HH Scott 299D - Power But No Audio

[gadget73]

it can possibly lead to low frequency instability problems. Not a guarantee but generally if the mfg picked a value, they had some decent reason for it.

 

[reverend13]

Thanks again gadget. Was curious what the potential result would be with the upper value cap. That answers it. Much appreciated.

 

[reverend13]

Phono Stage

Now that both the Tuner and Extra inputs are getting great sound, I've moved on to the Phono stage

I'm getting full sound from my TT in the right channel and minimal sound in the left channel. I've tried both MAG 1 and 2 for both pickups 1 & 2, getting the same result. What's strange is that even with the volume knob at zero, I get faint music in the left channel -- when I turn it up, sound only increases out of the right channel. I played with the switches and when I moved the tape switch from Out to In, the right channel mutes but the faint music stays in the left channel (is that weird?). I also moved the input knob from RIAA (with the tape switch Out) to NAB Tape and Mic, still getting the music from my TT playing in right channel (EQ sounds a little different but not as much as I expected) and faint in left channel.

I'm getting voltages to the V1 and V101 tubes. I tried swapping them, and then adding a new tube in V1, but the same problem persists. Also cleaned and retentioned the sockets in V1. The 82k resistor over V1 looks a little toast visually but getting the same readings as the one on V101. I also replaced the caps going to these tubes with new .022 uF caps.

I ran the TT in another amp and works fine. Reversed the RCA jacks L from R and still left channel super weak and right channel audible.

I've also had a few attempts cleaning all the switches, but perhaps this is still a problem?

Any other ideas? Could this be a problem with a capacitor or resistor away from the tubes and closer to the rear panel that's shared between these inputs (Pickups 1 & 2, MAG 1 & 2)?

Thanks,

 

[trainbuftony]

I'd start by cleaning the front pannel switch.
The signal goes through it before being applied to the first phono stage grid.
It's a very weak signal so the slightest corrosion in that switch will be too much to overcome.
You could actually jumper around it or route the signal from the working channel to the grid of the non working channel to prove out the switch.

 

[gadget73]

are the voltages similar at V1 and V101 and at least in the neighborhood of what the schematic says ? If they are, that probably rules out resistor problems. Only really 2 caps in that section that would totally kill the audio signal, C106 and C107 or their equivalents in the other channel. It does loop through the selector switch to set the EQ, but if that circuit were open you'd just get a lot of gain with no EQ curve.

 

[reverend13]

I couldn't find anything on the schematic for pins 4 and 5 but measured them anyway. Also not sure if I should be getting anything from pin 2 and 7?

Input Tube 12AX7 - V1 (left channel)

• Pin 1: 106.5v
• Pin 3: 0.805v
• Pin 4: -11.17v
• Pin 5: -0.002v
• Pin 6: 110v
• Pin 7: -0.005v
• Pin 8: 0.765v

Input Tube 12AX7 - V101 (right channel)

• Pin 1: 102.6v
• Pin 3: 0.859v
• Pin 4: -21.9v
• Pin 5: -11.15v
• Pin 6: 107v
• Pin 7: -0.005v
  
• Pin 8: 0.789v

Assuming when cleaning the front panel switch, this is referring to the input switch?

Thanks,

I'd start by cleaning the front pannel switch.
The signal goes through it before being applied to the first phono stage grid.
It's a very weak signal so the slightest corrosion in that switch will be too much to overcome.
You could actually jumper around it or route the signal from the working channel to the grid of the non working channel to prove out the switch.

are the voltages similar at V1 and V101 and at least in the neighborhood of what the schematic says ? If they are, that probably rules out resistor problems. Only really 2 caps in that section that would totally kill the audio signal, C106 and C107 or their equivalents in the other channel. It does loop through the selector switch to set the EQ, but if that circuit were open you'd just get a lot of gain with no EQ curve.

 

[trainbuftony]

Yeah.
Those dc voltage readings look good.
The nab/riaa switch is before the grid of the first tube iirc.
Just for giggles run a jumper between the p2 of each phonostage tube.
If both channels play, then you lost your signal prior to that point.
Remove the jumper and trace the circuit with a continuity beeper of your meter if 6ou have such a device.
If the channel doesn't come back, I'd look at things downstream from that point.

