Hi Friends;
There seem to be a lot of Yamaha CR- series receivers around, at least in this neck of the woods. They seem to have common problems in the regulated power supply for tone control etc. sections and bad solder joints in this area also. This one is being particularly stubborn.
I have completely recapped the power supply board #1 and replaced the 3 smaller caps on the main capacitor pcb.
There is a saw tooth ripple in the -25 volt output of ps pcb 1. When the load is removed (violet wire to tone control), the ripple goes away.
Still struggling with the power supply. With the violet wire (from the -25 terminal to the tone control board) lifted, both -NB and +NB rise from 34 to about 40 volts.
How can a load on the -25 side affect +BL?
Voltages on TR708 C: -34.9 B: -31.18 E: -33.4
I thought I was on to something as the voltage from B to E was 2.2 volts. The schematic calls for 0.8 v Is this not a significant difference? I removed TR708 and it tested good with my Fluke meter (diode checker mode).
TR714 C: -41.7 B: -34.8 E: -34.2
TR704 C: -34.4 B: -0.55 E: 0.0
There seem to be a lot of Yamaha CR- series receivers around, at least in this neck of the woods. They seem to have common problems in the regulated power supply for tone control etc. sections and bad solder joints in this area also. This one is being particularly stubborn.
I have completely recapped the power supply board #1 and replaced the 3 smaller caps on the main capacitor pcb.
There is a saw tooth ripple in the -25 volt output of ps pcb 1. When the load is removed (violet wire to tone control), the ripple goes away.
Still struggling with the power supply. With the violet wire (from the -25 terminal to the tone control board) lifted, both -NB and +NB rise from 34 to about 40 volts.
How can a load on the -25 side affect +BL?
Voltages on TR708 C: -34.9 B: -31.18 E: -33.4
I thought I was on to something as the voltage from B to E was 2.2 volts. The schematic calls for 0.8 v Is this not a significant difference? I removed TR708 and it tested good with my Fluke meter (diode checker mode).
TR714 C: -41.7 B: -34.8 E: -34.2
TR704 C: -34.4 B: -0.55 E: 0.0
Attachments
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Here's the schematic.
Thanks for any help. The folks here at Diy have helped me on so many projects! I have already checked the voltage dropping resistors on the capacitor board,
Regards, Peter in Canada
Thanks for any help. The folks here at Diy have helped me on so many projects! I have already checked the voltage dropping resistors on the capacitor board,
Regards, Peter in Canada
Probably a dumb question, but are all pots, selectors and switches properly cleaned?
Looking at the schematic (if I follow the right wire) they are all involved.
Looking at the schematic (if I follow the right wire) they are all involved.
Hi korpberget;
The pots could use a little cleaning but working all switches and pots had no effect on the saw tooth hum on the -25 volt terminal
The pots could use a little cleaning but working all switches and pots had no effect on the saw tooth hum on the -25 volt terminal
+BL or +BR?
+NB is regulated to +BR 25 volt. That supplies tho tone control board. A fault here could affect that voltage. Have you checked the voltages on the tone board?
+NB is regulated to +BR 25 volt. That supplies tho tone control board. A fault here could affect that voltage. Have you checked the voltages on the tone board?
It is the -25 volt supply that has the issue. But, yes it also supplies the tone controls. On the tone control board (PRE C. Board 1), there are various smaller pcb's (modules A ~ E) attached on edge. The Low filter increases the hum dramatically ( 2 volts on speaker with switch at 70 Hz, 0.4 VAC when switched to OFF).
Also the TO-92 transistors at the top of module E, are extremely hot. Looks like a major project to remove that module for access.
Also the TO-92 transistors at the top of module E, are extremely hot. Looks like a major project to remove that module for access.
It seems to me that the -25V rail is being overloaded by one of the circuit modules. Moving on that possibility, I suggest disconnecting the -25V line from each module, one at a time, to identifying which module is the culprit as evidenced by the ripple on the -25V line being removed, and the line being a smooth -25VDC. Begin with module-E. After identifying the problem circuit module, you still will need to troubleshoot it to locate the specific defective component, unless you have access to a set of replacement modules you can swap in.
At the moment, the so-called -25 supply (violet wire ) measures -33 V (and no ripple). I have disconnected the -25 supply to modules D and E by interrupting the pcb traces and jumpering power to the remaining modules
With modules D and E disconnected, the hum disappears and the unit works great! -----however, HF and LF filters not functioning.
With modules D and E disconnected, the hum disappears and the unit works great! -----however, HF and LF filters not functioning.
+1 Ken Newton .
