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Old 23rd May 2020, 4:48 pm   #1
eddie_ce
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Default Problems with MiniMod

Finally having got round to some homebrew work, I decided to build the MiniMod. Unfortunately without much success. The RF side of things is fine, the LC oscillator is stable, the output coil has a good defined peak.

However, the LM386 is extremely unstable and bursts into oscillation at the RF frequency. If I disconnect the 390R resistor to the BC109 the IC is somewhat more stable and does what it is supposed to do although it still does sporadically burst into oscillation. I have removed the capacitor between pins 1 and 8 with the aim to reduce gain, but the problem persists.

I have read elsewhere that the LM386 is prone to this sort of problem Does anyone have any tips on how to tame the beast. I have designed my own PCB, but can't see anything obvious, I have kept the input and output tracks as far apart as possible.

When in rogue oscillator mode the circuit draws 130mA and needless to say the IC gets hot after a short time. I am running it with a 9,5V supply. All decoupling is in place and as close to the IC as possible. I have soldered an extra 100nF C directly under the supply pins of the IC.
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Old 23rd May 2020, 10:14 pm   #2
ColinB
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Default Re: Problems with MiniMod

Single layer PCB, or double sided copper? Post a pic of the board (or better the CAD), we may be able to see what is happening...
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Old 24th May 2020, 2:45 am   #3
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Default Re: Problems with MiniMod

LM386's must have supply decoupling as close to their supply pins as possible.

Also try isolating the LM386 + supply from the rest of the circuit with a 10-100 ohm resistor followed by said decoupling caps.
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Old 24th May 2020, 8:58 am   #4
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Default Re: Problems with MiniMod

I'm not familiar with the MM cirduit - does it have the zobel network on the output and the RF bypass on the input? I've seen some terrible liberties taken with this little chip, even in the magazines!
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Old 24th May 2020, 9:32 am   #5
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Default Re: Problems with MiniMod

Hi Eddie,
Everything you ever wanted to know about the Minimod is in this thread

https://www.vintage-radio.net/forum/...d.php?t=149604

Including the original article from the BVWS Bulletin. There is a circuit diagram shown in post #4.

Luckily I didn't know there might be instability with the LM386, so I have not had the least problem with it!!

I've built three MM's, the first on a tiny piece of Veroboard with no thought for the layout, the other two on PCB's. They have all worked perfectly.

My thought would be to try another LM386 from a different source. IC's seem very variable these days, there seems to be loads of either poor spec or just plain fakes of many components out there.

It's a great little unit: I just need to finish repairing the radio it was meant for!

Cheers
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Old 24th May 2020, 10:22 am   #6
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Default Re: Problems with MiniMod

The LM386 gain extends into the MF area. The normal cure is to connect a 1nF disk
ceramic from pin 3 to ground e.g. pin 2. This value won't affect the audio range.
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Old 24th May 2020, 6:15 pm   #7
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Default Re: Problems with MiniMod

Guys, thanks for all replies.

I have tried additional decoupling and also a 1nF betweeen pins 2 and 3 (directly soldered to pins on the underside), all to no avail.

I have come to the conclusion that either my layout is somehow causing some unwanted feedback, although I can't find a cause or I have received a dodgy batch of ICs, these came from a german supplier who I consider to be reputable, but you never know. The markings on the ICs are very faint and difficult to read. They are marked as LM386L.

I have a second board in the etching tank as I write. This is more or less the design in the Bulletin or as supplied by David (EBT). I will use this as a test board to find out what is going on.

I will keep you informed.
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Old 25th May 2020, 1:09 pm   #8
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Default Re: Problems with MiniMod

Hi Eddie, it sounds like you might be using a mains power supply. Have you tried a 9v battery instead? I built mine on veroboard and haven't had any probs. Cheers, Jerry
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Old 25th May 2020, 3:32 pm   #9
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Default Re: Problems with MiniMod

I have made a second, unsuccessful attempt with a PCB layout based on the one published in the Bulletin. Again, the oscillator and output coil are fine with a nice peak.

The whole circuit draws approx. 13mA when no signal is applied.

When I apply a signal, it takes only a few seconds and everything takes off and the current rises to about 130-150mA and the IC gets too hot to touch.

CR57 - Yes, I am driving it from a mains supply. I have tried 2 different 9V supplies without success.

