It is my pleasure to have on the bench today an amplifier from a company not usually that well known for Hifi equipment. It is made by a sub-division of Sharp, named Optonica, introduced in 1976 to compete with some of the other well known brands of the time. The build quality and performance are very good indeed, and it will be interesting to measure how this amplifier performs when working. It belongs to the owner of the company where I work, and I have been asked to check it over.
Some views of the amplifier.....
To understand the black art of electronics is to understand witchcraft. Andrew.
Now, as with any vintage equipment, I never connect straight to the mains without some cold checks first. A few basic tests for any shorts or anything obviously wrong proved ok, so I introduced mains via a lamp limiter to allow some current limiting in the event of a major fault. The thing I noticed was that the protection LED on the front panel was remaining red. It should turn green as the relay clicks to connect the power amp sections to the loudspeakers. So, we have a fault.
Fortunately, a proper service manual is included along with a couple of spare power-amp modules.
To understand the black art of electronics is to understand witchcraft. Andrew.
I see you have the service manual but if anyone wants to play along, I've just uploaded the same to the data library.
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Well it looks nice and clean. Thanks Chris, just downloaded, usual comprehensive manual from Sharp.
Frank
Yes, the service manual is essential to enable proper understanding of the circuit operation and to enable relatively straight-forward fault-finding.
So, let's have a look at why the protection is operating.
Below is the protection circuit including the relay that connects the loudspeakers to the power outputs.
After a few voltage measurements, it was found to have about 7 vdc at the junction of two 6k8 sense resistors (R515/6) from the main power amp L+R output's. This can only mean that there is DC on one of the output amplifiers.
To understand the black art of electronics is to understand witchcraft. Andrew.
Sure enough, it was discovered that the left channel power amp module has a rather nasty 40vdc on the loudspeaker output. Now you see why protection is necessary as imagine if this was fed into your expensive loudspeakers. A puff of smoke, and it would be game over.
Here, the output of the left channel is shown, followed by the voltage reading and the actual device itself.....
To understand the black art of electronics is to understand witchcraft. Andrew.
So, it looks like the module is faulty, but this morning I will perform a few checks around the circuit to be more certain before I replace it.
To understand the black art of electronics is to understand witchcraft. Andrew.
That's one of two things (but you don't need me to tell you that!) - either (a) one power rail is missing or (b) the output module has failed. In some cases I've found an amp (a *huge* Sony 7x70W thing) had lost an emitter resistor so it had a couple of volts of DC on the output - the DC feedback doing its level best to keep the output at 0VDC !!
Should be an easy fix for you - got some heat transfer compound? Certainly an interesting circuit: I'm wondering what purpose 2pF and 4pF caps have in an audio amplifier.
I remember Optonica stuff (a school friend had some): it sort of capitalised on the growth of the cheaper end of hi-fi and the 'rack system' with lots of silver and lights but from memory, didn't last an awful long time before the name disappeared.
I'm pleased to say that replacement of the power module will not be necessary. The supply rails were correct in feeding the module, but the -42v rail for the driver stage of the circuit was absent. This turned out to be an open circuit 470r fusible resistor. Looking at the resistor, it seems not to have suffered any dramatic breakdown and a quick check for any shorts in the circuit following the resistor proved ok. So, after fitting a new resistor I applied power and was rewarded by normal operation and no DC on the output ! ?
To understand the black art of electronics is to understand witchcraft. Andrew.
So technically, right first time ?
I would however be casting an eye towards that 1uF cap (C405) decoupling the feed from said fusible. Might be worth running some signal through the amp and measuring the drop across the resistor, just in case.
That is very true Mike, also that capacitor has seen a reverse polarity voltage of about 19vdc during the fault condition.
I have briefly tested the power amp section and it can provide a shade under 75wpc rms into a resistive load of 8 ohms just before the onset of clipping. Note my series lamp limiter doing it's job as the input is increased ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Excellent result Andrew, worth a few extra brownie points with the boss too ?
There's something quite therapeutic about following amplifier repairs. I've been following Simon Spiers youtube channel for a while now and recently he's had a fair few 70's amps up on the bench, with some very interesting faults.
If you fancy some interesting Vlogs to follow then have a look here :-
https://www.youtube.com/user/simonspiers/videos
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Now, the "Sharp" eyed among you will have spotted an error in the circuit diagram......The left channel shows a 1uf decoupling capacitor C429 being applied to the audio input. That would reduce the treble quite a bit I would say. It should in fact go to the +42v rail as can be seen in the right channel diagram, C430 in this case, extreme bottom of picture.
To understand the black art of electronics is to understand witchcraft. Andrew.
It's time to take some measurements of the power amp section to see if it meets the spec.
Now, let's start off at 1 watt output so nice and safe with 100mV 1KHZ, directly to the power amp inputs. The distortion here is the same for both channels at just over 0.02 percent THD, 8 ohms resistive load.
The published spec calls for less than 0.05 percent at 70 watts output each channel. This requires 800mV output from the analyser and it can be seen that at just over 0.006 percent, we are well within spec. (Both channels are again very closely matched). As my load resistors are rated at a maximum of 50 watts, I certainly can't sit here for too long at full power. Besides, there are better way's of heating the workshop ?
To understand the black art of electronics is to understand witchcraft. Andrew.
Nice looking amplifier. I quite like working on this sort of thing too, probably because I can make a noise with it when it works!
I had a similar failure on one of my Sony FH-7's, fusible resistor had gone open for no obvious reason, I replaced it and it has been working fine since.
Regards,
Lloyd
Thanks Lloyd, well this amplifier has all the trimmings to make a nice noise with....
So, without further ado, let's get my test LP on the turntable and say "Bye Bye Blues" with Bert Kaempfert..... ? erm yes, I could choose something better perhaps.
To understand the black art of electronics is to understand witchcraft. Andrew.
The numerous switches were rather noisy and responded well to a squirt of switch-cleaner. The volume control pot and tone controls are very good quality items and needed no such treatment.
To understand the black art of electronics is to understand witchcraft. Andrew.
Only 75W per channel though. Pffft ?
Only 75W per channel though. Pffft ?
Lol, well I guess it depends upon what loudspeakers you are using and how you value your neighbours.
To understand the black art of electronics is to understand witchcraft. Andrew.
My speakers will take 600W... I did think of a Musical Fidelity M6-500, but the price was a wee bit on the high side. So I will have to live with a piddling 220W per channel ?
