DrumminDaddy
Hit it, Baby !!
Before I get into describing the situation with the 700m, just a brief bio on myself.
I am a machinist by trade with some years spent as a CNC repair tech. Most machine tool electrical faults involve switches and relays. The modus operandi for troubleshooting board level faults is to determine correct inputs/outputs. If the correct readings aren't seen, the board gets replaced. We aren't trained to look at discrete components on the boards. If the chips ain't flyin', the machine's not making money. The cost of a whole new board pales in comparison to the down time. Get in / get out. $$$. Obviously, that's not an option here. I have never done a repair on an amplifier before other than to clean some scratchy switches. So I decide that the 700m is going to be my first project (somebody stop me !! ).
That being said, my goals are simply:
1) Do no more harm.
2) Learn, have fun at it.
3) Achieve a working amp. (Perhaps the full monty by EchoWars at a later time...)
The scenario:
Upon recieving my 'new' amp, in my zeal to listen to this new toy, I turn it on and a couple seconds later hear the speaker relays click in. Titties!
Well, not so fast. Just wanting to hear some music through it, I plug in my headphones, and poof! Out comes a whisp of the magic smoke. Nuts!! (Well, that's not quite how I put it at the time.)
So off comes the cover and looking around, I do believe that I may be the first one inside (feeling a little naughty!).
I see R114 and R113 to the headphone jack L/R are roasty well done. Put a meter on the outputs and read 70vdc on both channels.
Lesson learned:
Check the DC offset on any unknown amp BEFORE hooking a load to it!
On EW's recommendation, I purchased the service manual from www.stereomanuals.com. Also had a brief exchange with him in another thread regarding dbt and the operation of the soft start circuit. He cautioned against using less than 100w bulb as to inhibit the soft start relay coil from operating.
I've been studying the schematic and I've got some idea what's happening in certain aspects, in other areas, quite ignorant.
What I've done/seen so far:
Fired up on dbt. Bulb lights bright for a second then dulls down.
Panel lights up, speaker output relays engage.
Disconnect meter board, no change. Still 70vdc on speaker outputs.
Disconnect output boards (leaving STV-3H varistors connected per EW), no change.
At one point I realize that the soft start relay (RL3) isn't even engaging. At first I'm a bit perplexed by seeing line voltage on the NO contacts. Studying this circuit, I've come to the conclusion that the NO contacts have voltage because the amp must be running all through the power resistor R301, which runs in parallel with the contacts. I suspect the reason R301 hasn't gone open is because whatever is shorting voltage to the outputs isn't drawing a load. (Unless, of course, I give it one. Like my headphones! )
I've removed the protection board from the chassis and determined that the coil for RL3 reads open and it shows heat stress.
There is also signs of heat stress on the PCB at Rp1, and on connector A-2 pin14. I suspect the heat mark on the pin connector was caused by the resistor. It was pressed tight against the connector. (Why wouldn't it be?). I conclude this because the heat mark is only on the side the resistor was touching, but I may be missing something. Rp1 (12k resistor) measures 1248 ohms.
I am perplexed as to why the protection circuit is engaging the speaker relays with this kind of voltage present on the outputs. Could the source of this voltage be the protection circuit itself?
I have a very cursory understanding of the operation of the driver boards, but it can be summed up as: signal in<gain out.
Can this amp be placed under power with the driver boards removed without doing any harm?
Any suggestions at this point which could help in isolating the cause of this output voltage would be greatly appreciated.
Regards,
Terry
I am a machinist by trade with some years spent as a CNC repair tech. Most machine tool electrical faults involve switches and relays. The modus operandi for troubleshooting board level faults is to determine correct inputs/outputs. If the correct readings aren't seen, the board gets replaced. We aren't trained to look at discrete components on the boards. If the chips ain't flyin', the machine's not making money. The cost of a whole new board pales in comparison to the down time. Get in / get out. $$$. Obviously, that's not an option here. I have never done a repair on an amplifier before other than to clean some scratchy switches. So I decide that the 700m is going to be my first project (somebody stop me !! ).
That being said, my goals are simply:
1) Do no more harm.
2) Learn, have fun at it.
3) Achieve a working amp. (Perhaps the full monty by EchoWars at a later time...)
The scenario:
Upon recieving my 'new' amp, in my zeal to listen to this new toy, I turn it on and a couple seconds later hear the speaker relays click in. Titties!
Well, not so fast. Just wanting to hear some music through it, I plug in my headphones, and poof! Out comes a whisp of the magic smoke. Nuts!! (Well, that's not quite how I put it at the time.)
So off comes the cover and looking around, I do believe that I may be the first one inside (feeling a little naughty!).
I see R114 and R113 to the headphone jack L/R are roasty well done. Put a meter on the outputs and read 70vdc on both channels.
Lesson learned:
Check the DC offset on any unknown amp BEFORE hooking a load to it!
On EW's recommendation, I purchased the service manual from www.stereomanuals.com. Also had a brief exchange with him in another thread regarding dbt and the operation of the soft start circuit. He cautioned against using less than 100w bulb as to inhibit the soft start relay coil from operating.
I've been studying the schematic and I've got some idea what's happening in certain aspects, in other areas, quite ignorant.
What I've done/seen so far:
Fired up on dbt. Bulb lights bright for a second then dulls down.
Panel lights up, speaker output relays engage.
Disconnect meter board, no change. Still 70vdc on speaker outputs.
Disconnect output boards (leaving STV-3H varistors connected per EW), no change.
At one point I realize that the soft start relay (RL3) isn't even engaging. At first I'm a bit perplexed by seeing line voltage on the NO contacts. Studying this circuit, I've come to the conclusion that the NO contacts have voltage because the amp must be running all through the power resistor R301, which runs in parallel with the contacts. I suspect the reason R301 hasn't gone open is because whatever is shorting voltage to the outputs isn't drawing a load. (Unless, of course, I give it one. Like my headphones! )
I've removed the protection board from the chassis and determined that the coil for RL3 reads open and it shows heat stress.
There is also signs of heat stress on the PCB at Rp1, and on connector A-2 pin14. I suspect the heat mark on the pin connector was caused by the resistor. It was pressed tight against the connector. (Why wouldn't it be?). I conclude this because the heat mark is only on the side the resistor was touching, but I may be missing something. Rp1 (12k resistor) measures 1248 ohms.
I am perplexed as to why the protection circuit is engaging the speaker relays with this kind of voltage present on the outputs. Could the source of this voltage be the protection circuit itself?
I have a very cursory understanding of the operation of the driver boards, but it can be summed up as: signal in<gain out.
Can this amp be placed under power with the driver boards removed without doing any harm?
Any suggestions at this point which could help in isolating the cause of this output voltage would be greatly appreciated.
Regards,
Terry