This mini amplifier is the next item from the pile of free gear I got to tinker with. Patreon www.patreon.co... Email: johnaudiotech@gmail.com Thank You!
Now that's a personal memory from my past. My wife inherited that amp and its matching speakers (single oval cone) from her brother in the late seventies. Used it for years as the "back room" music system. Now I wonder where did it go, when did I toss it, did I give it away ....
In Europe they want a rule that say the equipment must have schematic and repairable cause to have less waste. Good idea we look out for this like in the good old days 😀
@@HillsWorkbench I'm agree with you, but now it is easy from a shop to say that you must buy a new one. If it is cheap stuff, no problem but hifi, tv, washing machine or PC, 🤦♂️. We have a lack on qualified technicians cause of bad politics. Some people asking several shops, it is not repaired and you must pay for it. Popup repairshop are better but if the schematic is available or no Chinese parts it will be better.
Cool that you comment on a fellow You Tuber. I remember when going to college in Europe to get my Electronics papers we had to hand design PCB's. I was scrutinized for putting resisters upright. My attempts to argue that this is common practice with Japanese equipment got me no where. The reply was that this is proof of how bad that equipment is. Unfortunately when this happened I had not yet come across the inner workings or a Motorola Walkie-Talkie so I ran out of luck.
Yes, sure they certainly did. Thanks Nico. I realised that as soon as I wrote it..But by the beginning of the 70s most amps had standardised line level to .775v. This amp is weird! I assume you mean the output stage had no voltage gain? It certainly has plenty of power gain. For anyone else interested the output stage is usually only a follower with unity voltage gain the voltage being raised to drive the output stage by the preceding devices.
If I may propose an enhancement, bring the feedback by R160 from the negative of C142. This compensates the distortion of the output capacitor and decreases the low frequency response at low power levels.
At least there is a tape recorder output. You could use this as a line input that would effectively bypass all of the nonsense taking place at the input stage. It would be interesting to see how the unit measured under these conditions.
I made the same comments on that similarly designed RS equalizer when I went through it, a pretty strange way to do the phono Eq! Same engineer perhaps.
i am modifiering {sleeper junker}lol vintage realistic receiver its the big old unit with one cassette deck vu meters and no power lol 5 to 10 watts max >? l be unseeing a sure amp board irs mosfet 250x2 max i think from memory i tested it i love that thing i changed caps to nichycon i herd they use better Caps on them now i had this for 5 years .. i like it has trim pot to dial in switching frequency witch i needed to do after cap swap and they even have leds on each channel to let you know wens it right .i am a novice at electronics by the way...i looked it up its a Wondom brand same as Sure. now they make nice stuff we fer china stuff 2 x 250Watt Class D Audio Amplifier Board - IRS2092 store.sure-electronics.com/product/AA-AB32291
I remember these when they were new and when our local Radio Shack was clearing the last of these out. Not a big seller in my university town since most students in the 1970's could afford a higher end model. I remember Radio Shack with great fondness. Retail wise in Canada we lost Zellers, Eaton's, Simson Sears, Metropolitan Stores, Handy Andy, and from the great state of Texas, Radio Shack. Thank heavens we still have Canadian Tire. If the pandemic continues we might loose Walmart as well. Ordering everything on line is very dull and uninteresting. Also hard to judge quality although almost all Chinese assembly is pretty sloppy anyway. The words Chinese & Quality don't seem to go together. Radio Shack and our Technics Dealer had real showrooms and they always carried the top end models within their inventory. in the 60's & 70's all models were made in Japan, Quality. Now the audio showroom was really something special.
The later SA150 and SA155 also have that weird 'everything goes through the phono stage' design. I think the only thing different about the SA150/SA155 is that they use an LA4440 IC chip and they're rated at 1.8W/channel. The SA155 is basically just an SA150 with a few extra tweaks, like a front-panel input and a 1/8" headphone jack (instead of the 1/4").
