After the vacation, I opened the transceiver. An inspection and preliminary measurements were made to determine a possible malfunction. Assumption - failure of the TDA 7056 low-frequency amplifier IC. As soon as the new IC was delivered, I began repairing the radio.
Without tightening all the numerous screws, I connected the power and turned on the radio. The sound in the speaker appeared! The diagnosis was accurate.
I would like to immediately note that the sound quality has noticeably improved. Especially on loud signals. Previously, distortions were noticeable. I would like to share some thoughts:
The manufacturer made a good radio, but does not pay the required attention to the quality of the components. After all, the transceiver was purchased quite recently by my friend and was practically not used in harsh activation conditions!
I am completely dissatisfied with the lack of diagrams and other service documentation in the public domain. Apparently, in a radio assembled from standard and widely known ICs, there is some special, secret circuit design? Or maybe they are hiding something else…
In the advertising brochure, this radio is designated as waterproof. I was confused by the lack of an indication of the degree of protection, for example, IP-67 is assigned to many waterproof transceivers. Here, the IP is not indicated at all. Upon opening, rubber sealing rings were found only on the encoders. The panels of the excellent milled aluminum case fit tightly together, but this is not enough to ensure tightness. There are no rubber gaskets or sealant here…
It seemed to me completely unjustified to use a whole bunch of massive stainless steel screws. Of course, this adds brutality and bulletproof strength. But in this case, I would prefer a lighter fastener. A significant percentage of the weight could be saved. This is important for portable equipment!
A completely useless spectroscope for a telegraph operator. A fixed viewing band of 48 kHz makes the CW signal one pixel thick. Weak signals are completely invisible. Maybe SSB - operators will disagree with this. But this is not the cheapest radio positioned as universal. And the manufacturer has paid a lot of attention to processing CW signals. It can be admitted that the telegraph sounds excellent in this radio, perhaps. It really costs money. But was there really no software option to make the viewing band in the spectroscope at least switchable (for example, 48/4 kHz)?
The transceiver I used did not come with a BP-500 battery pack. I did not regret it. Instead of a 350 gram metal block that is attached to the back of the radio, I would have preferred an ATU. But the manufacturer does not offer or even advertise an ATU.
As a summary:
The Discovery TX-500 radio is a very interesting and solid product on the market of portable transceivers. The manufacturer has made a very memorable concept. Much attention has been paid to the quality of the receiver - this is a huge plus!
But, it seems to me that the authors have overdone it a little with the abundance of metal. The transceiver could have been even lighter!
73
Vlad
RX9WT
P.S.: Be careful if you hear distortions on loud signals in your TX-500 receiver. Do not wait for the radio to fail at the summit. Replace the power amplifier chip in advance. Most likely it was a whole batch with defective TDA 7056…
Thank you for the detailed report. You were lucky that the failure was TDA7056 and not anything serious. I personally don’t like buying electronics without the schematic diagrams (and preferably the entire service manual) found on the internet for same reasons. (There were also cases where I liked the products until I studied the schematics and other info closely, and then lost interest in those transceivers… due to the designer’s shallow RF engineering.)
Most disappointing radio in recent times, it was vaporware for such a long time, eventually when it was released I pre-signed for one and paid full price in the erroneous belief it had been tested to exhaustion… Decided to test it out fully in my back garden before heading to Belize in Central America (V31HT) Setup with a Alex-loop made two contacts and the audio disappeared… nightmare ensues, went through the usual including the unusual of reflashing after contact with seller, confusion abound as to a such a new radio could fail so soon (What did the G4 do to make it happen). Sent back, waiting for chip (Audio) waited waited, went to Belize with my trusty KX2. On my return I was sent the repaired one, test time again before heading to Madeira Island, on my normal home dipole, sound failed again, sent away again, went on holiday with my trusty KX2/G90. I waited another period of time and the seller(well known UK) said they were having problems sourcing chips so they sent me a brand new one (nice). Testing ensued again before a trip to the Dominican Republic, yep you guessed it sound failed again… So I can’t trust the radio, grabbed my KX2 & a Chinese SDR for fun in the DR. Sent back and got a full refund. It promised so much, I bought a 705 to lick my wounds but still reach for my KX2 ( & FX4CR (great radio)) as the KX2 is bulletproof(with lots of care of course) now to add the internal recharge circuit.
