from the output of the Mixer/Oscillator circuit to the input of the Detector circuit. The Mixer/Oscillator circuit takes a AM radio station signal and converts it to a 455Khz AM (Amplitude Modulated) signal. For testing we are feeding this 455Khz AM signal into the Detector circuit that converts it from a Amplitude Modulated RF(Radio Frequency) signal into a weak audio signal which is then amplified by the Audio Preamplifier and Audio Amplifier circuits. As you rotate the tuning dial you should hear at least one strong radio station from the speaker.

IF Amplifier Sections

The Graymark Model 536 AM Radio incorporates two IF Amplifier sections. The manual has you assemble each IF Amplifier section on the printed circuit board then test. Below is the finished printed circuit board with Mixer, Oscillator, IF Amplifiers, Detector, Audio Preamplifier, and Audio Amplifier sections installed.

The IF Amplifiers are special RF amplifiers designed to amplify only 455Khz signals coming from the Mixer/Oscillator section. The amplified IF signal is fed into the Detector section. Testing of the First IF Amplifier is accomplished by using a .02mfd capacitor to couple the IF output of the First IF Amplifier to the input of the Detector circuit. You should hear a couple AM stations out of the speaker when the radio is powered on. 

The Problem

I installed the components for the Second IF Amplifier onto the printed circuit board and then soldered them to the copper pads. At this point this should be a fully working AM Radio!  I connected the battery then turned the volume control to the ON position. I got a strange squealing sound from the speaker instead of getting the hiss of atmospheric noise or the voice of an announcer from an AM station. There was something wrong with the Second IF Amplifier section of the radio at it was time to troubleshoot. After close inspection of the copper circuit side of the printed circuit board I discovered that one of the leads of an IF Transformer in the Second IF Amplifier section had a cold solder joint and wasn't making a good connection. This was easily cured with the solder iron and the addition of new solder to the joint. Success! I could immediately hear an AM station once the power was applied to the radio. 

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Alignment

​Each IF Amplifier is coupled with a transformers that has an adjustable Ferrite slug. The Ferrite slug allows you to adjust the transformer's frequency response and that of the tuned amplifier circuit. In addition, the oscillator circuit has a transformer with an adjustable Ferrite slug used to tune the circuit to exactly 455Khz above the frequency on the tuner dial. The last two adjustments are located on the back up the tuner capacitor, they are the Antenna Trimmer, used adjust the antenna to resonance in the middle of the standard AM broadcast band and the Local Oscillator Trimmer which is used to adjust the oscillator circuit for the high side of the AM broadcast band.  

I use an old Eico Model 324 Vacuum Tube RF Generator for alignment purposes. I let it warm up for a least a half hour before using it at its frequency output tends to drift before being fully warmed up. The tuning dial on the Model 324 is not very accurate so I also use a Yaesu FRG-7700 Shortwave Radio Receiver (perched over my workbench) to accurately set the dial on the RF Generator. I set the FRG-7700 to the desired frequency then vary the frequency of the RF Generator until I can hear the 400Hz Amplitude Modulated signal from the RF generator on the FRG-7700.

Dead silence is heard from the FRG-7700 if the RF Generator is sending an unmodulated carrier only. 

Below is the order of the alignment tasks:

1. IF Alignment - A 455Khz unmodulated carrier is injected into the input of the 1st IF Amplifier using a RF Signal Generator.  An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Ferrite slugs of the IF Transformers are adjusted for maximum amplitude on the Oscilloscope.

2. Oscillator Alignment - The tuning capacitor is rotated fully counter-clockwise. A 540Khz amplitude modulated carrier is injected into the input of the 1st IF Amplifier using a RF Generator.  An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Ferrite slug of the Oscillator Coil is adjusted for maximum amplitude on the Oscilloscope. The second part of this procedure involves rotating the tuning capacitor fully clockwise and then injecting a 1600Khz amplitude modulated carrier into the input of the 1st IF Amplifier using a RF Generator.  An Oscilloscope is attached to the output of the 2nd IF Amplifier. The Local Oscillator Trimmer on the back of the Tuning Capacitor is adjusted for maximum amplitude on the Oscilloscope.

