Rivera 55 12 help

[LarBal]

I need some help with this one .
Amp turns on, power tubes are pulling current, but no sound. I am getting hiss from both channels with volume knob adjusted, though.
Amp seems to be working fine other than it's not passing signal

Does anyone have a schematic? Can't seem to find one anywhere

I've swapped all preamp tubes to no avail. A little digging tells me there is some complex +/- 18VDC filament system in place?
With no preamp tube in V1(parallel triodes), I'm getting +18VDC on pin 4, as is printed on the circuit board as a test point. Good.
With a 12ax7 inserted, pin 4 reads around 4VDC, pin 5 reads 1VDC and pin 9 is 2VDC, or somewhere thereabouts. This confuses me, as I have no idea what's pulling down the heater voltage when tube is inserted.

Also, of course there is no voltage drop across the shared 47K Rp, or the Rk 680R. Both read in spec.

The fact that I don't understand this heater string is not helping, but I suspect the heater issue is preventing the V1 tube from conducting.

Any help is appreciated!

[g1]

Not sure if you have all the preamp tubes in place, but it is probably a series arrangement so you will get weird results unless you have them all in there.
I think the Knucklehead has series DC heaters, so you might look at that schematic for some ideas.

[LarBal]

Yep I had them all in when I got the odd 4VDC on pin 4. I'm not sure if the "18VDC" printed on the board is supposed to be with all tubes in or out though. V1 is definitely not conducting though

[Enzo]

Don't measure heater voltage to ground. Your 12AX7s either have pins 4 and 5 wired together with 6v between them and pin 9, or there is 12v between pins 4 and 5 for 12v operation. 18vDC suggests to me tubes wired for 6v and then three of them in series. If the amp has +18 and -18, we could have six small tubes, maybe? But what matters is the voltage across each tube, not the voltage to ground.

Nobody puts voltages on boards or schematics for "all tubes out". Any specified voltages ALWAYS assume all tubes present.

But here is the test, look in the tubes, are the heaters glowing? If they are, then the voltages are right, whatever your measurements say. If some tubes are not glowing, find out

You get hiss in the speaker? Then the amp is amplifying. You should be able to find the input tube without a drawing. Turn it on, turn the volume and gain up, and touch a meter probe to pin 2 and then pin 7. Or use a small screwdriver. The probe or tool will act like a little antenna and inject hum into the amp. If you hear than, then the amp is working from that point on.

Another trick is to just go down the row of 12AX7s, or 12AT7s, or other similar, and measure voltage on pins 1 and 6, the plates. You should have B+ voltage present. If zero, then the B+ isn't reaching the tube. If the B+ reads high, like all the way up to the supply voltage, then that triode is likely not conducting. SO we also check pins 3 and 8, the cathodes, we expect a volt or two there from tube current. Again, if zero the tube is not conducting. I can check all of them in a minute or two, no schematic needed.

But clear up your V1, measure voltage from 4 to 5, and from 9 to either. That is what matters, not to ground. And with ALL tubes in.

[LarBal]

Ok great, thanks. There are 5 little tubes in this amp.
I'll list all voltages, but in brief, a visual inspection shows absolutely nothing lighting up in V1, and only half of V2 lighting up. Filaments and plates. Voltages confirm this.
Looks like neither triode in V1 is conducting, as well and the 1st half of V2.

For the audio test, working backwards from the PI(V5), and get extra hum when touching the grids on pin2/7 of V5, as well as V4 and V3(in high gain channel mode). Also in the second 1/2 of V2.
There is nothing at pin 2 of V2, and then of course nothing at the V1 grids.

So V1 and V2a aren't conducting at all.

Now that I think about it I guess there's no need to list all voltages on conducting tubes. There is a normal voltage drop acroos the plate and cathode resistors on those.
I'll just list voltage from 4 to 5, and from 9 to either.
I'll also list 4,5,9 to ground, as some of the test points on the board are clearly intended for measurement that way, as those on the conducting tubes are right on the money.


