Today, I replaced the Bridge Recifier, the C76 and C77 capacitor. The voltage now rise to 14v with immeditely with the old setting (so I adjusted it to 12v). The original capacitor C76 measured 580uf and C77 is, of course, 8400uf. Both repalced with 470uf 40v and 1000uf 16v capacitor.
Athough the voltage output is probably good now, Hum still exist. So there is something else not so well. Beside, the 25ohm adjustable resistor now become extremely hot after 2 minutes. So hot that I cannot touch it. I think there is something shorted follow the line? I will check the line around 7025 as well as the 6AN8.
Athough the voltage output is probably good now, Hum still exist. So there is something else not so well. Beside, the 25ohm adjustable resistor now become extremely hot after 2 minutes. So hot that I cannot touch it. I think there is something shorted follow the line? I will check the line around 7025 as well as the 6AN8.
Pulled out both 6AN8 and the votalge jumped to 17v . Those 6AN8 was drawing too much current I think.
17V on the 7025 is much to high and will damage them. Do NOT pull the 6AN8 without increasing the serial resistance. The 6AN8 are serialconnected, you measured earlier 2V on each heater, so the heaters are equal and almost certain ok. Would be very unlikely that both would be faulty exactly the same way and draw to much current. Put all tubes back, adjust the serial resistor to max resistance. Make sure you can read the dc heatervoltage on your meter during adjustment of the serial resistor. Switch on the power and adjust the resistor until you get 12,6V across the heaters of the 7025 tubes. Then çheck the voltage across each 6AN8 heater, it should be half of 12,6 = 6,3V on each. If you after that still got a hum, measure the voltages as adviced earlier, and come back with that info.
Ahh... Thank you for the information. I will try your suggestion and see what I find. I also ordered a 10w 10ohm resistor in case it is needed.
The unbypassed 1kΩ cathode resistor on the 1st stage 7025 can be an opportunity for hum...
For example, if 1µA of 50Hz leakage current can leak through the heater insulation/capacitance, 1mV of noise appears. In practice, a very good sample of 7025 is needed. The heater transformer will leak common-mode 50Hz current, so even DC heating is at risk of this.
Checking whether the hum is 50Hz or 100Hz indicates whether this is worth investigating. (use REW, or even a phone spectrum analyser app, if no scope available).
Bypassing the Rk is not possible here (other than for a quick test) the Rk is part of the EQ.
For example, if 1µA of 50Hz leakage current can leak through the heater insulation/capacitance, 1mV of noise appears. In practice, a very good sample of 7025 is needed. The heater transformer will leak common-mode 50Hz current, so even DC heating is at risk of this.
Checking whether the hum is 50Hz or 100Hz indicates whether this is worth investigating. (use REW, or even a phone spectrum analyser app, if no scope available).
Bypassing the Rk is not possible here (other than for a quick test) the Rk is part of the EQ.
Thanks. I put the 6AN8 back, adjusted the voltage to 12.6v (pin 4 to pin 5 7025) and measured the voltage. The 6AN8 voltage tested correct (6.3v from pin 4 or -6.3v from pin 5 to ground)
I also attached 10ohm resistor and removed the 6AN8 and repeated the same test.
(The Tape heads were disconnected so there should no signal input at all)
Hum existed and seems to be high frequency. It become louder with volume knob turning. If I remove 7025, then there is no hum.
Here are the results:
with 6an8
across R46 110v
across R49 2-5v mostly 3.5v (this voltage kept changing although most time is around 3.5v)
across C20 210v
without 6an8
across R46 120v
across R49 1.5-2.5v (this voltage also kept changing but range seems smaller)
across C20 220v
I also tested the voltage across the adjusted resistor R111. When I have both 7025 and 6AN8 and the correct 12v voltage output:
I've got 9v across R111 with a resistence of 14ohm
Thanks. The hum seems high freuqncy. It is shown in the following picture. I also attached a video. The freuqncy is ranging from 1k to 5k.
This is measured at the input of 12AT7 pin 7 with AC coupling
C20? Is it the voltage across the 20uF cap C26?
Just to make sure, the datasheet shows two 7025 heaters, do you have 2 of those working in the same mono channel, or is it stereo with 2 equal channels? Anyway, looks like you got two of those 7025, did you try to swap them? In case you have 2 channels, is there a equal problem in both? As to the heater, at 12,6V the heater current should be 150mA per 7025 and 450mA through the 6AN8, giving a total of 750mA. Your measured 9V across 14 ohm seems a bit low at 643mA but is hardly the source of your problem. As to replacing the 6AN8 with a resistor, 10 ohm is a bit too low, you need 6,3V/0,45A = 14 ohm per tube. I will look at the rest of ýour "hum" problem later.
Last edited:
Is this still the case? If so, you'll need to either reconnect them or short the wiring.
All good fortune,
Chris
As Chris said, make sure your input is not left open!
Voltage at R46 is ok, voltage at R49 should be about the same but is not measured correctly because of the 5kHz interference your scope shows.
Do (unplugged, cooled down) taken measurements confirm the correct values of R48 and R49?
Did you renew the decoupling caps C26, C27? If not, you should before going any further.
Given the old age of this thing, fully recapping may be the smartest thing to do.
If you replace couplingcaps, like C23 for instance, make sure the outer foil of the cap ( black ring on one side of the cap or some other form of indication) is connected to the anode and NOT to the grid.
Always connect the outer foil to the point of lowest impedance.
Voltage at R46 is ok, voltage at R49 should be about the same but is not measured correctly because of the 5kHz interference your scope shows.
Do (unplugged, cooled down) taken measurements confirm the correct values of R48 and R49?
Did you renew the decoupling caps C26, C27? If not, you should before going any further.
Given the old age of this thing, fully recapping may be the smartest thing to do.
If you replace couplingcaps, like C23 for instance, make sure the outer foil of the cap ( black ring on one side of the cap or some other form of indication) is connected to the anode and NOT to the grid.
Always connect the outer foil to the point of lowest impedance.
Last edited:
Thanks. I will try these tests.
The C20 is my mistake. I did meant the 20uf/450v cap i.e. C26 C27
The C20 is my mistake. I did meant the 20uf/450v cap i.e. C26 C27
Thanks for this advice. Once I plug the head wire back, the high frequence noise is gone. I am so sorry for my Ignorance caused this big confiusing. The deck now plays fine.
But just you suggested, I will replace all the capacitors of this deck.