I have a question to those who are familiar with the Yamaha A-700 / A-1000 line.
I've been working on an A-700 on and off for a few weeks now, which was going in and out of protection. It was also not balancing correctly in tape monitor mode / only working in Mono mode. On first looks, I saw the glue had corroded a bunch of resistors and diodes around the 2 680uf caps, so my first instinct was to pull those, test them, and either replace or reinstall. I opted to replace the resistors since that's a nominal cost, and I replaced the caps as well. Left the diodes alone as they measured out fine. Once I powered it back up I had no power to the LED boards on the front panel. Screwed around with it for a while, back-tracing my work, and could not find anything obvious. Then I started seeing all the dry solder joints and literally moving transistors. So I went through the board, re-soldered all the transistors that felt wiggly and also some just because, might as well. I now have power to the front panel, but still no relay. I am getting voltage at the hot side of the relay (51V -on the dim bulb) but something is preventing the relay from engaging.
I have attached the section of the schematic I am concentrating on, which revolves around the relay itself. I am getting 51-52v at the collectors of TR151 and TR152, but I'm not getting the supposed 4.4v at the base of TR151. TR150 I get nothing, and I wouldn't expect to. Logic would tell me the transistor is bad or the solder joint is bad, but I removed and tested the transistors (all 3) on my Peak meter and they all check out with no shorts.
If anyone has any insight on this, please let me know. I am getting B+ voltages at other parts of the board where noted, so there shouldn't be any issue with the supply.
I've been working on an A-700 on and off for a few weeks now, which was going in and out of protection. It was also not balancing correctly in tape monitor mode / only working in Mono mode. On first looks, I saw the glue had corroded a bunch of resistors and diodes around the 2 680uf caps, so my first instinct was to pull those, test them, and either replace or reinstall. I opted to replace the resistors since that's a nominal cost, and I replaced the caps as well. Left the diodes alone as they measured out fine. Once I powered it back up I had no power to the LED boards on the front panel. Screwed around with it for a while, back-tracing my work, and could not find anything obvious. Then I started seeing all the dry solder joints and literally moving transistors. So I went through the board, re-soldered all the transistors that felt wiggly and also some just because, might as well. I now have power to the front panel, but still no relay. I am getting voltage at the hot side of the relay (51V -on the dim bulb) but something is preventing the relay from engaging.
I have attached the section of the schematic I am concentrating on, which revolves around the relay itself. I am getting 51-52v at the collectors of TR151 and TR152, but I'm not getting the supposed 4.4v at the base of TR151. TR150 I get nothing, and I wouldn't expect to. Logic would tell me the transistor is bad or the solder joint is bad, but I removed and tested the transistors (all 3) on my Peak meter and they all check out with no shorts.
If anyone has any insight on this, please let me know. I am getting B+ voltages at other parts of the board where noted, so there shouldn't be any issue with the supply.
Looks like the schematic is not 100%, eg, those 2 WHite wires that go to half wave rectified supply and CT...
What is TR152 Vbe, schematic shows zero for relay open, expect 0.65V or abouts to close the relay.
Also check TR151 Vb. Assume you've checked voltage between the emitter resistors (both channels) to ensure you don't have excessive Vdc, expect a few mV.
What is TR152 Vbe, schematic shows zero for relay open, expect 0.65V or abouts to close the relay.
Also check TR151 Vb. Assume you've checked voltage between the emitter resistors (both channels) to ensure you don't have excessive Vdc, expect a few mV.
Basic Semiconductor theory would suggest that the legs have been ripped off of TR151 and/or TR152 - you can't get (+)4.4V (the polarity indicator is NEVER optional - YOU omitted the polarity indicator) at the BASE of TR151. However, the erroneous voltage that you've quoted is actually -4.4V on the circuit diagram. Impossible without the removal of 5 x components (minimum) OR a very large NEGATIVE DC offset from either channel.
You have been informed elsewhere that the voltages on the circuit diagram are incorrect.
Please don't use them as reference voltages - THEY'RE INCORRECT!
This is what I'd expect to see under NORMAL operation conditions: i.e. NO FAULT DETECTED.
N.B. These voltages are not definitive - they are for guidance purposes only.
TR149 & TR150 can cut-off the BASE drive to TR151. You need to determine which transistor is operational and the reason why...
There are many possibilities and they can be determined via careful measurement.
REMEMBER: 'NOTHING' is not a valid measurement. There are always numbers and polarity indicators.
Good Luck!
ADDENDUM: Don't forget to resolder the 4x 1k5, 5Watt dropper resistors - they're loose! HMP solder is a good choice.
Apologies for the hairdressing/beard-grooming salon speak. I should've stated that the resistors I'm referring to are R359-R362.
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