Ok, based on that testing it would seem that C17 is doing its job "no" ripple the negative output of the bias supply.
Marantz 8B red plate
- Thread starter Mike Chan
- Start date
In this picture the 1W carbon comp screen grid resistor has been stressed. I wouldn't be surprised if that resistor is badly drifted or cracks in half when manipulated. Is that the one that you have already replaced? The screen grid resistors are not all the same so I would think this issue has happened before on this amp.
I am starting to think that you may have multiple bias controls making poor contact. Do you have a variac? My suggestion is to remove the EL34s, then power the amp on with something like 10VAC mains. Then put you DMM with black to ground and red to Pin 5 on each of the output tube sockets. Slowly rotate the bias control for the associated socket and watch the negative DC voltage. if the voltage suddenly spikes or disappears completely that would explain your symptom. This would need to be done on each socket.
Its full of bumblebee caps and those plastic cased Good-Alls. I'd bet thats the problem. These things are just paper caps encapsulated in plastic. THey are very prone to becoming electrically leaky. When that happens it shifts the grid voltage less-negative which increases current flow through the tube. This appears to be very original from the early 60s, and its well overdue for a refresh.
If you want to confirm that, measure across the 100k resistor from the wiper of each bias pot to the tube socket. If the caps are good you'll get 0 volts +/- a few millivolts. If its more than about 25mv, the caps are leaking, and the higher it goes the worse they are. You can do this with the EL34 removed to prevent damage.
If you want to confirm that, measure across the 100k resistor from the wiper of each bias pot to the tube socket. If the caps are good you'll get 0 volts +/- a few millivolts. If its more than about 25mv, the caps are leaking, and the higher it goes the worse they are. You can do this with the EL34 removed to prevent damage.
I will go to get the Variac to try. Is 500VA Auto Variable Voltage Transformer AC Voltage Regulator OK for this testing or 1000VA?
There will be almost no current flowing for this test so a small variac is fine. It’s just to check that the bias controls are working.
It could ve
The plastic good alls are Mylar film according to the service literature. I very rarely find that these are leaky but it’s definitely possible.
Are you mention to measure voltage from middle leg of the 5K bias control to tube socket (ground) ? DC or AC?
The tube socket isn't ground, its the grid connection. DC volts, and its the 100k resistor from the center of the bias control to the socket where it joins with a 1K resistor. Leakage through the capacitor will show up as voltage drop across that 100k.
You're also quite likely right on the bias pot problem. I nearly lost a set of tubes in my Pilot SA-260 from that, which is why the Pilot now has new bias pots. This uses the same bias control setup that most things back then used, and if the wiper lifts it goes to zero bias. Not hard to make it more reliable, a 47k resistor from the negative end of the pot to the wiper would run the grid full negative if the wiper lifts without having much effect on the pot's adjustment range. Why none of the mfg did this I don't honestly know.
a lot of the issue is that this is just old, and any number of things can be at fault here. Loose connections, ailing pots, off-value components, dying components, etc.
Interesting, they look like some paper ones I've had. Mylar is usually pretty reliable though. I have a number of 1960s mylar caps still in use which are perfectly fine. All epoxy-dipped types though, not these specifically.
You're also quite likely right on the bias pot problem. I nearly lost a set of tubes in my Pilot SA-260 from that, which is why the Pilot now has new bias pots. This uses the same bias control setup that most things back then used, and if the wiper lifts it goes to zero bias. Not hard to make it more reliable, a 47k resistor from the negative end of the pot to the wiper would run the grid full negative if the wiper lifts without having much effect on the pot's adjustment range. Why none of the mfg did this I don't honestly know.
a lot of the issue is that this is just old, and any number of things can be at fault here. Loose connections, ailing pots, off-value components, dying components, etc.
Follow your suggestion, I measure the voltage between the middle leg of 5 K bias volume to the pin 5 of EL34 (on the side of R18A, R18B). It doesn't show any voltage.
The voltage doubler cap and the bias supply cap have a sheathing on them that crystallizes and looks bad- miraculously the caps are often ok.
Having said that,you need to rebuild the bias supply c17 and the diode at a minimum after validating your B+--- I would change the diodes in the B+ as well. The bumble bees are in the filtered input so you can use the test input to avoid them.
Your red plating could very well be the socket and loss of tension as you have validated the coupling caps are good...those caps are almost always 99.9% ok..... While marantz sockets are of high quality- no telling what the last 50 years has brought on.... you need to clean and retension the sockets.
Make no assumptions about your tubes either.
Check pin 5 of each output for a steady -36v you can pull the tubes- things will be a bit elevated but you should be in the ball park.
Let's hope its not the output transformer. Good Luck.
Having said that,you need to rebuild the bias supply c17 and the diode at a minimum after validating your B+--- I would change the diodes in the B+ as well. The bumble bees are in the filtered input so you can use the test input to avoid them.