3. Antenna Alignment - This alignment procedure is done without any external test equipment. The Tuning Capacitor is rotated fully counter-clockwise then rotated clockwise until the first AM station is heard. The antenna coil is then moved along the Ferrite core for maximum station loudness. In the second part of this procedure, the Tuning Capacitor is rotated midway through its full rotation until an AM station is heard. Then the Antenna Trimmer on the back of the Tuning Capacitor is rotated for maximum station loudness.

​

Final Assembly

You can see all of the solder flux residue on the copper clad side of the printed circuit board from soldering the components in place. Very unsightly and unprofessional!

I use a spray can of Flux Remover and an old toothbrush to remove the solder flux. Pressurized air from my air compressor quickly dried the printed circuit board. Be sure to use the Flux Remover in a well ventilated area. I typically deflux outside during the warm Ohio months.

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Time to mount the tuning knob. It is held to the shaft of the Tuning Capacitor with a small machine screw.

It is important to properly align the tuning dial faceplate. The number 53 should be pointed towards the Ferrite loop antenna when the Tuning Capacitor is rotated fully counter-clock wise.

Now its time to mount the thumbwheel for the Volume control. The thumbwheel is held to the shaft of the Volume potentiometer with a small machine screw.

There is a groove on the edge of the thumbwheel that indicates the OFF position. This groove should be pointing away from the circuit board when the Volume potentiometer is rotated fully counter-clockwise.

I install the self tapping screws into the standoffs in the front cabinet before mounting the printed circuit board and then remove them. This will make it a little easier to screw them in, and its reduces the chance of the screwdriver slipping and damaging components on the printed circuit board during final assembly. 

When I test fit the printed circuit board in the front cabinet, I had a alignment problem with the Volume Control thumbwheel making contact with the opening. See area circled in the picture below.

I used an Xacto knife to trim the Volume Control opening in the front cabinet until the thumbwheel no longer made contact.

The face of the tuning knob was also making contact with the front cabinet when the printed circuit board was mounted inside. I used a file to remove some of the plastic from the opening until the tuning knob was no longer making contact. 

The printed circuit board is held in place inside the front cabinet with four self-taping screws circled below. I had to substitute small wood screws as the original screws had been lost years ago.

The rear cabinet of the AM radio snap fits to the front. It was impossible to get the rear cabinet snapped in place until I used an Xacto knife to trim the tabs.

Here is a picture of my finished Graymark Model 536 AM Radio, only took me 30 years to complete the job ;-) 

My Graymark Model 536 AM Radio in Action!

Elenco Radio Build: Part III

 October 18, 2024 · by Budi Isdiyanto

The next part of the build is to assemble and test the AM detector and automatic gain control circuits:

AM Detector

The “detector” is surprisingly simple, it’s just a 5 KHz low pass filter with a diode to chop off the positive side of the IF signal.

The IF (Intermediate Frequency) is a copy of the original transmitted signal but using a fixed carrier frequency of 455 KHz rather than the original carrier frequency of the station. The signal is created in the initial stage of the receiver so that the rest of the pipeline (i.e. the amplifiers and bandpass filters) can be tuned to this fixed frequency band, i.e. 455 KHz +/- 5 KHz.

Testing the detector is simple enough: Pass in a 1 KHz modulated 455 KHz carrier, using a signal generator, and check that the output to the audio amplifier is a 1 KHz tone. Then increase the 1 KHz signal until you get -3dB attenuation of the output to find the bandwidth of the filter. It should be greater than 5 KHz, otherwise you’ll be losing part of the audio.

In my case the bandwidth was only 4.4 KHz as the installed C38 capacitor was actually closer to 20 nF than the 10 nF it was rated at. Replacing it with one that was around 11 nF got the bandwidth up to 6 KHz exactly.

Automatic Gain Control

Another simple and ingenious circuit is the AGC which is a feed back loop from the AM detector output to the input of the first signal amplifier via a really low pass filter. The two things to note here is that the low pass filter creates a DC “averaging” of the detector’s output signal, and that the output signal, due to the orientation of the diode D4, is negative. This means that if the audio’s overall output level increases then the averaged negative voltage from the AGC also increases on the first amplifier’s input (the base of Q8) which will attenuate the gain of Q8. This negative feedback loop ensures that the audio output from the AM detector will stay at a constant level even if the signal received from the radio station changes in strength.

