Beomaster 2400 Type 2902: Problems found during performance testing

This Beomaster 2400 had breezed through the early burn-in tests and listening to it during that time sounded like any other restored Beomaster 1900/2400 amplifier. To really compare the performance however we like to take some THD (total harmonic distortion) tests and look at a frequency response plot.

The early THD tests showed nice values at close to 6 Watts of output into 8Ω dummy speaker loads.

As I continued measuring the performance while increasing power the output began to deteriorate to where I was getting out of spec THD prior to reaching the rated 20W power output. Actually, my Beomaster 2400 Type 2902 claims 25W continuous power into an 8Ω load. The Type 2901 service manual claims 20W continuous power into an 8Ω load.

I stopped and rechecked the no-load current settings.

Those values look good so I did another physical check.

I could see some cracks in the emitter resistors. Pulling them out and measuring them didn't reveal any problem with their value though but I changed them anyway. Because the Beomaster still plays decently this is a case where it would be helpful for a hard over failure.  As it is I will have to start replacing and retesting parts in this Beomaster output amplifier. Pulling out transistors and measuring them while they are not under load will likely not reveal a bad transistor.

This will take a bit of time and I already scheduled a week or two off so I will pick this amplifier back up later.