I hope your repair has finally solved the below parr audio problem as I did like this radio a lot even if a tad heavy but I have to trust the radio especially as I travel a lot. 73. Best. G4LHT/V31HT
I don’t own a TX-500 but do you know whether TDA7056AT fails during receive (e.g. excessive received signal audio level) or during transmit? I’m curious.
That is because with an external speaker, common mode RF current flowing through the speaker (or earphone) cord is very common, especially in portable situations where the operating position tends to be near the antenna. (The same problem occurs with the key/paddle interface and to some extent the power cord.) Some manufacturers are very cognizant of this type of issues and do pretty good job in preventing it, but a lot of QRP-only manufacturers (and kit makers!) are very often bad at this.
I thought about listing the names of the brands but I’ll save that for another occasion.
TDA7056AT was made obsolete by the manufacturer so whatever in circulation must be deadstock or counterfeit… another concern.
Ryuji, your thoughts coincided with my assumptions. Several years ago on QRP.RU there was an active discussion about distortions in the audio amplifier in this transceiver. The manufacturer paid attention to the comments of active users, including well-known specialists in the field of sound and signal processing. The fact of the presence of distortions was acknowledged by the manufacturer. Measures were taken to reduce THD and intermodulation. The manufacturer reported on the regulation (reduction) of the signal level coming to the input of the low-frequency amplifier and on the adjustment of the modes. All this brought a positive result. But, perhaps, did not solve the problem completely. In my opinion, it was necessary to simply abandon the outdated IC. Put something more modern and high-quality. True, it would have been necessary to change the production process and have costs. And now the problem of counterfeit, “gray” ICs has not gone away.
I tried to describe in detail how the radio failed in my story about the unsuccessful activation.https://reflector.sota.org.uk/t/unsuccessful-activation-of-r9u-so-030-video/36345?u=rx9wt
I will no longer be able to add anything or clarify the reason. But I can say that I also thought about the possibility of damage to the low-frequency amplifier from the common-mode high-frequency currents. When I was diagnosing this radio, I did not see any chokes in the output circuits of the low-frequency amplifier. In the place where they could be, there are only 0 Ohm resistors. Just jumpers.
Damian, thanks for your support! I didn’t expect that the problem with sound loss was so relevant in this radio.
When choosing an IC supplier for repairs, I didn’t have much choice. For obvious reasons. But I still managed to buy a copy manufactured by NXP from an official and trusted supplier of components in the Russian Federation. Although it cost several times more than on Aliexpress. I really hope that the quality will be sufficient this time. I already wrote that after the replacement, the sound became better.
Our respected colleague Ryuji very professionally outlined one of the possible problems associated with the reliability of this radio. And possible revision now seems quite relevant to me.
Of course, it is better if the manufacturer does all this.
73!
Vlad
Very interesting comment, Ryuji, as when I began using the TX-500 I had reports of RF feedback distorting the audio. This was mainly on 60m and when using office type headphones with a small boom mike attached. The reports stopped when I began using a clip-on lapel mike and better quality headphones. At that time I was using an inverted-V linked dipole with a long RG58 feeder, so no problem keeping it clear of the audio leads.
I purchased this radio for its water resistance as it was evidently only a matter of time before the KX3 suffered damage from rainwater. Although it lacks some of the flexibility of the KX3 the TX-500 has several useful features of it own (e.g. the wide range SWR check) and has served me very well, so I hope it continues to do so.
I have one serious complaint - it seems to be totally deaf on 10m. Reported by G8ADD (when I changed band) telling me he could hear me really well on 10m - why did I not reply? Disappointing to discover on my first attempt at the 10m Challenge but almost certainly correct as there is never any activity on the band when I listen around.
73,
Rod
Rod, I think all doubts will disappear. If you measure the sensitivity of your transceiver. Most likely everything is normal there. Several times, on the 10 m range, I observed the effect of one-way propagation of radio waves.
73
Vlad
Vlad, if you could replace those 0 ohm jumpers with 22uH or greater RFCs/inductors with self-resonance frequency above 30MHz and enough DC/AF current capacity to drive the speaker, it may prevent the future problem, if the failure is indeed caused by the common mode RF current. You could wind the external speaker wire on a 31 ferrite material several times, if you don’t want to open it up again. Equally importantly, I would put a ferrite chokes (31 material best) on the key line, DC power line and the feedline coax. It is particularly important to make sure the RF impedance is higher than 1000 ohm and better yet 2000 ohm on the feedline choke. I personally find it best place a meter or two away from the antenna feedpoint, but with some transceivers (with poor RFI hardening) it is useful to have a second choke (smaller impedance is often adequate) right by the transceiver. If the feedline choke becomes warm, that means you don’t have enough choke impedance or the choke is interacting with the antenna fields unfavorably or both.
