About a year ago, I purchased a reel-to-reel tape deck on eBay.

Akhai GX-625 Uncleaned Tape Head Stack and Akhai GX-625 Front

Unfortunately, it was defective, and I luckily got a refund. Then, I purchased an Akai GX-625 reel-to-reel tape deck. It also turned out to be defective. The right channel was fine, but the left channel was very weak. I tried to have it repaired, but it sat at the first repair shop for five months. The technician said he was having trouble finding parts. He said he tried two different sources with no luck, so I had him return the deck to me.

Secrets Sponsor

Once I got the Akai GX-625 tape back deck from the repair station that kept it for five months without repairing it, I found and sent the deck to another repair station on the East Coast. He repaired it in one week, saying that the parts were generic and easily obtainable. He replaced three transistors, fixed the electronic speed control, lubricated appropriate parts, cleaned everything, and fully calibrated it for ATR tape. It is now in superb condition. So, it appears that checking out the potential repair station where you might be sending your vintage reel-to-reel tape deck is absolutely critically important.

He returned the deck to me. It cost $225 for the repairs. Shipping to and from the East Coast where the repairs were done was about $135 each way (I live in California). The parts were generic, so there is no reason why the first repair guy I sent the deck to in the middle of the country (USA) should have had the deck for five months without repairing it. He said the parts were on back order. I don’t think so.

I went around in circles getting the insurance that I bought at the same time to agree to pay the repair claim. When I purchased the deck, the company selling the deck said it was serviced. The deck originally arrived with brown tape residue all over the guides and heads (photo below).

Akhai GX-625 Uncleaned Tape Head Stack

The left channel did not work. I don’t believe this deck was serviced at all. If you plan to purchase a used deck, which just about all tape decks are, you have to be really careful, and expect that it will need repairs. Repair parts on the Akai were easily available. This is not the case for all brands of tape decks. Check for availability of parts before purchasing the deck. Ask about the brand and model number on reel-to-reel tape deck forums as to the quality of the deck and any known design problems. If you purchase a deck, and it needs repairs, go to reel-to-reel forums and ask for opinions on the best repair shops for your brand and model. Ask about any negative experiences with the repair shop you are considering.

Secrets Sponsor

I had our resident Ph.D. electrical engineer, Dr. David Rich, advising me. If I had not had his advice, I would have been screwed because I might have ended up with a deck that has reliability issues. The Akai GX-625 is supposed to be one of the best consumer reel-to-reel decks that Akai manufactured. It is not a fancy deck with a lot of features. Perhaps that is why it is so good. Not many things to go wrong.

I found information stating that certain brands and models had reliability issues such as motors, capacitors leaking or exploding, etc. I had originally considered a deck with 15 IPS speed capability. I was told that such decks were used by radio stations and got heavy use, and thus, the heads are usually very worn (they look flattened) and may require what is called re-lapping, which means polishing them so that the surfaces are not flattened. Expensive to have done.

Here is a photo of the repaired GX-625 deck.

Akhai GX-625 Front

March 12, 2021 – For ATR Magnetics Master Tape, shown below, make sure the brown side of the tape faces the tape heads. The black side is the backing. I have seen comments on forums where one person says the shiny side faces the heads and another person says the dull side faces the heads. Both sides are shiny, so there can be confusion. The brown side faces the heads.

Here is a photo of both sides of ATR Magnetics Master Tape. The brown side is shown on the bottom, and the black side is shown on the top. The brown side faces the tape heads.

LPR35 tape from Mulann Industries also should have the brown side facing the tape heads.

March 16, 2021 – When using the deck, including winding and re-winding, make sure the tape is wound smoothly with no visible ridges of the edges of the tape (photo below). If there are visible ridges, this can cause tape damage over time. The ridges appear as an irregular surface rather than smooth.

