The classic reasons for gain reduction in vintage audio equipment could be failing electronic capacitors and transistors. Usually, older silicon transistors are bound to fail over time, crackling sounds and loss of gain being some of the symptoms. It would be good to keep an eye on the hFE readings while checking these. Folks who do complete restorations of vintage equipment (in general) usually ensure:
- All electronic caps are replaced but the PFs are not ignored, they are checked and replaced as well
- All darkened or brittle resistors are replaced
- All silicon transistors are replaced
- All voltage regulators or regulating transistors that get warm/hot while in use, are replaced
- All power transistors are removed from heatsink, residual heatsink compound should be cleaned, replaced with a new coat, check the thermal insulators and install new ones if needed
- All pots, switches and sockets need to be cleaned, or replaced
In addition to the above, check voltages across the PSU stages and keep an eye on the rectifier diodes and zener diodes. If present, check bias voltages.
Once you tick all these boxes, you will have a piece of equipment which is as good as new.
Ps: some restorations include upgrades to lighting as vintage gear features small incandescent bulbs. These tend to cause hum especially if they are not feeding of separate and dedicated windings from the power transformer. However, when using LEDs keep an eye on interference.
Resources:
hFE stands for:
Hybrid parameter
forward current gain, common
emitter
Good resource for checking Transistor data:
https://alltransistors.com/
How to run a hFE test:
https://www.testmultimeter.com/2020/04/blog-post_84.html