V1:
1 : 385
2 : 0
3 : 0
4 : 3.6 (labled TP +18VDC)
5 : 1.1
6 : 386
7 : 0
8 : 0
9 : 2.4

4-5 : -2.4VDC
4-9 : -1.1VDC
5-9 : +1.2 VDC


V2
1 : 382
2 : 0
3 : .012
4 : 1.1
5 : -5.6
6: 265
7 : 0
8 :1.8
9 : -.001 (labeled GROUND)

4-5: -6.7VDC
4-9: -1.1VDC
5-9 : 5.6VDC

V3
(to ground)
4: -5.6
5: -18 (Labled TP -18V)
9: -12

4-5 : -12.5
4-9 : 6.2
5-9 6.2


V4
(to ground)
4: 0
5: -13.5
9: -6.7

4-5 : -13.4
4-9 : 6.7
5-9 : 6.7


V5
4: -13
5: -26 (labeled TP -24)
9: -20

4-5 : -13
4-9 : -6.3
5-9 : 6.5

This amp is looking for 115VAC in, btw

[loudthud]

I fixed one of these or similar recently. The heaters are run in series from the +/- 18V regulators. One tube has pin 9 grounded and half runs from the +18, the other half runs from the -18V with a whole tube in series from the regulator. You will need to replace one or both regulators IC1, IC2. It might be possible to replace the regulators without removing the PCB, but to insure good clean solder connections, I lifted the PCB so I could clean the underside of the double sided PCB with solder wick. PITA!

Is the attached the correct schematic for your amp? Some of the ones I found weren't clear scans and there is a lot of stuff crammed onto the page. Takes a while to find your way around.

Edit: It looks like the +18 regulator failed, the -18 seems to be working.

[LarBal]

Ok Great! Thanks a bunch. A quick glance at the schematic, and yes, this looks like my amp, but with 2 outputs. Ninja boost and all...
Ok so the regulators were my suspicion as well, and yes I'm usually ok with pulling a board but this would be a nightmare! It looks like I can easily remove the old one and get the new one in with no problem...hopefully.
Can you help me with the part? This is the one I'm thinking of, but it's 1A, the original(I think, was 2.2A) https://www.mouser.com/ProductDetail...GL8jcJorZLE%3d

[Enzo]

A 7818 has certain specs. it doesn't matter if one company calls it a 1.5A part and another calls it a 2A part. Anything that is a 7818 will work there. Not 78M18, Not 78L18. But 7818, yes. Any brand. All it powers is a couple small ICs and a few LEDs.

[LarBal]

Ok great, thanks for the help!

[LarBal]

Ok back to this amp. To refresh, V1 and V2a aren't conducting. No filament glow. This amp has the funky +/- 18V heater supply.

The positive side is the side I'm having problems with. I replaced IC1 , 7818, and nothing changed. Exact same readings as before(above).

I'm getting 18V at TP43, with no tube in, but when I insert a tube in V1 it drops to ~4V. Same as before. Same with the other pin4,5,9 readings. So the regulator is/has been working.

Any other thoughts? IC3 has +18V even with the tube in. Bad diode?

[Enzo]

Diode? What diode? D8? D8 has no way to know if there is a tube or not.

When V1 is inserted, it completes the circuit through V2 at the same time.. Pull V1 and V2. Power off. Look carefully to verify the sockets of V1 and V2 are wired as the schematic. V1 has no connection to pin 9. Verify pin 9 of V2 is grounded. Now verify with an ohm meter. Remember, no tubes. Resistance to ground from pins 4,5,9 of V1, and 5, and 9 of V2. Pin 5 of V1 and pin 5 of V2 OUGHT to be wired together - zero ohms - but to nothing else, so open to ground. V1 pin 9 also open to ground. Pin 4 of V1 ought to read the same as TP43, whatever that is.

You said TP45 stays +18 whether there is a tube or not, so I have to assume the +24v is OK. I notice the schematic lacks a line from +24 to IC3, it ought to be there. But pin 1 of IC1 and IC3 should be connected together, ar they? If ther is a resistance ionthere it could be preventing the IC1 from working. Measure the +24v right at pin 1 of IC1 with and without a tube, does the +24 stay or drop?