Actually, some bigger units of the era did similar things. The Pioneer SX-626, which is at the heart of my home stereo, runs everything through the phono preamp. Rather than use an inverse RIAA circuit, they simply replaced the RIAA feedback circuit with a resistor and stabilization cap, which set the response flat through the phono pre.
Now specs are all bullshit as everything from the china. I like how they purposely act like they don't know the translation but always lie on everything always in their favor. They take a 20 watt rms amp 20-20k .3%THD and go peak to peak at 20%thd then add on 500% and it's now 1000watts.
Always good practice to use a capacitor as a heat sink for the drivers. Maybe as the cap heats up it changes the bias? I must try this some time. I think I would (as an exercise) rewire the inputs so only the mag would go though the pre amp and take the line directly to the amplifier. Another option would be to remove all the anti RIAA from the line inputs and switch out the RIAA in the feedback loop with a resistors to lower the gain. Apart from getting something approaching a flat response the lower gain will kill the hiss completely. Another option is too just throw it in the bin! Amazing to think all that quite complex circuitry could be replaced by a single chip costing a couple of $ and will probably sound better.
Nah, keep it original as time capsule. This is an interesting and rare amplifier because many have done exactly what you suggested as alternative option.
I think even up until the 90's Radio Shack were including schematics in the users manual. Back in 1997 I got my first TV which was a Radio Shack TV, and the manual had the full schematic in it.
Used to spend hours browsing through the Radio Shack (Tandy here in the UK) catalogue in my younger days, I remember that amp and it's little brother. Spent a lot of my allowance in the local store and another independent electronics shop, both long gone.
Back in college, I used to show movies in a small (110 seat) theater. The sound system was a Pioneer Spec 2 driving a pair of JBL speakers. When setting up, I'd set the sound level to 1W/channel. If it went to 10W, people complained it was too loud. Don't underestimate what a few watts can do in a home setting.
My first hi-fi was a RS Modulette 949 Receiver/Cassette Stereo from 1979 that had the same simulated woodgrain finish, pressboard back, same font and RCA speaker connectors. I needed to get a preamp for my turntable though It even had the schematic glued inside the unit. It was used for many, many years until I Iet it go about 5 years ago. I'm sorry I did that. It wasn't the best sounding, but at least I could pick up Casey Kasem's show each week! Give Snickers a can of sardines for me!
This one is sort of a cross between a transistor radio and a "proper" hi-fi integrated amp... I guess that's about all you could expect at the price, too. The smallest models made by Pioneer, Kenwood or Marantz in those days would be downright powerhouses at 10-15 Wpc with substantially more complex circuitry and bigger everything.
Can you test tba 810 IC? This is a very old IC used in old cassette amplifiers/Televisions. Also keep posting videos about “Ic’s done right series”. Eagerly waiting for next IC video.
I've got one that still has the sticker on it saying "Simulated Walnut Finish". I kept it on there because I couldn't tell if they were treating it as a feature or a disclaimer. Or a joke.
Cool, wood grain realistic. I found a General Electric Model RA200A at the Goodwill. It is in excellent condition. I cannot believe that placed $12.00 on it. I have to clean the potentiometers on it & replace 1 bulb. Other than that, it is a beautiful vintage stereo! It is rare to find stereo systems as old as me or older in great condition!
I'm going to guess that BMS is something like Burst Modulated Sine. A classic method of inflating power figured is to measure it with short bursts of signal so the PSU filter cap voltage doesn't have a chance to sag very far.
if you were to take the FFT transform of the input signal and the Impulse response of the RIAA network then convolve the two run the result through the IFFT does that not yield the wanted signal?
Oh the "glory" ( :\ ) of the low end Radio Shack stuff. At least this one was all discrete. My youngest bought a Clarinette 16 at the local Goodwill last December. Believe it or not, everything worked. Only real work needed was cleaning the pots and adding a 1/8" stereo input. For it's output amplifier stage, it had a 8 pin stereo chip, about the size of an LM386. And get this, the output caps are 220 uF for the 8 ohm load. Yeah, it is rather light on the bottom end, and the max output is, I'm guessing, between 1 and 2 watts per channel. I've got a write up on it on ToidsDIYAudio forum and also the Nuts-n-Volts forum. Do a Google search for either of these and then search the model # to find it. And speaking of the LM386, I bought a pair of the kits from a local RS before they disappeared and the output caps are Wait for it 2.2uF! I do believe there's room for improvement there. LOL Good to see you again. My best to you and Snickers.
