Right at the wires where they come out of the transformer, and you'll be measuring in AC.
HR Deville low volume and distorted
- Thread starter STR0000087
- Start date
Basically, across cap 53.
or
where rectifier diodes CR6 and CR8 meet to where rectifier diodes CR7 and CR9 meet.
or
where rectifier diodes CR6 and CR8 meet to the standby switch.
Cap 31 is after the diode rectifier.
377 VDC is supposed to be 485 VDC, a small problem.
847 VAC in this location is a large problem.
the rectifier converts AC energy into DC energy with some small amount of ripple AC remaining.
EDIT: I see you probably reversed the VDC numbers on + and - 32. Now that I understand that I will look at the other numbers for clues.
the numbers are consistent across the board. Way too much AC every where.
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andrewRneumann
Tele-Afflicted
You’ve got me confused. Do you have a diode test on a DMM? If so, with the amp off, unplugged, and filter caps discharged, go through CR6 through CR9 (the diode bridge connected to the HT) and probe each diode. Keep the standby switch open to keep the PT secondary out of the circuit.
Swap the probe leads on each diode—measure both ways. One way you should get a reading of .5-.7V. The other way it should read OL or open circuit. You may see some variation in the reading as the capacitance in the supply charges up, but it should stabilize.
If you can’t get a good read because of the rest of the circuit, try reading the resistance across each diode. Swap the leads for each diode and verify the reading.
If there is a bad one, hopefully we can pinpoint it without desoldering anything.
STR0000087
TDPRI Member
Yes i reversed numbers on c32 +&-
Readings from the + side of C31 cap to the stby switch 422AC
+ side of C31 cap to CR9(diode) cathode 422AC
+ side of C31 cap to CR9 anode 840AC
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STR0000087
TDPRI Member
Diodes CR6 to CR9 all measure black lead to cathode 0.50v. Red lead to cathode OL.
STR0000087
TDPRI Member
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STR0000087
TDPRI Member
Reading on filter caps using ESR meter:
C31 OL
C32 0.761ohm
C33 1.04ohm
C34 3.36ohm
C35 OL
C36 2.0ohm
Could the loss of plate voltage be due to filter caps gone bad specifically C31 & C35?
C31 OL
C32 0.761ohm
C33 1.04ohm
C34 3.36ohm
C35 OL
C36 2.0ohm
Could the loss of plate voltage be due to filter caps gone bad specifically C31 & C35?
andrewRneumann
Tele-Afflicted
Ok so diodes appear intact.
Let’s summarize. You are getting a large loss of B+ voltage, plus a lot of hum even when no tubes are installed. Furthermore, with the tubes installed, you are getting hum, but apparently not from the tubes because they are biased in cut-off. They aren’t even conducting, so how can they be amplifying hum?
To get hum out of the speaker, AC current has to flow in the OT. With no tubes installed, that usually occurs when the OT magnetically couples to leak magnetic flux from the PT. Under normal circumstances, this produces minimal hum, but if the PT saturates—say because it is only conducting during half a cycle—it will throw off a lot more leakage flux which could be picked up by the OT. This is why I suggested checking the diodes.
Another way for hum currents to get into the OT with no tubes is some kind of AC short on the OT primary side. Let’s have a look at the power tube valve sockets and make sure there aren’t any shorts there. Measure resistance between the pins with no tubes installed. Make sure they are clean with no obvious signs of distress.
Even if a capacitor was bad, that doesn’t seem to explain the hum with no tubes, unless there is another problem like I described above.
That’s all I got for now. I’m more of a theoretical troubleshooter than a real tech like @Wally or @Jon Snell @tubedude @D'tar et al. Maybe we can get their attention.
Let’s summarize. You are getting a large loss of B+ voltage, plus a lot of hum even when no tubes are installed. Furthermore, with the tubes installed, you are getting hum, but apparently not from the tubes because they are biased in cut-off. They aren’t even conducting, so how can they be amplifying hum?
To get hum out of the speaker, AC current has to flow in the OT. With no tubes installed, that usually occurs when the OT magnetically couples to leak magnetic flux from the PT. Under normal circumstances, this produces minimal hum, but if the PT saturates—say because it is only conducting during half a cycle—it will throw off a lot more leakage flux which could be picked up by the OT. This is why I suggested checking the diodes.
Another way for hum currents to get into the OT with no tubes is some kind of AC short on the OT primary side. Let’s have a look at the power tube valve sockets and make sure there aren’t any shorts there. Measure resistance between the pins with no tubes installed. Make sure they are clean with no obvious signs of distress.
Even if a capacitor was bad, that doesn’t seem to explain the hum with no tubes, unless there is another problem like I described above.
That’s all I got for now. I’m more of a theoretical troubleshooter than a real tech like @Wally or @Jon Snell @tubedude @D'tar et al. Maybe we can get their attention.
it could be the cause of your problem.
high AC voltage on your filter caps is going to put high AC voltage on the plates of all tubes. Have you checked the AC voltage on the plates?
it looks like the PT is putting out high voltage AC like it should, although I don’t see that you have measured the secondary. Your recent measurements, including C31 are down stream of the rectifier. It would be good to know what AC voltage the PT is supplying the rectifier, but like I said, it does not look like that is the problem. And the PT is more important and harder to replace ($) than the filter caps and rectifier diodes.