When you insert V1 the voltage drops. If you pull V2 but install V1, does it still drop or does it now stay +18 at TP43? Without V2, there should be no current path through V1.

And just for tests, the tube draws 150ma from the 18v. Unless I am batty, that equates to a 120 ohm resistor. You got a 120 or 150 ohm resistor? Ground one end, and clip the other to TP43 - perhaps at the V1 socket. DO this with V1, V2 removed. Does TP43 stay up at 18v or does it drop to 4v?

[loudthud]

Check that filter capacitor on the +24. If it is low in value or high in ESR, the regulator will appear to work at low current, but at some higher current it will look like the regulator isn't working because of huge ripple Voltage. Is there a huge 120Hz ripple at the +18V line when V1 and V2 are installed?

[LarBal]

Diode? What diode? D8? D8 has no way to know if there is a tube or not.

When V1 is inserted, it completes the circuit through V2 at the same time.. Pull V1 and V2. Power off. Look carefully to verify the sockets of V1 and V2 are wired as the schematic. V1 has no connection to pin 9. Verify pin 9 of V2 is grounded. Now verify with an ohm meter. Remember, no tubes. Resistance to ground from pins 4,5,9 of V1, and 5, and 9 of V2. Pin 5 of V1 and pin 5 of V2 OUGHT to be wired together - zero ohms - but to nothing else, so open to ground. V1 pin 9 also open to ground. Pin 4 of V1 ought to read the same as TP43, whatever that is.

You said TP45 stays +18 whether there is a tube or not, so I have to assume the +24v is OK. I notice the schematic lacks a line from +24 to IC3, it ought to be there. But pin 1 of IC1 and IC3 should be connected together, ar they? If ther is a resistance ionthere it could be preventing the IC1 from working. Measure the +24v right at pin 1 of IC1 with and without a tube, does the +24 stay or drop?


When you insert V1 the voltage drops. If you pull V2 but install V1, does it still drop or does it now stay +18 at TP43? Without V2, there should be no current path through V1.

And just for tests, the tube draws 150ma from the 18v. Unless I am batty, that equates to a 120 ohm resistor. You got a 120 or 150 ohm resistor? Ground one end, and clip the other to TP43 - perhaps at the V1 socket. DO this with V1, V2 removed. Does TP43 stay up at 18v or does it drop to 4v?

V1 and V2 tubes out, amp off
There is clearly a trace on the board between V1 pin5 and V2 pin 4, which is not what the scheme shows. Confirmed no connections to V1 pin 9, V2 pin 9 is grounded.
Resistance to ground V1:
pin 4- 9Mohms (note that pin1 on IC1 shows 15M)
pin 5- 0 ohms, open
pin 9- 0 ohms, open

Pin 4 of V1 reads same as TP43, always

Yes TP45 stays 18V no matter what. Yes I believe the +24 rail is fine, but it's actually 28V. But regulated to 18V just fine it seems.

If I pull V2 but leave V1 in, TP43 now stays +18V. Interesting!

Pin 1 of IC1 and IC3 are connected together

At IC1 pin 1, the +28V stays constant in any combination of removal and replacement of V1 + V2.

That is as far as I've gotten. I'll do the resistor test in a bit

[Enzo]

OK, so the drawing reversed pins 4 and 5 on V2. No big, just make the mental adjustment.

Pulling V2 leaves +18, meaning the V1 socket is not likely involved. At least not directly.

I am interested in the resistor test.

[LarBal]

OK, so the drawing reversed pins 4 and 5 on V2. No big, just make the mental adjustment.

Pulling V2 leaves +18, meaning the V1 socket is not likely involved. At least not directly.

I am interested in the resistor test.

Resistor test with 130r from V1 pin 4 to ground

Both V1 and V2 pulled , TP43 (pin4) drops from 18V to 11V when 130r is grounded

With just V1 pulled, same thing