How To Build STK435 Amplifier IC Tester
A technician friend of mine called me up and asked my help to trouble shoot a National Component system, in which STK435 was used as the Audio Amplifier. The problem was that some other technician or technicians have tampered the circuit, with a lot of track cuts joined by wires. The board was in a total mess. The complaint was that one channel is short and IC gets hot. I went to his shop and studied the whole circuit and saw a demo of the complaint. Since my workshop was full and I already had enough backlog, adding this unit also for my attention, requested by my friend, could not be heeded to. Moreover, his customer wanted it urgently.
Though I had one STK435 with me, I did not want to take the risk of putting it on that set and possibly damaging this too! So, I came up with the idea of making a separate Testing Board for this purpose, using IC sockets. I always love to take a break from my monotonous service work, to assemble something new! I downloaded the STK435 Amplifier Circuit from the web (www.electrtonica.ro):
The components and accessories required for assembly are listed below:
Electrolytic Capacitors: (One 100K stereo volume control pot – See explanation of this at the last, as you cannot see it in the schematic) 1000/63V – 2 Nos, 0.47/63V – 2 Nos, 100/63V – 1 No, 10/63V – 2 Nos, 47/63V – 2 Nos, 220/63V – 2 Nos. Fixed Capacitors: 470pf – 2 Nos, 0.1mfd (100 Nano) – 2 Nos, Resistors (all quarter watt except 4.7 Ohms, which could be ½ watt or more: 100 Ohms – 1 No, 220K – 4 Nos, 390K – 2 Nos, 1K – 2 Nos, 120 Ohms – 2 Nos, 12K – 2 Nos, 4.7Ohms – 2 Nos, STK435 IC – 1 No, Small crocodile Clips Red and Black – 1 each, Speaker Output Sockets – 1 (set of two Reds and two Blacks), RC Sockets – 2 Nos.(White (For Left Channel) and Red (For Right Channel)), preferably straight soldering type (Not on sides with a soldering tag and nut), IC Sockets cut from a readily available dual line 18 Pin IC socket, gaps of which should suit the gaps of pins on the IC, 1+1 shielded wires and other single strand wires or cut leads of components, and finally a common PCB with independent copper spots, instead of linked lines. I prefer this, as we can interlink and place components easily for inter connections, using jumper wires or cut leads of components, as we like it.
There are two types, one just copper and another with solder coated. Solder coated is easier to use, as it does not require cleaning the copper side and the solder will bond very well. See pictures below: (1) Shows independent spots and linked spots, both solder coated (2) Independent spots without solder coating. (3) Directly soldering type RC Sockets (salvaged from old boards) (4) IC sockets (marked with arrow). Please see the difference in gap of IC pin inserts in various sockets. The one I used is marked with an arrow.
These are the occasions that we can make use of salvaged components and accessories. I used RC sockets and speaker connector, which had one black socket broken. As the speaker outputs from the IC have common ground, I need only one Black! Since I am going to use this board as a Tester, I placed the IC sockets in the middle of the PCB, so that I can get both sides to place components on each pin and avoid congestion or cramming. If the PCB is assembled for using it as a permanent amplifier, we need to place the IC at one end, positioning the heatsink fitting side facing out. For such purposes, the interlinked(interlined) PCB will be better for using, as we can get one line for each pin to fit components.
Picked up components and put it in a tray, cleaned the leads using tweezer thoroughly, and soldered the IC sockets first. If you watch in the picture of finished PCB, you can see that I soldered 9-socket-side one way and the rest of the 6 the other way. This is to ensure that the IC seats in properly when pushed, without which, it might slide out. This zig-zag arrangement will hold it in because of the spring action of the STK pins. All these are learnt from experience. Finished populating the components, taking extreme care in picking up resistors, as the color codes on these are many times misread, leading to malfunctioning or damage to ICs. The advantage of keeping the components in a tray and fixing components of one channel first, is that when we progress finishing the assembly one by one, if we find any component in the tray which are extra or not there, we can be sure that we have made some mistakes somewhere, and then recheck and rectify it. In my case, I placed 0.47Mfd in place of 47mfd and found this out when I was fixing the components for the second channel. So, this systematic method will eliminate human errors. Fixed a 100K stereo volume control potentiometer at the inputs; i.e., the input wires (shielded wires) were soldered at the centre pins of the volume control, and the wires from the input RC sockets were soldered to hot points, grounding the other two points. Fixed the speaker connectors, input RC sockets, power supply connecting wires with crocodile clips (Red wire with Red Clip for positive and Black wire with Black Clip for Ground) soldered on the tips, and finished grounding the various points using a single strand wire, without cutting it, i.e., I removed the insulation only at appropriate places of the black wire. Inspected all connections visually and ensured that these are ok. Please see pictures below:
Connected output speakers and power supply, fed audio input from a USB player, adjusted the input volume to a pleasing level. Wow, the output was superb!
I asked my friend to remove the STK435 from the amp and bring it, after cleaning the leads of solder. Inserted the IC and as expected, the IC was perfectly working. So, the problem is on the board, which he has to trouble shoot. I gave him a copy of the schematic.
Another different job done to my satisfaction!
For knowing the supply voltage requirement and pin details, please visit this link:
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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