Good Will Instruments Oscilloscope Repairs

I was very fortunate to acquire this oscilloscope from ZS4SF om Monk along with some additional test instruments. The list being as follows:\

1. Good Will Instrument Company 20MHz Dual Trace Oscilloscope Model GOS623
2. Keithly Model 179 Digital Multimeter
3. Good Will Instrument Company Function Generator Model GFG 8015G
4. Tabor Electronics 100MHz Counter Timer Model 6003

These units are all of a high quality and are worth looking after. They were manufactured in the 1970's. I noted two issues with the scope. The first being Channel 1 not working correctly on the Vertical axis with noise and no ability to connect via the DC switch. Also an intermittent fault whereby the beam would fade after a period of operation of about 45minutes to an hour. The scope would then take about the same length of time to become operational. Both these faults turned out to be a challenge for me to troubleshoot. They both turned out to be dry solder joints which required a total strip down of the scope's sub assemblies to identify.

Since I could not find the Service Manual on the internet I called the GW office in Taiwan. They were very helpful and sent me the full service manual. Without that I would never have been able to fix the scope. Thanks also to Monk who provided input and suggestions throughout the process.

The attached diagrams and notes show the issues and solutions. I am putting this information on my blog for ease of future reference in the event that I have additional issues.

Channel 1 input to Attenuator

In Fault condition showing no HT Voltage

No Fault condition showing HV available. 

High Voltage Circuit for CRT supply showing the Oscillator and Step-Up transformer

Voltage Divider Calculations. How to measure 2100 volts with a 1000 volt DMM