Lead Solder

In Offset Voltage’s videos he states that the PCB will not take to lead based solder. However I decided to try it and it worked fine. The large traces and pads still made achieving good flow hard but being able to work at 650F rather than 750F is a big plus. I found I still needed to use a lot of solder on each joint and not really happy with them still. Might try more heat in future.

Second Amplifier Stage and Filter

Next was to assemble and test the second amplifier circuit:

The output of the second amplifier goes through T8, which is a bandpass filter, and then on to the AM detector. The manual describes how to test the gain of the transistor and how to tune T8 for 455 KHz, and to test its bandwidth.

Here I got a some interesting results. The gain came out at 256 which is high but I guess transistors can be between 100 to 300. The bandwidth of T8, tuned to 455 KHz went from 413 KHz to 459 KhHz which seems very lopsided. In the manual it notes that the bandwidth should be around 445 KHz to 465 KHz for a single stage.

It also states that the input capacitance of the oscilloscope must be 12 pF or less. Mine are rated at 17pF so not sure if that is messing things up. It might also have to do with the transformer itself. Maybe the ones in the kit are not the highest quality?

First Amplifier Stage and Filter

Next was to assemble and test the first amplifier circuit:

This is identical to the second amplifier but with T7 as the bandpass filter and the base is adjusted by the AGC output.

Here I got a gain of 400 for Q8 which seems crazy high.

The AGC is disabled during testing. When the AGC is enabled again it should cause a 4 Vpp signal to drop to 0.8 Vpp. In my case it dropped to 0.2 Vpp which, judging from the extreme gains of the amplifiers, is probably correct. Still, at this point I’m a little nervous that there might be a problem with the build… Is so much gain going to increase noise?

Oscillator, Mixer and Antenna

Final assembly is the oscillator/mixer circuit, with antenna:

And large tuning capacitor C1 that has the dial on it:

From watching Offset Voltage’s videos I have a fairly good understanding of how this section of the radio works, though it’s not entirely clear to me yet how the “oscillations” of the oscillator happen. Nevertheless Q7 oscillates and its frequency is determined by combined inductance of L5 and capacitance of C1. C1 is also used with the antenna coil to create a bandpass filter that filters out all frequencies except the one we select with the dial.

The trick here is to tune the tuning capacitor C1, using two trimmers on its back and L5, to not only to reject all the frequencies except the one selected on the dial, but also to set the oscillator to oscillate at exactly 455 KHz above that selected frequency.

So if you turn the dial to 1000 KHz then the frequencies coming from the antenna to Q7’s base would only be in the range 995 KHz to 1005 KHz, i.e. we are only selecting the frequencies that belong to the station broadcasting at 1000 KHz. However, Q7 is also oscillating at 455 KHz above 1000 KHz at 1455 KHz and so the station frequencies are getting mixed mixed together with the 1455 KHz singal.

When you mix two frequencies together you get the original two frequencies plus two new frequencies; the sum of the two originals and the difference of the originals. So on the collector of Q7 we now have:

• The original station signal (which is the 5 KHz audio signal @ 1000 KHz)
• The oscillator signal (which is 1455 KHZ)
• The sum of two (which is the 5 KHz audio signal @ 2455 KHz)
• The difference of the two (which is the 5 KHz audio signal @ 455 KHz)

The neat thing here is that regardless of the frequency of the station there is always a 455 KHz version of the signal at the collector of Q7, i.e. the difference of the original signal and the oscillator.

T6 is another bandpass filter that only passes the audio signal @ 455 KHz on to the amplifiers. Pretty neat!

The manual details how to achieve all this tuning which is definitely easier if you have a signal generator that can do AM modulation.

I was able to tune the oscillator however it became clear that my amplifier transistors have way too much gain, to the point where the radio is unusable. Turning up the volume control just a small amount causes distortion and extreme loudness. Need to fix this problem first before I can complete the tuning and have a functioning AM radio. So close….