Your red plating could very well be the socket and loss of tension as you have validated the coupling caps are good...those caps are almost always 99.9% ok..... While marantz sockets are of high quality- no telling what the last 50 years has brought on.... you need to clean and retension the sockets.
Make no assumptions about your tubes either.
Check pin 5 of each output for a steady -36v you can pull the tubes- things will be a bit elevated but you should be in the ball park.
Let's hope its not the output transformer. Good Luck.
I have ordered a variac to check the 5 k ohm bias volumn and waiting for its arrival. What I concern is the output transformer as it will be no parts to replace if it turned bad! By the way, how to check this transformer is good or not.
The Variac arrives today. I follow your instruction to measure the DC voltage with Multimeter Black Probe on Ground and Red Probe on Pin 5 EL34, but the pin goes to left direction as it is a negative voltage, so I have to change by Red Probe to Ground and Black Probe to Pin 5 EL34 for measurement reading. The result is as below:
Turn the output of the Variac to 10v AC
A1 - turn the knob all the way to left - it reads -3.2V, then start to turn from left to right - the reading drops to - 2V on the right end
A2 - turn the knob all the way to left - it reads 0V - when starting to turn just a little, it jumps to -3.2V immediately - when turn it to near the right end, it drops to 0V - then I turn it a little back to left, it shows in between -2V ~ -1V (unstable)
B1 - turn the knob all the way to left - it reads -3.1V - then start to turn from left to right - the reading drops to - 2V on the right end
B2 - turn the knob all the way to left - it reads 0V - when starting to turn just a little, it jumps to -3.2V immediately - when turn it to near the right end, it drops to 0V
Hopefully this information can give you some ideas on the amp's situation.
By the way, Is there any way to check the transformer is good or not?
ohm check it from the center tap to each screen and each plate. If its not open, its probably fine. The other fail mode is an internal short which you'd have to apply AC voltage to confirm. Output transformer failure is uncommon, though not impossible. I've had way more problems with bad pots and caps than failed transformers, though there is a bad output transformer sitting on my shelf and I have an amp rebuilt with completely different transformers because of it. That one has an open circuit to the plate.
I also sold an amplifier to someone who had a transformer with an insulation short. When power was applied the winding would arc to the transformer frame because the original construction didn't include any chafe protection for the windings and the enamel failed where it was pressed against the steel frame.
A2 and B2 both sound questionable to me if the reading is jumpy and unstable. Warrants at least a good cleaning and a re-test.
I also sold an amplifier to someone who had a transformer with an insulation short. When power was applied the winding would arc to the transformer frame because the original construction didn't include any chafe protection for the windings and the enamel failed where it was pressed against the steel frame.
A2 and B2 both sound questionable to me if the reading is jumpy and unstable. Warrants at least a good cleaning and a re-test.
Besides, the jumpy and unstable. Can you tell me that the reading of the voltage from the Pin 5 is normal or not?
It is not possible, with only 10vac input to the amplifier, to tell you definitively that your measured voltage at pin 5 on one of the EL34 tubes "is normal or not" as this is not a normal operating condition (10vac input) for the amplifier. The jumping voltage reading in most switch positions is of concern. Given that this wafer switch is 60+ years old and only is used to set bias (not a daily/weekly or monthly task) it is probably dirty. Also possible is the aged condition of C17, which is 60+ years old. In my opinion, given that the amplifier is unrestored, problems will continue to happen regularly because of component failure due to age. Your concern about output transformer failure, not currently supported by the present symptoms, could happen if you continue to use it in it's present condition. My opinion.
at 10v input that should be about 10% of normal run voltage, so not completely unreasonable but no guarantee. really sort of need to see it at full voltage
but I agree with the above, its likely to continue to have random goofy problems until it gets serviced, and its possible one of those problems will turn very expensive.
but I agree with the above, its likely to continue to have random goofy problems until it gets serviced, and its possible one of those problems will turn very expensive.
To me it looks like A2 and B2 pots are intermittent throughout their range. Any time you have drops in the negative voltage during rotation the pot is making bad contact. My guess is that the wipers are losing contact depending on the pot position causing loss of bias and red plating at random.
You can try to work the pots back and forth many times and see if you start to get consistent results. If this ends up being the root cause of the issue the pots would need to be opened up and cleaned which is not very straight forward.
Thank you for your reply. I already order two set of 5 K Ohm potentiometer ahead and waiting for their delivery. I will also try your method to see if it can be solved or not.
BTW, do you still remember at the early beginning I also found that:
The voltage in between R31 & R32 is 470volt instead of 408 volt.
The voltage on the other end of R32 is 450volt instead of 390 volt.
Can you tell me why these voltages are so high? Is it normal?