Repairing a HK1400 Amplifier

[Mark Hennessy]

I'm a member of HiFi Engine and have already downloaded the service manual, studying the circuit.

Perhaps my hint was too subtle

I wasn't proving the link for your benefit; rather, for anyone else who fancied getting involved.

So, speaking very broadly, I was trying to suggest that it would be nice if the OP could provide the link (or attachment) in these sorts of threads. I'm finding that this is a recurring theme across forums in general. In short, when asking for help, providing easy access to the relevant data - along with anything else that might tempt someone to get involved - will maximise the chance of receiving help. Yes, we can all use Google, but why not help people to help you?

AS for current limited PSUs, I agree that they are very useful, but I only have two, one goes up to +22v @ 2A max, the other up to ca. +18v @ 10A Max. The latter is an ex car radio demo stand PSU, the former I built many years ago to a design published in PW. Since I mainly work on portables & car radios, these are adequate for those jobs, though I have succesfully repaired quite a few HiFi amps. over the last 10 years or so without the problems I'm having with the HK1400.

I would not describe them as "useful". They are essential. Absolutely essential.

And I should clarify what I mean by "current limited". Any half-decent power supply will have current limiting, but that value of current is fixed - perhaps with "foldback" - and is there to protect the PSU from abuse like a short-circuit across the output.

But that's not what we need here. Rather, we need power supplies that have an adjustable current limit, right down to milliamps. A current limit that you have complete control of from the front panel. To protect the device under test.

These are the norm in electronics bench work. There are exceptions (e.g., the Farnell E series), but the vast majority should be properly adjustable, like the examples I gave earlier.

No idea if your 2 supplies fit that category, but if they don't, current sources or light bulbs can easily be added.

As for the voltage, audio amplifiers built around the basic "Lin" topology are pretty tolerant of varying supply voltages, and will happily work down at a much lower level than usual. I usually start at around +/-15V (current limit set to 50 or 100mA).

As to what is termed 'blanket replacement', I'm sure I read somewhere, maybe on this forum or on UKVRRR that, because, as I well know, an o/p stage failure can damage transistors upstream of the output, so replacement of all Transistors in the defective channel is the way to go.  I'm sure someone said that, in his days as a service engineer, this was less time consuming than any other way to proceed.

It's a commercial decision. I've done it in the past when devices were readily available and time was short. But here, you have to factor in the time it'll take you to identify and source the equivalents...

With current-limiting provided (in whatever form you can - even via a lamp limiter), you are able to analyse the problem and fix the thing that started it off, along with all the casualties taken along the way, and perhaps a few other parts that you fear might have suffered during the incident. That's the intellectually satisfying approach, and quite possibly the most time effective approach in this situation. Or perhaps not? Only you can decide, as I said before. I'm not against "blanket replacements" as such, but it carries the risk of creating more problems and is not intellectually satisfying. But each to their own - every situation is different.

Whatever route you take, good luck and keep us updated