Hi, I am working on an Niko Alpha II amplifier it is an older amp with the two large VU meters on the front. There is a problem when powering up that one chanel may not work. I have tracked this down to a relay that is in the speaker output path and is 24volt dc with two normally open contacts. It is on a printed circuite board and I wish to change it as cleaning did not help and I am afraid it may be degrading sound when working if not providing a good path. I uderstand this relay helps prevent thump on powerup and the sound from dithering out on shut down. Thank You for any advice.
Dsrjohn, AC voltage from one of the transformers taps produces positive voltage (D309,C309) for the status LED and the negative voltage (D308, C306, C307) to open transistor Q305. When all for transistors Q302, Q305, Q303, Q304 are open then voltage on C308 builds up (to +21V) turning on transistor Q306 that activates output relay. It appears that Q305 detects presence of AC voltage, Q303 and Q304 detect presence of DC voltage on the output (average of both outputs), Q303 reacting to positive and Q304 to negative output voltage, with time constant 1.76s. Q302 appears to be controlled by Q301 that detects overcurrent in either channel. When voltage on either of 0.22ohm resistors R301, R302 is greater than 1.3V (two junctions) then Q301 activates Q302 turning off Q306 hence output relay. It happens at 1.3V/0.22ohm=5.9A current draw from either output positive rail.
I'm afraid I did not simplify anything for you, but wanted to show 4 possible reasons: no AC voltage, DC on either output or overcurrent on either output. It can be also faulty component - something is holding down voltage on C308 (something that activates one of four mentioned transistors). Since you have schematics you can start measuring voltages (be careful). Measure output DC voltage - it has to be more than +/-0.6V to trigger fault condition. Next voltage on C308 should be around 21V (voltages on schematic are in normal operating condition). If it is, then problem is around Q306 or even relay itself (or shorted diode across it - D307). If it is low, then something triggers it. Check the voltage on the Base of Q302 and Q305 - should be less than 0.6V (most likely close to zero). If it is then you most likely have bad transistor. Measure what you can and post your findings. Again, be careful.
Tried replacing cap C308 with several dif 100 uF caps but doesn't work. What would be the next thing to try? If I manually join the contacts in the relay, I get sound out of the speakers so it is something upstream from the relay. Maybe the relay needs replacing?
A friend has a Sony TA-F3A amp. She says it was acting up - i.e. after
powering up, it would take quite a while before she would hear the
characteristic click and she would get sound but now it never clicks.
Try replace C308 (100uF 25V) may solve the problem
Realize that about the fuse. Just wonder if it is a relay issue or something upstream that causes the relay to engage. The contacts on the relay look perfect - gold plated and not dirty at all.
What is the problem with simply replacing the relay?
Also, I've had similar issues with Yamaha's. While I was upgrading parts, I also used a relay socket. This lets you swap relays in/out quickly without resoldering. Relays have been standardized for a long time, so with a little luck you can find an exact fit.
The old relays I saw used convex curves which minimized contact area and had more reliability issues. More modern relays have matching concave / convex contacts which supply much better contact surface.
Not sure anyone will respond but...A friend has a Sony TA-F3A amp. She says it was acting up - i.e. after powering up, it would take quite a while before she would hear the characteristic click and she would get sound but now it never clicks. Sounded like a relay problem. So I opened up the relay, blew some contact cleaner and that did not help. The contacts look to be gold plated so they shouldn’t even be dirty. I removed the little spring (see the pics) and sure enough, the contacts look very clean. Next test was to see if there was any sound - so I added a source and listened through earbuds. No sound. However, if I manually (using insulated pliers) flip the contacts in the relay so that they make contact, I get good sound out of the earbuds. So for some reason, the relay will not flip. It does not seem to be because of a problem in the circuit, as I get good sound out of it. So either the relay is gone or something upstream of the relay is gone. Any ideas?
Guy this is so easily fixed. All you need to do is burnish the relay's normally open contacts, which are now dirty & degraded from the years of use. Go to Radio Shack or check their website for a relay contact burnishing tool. These are like a very fine nail file, but are coated with diamond dust on both sides. You simply insert the spatula end in between the contacts, then apply finger pressure to the relay armature to hold the contacts against the tool, then file with an in/out motion. If both sides of the tool aren't abrasive (some are / some aren't) then turn it over & do the opposite contact. Spray or hand apply some good oilfree contact cleaner afterwards. Crocus cloth (a very fine abrasive) found in better hardware stores will also do a good job, & is cheaper too.
You can do much better without these relays and opt for smaller output fuses i.e. .5A instead of 2A. Thus you will increase resistance on the signal path but along with that elimintate the switch that impacts synergy much larger. You can get high quality ceramic fuses from www.mouser.com for pennies.
Thank You for your reply, I did not realize that this relay also may offer protection, Though I am sure it is a mechanicle/contact problem with the relay itself do to the fact that if I tap the relay on top the chanel restores. Thanks again, Guycom
I'm not familiar with the circuitry of your amp but I suggest checking more deeply arround the ciruit board for the following possible failures:
This relay faliure can be on more reasons than you might think of. Relay can also block an input stage thus blocking any signal that is about to enter the blown output stage.
I'd suggest checking output devices since often fuses can act too late to protect and Relay is acting only after the output stage is blown. Please note that relay in this case shunts the signal(and/or power supply to the output stage) through the load resistor and depending on time of such task can also be blown along with load resistor.
To check your output devices remove relay to connect directly and use dummy load 50W/10Ohms resustor to MEASURE DC from the binding posts. You can end upto 30V that can blow the speaker in the flash even after you replace relay wihtout checking out all possible failures.
Possible reasons for output stage failures: --Bad trim pots that adjust the emitter(or channel) bias. --Surge or voltage jump(s).
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