Here are some bench tests. I used the line-in and line-out RCA jacks on the rear panel. The phones output level control (red arrow) was turned up all the way. This is because that control affects the line-out as well as the headphones jack output. The left/right microphone input level (yellow arrow) was turned down all the way to reduce noise. The left/right line-input level (green arrow) was set to the desired dB VU when the test signal was recorded.

First a 1 kHz sine wave. The recording level was set at 0 dB on the VU meters (using the line input level control dial).

THD+N and THD were both less than 0.05%.

Next, 0 dB VU for the sine wave recorded and played from the tape. THD+N is high because of tape noise. THD, however, is still less than 0.2%. Notice that the noise floor of the tape spectrum is high compared to the noise floor of the source spectrum. That represents some of the tape “hiss”.

March 26, 2021 – I have had a number of phone calls about the name of the company where I had my tape deck repaired. It is as follows:

Precision Audio Services (AKA Precision Audio Restorations)

6589 Main Street

Williamsville, NY 14221

Phone: 716-634-8845

Web: https://precisionaudiorestorations.com/

April 2, 2021 – Frequency Response (FR) of the source vs. off-the-tape is shown below. Tape deck specifications for FR are usually at minus 10 dB VU, so let’s start with that. First, the source. I used an Impulse Response (IR) to derive the Frequency Response. This is produced by sending a short frequency sweep from the low end to high (20 Hz to 48 kHz).

As you can see, the FR is within about 2 dB. THD can also be derived from the IR along the complete frequency range, which is why the IR is such a powerful test procedure. THD for the source at -10 dB VU is between 0.05% and 0.15% from 20 Hz to 500 Hz and then about 0.04% up to 25 kHz.

Now, here is the FR and distortion for the -10 dB VU sweep off-the-tape. As a reminder, this is at 7-1/2 IPS (Inches per Second).

The FR is within about 7 dB between 20 Hz and 25 kHz. Distortion is high between 20 Hz and 35 Hz, and then it drops to between 0.1% and 1% for the rest of the FR. Notice that the highest signal output is in the midrange, between 100 Hz and 1 kHz. This may be what gives tape one of its desirable sound characteristic. Our hearing is most sensitive in this range. Also, the 2nd harmonic is the highest of the three harmonics measured here, i.e., 2nd, 3rd, and 4th. A high 2nd order harmonic is also a desirable characteristic.

So, what does the FR look like at 0 dB VU?

Here is the source. The FR is the same as it was at -10 dB VU. Distortion is lower, however, staying in the 0.1% to 0.008% range.

For the 0 dB VU off-the-tape sweep (below), the FR is now within 12 dB, 20 Hz to 20 kHz, instead of the within 5 dB at -10 dB VU. The midrange region is, again, prominent. Distortion is also quite a bit higher than at -10 dB VU. I imagine this is why tape deck specifications are usually stated at -10 dB VU. The 2nd harmonic is the principle one even at 0 dB VU, which is good.

For the Tape spectra, notice a small bump at about 50 Hz. This is typical of tape. Impulse Response-derived FR for tape decks have not been published before, as far as I can find. Nor have 2nd, 3rd, and 4th harmonic data for the complete frequency response range.

April 13, 2021 – Shown below is a frequency response measurement made by using white noise instead of an impulse response. The recording level was at 0 dB VU. The yellow line is the source frequency response. It is underneath the sound card frequency response (green and blue lines), so it is a little difficult to see. However, below 20 Hz and above 30 kHz, you can see that the source response drops off below the sound card response. Also, because I used a high resolution FFT, the sound card response declines 5 dB from the high frequencies to the low, so you have to take this into account when evaluating the source and tape frequency response. The tape response is shown in the red line. The response is boosted in the midrange, but drops off starting at 500 Hz, with a rapid decline above 10 kHz. The decline is larger than the tape response using an impulse response. What this tells us is that when there are a lot of simultaneous frequencies, the high frequencies are recorded at a lower level than when the frequencies are presented one at a time for longer periods. The white noise signal is more like what we would have when music is being played rather than with an impulse response or with singular frequency test signals. The boosted midrange is probably why we like the sound of analog tape so much, along with a high frequency rolloff which reduces the sharp “edges” of the sound.