@@JohnAudioTech No clue what the chip actually is. It's hidden under the stamped tin/steel heatsink that is soldered onto the board. It works, so I won't mess with it. Here's the link to the Nuts-n-Volts forum post: forum.nutsvolts.com/viewtopic.php?f=45&t=18144 There's a link to the Toid's DIY Audio post about 2/3 the way down that first page. Funny thing is that my posts are the only ones that show anything about this old low-end unit from Radio Shack. (Probably rightly so, it's weak - the stereo, not my writing lol)
Thanks for video John. ,,This video with that amp was very interesting. Yea, it was a little strange this input-arrangement --It looked like that both phono and the other inputs(tuner, tape, etc) traveled trough same level of amplification? ,,remembering yourself commented that those other "higher-level-signal" inputs(tuner, tape, aux orWhatEver) should been bypassed around this first phono-stage. Nevertheless, love to watch an listen when you contemplating trough schematics... ..Have nice weekend to Snickers, You, and your surroundings. /br, from norway. (You have good content on your channel.)
This thing has such bad design it is funny, what where they thinking with that reversed RIAA curve to run the line in trough the phono amp? I suspect this is a rebadged AudioSonic, the Crosley of it's era. Oh wait, that is rather an insult to AS because as cheap as it was, it was actually getting the job done.
Yeah, testing capacitors in circuit really tells you nothing! That component tester is your friend! I would go as far to say that the Vloss value in the top right corner is the most important detail when measuring capacitors! And yet most people ignore it. That tester is a bit on the basic side, it only gives a percentage of capacitor leakage instead of actual voltage but if it's all you have, it's definitely better than nothing! Always have a new capacitor to be able to compare to the old ones! Keep an eye on the Vloss value and always make sure that the figure is as low as possible, otherwise if it's leaking DC it's biasing up the next stage which is how components die! ESR is a less critical factor than Vloss! This can be tested, take a new capacitor and apply resistance of various values in series, then try the same thing but in parallel... This should be done in circuit to show the effects of both ESR and Vloss.. you will quickly learn which failure mode is more important than the other...
It might be that they directed the ceramic input through the RIAA section because there was insufficient gain at the main amp once a high value resistor for impedance requirements was placed in series to suit the cartridge.
@@1pcfred The gain has little consequence. I wrote my comment before seeing Johns tests - and the supply is very wimpy. An extra volt or two and a stiffer supply would likely raise output to over five watts a channel safely. I couldn't see the device numberings but they looked like RCA parts and I have similar packages from that time capable of dissipating well over 10W per device. Some in that package can be rated much higher if I remember right, with 40v collector at over an amp. Or are they Japanese??? Anyone? Edit Saw the devices on the schematic 2SA699? The camera was bouncing, anyhow they are 10W 35v 3A. So they sure are well under rated.
@@martinda7446 by the looks of the schematic this amp operates at about 21V. I really don't know why the output is so low at that voltage. I doubt another volt is going to help it any either. Those transistors probably have a gain of 6 or so. They're impressively weak by today's standards.
@@1pcfred Admittedly I was just thinking aloud. The maximum theoretical output from a push pull arrangement like this with 21V is just a tad over 6W. A push up to 25V would give the theoretical 10W I was after. The losses are due to a few things like the emitter losses, the uneven clipping etc. But mostly because of a very slack supply which droops under load. As I said a tighter supply with a couple more volts and 10W instead of 3 might be in order. With these output stages the capacitor on the output charges up to approx half supply and charges/discharges through the speakers, it is the effective power supply for the output stage.
@@1pcfred PPS Not sure why you are fixated on the gain, but I have plenty of this style transistor ad they all measure well. Hfe of between 20 and 50 is usual. Not that it makes much difference??