I would confirm that high voltage AC is in the B+ and reaching the plates. I find it hard to believe that high voltage is on the plates and that you don’t have a crazy amount of hum.
the filter caps are supposed to pass AC to ground. If the filter caps go open, then the AC cannot reach ground. If the filter caps short out, the fuse will probably blow.
if you find high AC voltage on the plates, then I believe your next step is all new filter caps.
That schematic in the OP is over 20years old. I suspect your amp is of this vintage and the peak into your video appears to have the stock caps. Job one IMO is a good service replacing all the electrolytics throughout to include bias caps. While you are there check for drifting resistors, clean jacks, proper fuse etc.
Have fun, be safe!
Have fun, be safe!
I need to correct what I wrote earlier about finding the expected AC voltage on the secondary of the PT from the expected B+ DC voltage.
first off, the HRD amps PT are not center tapped like the classic fenders.
It appears the relationship is this, depending on whether the PT is center tapped or not. The square root of two is 1.414.
Center tapped power transformer - full wave rectifier
Vct / 1.414 = B+ in VDC
non-center tapped power transformer - half wave rectifier
VAC * 1.414 = B+ in VDC
in the case of the hot rod de ville, you want this:
342 VAC on secondary of PT * 1.414 = 484 VDC B+
i had to just go thru the PT specs and schematics to figure this out.
I am surprised that the non center tapped, half wave rectifier appears to be more efficient in producing VAC than the non center tapped half wave rectifier.
EDIT: I see a non center tapped pt like the 290VX for the de ville, uses lower voltage and more current than a bassman, however, the hot rod deluxe has low voltage and regular current for a wimpy 77 W output.
first off, the HRD amps PT are not center tapped like the classic fenders.
It appears the relationship is this, depending on whether the PT is center tapped or not. The square root of two is 1.414.
Center tapped power transformer - full wave rectifier
Vct / 1.414 = B+ in VDC
non-center tapped power transformer - half wave rectifier
VAC * 1.414 = B+ in VDC
in the case of the hot rod de ville, you want this:
342 VAC on secondary of PT * 1.414 = 484 VDC B+
i had to just go thru the PT specs and schematics to figure this out.
I am surprised that the non center tapped, half wave rectifier appears to be more efficient in producing VAC than the non center tapped half wave rectifier.
EDIT: I see a non center tapped pt like the 290VX for the de ville, uses lower voltage and more current than a bassman, however, the hot rod deluxe has low voltage and regular current for a wimpy 77 W output.
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If my calculation is correct, above, that you want your PT to output 342 VAC, it is interesting to note that you have measured 343, 353 and 340 VAC, in the wrong places, but it could be a clue that the PT is doing what it is supposed to do.
STR0000087
TDPRI Member
i've been away this weekend.
Just checked all pins and look good and clean.
Resistance reading without tubes
Output tubes V4 & V5: have 51ohms between pin2 and pin8, all others are OL.
Phase inverter V3: 0 ohm between pins 4,5 & 9.
STR0000087
TDPRI Member
I just measured AC on the pins of the Output tubes (without the tubes).
V4 & V5 pins 3 & 4 have 1004 VAC. pin7 has 6 VAC
V3 pins 1 & 6 have 825 VAC
BTW my amp does't have C53 cap before the diodes like the schematic. Could you pinpoint exactly where to put the probes, since i can't find the exact location for the reading on the AC of the PT secondary.
Attachments
This is definitely a problem that must be addressed.
Where diodes 6 and 8 meet, on the left of the diodes on the schematic, to where diodes 7 and 9 meet, also to the left of the diodes, on the schematic.
I don’t think this measurement is critical until the high AC voltage is removed from the B+.
andrewRneumann
Tele-Afflicted
To read VAC on your PT secondary, put your probes here. Standby switch has to be closed.
Those VAC readings on the output tubes and PI are crazy and should be dismissed. You should be reading DC in those locations. Cheaper meters can't distinguish AC from DC and will flip out when you try to measure a small amount of AC on a large DC. I assume that is what is going on with your readings.
I'm not sure what the transformer is rated for, but I'm going to guess around 375VAC with no tubes installed. Let us know what you get. This is the highest VAC you should get anywhere in the amp.
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andrewRneumann
Tele-Afflicted
Did V4 & V5 pin 7 measure 51 ohms to pin 8 too? Did it also show 0 ohms to pin 2?
STR0000087
TDPRI Member
Ok, i'll be ordering new filter caps and changing them.
AC voltage of PT secondary is 363VAC.
STR0000087
TDPRI Member
I just rechecked VAC readings with a better DM(Sperry- 6450). I'm getting OL, so i'm guessing it's a high AC reading.
