Vintage Sep 1974
22 Watt @8 ohms
Service Manual uploaded
I am (still) the proud owner of a fully functional SA6200. I bought this amp new in 1974 at the tender age of 15, and it remained my front-line amplifier for about 10 years. Thereafter it was moved to the garage, and storage, etc. But every time my replacement amp needed fixing, somehow the trusty old SA6200 was brought back into the lounge.
Of recent times I noticed the left channel was much noisier than it should be, so I decided to give the amp a treat and refurbish it. It turned out to be quite straight forward: one thing you can say is that mid-70’s technology is a breeze to work on compared with more modern and integrated equipment.
Basically I replaced all the electrolytic caps throughout the amplifier with the equivalent capacitance in 105°C long-life caps, with a few exceptions as follows:
• The main reservoir cap (4700uF 63V) was replaced with 10,000uF 80 volts, because it fitted the clamp. As a follow on, I replaced the main rectifier diodes with 1N5408 diodes (3 amp continuous) to avoid blowing the old diodes due to the increased inrush current on turn on.
• The power amp output decoupling capacitors (3300uF 35V) were replaced with 4700uF 50V caps, again because that was what I could get at the time that fitted the clamps. I have not had any problems with these although I don’t thrash the amp the way I did when I was 15…..
• Any electrolytic cap of 4uF7 or less was replaced by the equivalent polyester cap. There were some issues fitting the physically larger polyester caps but for the most part there was no problem.
• C3 on the Switch Assembly Board B (470uF 35V) was replaced by 1000uF 50V. This was done to provide additional filtering for the Equaliser Amplifier which in the original design does grumble a bit due to the high gain at low frequencies. This change is acceptable: if you check the time constants of the successive supply smoothing networks for the Equaliser Amplifier, the dominant time constant is that formed by R1 and C4 on the Switch Assembly Board B. Further, the biasing of the transistors in the Equaliser Amp is such that they must turn on in a specific sequence that is marginally enhanced by the slower rise time of the power supply due to the increased value of C3. As far as I can see, there is no prospect of incorrect turn on in this stage. Odd as it may sound, it would seem that the original designers did know what they were doing!
• The noise problem was traced to Q1 in the Control Amp. This was replaced by a BC550C. I had no concerns about this substitution as this is a relatively low gain stage (4.7 x) and the high frequency characteristics of which ever transistor is used there (within reason) are totally swamped by the base-emitter capacitor C3 (47pF).
• As part of the fault-finding I replaced all of the resistors around Q1 with the equivalent value 0.5W metal film resistors.
• To keep things even, I made the same changes to the other channel of the Control Amp assembly.
• Finally, many years ago the transistors in the Equaliser Amp had blown and at the time I replaced them with BC548C transistors. Since I had a couple of spare BC550C transistors which are supposedly lower noise devices compared with the BC548C, I swapped out Q1and Q2. Stability is not a problem, but if I ever have time I will do a proper noise analysis to check that this change was justified. Sadly, I don’t expect to use the Equaliser Amp much: my old vinyl collection is no longer useable.
Apart from changing the capacitors, I made a point of leaving the Power Amp assembly alone. I have no information about the normal bias current setting on the output transistors so no resistors were changed anywhere in the Power Amps. The Power Amps were working correctly and acceptably quiet so why mess with them unnecessarily?
The result? Well everything works as it should. I don’t have any “before and after” test results but the excess noise is gone and sonically everything seems fine. I checked using my oscilloscope for any sign of oscillation or instability and no problems were noticed. All the bias voltages were as they should be as well. So hopefully I will get another 35+ years of service from this fine little amplifier and if I’m still able to hear anything at all at that point then maybe I’ll do it all again.
Design and Content © HiFi Engine 2006-2016
FAQ | Site Policy