Good morning,
Yesterday I returned to work on an old 50s valve tape recorder. It is a Brenell Mk 5, gold deck- three speed, with record/playback/amplifier modes. The valve compliment is EF86, ECC83, EL84 and EZ80. I hope this is in the correct forum, as it's not so much a tape issue as an issue with the valve amplifier.
I had it quite close to working in amplifier mode, being able to pass signal through the amp and the speaker. Yesterday, however, I turned it on after it having sat awhile on the bench and am getting really high screen and plate voltage on the EL84. I'm reading around 430VDC on both. The EL84 is also glowing much brighter than before. It looks like screen grid glow?
The power supply is the original 300-0-300 CT housed separately from the amp, and with a CRC filter of 47uf-470 ohm 5W-47uf. There is an additional 470 ohm 5W dropping resistor across the HT+ input line into the amp itself.
I tested all resistors (on EL84 and in HT line) and they are in tolerance and appear to be functioning properly. Also, all resistors were replaced with appropriate wattage/type during recap as many were out of tolerance.
I also tested all heater voltages and they are right on at 6.3V. The plate supply for the EZ80 is also good. As the heater current can handle it and I had one on hand, I decided to try subbing a known-good EZ81 to see if maybe the EZ80 was bad. But it actually produced a higher voltage, as expected, due to its lower voltage drop.
I've attached the relevant bit of the schematic. Am I looking at a failing PT or failed filter caps? No smoke, mess or stink anywhere and the PT isn't warming. I should note that early in my restoration the 6.3V tap for the EZ80 failed, but the rest of the transformer was soaked and tested and functioned properly. I replaced that winding with a 1.2A filament transformer I had and it was working after replacement fine. I appreciate any insight or comment. Thanks!
Yesterday I returned to work on an old 50s valve tape recorder. It is a Brenell Mk 5, gold deck- three speed, with record/playback/amplifier modes. The valve compliment is EF86, ECC83, EL84 and EZ80. I hope this is in the correct forum, as it's not so much a tape issue as an issue with the valve amplifier.
I had it quite close to working in amplifier mode, being able to pass signal through the amp and the speaker. Yesterday, however, I turned it on after it having sat awhile on the bench and am getting really high screen and plate voltage on the EL84. I'm reading around 430VDC on both. The EL84 is also glowing much brighter than before. It looks like screen grid glow?
The power supply is the original 300-0-300 CT housed separately from the amp, and with a CRC filter of 47uf-470 ohm 5W-47uf. There is an additional 470 ohm 5W dropping resistor across the HT+ input line into the amp itself.
I tested all resistors (on EL84 and in HT line) and they are in tolerance and appear to be functioning properly. Also, all resistors were replaced with appropriate wattage/type during recap as many were out of tolerance.
I also tested all heater voltages and they are right on at 6.3V. The plate supply for the EZ80 is also good. As the heater current can handle it and I had one on hand, I decided to try subbing a known-good EZ81 to see if maybe the EZ80 was bad. But it actually produced a higher voltage, as expected, due to its lower voltage drop.
I've attached the relevant bit of the schematic. Am I looking at a failing PT or failed filter caps? No smoke, mess or stink anywhere and the PT isn't warming. I should note that early in my restoration the 6.3V tap for the EZ80 failed, but the rest of the transformer was soaked and tested and functioned properly. I replaced that winding with a 1.2A filament transformer I had and it was working after replacement fine. I appreciate any insight or comment. Thanks!
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No1,
Is the mains input voltage selector set correctly?
Pull the rectifier tube and test the AC voltage across both secondary coils.
And check the working voltage of the supply caps..ie are you exceeding the working voltage?
What voltage do you have across C29?
Is there a connection between pin3 and the CT of the HT secondary? Do a resistance check to make sure..(when its discharged and dead of course)
NB remember the HT will stay up when its powered off there is no current draw on the PSU B+ rail.
Put a discharge resistor between pin 9 and Pin3 something like 500K 2 watt<<might save your life.
You need to be careful not to exceed the current draw on the rectifier.
Regards
M. Gregg
Is the mains input voltage selector set correctly?
Pull the rectifier tube and test the AC voltage across both secondary coils.
And check the working voltage of the supply caps..ie are you exceeding the working voltage?
What voltage do you have across C29?
Is there a connection between pin3 and the CT of the HT secondary? Do a resistance check to make sure..(when its discharged and dead of course)
NB remember the HT will stay up when its powered off there is no current draw on the PSU B+ rail.
Put a discharge resistor between pin 9 and Pin3 something like 500K 2 watt<<might save your life.
You need to be careful not to exceed the current draw on the rectifier.
Regards
M. Gregg
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The next thing is change the coupling cap for the G1 on the EL84.
