By Member: PYE625
This is really an introduction to good practice for amplifier repairs, certainly not an exhaustive guide.
Safety precautions already suggested within the Radio and Television repair guides will apply here, as the basic principals are very much the same. The possible faults encountered have a distinct similarity too.
First Steps & Checks
If you are looking at a vintage valve amplifier, be it from a record player, radiogram, tape recorder or HIFI separate such as this….
Then an important thing to check are the capacitors. These can become leaky by passing DC in exactly the same way as in radio and television sets. Here is an example of metal cased coupling capacitors neatly arranged on a board…..
I would say it is essential to obtain the circuit diagram at the very least, preferably a service manual. You need to become familiar with the layout and circuit first before going anywhere near the mains. This way, it can be ascertained if any repairs or alterations have been made before. More importantly, you must perform some cold tests for any shorts and a visual inspection for any burning or obvious damage. The transformer windings may be tested if you feel it necessary for continuity (or shorts where there should be none), and of course that the mains tapping is set correctly.
Assuming all is ok so far, you can then test any smoothing capacitors in the HT line. They will almost certainly need to be re-formed. This is vital to avoid any damage to valve or solid-state rectifiers upon powering up later.
This should be done out of circuit and an evaluation of their condition can be made. If necessary, any temporary replacements can be fitted to enable powering up of the equipment to allow further testing to be completed.
Note: DO NOT use raw mains.
An isolating transformer together with some limiting of mains current is essential. A lamp (say a 100w light-bulb) may be installed in series with the live of the mains and this will provide a degree of protection against damage to the equipment if a serious fault should be apparent. An example of a series lamp limiter.
Voltage Checks & Resistive Loads
At this point, a quick test of voltage conditions may be made in the equipment with reference to the circuit or manual. Depending upon the type of amplifier, HT can vary from 250vdc to over 450vdc, so BE CAREFUL.
There is no need for an input signal or a loudspeaker to be connected at this time, but of course later you will need an audio source of some description (preferably an audio oscillator) and a loudspeaker or a power resistor of a resistance and wattage to suit the output, but this does not have to be exact. Here are two load resistors of 8 ohms, 50 watts.
Remember that some audio valves are extremely expensive. You do not want to operate any amplifier with a fault condition for very long as damage may occur to these. You need to observe that the biasing of output valves are within sensible conditions or else these, and possibly the audio output transformer, may be damaged. In most amplifiers, cathode bias is used. This simply means that the first grid (or grids of a pair of valves) is held near to 0 vdc and a voltage across a cathode resistor will be apparent. A pair of valves may share a cathode resistor and this should be tested to see if it is the correct value before powering up.
The same goes if individual cathode resistors are used. Here is an example of a relatively newly fitted cathode resistor and it has overheated due to excessive current caused by a leaky coupling capacitor to the associated valve.” Any cathode bypass electrolytic capacitor will likely need to be replaced too due to age and heat.
As mentioned, one thing that often upset’s the operating conditions is DC leakage through a coupling capacitor to the output valve(s). THIS IS VITAL to sort out quickly, especially if the anode(s) of the output valves begin to glow red. It may be prudent to test/replace ANY signal coupling capacitors if in any doubt. Assuming all the DC conditions are approximately correct, it is now safe to feed in an audio signal having connected a suitable load to the output. A resistor as mentioned above, plus an oscilloscope to monitor the output is preferable, but of course you may use a loudspeaker. The signal input level will of course depend upon the sensitivity of the amplifier, but best to start very low at about 10mV and work up as necessary.
Scoping the Audio
If observing the audio on an oscilloscope, you can check for a good sinewave. If stereo, it is useful to observe both together.
Increase the input and see that the clipping point is reasonably even at the tips of the sinewave. It is good to do this at 1khz. You may check the frequency extremes (20hz-20khz) but it will be unlikely to obtain full power at, say 20hz.
This more or less concludes the power amplifier section.
The pre-amplifier section will normally be fairly simple with the main culprits for faults being leaky coupling capacitors, noisy switches and potentiometers. Again, the DC voltage conditions will need to be checked but any faults here do not usually spell disaster and can be sorted by replacing any wrong value resistor or defective capacitor. A source of intermittent operation can often be noisy switches etc, but valve bases and pins can be a cause of trouble too.
Forum post with pictures here