When I obtained the unit, the ouput protection relays chattered for the first couple of seconds after power up and then the output cut-out completely. Now there is no output at power on. With a scope I see the RF signal at the input to IC1 for both channels and see the signal at the left channel output of IC1( pin 6) but the right channel output of IC1 (pin 10) is sitting at -34 Vdc. I am looking for ideas on best actions to take to determine the problem and work to recovery. Thanks for any help offered.
Rich [email protected]
Don't bypass the relay contacts. They serve to disconnect the amp stages from the speaker if a DC condition is present, and in doing that, save your speakers form damage. Check for a DC offset condition on the junction of the emmitter stabilizer resistors to the STK0040 on each channel. They will be 0.47 ohm power resistors(2 per STK) connected to pins 3 and 8 of the STK. Anything substantially greater that 100mv here generally indicates a blown STK. Don't bother to change both, but just the one that is faulty. Another common problem for this age of amp. The high silver content in the legs of the transistors allows for "growth" between legs and internal connections of the transistor. Check for "black death" on the legs of transistors driving the STK. . . I tend to replace them if in doubt. If you need expert advice, comment me back. I am an audio specialist.
Its very hard to check darlingtons with just a junction barrier potential check(using a resistance meter or diode checker) because not all the junctions are available at the external pins. If in doubt, just replace the STK module. They are not the expensive, and readily available, and a common cause of failure in amps that use them. Discrete components still has the edge over thick film hybrids for reliability, Just cheaper to make a chip that use individual parts.
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Pleace schematic kenwood kr4010
chile
[email protected]
In those days I found that it was best to bypass the relays as they caused a lot of output transistors and speakers to blow.
it was some stupid hot idea back then.
I am still running 2 marantz amps since 78 today that are wired across the relays.
the normal issue with these amps is usually always that the outputs transistors or output amps if IC are blown because of this . replace outputs and try again.
use headphones while troubleshooting.
If you want me to I will look at the schematic if you have one. I sold all of mine and threw a bunch away 10 years ago.
if you know of one online send me a link
Jumping across the contact but from under the relay is the way.
as for the schematic its too small .
stick a larger one on imageshack.
I can only guess as to the external transistors after the IC.
but if those transistors are connected through the switch to the coupling capacitor and speakers they are the usual culprits.
do you know how to measure resistance in a transistor to test if shorted.?
set your meter Rx1 , Just remove them and check for 20 ohm
in one directions and open when you reverse the conection base to
emitter, collector to base ,
and should show open colllector to emitter
if you see any leakage between these readings replace the xistor.
an old analog meter is best when testing xistors.
As for jumping the relay ,your choise, I always do and haven't lost any
speakers or any more blown transistors since I have fixed over a
hundred of these 30 years ago. a lot of the units are still in service
including my own. when you power on the unit you have at least 5 sec to
turn it off/down before it blows as you will hear the issue. with the
relays working you can blow the speakers as they engage if the vol is
way up . or blow out your windows as I did. I cracked 4 double paned
windows in my old house. Lol . hurt more than my pocket book on
that one.
I had verrry large speakers.
Tech buddy allows a private chat with a long wait it seems. but that way you can exchange emails or messenger id.
I don't even remember doing it at all.
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Thanks to Coldcut for a very prompt reply and for the direction from his experience with my specific problem. I have inserted an image of the schematic with this comment along and have a couple more questions. Coldcut recommended replacing the outputs - are they the IC2 and IC3 (the STK-0040AT) amplifiers? And he recommended wiring across the relays - by just jumping across the contacts or should I do more? Thanks agai
n.
I do know how to check out the transistors for shorted condition and yes the STK's are the external transistors right before the switch and then to the speakers. You have have confirmed the recommendation without the schematic so unless I run into further problems, I won't upload on imageshack at this time. I will check out the transitors and replace as necessary. Thanks again.
Thanks for the detailed tip on the DC offset. I'll check that out and do the replacements. Thanks for sharing your high level of expertise.
I see now the check won't be that direct as the STK-0040AT output transistors each are a pair of darlingtons, an NPN and a PNP. I do have an analog meter and I'll only be able to tell if I have shorted through pairs of junctions.
By the way, I received an e-mail indicating a tech buddy request came from you. I did not respond to the tech buddy request because I do not know what it entails or requires and I could not find any info about it. Can you tell me where I can find out more? Thanks for your help.
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