Its not shown on the drawing you posted but it could be leaking.
Or is it DC coupled?<<if it is and the driver tube is not working the voltage on the G1 would be wrong.
This is assuming the B+ voltages are correct.
Regards
M. Gregg
Its not shown on the drawing you posted but it could be leaking.
Or is it DC coupled?<<if it is and the driver tube is not working the voltage on the G1 would be wrong.
This is assuming the B+ voltages are correct.
Regards
M. Gregg
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Next thing have you lost conduction on the anode of the EL84?
Sounds nuts but what voltage do you have across R33? (Cathode resistor)
If you have lost the anode line via the output transformer you will get grid current and not draw enough current to drop the B+ voltage.
You could do a resistance check for continuity from Pin 9 Ht input to the anode of the EL84.
Then test the anode voltage on the EL84 (remember to reset your meter from resistance) and discharge after power up each time.
In series with the anode of the EL84 you have the output transformer primary and the erase head oscillator. (+ the wiring and connections)
Obviously assuming R33 is good and not open circuit..
There is always the posibilty that you have lost connection on the anode on the tube socket of the EL84..
Regards
M. Gregg
Sounds nuts but what voltage do you have across R33? (Cathode resistor)
If you have lost the anode line via the output transformer you will get grid current and not draw enough current to drop the B+ voltage.
You could do a resistance check for continuity from Pin 9 Ht input to the anode of the EL84.
Then test the anode voltage on the EL84 (remember to reset your meter from resistance) and discharge after power up each time.
In series with the anode of the EL84 you have the output transformer primary and the erase head oscillator. (+ the wiring and connections)
Obviously assuming R33 is good and not open circuit..
There is always the posibilty that you have lost connection on the anode on the tube socket of the EL84..
Regards
M. Gregg
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So in a nutshell,
Is there a fault on the B+ supply transformer (overvoltage)
Or insufficient current draw to drop the B+. <<conduction issues.
Or is the B+ OK and the EL84 in over current?
Hopefully a few things to try..best of luck! Stay safe<<No1 add the doubt it is probably still live..
Regards
M. Gregg
Is there a fault on the B+ supply transformer (overvoltage)
Or insufficient current draw to drop the B+. <<conduction issues.
Or is the B+ OK and the EL84 in over current?
Hopefully a few things to try..best of luck!
Stay safe<<No1 add the doubt it is probably still live.. Regards
M. Gregg
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What do you mean? The orange glow of the heater? A blue glow inside the anode? A red glow indicating overdissipation of the anode? A pink glow indicating gas presence?
Well, oddly enough, it has started working again this morning.
The only thing I did was swap out the Sovtek EL84 I had for the older, original EL84 that came with the unit. This one is unmarked, but the previous owner had said the valves were original to the unit and the others are Mullards, so I have no reason to doubt the validity. I thought I had read a few other posts regarding class A amps and NOS vs new EL84s? This circuit is certainly based on the Mullard 3W tape amp circuit.
The glow I was referring to, looks like regular orange filament glow but much brighter. This is much less pronounced with the original EL84.
One thing I noticed is I can see the EZ80 filament at three points through the valve, whereas before when it was acting up, I could only really see one point. I'm wondering if the valve socket pins might be loose, broken or otherwise intermittent as you surmised. Or the rectifier is going? Don't they usually fail open or pitter out on their HT output rather than increase?
The EL84 screen and plate are down to 280V on my Ratschack analog multimeter, but there may be loading issues due to the values on the schematic being taken with older equipment.
I've been listening to the turtable this morning through the deck set to amp and it aounds reasonable for a relatively low-fi unit like this. The only odd thing is that a volume-dependent hum will sometimes develop after playing through a record. This doesn't occur everytime. Something intermittent going on here...
The only thing I did was swap out the Sovtek EL84 I had for the older, original EL84 that came with the unit. This one is unmarked, but the previous owner had said the valves were original to the unit and the others are Mullards, so I have no reason to doubt the validity. I thought I had read a few other posts regarding class A amps and NOS vs new EL84s? This circuit is certainly based on the Mullard 3W tape amp circuit.
The glow I was referring to, looks like regular orange filament glow but much brighter. This is much less pronounced with the original EL84.
One thing I noticed is I can see the EZ80 filament at three points through the valve, whereas before when it was acting up, I could only really see one point. I'm wondering if the valve socket pins might be loose, broken or otherwise intermittent as you surmised. Or the rectifier is going? Don't they usually fail open or pitter out on their HT output rather than increase?
The EL84 screen and plate are down to 280V on my Ratschack analog multimeter, but there may be loading issues due to the values on the schematic being taken with older equipment.