 

Elenco Radio Build: Part II

October 18, 2024 · by Budi Isdiyanto

While testing the radio’s audio amplifier in the last part, it was definitely annoying having to hearing the test tones and annoying for my family too. Offset Voltage recommended making a “dummy load” for the amplifier via the earphone plug with resistors attached to emulate a speaker’s 8Ω impedance, though my understanding is that a speaker’s impedance rating is a nominal value and in reality the impedance can change a lot depending on the frequencies going through it. Nevertheless it makes sense to have some load resistance for the amp to work against so why not choose 8Ω.

For the dummy load I used three 1/4 Watt resistors in parallel in order to handle the roughly 700 mW of max power that the amplifier can output. Using two 22Ω and one 27Ω resistors I was able to get a combined resistance of 8.6Ω.

The Dummy Load

Creating the dummy load was not so easy.

Using heat shrink to bind the three resistors together was the easy part and it helped making soldering the earphone wires to the resistor leads a lot more manageable.

The tricky part was dealing with the very fine earphone wires. I don’t know what kind of wires they are but when you strip the insulation they look like they are bare but are still coated in some kind of insulation. I used a flame to burn it off, as well as a blade to lightly scrape the surface, and this did the trick.

Next was soldering them to their respective ends of the resistors. Fiddly work but not too hard. However the wires are so fragile I felt it would be smart to use epoxy to protect them from mechanical wear. The epoxy along with a final heat shrink over the whole thing made for a very solid device. Very happy with how it turned out.

Next up was to assemble the alternative amplifier that uses discrete transistors to replace the LM386.

As an aside: I found that there was a version of the kit that only used discrete components and has since been discontinued: The AM/FM 108TK Rev-L

Wonder why it changed? Maybe educators wanted the option to use an LM386 to shorten build times for their students?

Transistor Amplifier

The transistor amplifier is on its own board and simple to assemble:

Figured the soldering for this board would be much easier than the main board as the traces and pads are much smaller, as you can see:

Nevertheless it was still challenging. The long header pins ended up cockeyed due to the heat melting their plastic holders. Not terrible but took a lot of bending and tweaking to get them aligned enough to all go together into the IC socket on the main board:

The other thing to note is that when the amplifier is installed it covers the earphone jack and even without my modification it would be impossible to have the earphone plugged in at the same time, so for testing I’m going to have to listen to the annoying tones again. When done with this section, the LM386 is going back in and the dummy load will be used for the rest of the build.

Fixing the Stand

Another suggestion from Offset Voltage was to use some tape on the supplied stand to stop the radio from slipping out so easily.

I used three layers of gaffer tape. The tape doesn’t need to go down far into the slot:

Just creating a small lip on the inside will keep the board from slipping out:

Testing the New Amplifier

The tests are the same as for the LM386, except for an additional transistor bias test and a DC gain test. The instructions for the DC gain test were not clear that you effectively need two power sources; the regular 9V as before and an adjustable power source that gets feed into test point 16 via a 1 meg resistor. The instructions seem to imply only one power source.

Here are the results for the transistor amp, along with the results for the LM386 for comparision:

Metric	Expected	Measured	LM368 Results
Idle Current	<10 mA	6.8 mA	4.4 mA
DC Gain	47.4	41.66	N/A
AC Gain	100-180	122	166
AC Bandwidth	>30 Khz	62 Khz	52 Khz
Clip Voltage	~	5.04 V	6.7 V
Max Power Out	> 200 mW	398 mW	714 mW
Max Source Current	~	109 mA	143 mA
Source Voltage	9 V	8.99 V	9 V
Max Source Power	~	0.98 W	1.29 W
Efficiency	~ 50 %	40 %	55 %

You can see the LM386 is quite a bit more efficient.

One interesting thing you can do with the transistor amp is bypass its biasing diode and view the effect of crossover on the output:

And doing the tests again with the biasing diode removed resulted in an efficiency of 45% compared to 40% with it in.

Next up is to put the LM386 back in and start on the AM detector and automatic gain control section… with the dummy load, yay!

