Aliexpress LoRa Amplifier AB-IOT-433

I've been experimenting with APRS using the T-Beam LoRa hardware, as described in Andreas Spiess HB9BLA Wireless' YouTube video here. (Andreas has two YouTube channels, both are very much worth subscribing to, here and here.)

Now the standard T-beam hardware outputs around 100mW, and I'm getting a range of around 9 miles, although one packet managed to make a 12 mile journey one morning!

Perhaps a little more output power would help.

AliExpress advertises a small RX/TX amplifier. A quick search for "Lora amplifier" pulls up a few, and I bought this one.

It arrives from the seller very promptly. 

It advertises a 2.3W output power at 5V supply, so you will need to have a suitable license to operate this. I have a ham radio license, so I'm good on 433 MHz.

I gave it a quick test on the bench, and it makes 2W happily at 5V, so a real improvement, however the second harmonic is not very well suppressed... 

We're going to need a simple filter. 

Using the tracking oscillator of my Spectrum analyser, I cut an open circuit length of coax to make a suitable stub filter, and once it's cut to the correct length, It's connected via an SMA T connector to the output. 

The output is measured again...


Much improved :) 

I've curled up the stub, which does not effect it's performance, and connected the amp up. It's probably best not to run the amplifier without an antenna or load connected. 

Lashed up in the car for testing... 

And the range now?

Almost no change! I'm not sure what I was expecting. UHF at these frequencies is very line-of-sight, and the range in this case is ultimately defined by terrain. A testament to the robustness of the LoRa protocol at just 100mW, and an interesting, but ultimately pointless experiment! 

Thanks to Jayne, M0JNE, for her help and thoughts on this project.

Audio Innovations Series 300 repair

Dave the Quad phoned...

"Got an Audio Innovations here, it's cracking in one channel. Can you take a look?"

Yeah .... why not?

It's a Series 300 amplifier from the late 80's early 90's. Not sure I care for it's styling to be honest ... anyway...

Off with the top, and there's a nicely laid out PCB, featuring solid state rectification, an ECC83 phono pre-amp, ECC83 pre-amps, and four triode-pentode ECL86's (the two triodes in each output pair forming the phase-splitter)

Connecting a set of speakers, and waiting for everything to warm up, soon reveals the fault. 

The third ECL86's tube base is flashing over... This can be indicative of output transformer failure, a failure in the valve itself, or the amplifier having been run with the speakers disconnected. Once a tube base has started to flash over, no amount of cleaning will sort it reliably. We'll change all four to be on the safe side. Let's hope the transformer is OK. Try as I might to get a photo of it... I couldn't.

Disassembly is a bit of a pain ... 

First off, check that reservoir capacitor has discharged to a low voltage. This particular amp rapidly sinks to a safe 14V or so after a few minutes off... but check it anyway!

Remove the knobs. They're secured to the shafts of the controls by a small grub screw with a hex head.

Unsolder the wiring to the gain control. A red, white and black wire. I wonder why this network is suspended like this? Added after to remove a bit of DC from the pot perhaps?

Undo the nuts securing the controls to the front panel, and gently lift clear. 

There are three M3 nuts, and a few plastic stand offs which must be released to allow access to the print side of the PCB. I removed the valves. Keep the ECL86's in the same order that they were removed.

I gave the PCB a blanket re-solder as a good few of the joints looked suspect, and replaced the four output valve holders.

Once everything's reassembled, the amp is tested, and, thankfully, the output transformer is OK. 

A lovely sounding amp, and absolutely silent.. no hum and almost no blow. Superb. 

It's soak tested with Matt Berry - The Blue Elephant.

Another saved from landfill! 

Martin's other Luxman R-1040

Having completed Martin's first Luxman R-1040 (and his L-80V, both documented on these pages), he wanted his second R-1040 re-capped and set up.

...why not ?

I wasn't going to do a post about this, as the job was going to be identical to the previous, but it turned out differently...

Well, after the arduous task of re-capping was complete, it was time for a test & to set the bias etc ...

This is the part where I wished I'd powered it up before I'd re-capped it ...

