Greetings Connor G et al.,
I recently acquired an HK770 whose power supplies have been completely recapped with what looks like the recommended Nichicon Gold 4700/50 and 2200/63. There's also a little recapping on the driver boards, and new one-turns for DC offset adjustment.
As yet, I've only hooked up the 770 to my (even older) HK-40 Stratophonics, and it satisfyingly kicks the hm out of them with a bit of Grace Jones. My whole lab rattles, so I'm not yet able to give the combination a real run for its money! I did, however, play some loud signal through it into a pair of 50W 8ohm resistors for several hours. This warmed up the 770 comfortably (~50C) while making the load's heat sink too hot to touch. A considerable bias mismatch (15mV R / 6mV L) explained a 10C difference in the heat sinks on either side. Note also how much lower this is than the manual's 24mV.
I'm finding that bias and offset settings are very touchy. Am I safe to replace the trim pots with multi-turn types (maybe 3 turns)?
I set the bias on both channels to ~18mV, and left the unit powered for a couple of days. When I looked again, both heat sinks are at an almost identical 40C (no load), *but* the bias has dropped to ~10mV on both sides! Is it likely that the bias trim pots could be to blame for this kind of drift?
Moreover, I'm noticing a fairly large difference in two transistors' bias on one side of the amp. I've measured TP6/TP4 at 17.5mV and TP6/TP5 at 22.8mV. More recently, TP6/TP4 = 13mV and TP6/TP5 = 19mV. I personally haven't had the unit (all) apart yet to examine the solder sides. I'm hoping the apparent transistor mismatch may be due to a dodgy/cold solder joint.
Does anyone have any ideas on the bias drift and the bias mismatch between two transistors on one side?
Regards,
Mike (The Plumber)
Hi Mike,
First of all, congrats on the purchase! The 770 is certainly a special amplifier that flies way under the radar (or at least used to...)
Regarding the bias drifting: I've found this behavior only with units that had dodgy soldering and original single-turn bias trimmers. It could also be an issue with the pre drivers surprisingly. I've since started replacing all TO-92L predrivers in all of the 770's I work on (2SD666 & 2SB646 respectively).
A more common issue I see with the 770 is a wildly fluctuating DC offset, usually caused by the ultra temperature-sensitive dual fet packages on the inputs of each channel. The DC offset usually oscillates between +/- 10mV on a typical 770, but with a few units I've had through, fluctuated as much as +/- 25mV. Granted, 25mV offset on a DC amplifier is still very good in my book, there doesn't seem to be rhyme or reason as to why some examples fluctuate more than others.
You shouldn't have to replace the bias trimmers with multi-turn parts, as the bias adjustment is somewhat rough and doesn't need to be a fine adjustment. If there is substantial drifting taking place, and the original single-turn trimmers were replaced with new, I would look elsewhere for your issue.
Also, as for the differential in bias on the outputs, I wouldn't worry about it as it is pretty minor. HK biased this amp higher than need be from the factory, so a variation of ~5mV from one output to the other shouldn't be of issue. I've seen this behavior in many of the examples I've worked on, both before and after restoration. If the differential was greater, I would look into the issue further.
Start by inspecting all solder joints, especially on the hot-running driver cards. These cards easily slide out of place when removing two screws, the input connector, and the thermal tracking varistor. Another area that needs attention is the bottom sides of each main board. The biggest offenders here are the corrosive glue which can eat through traces and cause high resistance paths where they ought not to be, as well as bad soldering on the driver card connectors which are soldered to the main boards. You can see more on these issues
here.
Good luck and enjoy! Let me know if I can help you any further