Best advice I ever got, was to "meat cleaver" the issue.
Pick a point, any point and compare at that point.
Is the problem before or after that point.
Now you have half the stuff to check.
Once more.
Drive the cleaver through it.
Metaphorically of course.
You've got a quarter of the stuff to check.
Now cut the problem area in half and in half until you have fewer possible failed components than fingers on your left hand.

 

[reverend13]

Using MAG 2, I’m getting continuity through the input switch and pickup switch to right before the 10k resistor (R8) leading to p2 of V1.

However I notice these 4 caps in the input switch. Are the ones that look a little bulgy to me below the .022 caps (C6?) the .047 caps (C1)? Could they be failing? Gets really super tight in there to work.

 

[therling]

When working on the areas around the front panel switches, I usually pull the top cover and remove the screws on the sides that secure the front panel assembly to the chassis, leaving one on each side so I can pivot that front chassis assembly forward to gain better access to components in there. You can similarly do so for the rear chassis as well. Just be careful when bending the wiring since it’s mostly solid wire and doesn’t like being flexed too much.

 

[gadget73]

those Ceracaps are somewhat prone to going open circuit when they fail, and by the schematic I think they may be the output coupling caps. C101 would be an 0.047 and C106 is an 0.022. Looks like the more buried one is a little fatter than the visible 0.022. The way these fail is the lead breaks inside the cap. You might be able to get it to re-connect by gently pushing on the end leads to see if it will come back. One end of that 0.022 is tied to the plate of V101 though so be aware of the high voltage. If it comes back to life when you push on the leads, you have your answer.

This would be a perfect spot for a signal tracer or a scope to see whats going on. Messing around in selector switches is a huge pain in the neck and carries significant risk of damaging the switch if you aren't very careful. This is one of those cases I'd be fine with a J-hook connection.

 

[reverend13]

I’ll take a crack at replacing the ceracaps. Will do the fish-eye/J-hook connection to not mess directly with the selector switch. Hopefully will get that done over the holiday weekend.

BTW - Forgot to mention how clearer the sound is now with the correct .1 caps in place of the 1.0’s I installed. Very happy with the sound.

 

[reverend13]

Yeah.
Just for giggles run a jumper between the p2 of each phonostage tube.
If both channels play, then you lost your signal prior to that point.
Remove the jumper and trace the circuit with a continuity beeper of your meter if 6ou have such a device.
If the channel doesn't come back, I'd look at things downstream from that point.

Update on this phono left channel issue. I ran a jumper from p2 of V1 to V101 and the left channel was still out, so perhaps this means the issue is after p2 on V1?

I also replaced both the two .022 and two .047 capacitors in the input selector switch and nothing changed other than the right channel sounding a little better — left channel still really faint.

Perhaps I need to take a another look at the .022 cap I replaced in C7, although I am getting roughly the correct voltage to p1. Not sure what I should be getting to p7.

Also double-checked the TT with another amp and works fine, just to rule out the cartridge and RCA cables. Even though I reversed the RCA cables previously to see if the faintness also switched channels, in which it stayed in the left channel. Also swapped and switched tubes again to still get the same result.

The resistors upstream read ok but perhaps I should disconnect some and remeasure all them going to the 20/450-20/450-25/25-25-25 electrolytic can capacitor?

A little stumped.

 

[gadget73]

if pin 2 to pin 2 does nothing, that means the problem is not between the input jacks and the grids. That rules out the pickup switch and the wiring between input jacks, the switch, and the grids.

Next quick and dirty check would be pin 7 to pin 7. I suspect this isn't going to do anything, but you never know.

pin 7 should be basically zero volts, it may show a fraction of a volt from grid leakage and whatnot. Would be worth confirming those 2.2M resistors are at least in the neighborhood of OK though.

If the plate voltages at V1 and v101 are in the ballpark of right, I don't think its going to be anything to do with any of the power supply resistors. Even if they're a little off value, the ~100+ volts you're getting is plenty enough for this circuit to function as expected.

 

[trainbuftony]

If you have a left over .022uf cap, and you should, then use a cheap RCA cable, solder the .022 to the end and insulate it.
Heat shrink or a pen body or both...
Plug it into the working channel and drive a speaker.
Plug a signal into the non working channel and trace it through the amp part by part until you lose it.
As you touch the grids and plates with the cap lead feeding the working channel you'll get your beep through the working channel and quickly troubleshoot the non working one..