If you want to be super cautious about the RF getting into the speaker line, you could use a shielded twin wire for the speaker line, in addition to the RFC or choke. Leave the shield open on the speaker side, but connect it to the transceiver’s chassis ground at the connector. Make sure the both + and - outputs of the BTL amplifier is completely isolated from the shield. This at least diverts majority of the RF currents directly to the chassis ground not to TDA7056AT.
I suspect the common mode RF current as the prime candidate for the problem because most other possible causes of the problems are easily reproducible by any decent engineer and they probably would be ironed out by now. The chip also has self-protection mechanisms built-in, and it is not too easy to destroy that type of chips through normal operation. The RF issues are very situational and most engineers don’t know that it is impossible to test the equipment in all possible conditions. So, it is more important to use theoretical understanding to imagine the worst case and prevent it before it happens or at least suppress it to a level that doesn’t cause permanent damage, especially if toughness and reliability are crucial for the brand and the equipment design.
When some symptom like RF feedback into the microphone line or any other erratic behavior is specific to some bands and not so on others, the problem is most likely RF common mode current and RF grounding problem. First thing is to organize the wires. The feedline should go one way, the others should go the other way. Then check RF grounding issue if there are peripheral devices. Then make sure RF chokes are adequate.
I should add that inadequate measures to prevent common mode RF current related malfunctions is very common with equipment sold by some well-known manufacturers (one of the less expensive and popular Japanese brands). It is also pretty bad with some well-known QRP kit brands. But I don’t really know of another case where AF amplifier chip is fried, so there is some small mystery there, but at the same time, I don’t know of other cases TDA7056 used in a modern transceiver. (Earlier, I was looking for the schematic and board pattern diagrams online without any luck, but Vlad’s closeup photo and comments indicated that the RF block inductors are missing, so that alone is concerning.)
Ryuji, thanks for the great recommendations! I think it’s like the 18 basic techniques in Aikido. Only this is EMC-Aikido. This will be useful to any mobile radio user. I recently had to solve a similar problem at work. To the techniques listed above I added one more - additional radials at the base of the antenna.
Proper radial length and placement are crucial for 1/4 wavelength and loaded (shortened) vertical antennas. Where quality of radials/ground plane is difficult to secure, 3/8 wavelength vertical is very effective but requires a matching circuit. But your copper tape solution should be very effective.
For SOTA and POTA activations, I find a loop radial to be very effective, simple/quick to set up and take down, so that technique is my current standard. Maybe we’ll discuss that some other time.
It’s a form of elevated radials. Instead of using 3 or 4 elevated radials, it uses one single loop, where both ends are connected to the feedpoint “ground” or the coax outer conductor. It has an advantage of self-balancing the RF current in the radials (with multiple discrete radials, the majority of the RF current often flows into just one radial due to the slightly uneven geometry, and the rest are not doing much). Another advantage of loop radial is relatively small footprint required.
For my typical SOTA activation, when I use a Carbon6 mast, the radiator is 5.2m long, and the loop is 6.5m long. I have a 1:1 4:1 9:1 tapped Ruthroff transformer at the feedpoint, so with that and the ATU in KX3, I can QSY 10/12/15/17/20/30m without changing the radiator or the radial. Note that I lifted up and insulated Carbon6 off the ground with a short section of fiberglass tube.
I’d be happy to talk more about that, but I also don’t want to highjack TX-500 thread.
You guys have me terrified. I bought my TX-500 when they were first released in the country and it’s been fine. I rarely use it with the mic instead I use a Heil headset and a couple of stereo to mono or mono to stereo plugs to fit the adapter. I honestly can’t remember which way round it was. My aerial has always been a carbon mast 6 - 10m and a SOTABEAMS bandspringer or bandhopper. If the audio goes I’ll be gutted, I don’t actually know what servicing is available but I do know it’s beyond my skillset.
Hi Paul.
If you have had no problems since you started using this radio, then you don’t need to worry too much. The component that failed in my case turned out to be of good quality in your transceiver. The risk of unexpected failure is minimal. I think so. But it is highly advisable to follow the simplest advice of the respected Ryuji. Install ferrite chokes on the power and headset wires. For example, in the form of special ferrite “latches”. This is relevant for work in the field and at summits.
73
Vlad
but almost certainly correct as there is never any activity on the band when I listen around.
