Hi!
Im quite new to troubleshooting of amps (though know the dangers and how to deload capacitors) and would need some help if I could solve the problem myself (and maybe learn something new!)
I have Yamamoto a08s and the rectifier tube had some electric lightning inside. At that time I didn't know much about these electronics, just thought that the tube needed to be changed. I remember to checked that the ampere meter at the deck showed 0. Every one of the tubes were glowing normally.
After changing the tube the electric lightnings started again. I opened the chassis and saw the diodes blowen (check the drawing). After changing the diodes they soon blowed again - and now it's time to think before doing anything else..
I measured every resistor and they seem to be fine. Also all the capacitors seem to increase their resistance as should when measured with a basic voltage meter. The rectifier tubes filament voltage is about 5.2V and the other pins measure 350V.
I would really appreciate if someone gave me tips what to measure/try to change next (and sorry for my bad English!) 🙂
Im quite new to troubleshooting of amps (though know the dangers and how to deload capacitors) and would need some help if I could solve the problem myself (and maybe learn something new!)
I have Yamamoto a08s and the rectifier tube had some electric lightning inside. At that time I didn't know much about these electronics, just thought that the tube needed to be changed. I remember to checked that the ampere meter at the deck showed 0. Every one of the tubes were glowing normally.
After changing the tube the electric lightnings started again. I opened the chassis and saw the diodes blowen (check the drawing). After changing the diodes they soon blowed again - and now it's time to think before doing anything else..
I measured every resistor and they seem to be fine. Also all the capacitors seem to increase their resistance as should when measured with a basic voltage meter. The rectifier tubes filament voltage is about 5.2V and the other pins measure 350V.
I would really appreciate if someone gave me tips what to measure/try to change next (and sorry for my bad English!) 🙂
An externally hosted image should be here but it was not working when we last tested it.
Last edited:
What's the capacitance of the first cap after the rectifier? If it's too large, the PS might be drawing too much current thru the rectifier, thus blowing it. Especially since you seem to be operating at the max rated voltage of the 80.
It could also be a faulty 717A or 45 Valve or Output Transformer.
I would tentatively suggest trying removing the two non-rectifier valves one at a time to see if the arcing stops, however, the unloaded B+ could exceed the ratring of the caps under no load.
The caps seem well specced, so it may be OK.
I would tentatively suggest trying removing the two non-rectifier valves one at a time to see if the arcing stops, however, the unloaded B+ could exceed the ratring of the caps under no load.
The caps seem well specced, so it may be OK.
Is it easy to check that the windings of the mains Transformer are all electrically isolated from each other. If you have a current path from HT to one of the heaters, the heaters will act as anodes. This may explain why the diodes have failed.
I removed the 45 tubes one at a time and no arching was seen - but this time the right channel's 717A tube was immediately glowing much much more than the other so I had to pull the power off soon since I had no idea what would happen. I checked that it happens with any 717A in that same channel.
With all the other tubes than the rectifier, both 717A's shine equally. The ampere meter at the deck shows 0 at all times.
Another stupid question from me, but can those caps change their values so that it could be the reason for this prob? The next cap after the rectifier is 5uF (now corrected).
Last edited:
WAY too large!!! For this type of tube, that cap should be more like 5-10uF, otherwise death of the tube will occur, sometimes rapidly, sometimes after a bit of use.
tovaskai,
Which diodes are you refering to?
The heater diodes or the Zeners on the 717a?
Regards
M. Gregg
Which diodes are you refering to?
The heater diodes or the Zeners on the 717a?
Regards
M. Gregg
The zener diodes on 717a:s, the were blowen when I opened the chassis first time, but after changing them they blowed again.
Check the cathode resistor on the 717a and 45. Also that the choke is not open circuit.
Also try removing the 717a replace the diodes and measure the voltage on the top of the zeners to Gnd. They may blow again however it may be that you have to high a voltage on the top of the 125V rail. The only other thing is grid current. The zeners should work if the voltage is correct! work on one channel so you don't blow the other set again.
This could be due to not enough current being drawn by the tubes.
Also try removing the 717a replace the diodes and measure the voltage on the top of the zeners to Gnd. They may blow again however it may be that you have to high a voltage on the top of the 125V rail. The only other thing is grid current. The zeners should work if the voltage is correct! work on one channel so you don't blow the other set again.
This could be due to not enough current being drawn by the tubes.
I suspect that the 717A is bad on that channel. Too high a voltage would affect the other tube since they share the same PS, there is a short in that tube.
Try swapping the tubes.
He says that both sets are blowing, I assume both channels in post #13.
Regards
M. Gregg
Doing some simple maths on the zeners around the 717A would tend to indicate that the 6K8 resistor or the 717A is faulty.
With good zeners, No 717A fitted and a good 6K8 resistor. The current through the zeners is only 2mA. This amounts to 66mW dissipation in the 33V zeners which is well below their limit of 400mW.
If the 717A is faulty and is applying a (FAULT) voltage at G2, this could cause the zeners to fail.
Similarly if the 6K8 resistor is short circuit then the zeners could fail.
I would remove the 717A and replace the 6K8 with a new resisitor. Then check that Pin 6 of the 717A is at 113V.
Just because you have swapped the 717As doesnt mean that both are not faulty....
Also if the 0.22uF / 400V interstage cap is leaking, the 45 will draw excessive current.
With good zeners, No 717A fitted and a good 6K8 resistor. The current through the zeners is only 2mA. This amounts to 66mW dissipation in the 33V zeners which is well below their limit of 400mW.
If the 717A is faulty and is applying a (FAULT) voltage at G2, this could cause the zeners to fail.
Similarly if the 6K8 resistor is short circuit then the zeners could fail.
I would remove the 717A and replace the 6K8 with a new resisitor. Then check that Pin 6 of the 717A is at 113V.
Just because you have swapped the 717As doesnt mean that both are not faulty....
Also if the 0.22uF / 400V interstage cap is leaking, the 45 will draw excessive current.
I'm not familiar with the 717A but most valves operate with almost negligeable Grid current.
If we assume that the grid current is ZERO, ie an infinite input impedance, then the only source of the destructive voltage for the zeners is the 6K8 resistor; or of course a faulty 717A.
If we assume that the grid current is ZERO, ie an infinite input impedance, then the only source of the destructive voltage for the zeners is the 6K8 resistor; or of course a faulty 717A.
Hi!
Thanks for everyone for helping with this, the problem was solved yesterday. The 100uF/450V capacitor was actually bad (changed it's value pretty much). I couldn't see that vith a voltage meter, got a good capacitance meter that measured it. Now everything works perfectly.
Thanks!
Thanks for everyone for helping with this, the problem was solved yesterday. The 100uF/450V capacitor was actually bad (changed it's value pretty much). I couldn't see that vith a voltage meter, got a good capacitance meter that measured it. Now everything works perfectly.
Thanks!
- Status
- Not open for further replies.