June 1, 2021 – I purchased an Otari MX-5050B-III-2 tape deck a few weeks ago. It was listed as being in near-mint condition, and I paid several thousand dollars for it. The seller (a website that specializes in tape decks) said it had been thoroughly cleaned, lubricated, and calibrated.

When I bench tested it, I found that I would need to recalibrate the bias for my ATR Magnetics Master Tape. So, using the 15 IPS tape speed, I measured the frequency response of each channel, and then I increased the bias a bit by using a small flat blade screwdriver to adjust the bias pots on the front lower panel beneath the VU meters.

There are two pots for this: RECORD BIAS CH 1 and RECORD BIAS CH 2. CH 1 is the left channel, and CH 2 is the right channel. I then remeasured the frequency response and made further adjustments until the response was flat out to about 10 kHz. At that point, I used the same screwdriver to increase the HIGH RECORD EQ pot (for the high frequencies) until the frequency response was as flat as I could get it out to 20 kHz. There are LOW RECORD EQ pots (for low frequencies) as well, but I did not need to adjust them.

Here are the adjusted frequency response graphs for the Source and Tape for each channel. The adjusted frequency response only applies to the Tape (playback from tape), not the Source (monitored from the input).

Although the high end FR is good, being down only a couple of dB at 25 kHz, the low end falls off below 50 Hz rather steeply. This is due to the 15 IPS speed. Low end FR is known to do this at the high tape speed. I would suppose that it is compensated for by using EQ during the editing process before cutting the master lacquer for making vinyl LPs.

In any case, this Otari MX-5050B-III-2 is a very good deck, and I am pleased that it turned out to be in such excellent condition. Its performance is dramatically different than my Akai GX-625 consumer deck. The Otari is considered to be a semi-pro deck, so the better frequency response is expected. The use of higher tape speed is part of the better response.

June 3, 2021 – The head stack on the Otari MX-505-B-III-2 needs to be cleaned. You can see debris on the guides (green arrows) and rollers (red arrows). Two additional photos for clarity are shown as well. I will go over how I cleaned them shortly.

June 4, 2021 – I cleaned the head stack, but I was mistaken as to what was tape debris. I used Q-Tips and 91% isopropyl alcohol. In the photo below, you can see my hand with a Q-Tip cleaning one of the guides.

Now, here is a photo after I cleaned the guides and heads. The yellow arrows indicate a bit of tape debris left on the guides. The green arrows point out the top and bottom of the guides. They are the same color as tape debris but are not tape debris. That is just the color of the top and bottom of the guides. This is not proper design. They should be chrome like the rest of the guides so that the user can see if they are clean. This particular photo was taken with flash.

This photo (below) is the same view, but with no flash.  A small bit of tape debris is visible on the right guide (yellow arrow), but the debris elsewhere that is visible with flash is not visible with this photo. So, if you are cleaning your head stack, take photos both with flash and without flash for examination. I took these photos by laying my cell phone beneath the head stack on the table with it facing upward in selfie mode so I could see the heads and guides. The deck was vertical on the table. I could not place it horizontally because of the cables connected on the rear panel. After taking the photos, I used my computer with a photo editor to copy each photo and paste it into a new window, then rotated it 1800, and then flipped it horizontally and saved it. If I didn’t do that, it would have been upside down and reversed side-to-side. This would make it difficult to interpret.

Anyway, I needed a lot of Q-Tips to remove all the tape debris, going over each guide several times. The heads were clean. It was the guides that were dirty.