If it is any easier to get to C714 and/or C719 -- 'failed, open' is kinda likely in a machine of this vintage. Both are too small to be outright current-supplying filters; they raise the AC gain of the regulator.
The drawback to the excessive-load hypothesis is two-fold. Lots of these circuits already have a hot TO-92 here and there, so that isn't always an indicator. And heavy loading would likely draw it down without the sawtooth -- or pop a load-sharing resistor feeding a pass-transistor Collector.
Cheers
edit: Yay! You and Ken beat me to it.
If it is any easier to get to C714 and/or C719 -- 'failed, open' is kinda likely in a machine of this vintage. Both are too small to be outright current-supplying filters; they raise the AC gain of the regulator.
The drawback to the excessive-load hypothesis is two-fold. Lots of these circuits already have a hot TO-92 here and there, so that isn't always an indicator. And heavy loading would likely draw it down without the sawtooth -- or pop a load-sharing resistor feeding a pass-transistor Collector.
Cheers
edit: Yay! You and Ken beat me to it.
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Okay, now reinstall module-D and check for any return of the -25V rail problem. If yes, problem is on module-D. If no, then install module-E to verify that the problem has returned. If it has returned, then you must troubleshoot module-E to locate the defective device.
With module D reconnected (Only module E disconnected) The hum returns slowly after aprox 10 seconds after powering up. Violet wire (-25 volt supply) measures -31.2 volts.
With only module D, disconnected hum is there immediately when relay pulls in. Voltage is -31.3 and there is a faint arcing or scratching sound.
With all modules reconnected, Voltage is 29.5
Looks like I'm pulling module E....
With only module D, disconnected hum is there immediately when relay pulls in. Voltage is -31.3 and there is a faint arcing or scratching sound.
With all modules reconnected, Voltage is 29.5
Looks like I'm pulling module E....
I think you were right, Rick;
I pulled module D and E. All 8 transistors tested good. Caps are only coupling, diodes gave sensible readings..... Back to to regulator board..
I pulled module D and E. All 8 transistors tested good. Caps are only coupling, diodes gave sensible readings..... Back to to regulator board..
I'd take a closer look at the abnormal TR706 Vbe. Does it normalise when you remove the load? R726 (15/1W) ok? TR715 b/c/e voltages to GND?
Fusible resistors are problematic in this series of amps.
Fusible resistors are problematic in this series of amps.
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Hi mbz;
Do you mean TR708 ? I could not say for certain that the '8' was not a '6' except that there is a TR706 elsewhere, driving the protection relay.
Do you mean TR708 ? I could not say for certain that the '8' was not a '6' except that there is a TR706 elsewhere, driving the protection relay.
All the modules are back in place and connected.
Does this mean anything; Voltage across R726 (neg supply 15 ohm resistor ) is 2.99 V
Voltage across R725 (pos supply) is 3.02 V
Can I assume the load is the same for both pos and neg sides of the supply? Yet only the neg side has a saw tooth.
I have yet again bypassed the fuse-resistors with 12 ohm power resistors. Also the hum/saw tooth only slowly appears about 10 seconds after a cold start.
Does this mean anything; Voltage across R726 (neg supply 15 ohm resistor ) is 2.99 V
Voltage across R725 (pos supply) is 3.02 V
Can I assume the load is the same for both pos and neg sides of the supply? Yet only the neg side has a saw tooth.
I have yet again bypassed the fuse-resistors with 12 ohm power resistors. Also the hum/saw tooth only slowly appears about 10 seconds after a cold start.
TR704 had a Vbe of 0.1 V I replaced this transistor, no difference.
I see that on start-up, the Vbe is about 0.5 V and there is no hum.
After 10 seconds, the hum comes back slowly---as the Vbe goes down.
I see that on start-up, the Vbe is about 0.5 V and there is no hum.
After 10 seconds, the hum comes back slowly---as the Vbe goes down.
Yeah, schematic is not real clear. Looking at thje transistor part of regulated supply for -25V, connects to TR714, 715. OK, I see TR706 in the protect circuit so interested in TR708, 714, 715, b/c/e voltages to GND
Appears there is another position, 15Hz? Try that. Maybe problem with mylar caps on that 70Hz branch, ie, C425, C429
Hi , looking at schematic , R725 ,R726 are used for current ptotection in power supply , so normally voltage drop should be less than 0,6 volts on each of them. You wrote about 3v drop ,thats indicate fault in load. Try to measure voltage drop on 10 ohm emitter resistors in modules D and E , should be about 50-60mv on each resistor .If you see much higher drop , it means preamp transistors like TR571 TR573 open too much .You said they all measure ok ,so then check r575 d571 ,diode is dual .also c571 c573 may shorting, and leaky .