 

Elenco Radio Build: Part I

October 18, 2024 · by Budi Isdiyanto

Elenco have this great AM/FM “Superhet” Radio Kit that I came across on a YouTube channel called The Offset Voltage that has a series of videos on building the kit, going carefully and thoroughly through all the steps detailed in the excellent manual, as well as explaining the operation and theory of each section. Highly recommend watching them if, like me, you have ever wondered how AM/FM works.

Of course had to build this kit myself as it looked super interesting and a good introduction to RF, a topic that was already on my radar.

You can find lots of reviews and unboxings online so I’m not going to go into any details on the kit itself. I only plan to document my experience with building and testing the radio. If you want to know more about the kit I recommend watching Voltage Offset’s introductory video as it is quite detailed and he gives some good suggestions and insights.

The kit includes everything needed, all nicely labeled and bagged. However you will also need:

1. Soldering iron
2. Lead trimmer
3. Multimeter
4. Philip’s screw driver and pliers
5. 9V Battery or equivalent power supply

Having an oscilloscope and/or signal generator will make testing and tuning the radio much easier and be more educational too, though not a requirement.

The only other thing I would point out, and it should be obvious from the PCB, is that this kit is meant for edification and not function. Don’t expect great performance out of it, though I am curious how well it will or won’t work.

Here is a shot of of what comes out of the box:

Everything is nicely packaged and labeled.

I rarely check parts lists for kits but strongly recommend doing so for this one:

I even tested all the resistors, diodes, large caps and transistors with a multimeter to minimize simple bugs later on. There are a lot of parts to go through but it also helps create a memory map that makes following the instructions easier.

The first step is to build the amplifier section using the LM386, which is what this post is really about:

Along with the battery holder and the on/off/volume potentiometer:

As well as the speaker and earphone jack:

In the last picture I included the whole backside of the PCB to show the huge sizes of the traces and pads. Just like the PCB for Elenco’s power supply kit it makes working with the supplied lead-free solder very challenging indeed. You’ll need a lot of heat from your iron. Ran mine at 750F and worked hard on getting the pads and leads up to temp so that the solder would flow well. It was a struggle and in places got partial delamination of the traces. Would recommend practicing on simpler kits first if your soldering skills are minimal.

Construction of this first section took just under two hours. Took it slow and focused on getting good solder joints.

The next night I did all the tests and measurements specified in the manual:

It’s these test and measure portions of the instructions, along with the explanation of the circuits, that makes this kit stand out, particularly for relative newbies like myself. You get to know how the circuit should work and how to check that it is working as it should. Really nice.

So how did mine turn out? Really well. It aced all its tests!

I did have one annoying issue and a funny one at that. When turning on the device you can hear very mild noise from the speaker as you’d expect. However, when connecting an oscilloscope lead to the J3 jumper (which is the input to the audio amplifier) I could hear a station playing! “Wow!”, I thought. “The kit’s working already!”. As amusing as that was it made it hard to accurately measure the mV signal levels going into the amplifier due to that interference. [I remember in Offset Voltage’s videos him mentioning how to minimize interference so need to look into that more.]

Was able to get around this somewhat by using the cursor feature of the oscilloscope:

Which was my first time using this feature in anger. You can see from the screen capture that the oscilloscope clocks the peak-to-peak voltage (of the 1 KHz signal) at around 7 mV (it was jumping about a lot). Using the cursors I could see that the level was more like 6mV.

Used the cursor feature again when doing the bandwidth test, in this case to show where -3dB was visually:

Doing all the testing and measuring took about another two hours. A lot of that was due to configuring and setting up equipment. It was also my first time using the signal generator I recently bought specifically for this project.

Have to say I’m really enjoying the kit so far. Am very curious how the AM and FM receivers will ultimately perform.

If you are following along, here are all my test results for the LM386 version of the amplifier:

Metric	Expected	Measured
Idle Current	<10 mA	4.4 mA
Gain	100-180	166
AC Bandwidth	>30 Khz	52 Khz
Clip Voltage	~	6.7 V
Max Power Out	> 200 mW	714 mW
Max Source Current	~	143 mA
Source Voltage	9 V	9 V
Max Source Power	~	1.29 W
Efficiency	~ 50 %	55 %
Fun Factor	100 %	100 %

The next step is to build a discrete version of the amplifier and test its performance.