June 6, 2021 – I want to talk about a tape deck factor called “Flux”. It represents the magnetization of the metal particles on the tape. The term used to put a number on flux is Reference Fluxivity, and it is the strength of magnetization that is used to record a signal (music) on the tape at 0 dB VU. The numerical value is the number followed by nWb/m, which stands for nano Weber per meter. Typical values are 185 nWb/m, 250 nWb/m, and 320 nWb/m. Back in the early days of tape, 185 nWb/m was the reference fluxivity that was used because the tape at that time could not be magnetized any stronger than that. And, NAB 0 (the American Standard) was set at 185 nWb/m. But now, 520 nWb/m can be used because tape quality is much better. This is + 9 dB over NAB 0.

On the rear panel of the Otari MX-5050B-III-2, there is a slider switch that is marked “Ref Flux”, and there are three values that can be selected: 185, 250, and 320. At each of these three settings, recording at 0 dB VU will create fluxivity (strength of magnetization of the metal tape particles) of 185 nWb/m, 250 nWb/m, and 320 nWb/m respectively.

I tested this at 250 nWb/m and 320 nWb/m.

Here are the results.

The first graph, shown below, is at 250 nWb/m.

Compare that with the following graph, at 320 nWb/m.

The difference is that increasing the Ref Flux lowers the noise floor by about 3 dB, but the harmonic distortion goes up by about 0.03%. So, you have to choose which is most important to you, the noise or the distortion. For me, it is the distortion, so I have chosen 250 nWb/m as my standard Ref Flux on this tape deck.

June 8, 2021 – When you purchase a used reel-to-reel (RTR) tape deck, one issue you have to deal with is the fact that the heads are worn, unless you purchase a restored RTR that includes heads that have been replaced with new ones or NOS (New Old Stock). Before you purchase a deck, be sure to request closeup photos of the head stack so you can see how much wear the heads have. In the photo below, from my Otari MX-5050B-III-2 RTR, the wear on the heads is indicated between the green arrows. I was very lucky as the wear is minimal. Click on the photo to see a larger version with more detail. It is not always easy to see where the wear region is demarcated (identifiable) since it is a flat area within the shiny chrome surface that contacts the tape, and photography includes reflections on this shiny surface. But, if the photos are taken as a slight angle, the worn surface is more easily detected. If the worn area (it will be flat) is about three times as wide as the flat area shown in the photo below, you may need to have the heads relapped, which means the head stack is removed and the heads are polished to make the surface rounded as the surface was when the heads were new. This can cost several hundred dollars, but it might be necessary if you want the deck to have a good high frequency response. A restored RTR may include head relapping.

Here are some drawings that show various types of head wear you might encounter.

June 12, 2021One of the things you need to know about when using a tape deck is the head alignment. Presumably, if you buy a deck that has been serviced before the seller sends it to you, the heads have been aligned. It is important because if the heads are not aligned, wear can be uneven as shown in the diagrams above. Secondly, if the azimuth is not correct, the signal on the left channel will be out of phase with respect to the right channel. The azimuth is the vertical angle. The left channel tape head gap should be exactly in line with the right channel head gap, vertically.

The azimuth is tested by recording and playing a sine wave, such as 1 kHz, in both channels and adding the outputs of both channels together. If the head gaps are aligned, the result will be a peak that has exactly twice the voltage of one channel, assuming that both channels have the same output voltage. This can be tested by using an oscilloscope, but it can also be tested using audio software that allows you to add both channels together. The latter is what I did to test the head alignment on the Otari MX-5050B-III-2 tape deck that I purchased recently.

So, here are the results using a 1 kHz sine wave in both channels.

First, I connected the sound card outputs to the sound card inputs and added the two channels together. The resulting peak is at 14.5 dBV.

Next, I set a 0.5 msec delay for the left channel, which made the left channel 1800 out of phase with respect to the right channel. The peak (shown below) was now at -42.13 dBV which is only a few thousandths of a volt. This is the point of maximum cancellation due to the out of phase waveforms. The reason there is still a peak is because the outputs of the two channels are very close, but not exactly the same. When I set the delay more or less than 0.5 msec, the peak was higher than -42.13 dBV because the left channel was only partly out of phase. This is to demonstrate that having the two channels out of phase causes the peak from adding the outputs of the two channels together to be lower than the peak when the two channels are exactly in phase.

