The owner runs the site as a hobby on a constrained connection, so there are significant limits. From the site FAQ:
Is there a limit on the number of files I can download?
Yes, this service is provided free of charge and our bandwidth is limited.
There are daily, weekly and monthly limits to the number of files you can download based on account name and IP address.
The limits are dynamic, constantly under review and based on a score determined by factors such as server load, IP address (some ranges are prone to high levels of abuse and are throttled), behaviour (using VPNs and proxy services may downgrade your account) and your level of contribution to the library.
Daily limits are based on activity in the previous 24 hours.
Registering additional accounts to bypass download limits will result in a permanent ban, without notice, and any IP addresses/ranges used by your accounts will be blocked from the download server.
Almost every manual or schematic available on hifiengine is available free elsewhere, e.g.,
https://elektrotanya.com