The bias is drifting around all over the place on the Right hand channel power amplifier. I set it up, and all appears OK, look at the meters a few minutes later and it's drifted. It's also very slightly crackling.

"That's bound to be the pot"

The bias pot, VR202b, is replaced, to no avail ... meh. VR201a (DC offset) also proves to be innocent. 

Checking the DC conditions around the amplifier, things are moving around a lot around Q204b, a 2SC945. Not having a replacement, I swap out the same transistor from the other amp. No change. 

I'm starting to think I may have swapped a cap out, and may have fitted a brand new, but faulty cap. Prime suspect being C205b and maybe C204b ... they both check out OK. 

I then decide to probe round the stage, under quiescent conditions with the oscilliscope. The voltage on the base of Q204b (and the collector of Q204b) is not steady. I scope the supply back. "Spiky" on both sides of R212b, but steady the other side of R211b ... R211b somehow noisy?? I've seen this happen with old carbon resistors in valve amps before. R211b (3.9K) is duly evicted and replaced. The supply is now steady and the amp quiet...

The bias is now set for 45mA per side, and remains rock sold. DC offset is also set, and doesn't budge.

The workshop MP3 player is hooked up, hoping to enjoy some International Teachers of Pop. No such luck ... well 50% luck. 

There's no audio on the left hand channel. Not from any source.

I put a tone into the left hand channel and trace the signal through the amp. It meanders it's way through the source switching, through the balance and volume control, then disappears across the volume control buffer PCB... 

The signal enters on P301a, via the volume pot VR301a, which, thankfully is innocent, as it's one of those 4 pin affairs (used for balance) and the signal appears at the base of Q301a. The DC conditions are, once again, off here. The -23V is fine at R309, but woefully low after R306a. The voltage on the output capacitor should be -16V, it's -2.3V. Thinking back to the amplifier, R306 is checked, and proves it's innocence. It's not going to be C302a, as that is fed by a 33K resistor, and isn't capable of pulling the rail down that far. Q301a ? Q302a?... I'm betting on Q302a, a 2SC1345 (it was easier to remove!) ... a quick diode test in circuit gives inconclusive results, so it's pulled out of circuit, and put on the Peak tester...

... eh ? Should be NPN...

... don't think so ...

... nah, it's sick as a dog ...

... and has given up having much in the way of gain!

Evil thing... 

Looking the in transistor drawer, I don't have a replacement, so a BC547 is used, making sure the legs are bent to ensure the base and collector go to the right holes!

DC conditions are restored, as is sound!

I think there was originally some foam tape against the chassis to provide some insulation between the volume control PCB and the front of the chassis. This has long since turned to dust and vanished, so a bit of capton tape is used when the PCB is refitted.

After the dial illumination is converted to LED, the amp is reassembled and soak tested with the most excellent Pop Gossip by The International Teachers of Pop 

Luxman L-80V repairs and renovations.

The ever-cheerful Martin called ...

"I'm chuffed with the R-1040 you did, can you do my other one?"

Yeah ... why not. 

.. and he brings me another R-1040 to do, and also a rather nice L-80V (which was unexpected, as somewhere our wires had got crossed!)

Luxman do make some nice looking kit.

Martin wants the amp serviced and re-capped. It's got a low-level hum to it.

Anyway , off with the wooden sleeve (in the same manner as the R-1040) and I'm greeted with a very familiar sight...

... it's the same as the L&G L2800 I did here all those years ago, or at least the main PCB is. 

It's also suffering with the same leakage... Let's get re-capping the thing. 

Almost as much of a slog as the R1040....

Most of the electrolytic capacitors are in a woeful state...

Much in the same way as the L&G 2800, output transistors are a mix of manufacturers, this time on the same channels!

To get at the capacitors on the tone control and loudness control boards, first remove the knobs.. 

Then remove the screws securing the front panel, both the aluminium and plastic sections. Theses are
located on the top and bottom of the front panel. 

With the front panel removed...

... undo the securing nuts holding the two tone control pots to the front panel. 