The problem doesn't seem to be the layout. I am wondering if I have been sent a duff batch of ICs. I have ordered some LM386N from a different supplier. I will have to wait for them to arrive before continuing.

My ICs are are labelled LM386L. I have seen in some adverts on a certain auction site that the voltage range of the LM386 is given as 4-6V, could this be a clue?
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Old 25th May 2020, 5:44 pm   #10
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Default Re: Problems with MiniMod

Well, it seems that Cathoderay57 has hit the nail on the head.

It seems that the internal resistance of my power supply is too low. If I place a 330R resistor between the PSU and the MiniMod all seems fine and dandy. It has been running now for about 20 minutes drawing a stable current of about 8mA.

No discernable distortion. I have placed a resistor of 120R between Pin1 of the IC and C9, this lowers the gain of the amplifier somewhat which better fits the default volume level of my MP3 player and prevents overmodulation.

So it looks as if I now have 2 Minimods.

Thanks for all your replies.
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Old 26th May 2020, 12:55 am   #11
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Default Re: Problems with MiniMod

Quote:
Originally Posted by eddie_ce View Post

My ICs are are labelled LM386L. I have seen in some adverts on a certain auction site that the voltage range of the LM386 is given as 4-6V, could this be a clue?
Supply voltage - 4-12V for M & L & no suffix versions and 5-18V for the N-4 version.
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Old 29th May 2020, 9:23 am   #12
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Default Re: Problems with MiniMod

Just to bring you all up to date.

I have received a new batch of LM386 with the suffix N from a different supplier. I replaced the LM386L with these new IC's and what should I say, everything is working as it should with no 330R resistor in series with the supply. Incidentally, the resistor was also required when powering the MiniMod from a PP3.

So either the LM386L came from a dodgy batch or the LM386L has different characteristics to the LM386N. Although I am led to believe that the only difference is that the LM386L is RoHS conform. Maybe somebody could put me right on this.
I hope this information might be of use to someone building the MinMod, which I must say is a great design and thanks go to its designer.

All I have to do now is sort out the modulation hum.
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Old 4th Jun 2020, 3:52 pm   #13
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Default Re: Problems with MiniMod

Just an update.

I have just finished restoring my Ultra T401 and using the MiniMod I have just listened to the program featuring Gerry Wells which was broadcast on the BBC World Service some while ago. I thought the right program for the MiniMod's first outing.

Audio is excellent, very pleased with the two I have built.
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Old 20th Jul 2020, 11:13 am   #14
Ian - G4JQT
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Default Re: Problems with MiniMod

I seem to have missed this thread. (I wrote the Bulletin MiniMod article.)

I can't imagine a power supply having too low an internal impedance being a problem, so it does look like you had a duff LM386, although what internal fault could cause such a problem I don't know. Anyway, very pleased you sorted it out.

The only other problem I've heard of with the MiniMod is drift and severe FMing. With such a simple design a little FMing is inevitable, but not noticeable normally.

The severe drift I believe was due to a bad batch of inductors sent to Spectrum Communications.

Anyway, glad to read the little project is still being used.

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Old 20th Jul 2020, 11:56 am   #15
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Default Re: Problems with MiniMod

The MiniMod is a fine piece of equipment, performs extremely and such a simple design.
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Old 20th Jul 2020, 12:20 pm   #16
M0TGX Terry
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Default Re: Problems with MiniMod

I use my mini-mod almost every day, and if my 2 year old grandson comes here and there's no big band 1940s music playing through one or other of my radios, he stands in front of the group of sets and shouts "Moooosic" until the situation is rectified! The Mini Mod is amazing (just ask my grandson).
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Old 20th Jul 2020, 12:36 pm   #17
eddie_ce
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Default Re: Problems with MiniMod

Hi Ian,
before I continue just let me congratulate you on a fine design. As others have stated on numerous occasions, for a basic design it works very well.

My problems were indeed due to duff LM386s. A batch from another supplier performed perfectly. This whole fakery malarky seems to be getting worse, if indeed they were fakes. I ordered my coils a long time ago from Spectrum and never got round to making the MiniMod until recently and have abslouely no problems with drift. After initial drift of a couple of hundred Hz at switch-on, it is very stable, possibly due to the high quality mica caps used in the tuned circuits. I have two MiniMods in regular use now.

Thanks for replying to my original post.
Regards
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