Golden Age Project PRE73 MK III - Issue

[StaggCymbalsMapexDrums]

Hi everyone,
I recently sent my 'Golden Age Project PRE73 MK III' to a local electrician for repair, could anyone recognize the fault by hearing a short clip please?
The lad who is willing to take a look at it, 'modded' 4 of these Pre's in total for me, upgrading the transformers to Carnhill's, as well as installing vintage Motorola 2N3055's. The other 3 modded Pre's are still working excellently well, but this one, which was setup for my 'O/H Floor' mic channel, developed the fault recently. I've attached the drum clip, which clearly shows a break in the recorded signal, which progressively gets worse.

I can't work out if it is maybe to do with one of the vintage 'Motorola 2N3055', or possibly the Carnhill transformer mod, but basically it can record well to start with,
albeit a lower signal, but then usually after a few minutes, it begins to lose signal, and crack/distort and just drop out altogether in the end.
I have attached a google drive link of a small MONO CHANNEL MP3 audio clip, to try and demonstrate the issue. It was recorded as usual @ around -12db.

Cheers,
Dan. (Wales, UK)

https://drive.google.com/file/d/1fUW_Mh ... drive_link

[Hugh Robjohns]

Changing over time like this suggests changing biasing conditions (due to heat or dodgy caps), or unhappy transistors. It should easy to figure out what's going on by comparing the DC conditions between the faulty unit and a good one to start with. I'd also check for ultrasonic oscillation around the 3055.

[StaggCymbalsMapexDrums]

Hi Hugh,
many thanks for the informative reply mate, much appreciated
I will pass this onto the electrician, cheers!