Hi,
I'm a semi-newbie at electronics.
I have an old HK770 amplifier. It sounds okay (not great) but the heatsink on one channel get scorching hot within a few minutes. So I shut it off before any output devices fail.
Does this sound like it might be oscillating, or thermal tracking transistor failure, or ??
I've also noticed the speakers make a slight popping noise when connecting them to this amp. Too much DC offset? I'm not 100% sure how to measure that. Across the output terminals with no input?
Does anyone have a service manual or schematic for this amp?
Thanks!
-- Todd
I'm a semi-newbie at electronics.
I have an old HK770 amplifier. It sounds okay (not great) but the heatsink on one channel get scorching hot within a few minutes. So I shut it off before any output devices fail.
Does this sound like it might be oscillating, or thermal tracking transistor failure, or ??
I've also noticed the speakers make a slight popping noise when connecting them to this amp. Too much DC offset? I'm not 100% sure how to measure that. Across the output terminals with no input?
Does anyone have a service manual or schematic for this amp?
Thanks!
-- Todd
anatech said:
Thanks Chris,
Across the speaker terminals I measure -6.6v on the hot channel, and +.09v on the cool channel. I assume it's DC voltage I'm measuring here right? Not current? I measure no current on either channel.
And I can probably also assume -6.6v is not expected or appreciated?
Looks like I'm going to have to open it up and start looking for differences between the two channels.
Is there a way to test for oscillations without a scope?
..Todd
A highly innacurate test for amp HF oscillations is to just put a resistor across the speaker terminals and check if it heats up. Or a small lamp and check if it lights up (the lamp will integrate the HF). Or use a small signal diode and capacitor...
6.6V across the speaker terminal? TURN IT OFF quickly!!!!!!
Read this: http://audiokarma.org/forums/showthread.php?t=5634
6.6V across the speaker terminal? TURN IT OFF quickly!!!!!!
Read this: http://audiokarma.org/forums/showthread.php?t=5634
Thanks Chris, for the email (on both counts!) My email link should be live now. I thought I fixed that before. 
Apparently not.
So, this amp has bias adjust but no offset adjust, and it sounds like it's far beyond adjustability anyway. I'll just follow the service manual's voltage measurements until I find something awry. I hope it's not those funky dual diff amp transistors. I suspect they may be hard to find, and replace.
..Todd
Apparently not.So, this amp has bias adjust but no offset adjust, and it sounds like it's far beyond adjustability anyway. I'll just follow the service manual's voltage measurements until I find something awry. I hope it's not those funky dual diff amp transistors. I suspect they may be hard to find, and replace.
..Todd
Hi Todd,
Look up the specs on the dual transistors and substitute single ones in there that you will hand match. Thermally couple them.
Before getting into those (you can always simply measure the hFE on each section), check transistors 410,412,414 and 416 for leakage. Often HK will run a transistor close to it's limits both thermally and voltage wise. The result is odd faults. On many HK amps I just replace the lot of transistors with parts having higher ratings. Look for bad solder joints also. Dark brown areas indicate heat. Fix the fault first, then go over the whole thing. Look up each transistor's rating and compare it with your supply voltages.
-Chris
Look up the specs on the dual transistors and substitute single ones in there that you will hand match. Thermally couple them.
Before getting into those (you can always simply measure the hFE on each section), check transistors 410,412,414 and 416 for leakage. Often HK will run a transistor close to it's limits both thermally and voltage wise. The result is odd faults. On many HK amps I just replace the lot of transistors with parts having higher ratings. Look for bad solder joints also. Dark brown areas indicate heat. Fix the fault first, then go over the whole thing. Look up each transistor's rating and compare it with your supply voltages.
-Chris
anatech said:Hi Todd,
I guess we won't hear from you for a while then .............
-Chris
What? You kiddin me?
There is no end to the questions a newbie can come up with, while in over his head.For example:
When replacing these dual transistors with something compatible, am I on the right track by substituting in MPS8055/8955 for these?
Dual UPA75V (UPA74V-NPN is essentially the same specs)
-----------------------------------------
Description = Si PNP LP HF Bipolar Junction Transistor
Manufacturer = NEC
V(BR)CEO (V) = 80
V(BR)CBO (V) = 80
I(C) Abs.(A) Collector Current = 50m
Absolute Max. Power Diss. (W) = 300m
I(CBO) Max. (A) = 1.0u
h(FE) Min. Static Current Gain = 150
@I(C) (A) (Test Condition) = 0.1
@V(CE) (V) (Test Condition) = 6.0
f(T) Min. (Hz) Transition Freq = 300M
C(obo) (Max) (F) = 4.0p
Status = Discontinued
Package = ZIP
Military = N
Just happens I'm building a Leach amp and will be ordering a bunch of MPS8099/8599s anyway. So if they work, I'll buy a much larger batch for matching.
..Todd
anatech said:
I had those transistors on my list to check out, but I couldn't find a Canadian source for them. Digikey didn't respond with any search results. Not sure where else to look. Border crossing isn't fun for little orders like that.
Any more good Canadian electronic parts resources I should know about?
..Todd
anatech said:
Well, I found a local source (two actually) here in Vancouver, for these transistors.
http://www.mainelectronics.com/
http://www.pacificsemi.com/
But I'd much rather use something off the shelf that I can get lots of (easily/cheaply) and use for everything. Would a 2n5551/5401 pair work here?
..Todd
HK schematic
Here's a link to a schematic; hope it helps:
http://manuals.harman.com/hk/Service Manual/hk770 sm.pdf
Here's a link to a schematic; hope it helps:
http://manuals.harman.com/hk/Service Manual/hk770 sm.pdf
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