Commercial Equipment Repairs

1. HP1741a Oscilloscope

I have had two of these instruments on my test bench, both with the same problem - a faulty power supply board. In both cases one or more of the 723H regulator ICs had failed. One of the units had been run on the UK mains with the voltage selector set to US mains (120v) so it is a testament to the designers that the power supply did not fail immediately. One of the stabilised supply rails is used as a voltage reference for the others so fault location can be more difficult. It is important that replacement 723H metal can devices are used and not the standard 723 regulators. While the unit is on the bench do check the small bridge rectifiers on the same board for continuity and replace any that are faulty. All components that look as though they have become swollen or over-heated should also be investigated.

So far my own 1741A scope is working well.

Heathkit SB-Line Units

a) One of the most common faults with these units that were mostly manufactured on the 1960s is solid carbon resistors that have aged high over the years. The only solution is to replace all that are out of specification with new carbon film parts of the same power ratings - most are half watt and the modern replacement parts are normally 5% tolerance. There is no need to use the more expensive metal film types.

b) Electrolytic capacitors do dry out over long periods of time, particularly if located next to a valve or other heat source as is the case in some of the receiver audio amplifier boards. If they do not reform correctly or show a significantly different value or high series resistance on a component tester then they should be replaced. See here for information on reforming capacitors.

c) The pressed pin sockets in some valveholders lose their tension after years of being heated and cooled and become intermittent. Best solution is to change the valveholder if a new one is available otherwise it is possible to tighten each socket by gently bending the edge in a little with a sharp scriber.

d) The plastic centre of the 0-100KHz dial drive mechanism often shows cracking or crazing due to the material becoming brittle with age or being over-stressed by setting the central friction clutch mechanism too tight. Ideally the item should be replaced but they are difficult to locate except from a parts unit so the next best option is drop the front panel forwards a little and disassemble the drive mechanism. Apply some thin acrylic or polystyrene glue in a small pool in the central plastic area but not overlapping the shaft mechanism. Then very gently push the centre shaft area upwards to allow the glue to penetrate into the cracks, push back downwards to close the cracks and leave somewhere (not too warm) to allow the glue to partially dissolve into the plastic and dry - this may take several days to harden so do not rush it. When dry and hard, re-assemble the drive mechanism, refit the front panel and adjust the clutch for sufficient pressure to just obtain smooth tuning control and no more.

I use Revell Contacta Professional liquid glue for plastics for this purpose which pools easily and is commonly available from model shops in the UK and Europe. This glue will not be accepted for air-mail due to its solvent contents but may be fine for surface mail.

PS-120 Power Supply

See this site page

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