... and, whilst access is still limited, you can now get at the caps on the tone control board

Similar process with the loudness board. Remove the four securing screws... 

... and re-cap the board.

The main smoothing caps, whilst looking in good condition, are both reading low in value. This is probably the cause of the hum. Sadly the caps I used in Martin's R-1040 are now on a long back-order time. Some other Nichicon caps are in stock, so these are ordered, but are of smaller diameter than the originals, so some sleeves are 3D printed, and the caps replaced. I've printed four, because I'm going to need 4 for Martin's other R-1040

Snug...

With the re-cap complete, the bias is set, and the amplifier is a stunning performer. 

It's given a good soak test with the most excellent International Teachers of Pop 

https://www.youtube.com/watch?v=9r_Gqx_GFEg

Now to do Martin's other R-1040 !

Aurex (Toshiba) SB-A10B

The ever-cheerful Matin's been badgering me to do a couple of amps for him, a beautiful Luxman R-1040, and an awful Aurex SB-A10B ... he's off on holiday, so dropped them off on his way past...

The label helpfully has the fault description "As they say in Brum, it woo gooo!" which roughly translated means it's doesn't work.

Fancying a quick win before tackling the enjoyable, but somewhat arduous task, of re-capping his Luxman, we'll have a quick look.

It's one of those small form factor separates, and quite well regarded (by those who don't have to fix them) Aurex was Toshiba's "posh" range of hifi.

Something's nagging me in the back of my mind, that last time I did one of these it didn't end well... 

The first challenge is to get the damn thing apart. Remove the Bass, Treble and Volume knobs, and every damn screw you can see... there are plenty. Don't forget the two recessed ones.

Now start wiggling the case around until frustration builds to an almost intolerable level. 

Eventually the top should "ease" forward. Not much room in there....

We need to get the bottom off now....

.. and at this point it's worth noting that the service manual tells lies. At no point does it mention that you need to remove this screw. Remove it. 

Now spend another 5 minutes wiggling the case like a demented fool, only to discover you can't until you remove the two plastic tubes that surrounded those two recessed screws from earlier.

... remove the two plastic tubes, take a deep breath and start wriggling the case like a Tory MP in front of a select committee, until it finally yields. 

You are now faced with a densely packed lump of electronics. Complete with surface mount resistors (in 1979!) ...

The mains fuse is located under a small plastic cover by the on-off switch. It's visibly blown, and looks to have failed with purpose.

It's a T1A fuse, and is duly replaced.

Voltage is gently increased on the variac, whilst monitoring the current, and it's very clear something is under a lot of load. 

Measuring the output transistors on the left channel, and all is well... measuring the right hand channel and the output transistors (2SB595 and 2SD525 respectively)  are short circuit, well bang goes my quick fix... and they're made from unobtainium (enter the whole of the internet saying they're still available on eBay, off you go and order them then, good luck if they're shot or fakes). Great. 

Some discussions are had with Martin, ruining an otherwise pleasant holiday no doubt, and I reckon an MJE15030 and MJE15031 will do as sunstitues, so some are ordered from a reputable supplier. I will not be beaten by mere machinery. 

I have, however, got an awful sinking feeling... 

... eventually the Luxman is finished (phew!) ... so it's back to this ... 

The heatsink and output transistors are removed as one piece... Both transistors are lose on the heatsink! No small wonder they failed.

The NPN device has been especially warm!

Look at the state of the insulator! We'll have a nice new one.

The MJE15030G and MJE13031G substitutes are pressed into service, after checking the driver transistors are undamaged, which they are...

Both channel's screws are locked into place with a dab of thread lock.

The unit is lashed up naked on the bench, and power is supplied gently via the variac ... after a few seconds the speaker protection relay clicks, indicating there's no DC on the speaker outputs.. Good.

The bias is set up as per the service manual, and speakers connected up... 

... and knock me down with a feather, it all works and sounds great! It's left on test for a while playing The Lasters by Fred Deakin. 

Now the fun and games shoehorning it all back into the case...

... and after much cussing and swearing ... 

... another saved from landfill!