Headache

About two weeks ago I staggered into my local friendly music store and saw an HH Performer head stuck in a dark corner. When I asked the manager (Jeff — shameless name dropping I know, but what the hell?) how much he wanted for it, his usual sunny disposition clouded a bit. Apparently he'd taken it in part exchange and had not spotted that one of the channels was all but dead. Taking a chance, I offered to take it off his hands provided the price was right. After a bit of animated haggling I walked out of the shop with a slightly defective amp head that had one saving grace — I got it for a song.

Having worked on these amps before I was well aware that a fairly common fault was the blowing of the input transistor. The fact that the other channel worked perfectly gave the power amp section a clean bill of health and so it was a pretty fair assumption. This is probably the most common reason for any transistor amp going on the blink, so a few words about fixing such a problem would seem to be in order.

Testing the Input

The first problem is to find out which of the many transistors in the circuit is the input one. The easiest way of going about this is to look for certain recognisable features in the input circuitry. If you take the cover off the amplifier you can spot the input jack socket of the channel concerned. Almost certainly this will be connected via a piece of co-ax to a printed circuit board. Looking at the underside of the PCB you should be able to follow the printed track from the co-ax centre lead to see where it goes. This will link to an electrolytic capacitor which, in turn, will either be connected directly, or through a resistor, to the base of a transistor. Figure One shows an example layout. In this example the input comes through C5 and R8 to the centre leg of TR2, the input transistor. (Transistors are easily recognisable by the fact that they have three legs). If you look at Figure One you should be able to trace the input through to TR2 using the copper tracks and components.

A couple of points are worth mentioning: firstly, in my particular example, the first transistor is labelled TR2 — you can't rely on the numbering system; secondly don't asume that just because you've found the input transistor that the fault must automatically lie there. Before going any further you should make sure.

To test the circuit, connect up a speaker and switch the amp on. Turn up the volume and tone controls and touch the core connection of the input co-ax at the point where it joins the circuit board with the tip of your finger. This is perfectly safe, but make sure you don't accidentally touch the mains or power supply whilst you're doing it. Systematically work your way through to the input transistor by touching the bottom and then the top lead of C5, then the top and bottom lead of R8, in my example. If you can't get your finger to any of the connections then touch them with a piece of uninsulated wire held in your hand. The point where a very loud, rattling hum comes from the speaker shows the point from which the circuit is working. The components before this point include the dud (which could be a crack in the printed track). In the example circuit a buzz from R12 would indicate that TR2 is dud whereas a buzz from C7 shows that TR1 or TR2 or both is/are dead.

Life is made a lot easier if you can get hold of a circuit diagram (HH were very helpful and supplied one for the cost of a SAE). Figure Two shows a section of the one circuit diagram which relates to the board in Figure One. Diagrams are usually read from left to right so the signal path through C5, R8,TR2, R12, TR1 and out through C7 becomes a little more obvious. The majority of manufacturers also print the component locations on the component side of the PCB whilst the circuit diagram should give the component size/type. If the diagram is not available then copy the numbers from the faulty transistor and take this information to the shop when you go to buy the replacement.

All Change

Once you've bought your replacement, all you have to do is fit it. In most cases you will have to unbolt the circuit board from the case to gain access to the copper track side. Some manufacturers fit PCB mounted pots and use these to anchor the board, in which case you will have to remove the knobs and fixing nuts in order to move out the panel.

With a hot, clean soldering iron touch each of the transistors older joints in turn whilst pulling gently at the transistor with a pair of pliers. Repeat this process until the transistor comes out. Put in the replacement transistor making sure it is the same way round and flow fresh solder on each of the joints. Test out the amp before you reassemble just to make sure that you have actually cured the problem.

For those of you with Performer amps I'll slip you a little extra information which may make life a bit easier. On my particular amp the sustain input on channel 2 was very much quieter than the channel 2 normal input. This indicated a fault in one of the two transistors in the sustain section. As in Figure Two it turned out that TR2 was the first in line with the input so I put my finger on TR1 input and was rewarded with a buzz. Having pinned down TR2 as the villain I removed the circuit board by taking off the knobs and loosening the clamping nuts of the pots using needle nosed pliers to locate in the slotted, circular nuts. TR2 was desoldered from the board and turned out to be a BF245C. None of my local electronics shops had one but I was able to get hold of a BF245B (which has a slightly lower gain) which seemed to work okay.

Total cost of the repair? — 69p. The cost of the repair through a shop would be unlikely to give much change from £15 and would end up taking considerably longer than the half hour the job actually took.

For those of you with Performer amps the information shown in Table One could be useful in the future so it's worth cutting it out and sticking it inside your amp case.