I've been listening to the turtable this morning through the deck set to amp and it aounds reasonable for a relatively low-fi unit like this. The only odd thing is that a volume-dependent hum will sometimes develop after playing through a record. This doesn't occur everytime. Something intermittent going on here...
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Possible dry joint or bad socket.
I don't know how hard it is to do, however I would change the socket for the EL84 and the rectifier. Then they are eliminated from the situation.
But in preference I would do the EL84 first! Its unlikely a component failure but not impossible. I have seen components get intermittent contact even fuses.. <<I don't want to go there but a car fuse had me going for 3 hours..LMAO..it would go open and remake and weld for days at a time..
I have seen transistors work with the tap of a terminal screwdriver..thought it was a dry joint but no it was an OC in the transistor. Solder the legs and it would work all day then fail the next!
I have seen lots of wire wrapped joints in old radios that should have been soldered and were never soldered from they they were made..so now you know the old fable give it a smack and it works..
Cracked PCB's lets not go there..many a happy hour with scopes all day to find a hair line crack breaking tracks intermittently.
Regards
M. Gregg
I don't know how hard it is to do, however I would change the socket for the EL84 and the rectifier. Then they are eliminated from the situation.
But in preference I would do the EL84 first! Its unlikely a component failure but not impossible. I have seen components get intermittent contact even fuses..
<<I don't want to go there but a car fuse had me going for 3 hours..LMAO..it would go open and remake and weld for days at a time..I have seen transistors work with the tap of a terminal screwdriver..thought it was a dry joint but no it was an OC in the transistor. Solder the legs and it would work all day then fail the next!
I have seen lots of wire wrapped joints in old radios that should have been soldered and were never soldered from they they were made..so now you know the old fable give it a smack and it works..
Cracked PCB's lets not go there..many a happy hour with scopes all day to find a hair line crack breaking tracks intermittently.
Regards
M. Gregg
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Thanks for the help. It's much appreciated and I'm always up for learning more.
I can get to both the EL84 and EZ80 sockets fairly easily. I'll try reflowing the joints and seeing if that doesn't work, but I had done that before when I recapped. Apparently the unit had been stored in an attic in the UK for a number of years so it seems likely one or more of the sockets could have bit it.
And that fuse bit sounds awful. What a pain in the neck.
I can get to both the EL84 and EZ80 sockets fairly easily. I'll try reflowing the joints and seeing if that doesn't work, but I had done that before when I recapped. Apparently the unit had been stored in an attic in the UK for a number of years so it seems likely one or more of the sockets could have bit it.
And that fuse bit sounds awful. What a pain in the neck.
Thanks for the help. It's much appreciated and I'm always up for learning more.
I can get to both the EL84 and EZ80 sockets fairly easily. I'll try reflowing the joints and seeing if that doesn't work, but I had done that before when I recapped. Apparently the unit had been stored in an attic in the UK for a number of years so it seems likely one or more of the sockets could have bit it.
And that fuse bit sounds awful. What a pain in the neck.[/QUOTE]
It was through the winter snow and ice early mornings and some getting home after night shifts. I would turn the key and it would just turn over and not start!
So I would go home via taxi and go back after getting up early and turn the key and BRUMM first turn it would run for a few days. Then go to it in the morning in the dark and it would just turn over again and not fire. I took it in the end to specialists and it would never fail. Left it with them for two weeks and they changed the injectors. I paid the man drive half way home and it cut out at the lights.
I could write a book about it I took every part out of the car and tested it. Then in desperation I changed all the fuses every one. The fault vanished I put the fuses back one at a time and after a couple it failed at 5 in the morning changed the fuse again and it worked. got home and tested the fuse it was OK..so I broke the fuse open and it was cracked where it met the blade no burn marks just a crack and some discolouration where the element met the blade.
It had gone to 5 specialists and three said it was me if there was a fault it would show up on the computer fail flags and there was no history of Failed-start..
It drove me nuts..I got to the point where I would hesitate to turn the key.
Regards
M. Gregg
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Isn't the weather aleays miserable just when things want to fail hard? Sounds like a freak incident though with it rewelding.
On a side note, I've had this thing off and on listening to records all morning now. I noticed when I turned it up, the volume and treble would cut out. I small smack near the tone pot is th culprit. It being the only pot in this thing I didn't clean! I think this will clean up the intermittent hum issue as it's injected when the volume drops out or fooling with the tone pot knob.
I'm also gonna try rolling a few other valves through V1 and V2. Much obliged again!
On a side note, I've had this thing off and on listening to records all morning now. I noticed when I turned it up, the volume and treble would cut out. I small smack near the tone pot is th culprit. It being the only pot in this thing I didn't clean! I think this will clean up the intermittent hum issue as it's injected when the volume drops out or fooling with the tone pot knob.
I'm also gonna try rolling a few other valves through V1 and V2. Much obliged again!
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