OK, so let’s look at the spectrum from the Otari tape deck source monitor when I input 1 kHz at 0 dB VU (below) and added them together. The output was -2.29 dBV (0.77 Volts) which is twice the output of one channel. The source is in phase for the two channels as it should be.

Now, the spectrum from the 1 kHz tape playback at 0 dB VU (below). The peak is also at -2.29 dBV, indicating that the azimuth of the record and playback head are properly aligned. If the azimuth was not in alignment, the peak would have been less than -2.29 dBV. So, recording and playback with this Otari deck is in correct alignment.

The next step would be to obtain an outside sourced test tape with sine waves recorded on it using a precisely calibrated (azimuth aligned) tape deck. I am trying to find one, but it is not easy.

June 13, 2021For a second alignment test, I measured the left plus right channel at 10 kHz. Here is the spectrum for the source. The output was -2.19 dBV.

Now the spectrum from the tape. The output was -3.63 dBV which indicates a slight phase misalignment. However, this may be due to phase shift caused by the EQ that is done to flatten the frequency response. The EQ does not affect the response at 1 kHz and therefore does not cause phase shift at that frequency. However, EQ does affect the response at 10 kHz. The fact that the alignment test showed accurate alignment at 1 kHz but not at 10 kHz does suggest that EQ may be the cause.

I have found a potential source for a professionally produced alignment test tape and am in the process of researching whether or not this test tape will be the one that I need. I will report on this later, but suppose the alignment results shown here are due to phase shift caused by the EQ settings (EQ for each channel is slightly different). Further alignment using a test tape at 10 kHz might throw off the true physical head alignment. That might be fine for recording and playback of tapes made with this specific tape deck, but playback of commercially available music tapes made with other decks would not have proper phase alignment. On the other hand, assuming the test tape was created with no EQ adjustments at all, using the test tape for alignment could produce a proper physical alignment, but with the EQ on my deck, playback could have worse phase alignment due to adjusting the heads based on a tape recorded on a different deck.

July 4, 2021 – Some months ago, I negotiated with an individual who markets copies of audio master tapes to send me some of his tapes to review. I sent him several blank reels of tape so it would not cost him anything but some time to push Play on one deck and Record on the other. He said it would take a week. Time went on, and excuse after excuse, no tapes. It was months ago that he agreed. Last week, I told him to forget the project as I no longer trusted him. I did not hear from him again, even to ask if I wanted him to return the $150 worth of blank reels of tape. So, my experiences thus far with buying used tape decks and associated items have resulted in 90% failures in obtaining what I had expected to obtain, including two decks that had been advertised as in working condition and which turned out to be defective. The first one was returned for refund. As you might remember, one of the repair guys kept the second deck for six months with no repairs. The second repair guy, whose address is listed in one of the posts above, did a great job and repaired the deck in a week, stating that the parts were generic and easily available.

My conclusion is that when dealing with an individual whom you do not know, and with whom you are considering doing business to purchase a used product over the Internet, such as a tape deck, probably expect that 60% (conservative estimate) of the time, you will not get the quality you expected. When considering purchase of a deck, request closeup photos of the tape heads. Look for wear (a flat region at the center of the heads) and cleanliness (lack of brown tape debris). Also request to see a video of the various tape functions in action, including record, play, fast forward, and rewind. When in record and play, make sure you can see the meters responding. Ask to see a record/playback frequency response curve at 7.5 IPS (0 dB VU and minus 10 dB VU), and, if 15 IPS is a speed on that deck, the response curve for that speed too. Ask for a written confirmation that the deck has been cleaned, lubricated, and calibrated if it is stated that the deck has been serviced, refurbished, or restored.