 

[reverend13]

Hmmm….so going back to my initial readings, I’m looking at pin 5 for both V1 (left channel) and V101 (right channel) —

Phono Input Tube 12AX7 - V1 (left channel)

• Pin 5: -0.002v

Phono Input Tube 12AX7 - V101 (right channel)

• Pin 5: -11.15v

Apologies in advance if I’m asking the wrong questions due to my lack of experience. Since these are the heaters for the 12ax7, shouldn’t they be AC and minimal DC? Should the .022 cap block the DC (even if it’s negative like it is for V101)? If that’s the case where VDC should be minimal at p5 like it is in V1, why is that the channel I’m getting very little sound and V101 at -11.15 VDC at p5 is where I’m getting sound?

 

[trainbuftony]

Pins 4 and 5 are the heater you're correct, and usually they would be run by AC.
Scott didn't do things that way.
They had 45 or so volts of bias supply at sufficient current to light the ribes which when in series want to see about 50v.
Ac/dc either will light the tube, they had a source of dc which is quieter and choose to use that for the 4 most sensitive ones.
Pretty smart if you ask me.
For the problem you're experiencing ignore the heaters if they work, that's all they need to do. You're losing your signal somewhere along the signal path in that channel.
Connecting the grid that's working to the grid of the non working channel didn't bring back the failed channel. One could assume then that whatever open circuit that's preventing the unit from running is after that point.
Your phonostage voltages all look great to me, so I'm going to guess that the channel drops after the 12AX7 phonostage tubes.
I can't for the life of me remember what component comes after the second plate. Probably a selector switch...
Let me pull the schematic up...

Also noteworthy: touching the grids/plates with your probe will produce an audible click in the speaker which you can use to troubleshoot, because obviously the busted channel won't do it prior to the open, but will click after that point.
You're so close I can smell the rosin burning off stick with it, keep at it, you're nearly there.

 

[gadget73]

what he said, ~11v is plenty to light the tube, and often phono tubes were run a little bit on the low side for noise reasons. DC heat is purely for noise reasons, and its a smart place to want to reduce noise as much as possible.

 

[therling]

The heaters for the 12AX7 preamp tubes are DC which is less noisy than AC, which is important because of the low level of signal they amplify. The heater pins on the 12AX7 are numbers 4 and 5. That DC comes from the bias supply rectifier, which is why it’s a negative voltage.

If you follow the preamp DC heater circuit, it starts at around -50VDC on V102 pin 5. The preamp heaters are wired in series so each tube’s heater will drop the voltage by about 12VDC. At V102 the voltage should be about 38V on pin 4 (and pin 5 on the next, V2), after that 26V, and so on, down to zero at the end, which in this case should be pin 5 on V1, which is connected to ground.

Thanks assuming that your model is wired as it shows in the schematic. What’s odd is that it shows the sequence as 5-4, 5-4, 5-4 and then on the last one, V1, it’s 4-5. I don’t know why they did it that way.

So your voltage readings are correct for the preamp heater circuit.

Where I see a problem is your voltages on pin 2 of V4,V5, V104, V105. Those should be about 6V AC for the heaters Did you change your DMM to AC volts? DC will show almost zero.

Second is the low voltage on pin 6 of V4. That should be about equal to the others, around -17 to -18 DC. That’s the bias voltage that should keep V4 from redplating.

It’s often a matter of diagnosis by exclusion. Rule out each variable one at a time until you’re left with the problem area.

 

[therling]

Oh, and about hearing faintly on the dead channel when the volume is all the way up is likely some voltage bleeding through, also referred to as crosstalk, from the one channel to another. Both sides share the same power supplies so there’s bound to be a little leakage somewhere.

 

[trainbuftony]

I just pulled a schematic from our very own AK data base.

What a jem that is!

The schematic confirmed my suspicion that after the phono stage the signal goes into the selector switch.
Proceed with great care, if something does move don't force it.
You can unbolt the face on this from the main chassis and tilt it to gain access to the back for troubleshooting.
I honestly don't see any way around doing so.
Be extremely careful and meticulous, and clean clean clean.
You've got to clean the contacts, and trace the signal through the whole thing and I don't envy you at all.
I'll forever Remember what the old motorcycle machinist said years ago:
"Nobody ever lost a race because their ______ was too clean"
(If you going in anyway, service everything and leave it spotlessly clean. You wont regret it.)