6922 tubes in an Avalon Vt-737sp

[studiotech]

I've got an Avalon VT-737sp channel strip from our studio and it's got no output. Main fuse is fine and power supply outputs all of it's voltages 190, 6.3, 12, +-34. They make it down to the main PCB from the power supply unit. I get the +-34 voltage where it should be, and see it further knocked down to +-15 at the opamps. ALL LEDs work and all relays are functioning as features are selected.

However, as I plug the tubes into their sockets, one by one, the heater voltage of 6.3 drops each time, so by the time all tubes are seated I'm only reading 2.9V. Even in a darkened room, by the time all 4 tubes are seated, I can't even see any glow inside the tubes.

Additionally, based on the spec sheet I've seen for 6922 tubes, I feel like we're not getting voltages in the correct pins. I'm seeing 190V on pins 1, 4 and 9 on each tube socket. I only get the heater voltage on pin 5. Does this sound correct? I don't see any faulty connections or signs of burnt resistors feeding the tubes. No signs of any overheating on the tube socket PCB.

Avalon will not give out schematics, and there's a lot going on inside this unit, but it seems as if all the Opamp based sections of the circuits are operating correctly. Any hints about the 6922 tubes and what I SHOULD be seeing at the tube sockets?

Thanks so much!

Greg

 

[ruffrecords]

6922 heater pins are 4 and 5. Pin 9 is a screen between the two triodes that is usually connected to analogue 0V.

When you say you only get heater volts on pin 5, what is that relative to?

Cheers

IAn

 

[gyraf]

faulty rectifier diode in heater supply is probably the most probable cause of weak heater voltages?

What were the tube type originally used in there?

/Jakob E.

 

[pucho812]

I am quoting myself first and then adding to it.

For starters the valves/tubes used in the units are 6922. I believe the euro version is an E88CC.

From there the PSU outputs 5 X different voltages B+, heaters, 12V for relays, +/-34VDC for the class A circuits. There is further onboard regulation on the main board to give around a +/-18VDC for the IC's that do the meter drivers and DC offset. Further more, phantom power voltage is taken from the B+ feed.
It's pretty neat how that is done.

Anyway, 2 ways to check the PSU without a load and with a load. Because of the design, we can measure the PSU unloaded and see the expected voltages when powered on but not connected.
the color code for the 8 pin molex is as follows
green and blue wire is ground
Red = +34VDC(class A circuits V+)
Black = -34VDC(Class A circuits v-)
Orange = +12VDC(Relays, leds, vu buls)
Yellow = +6.X, usually around 6.2 - 6.3(heaters)
White = B+(usually around 180VDC)
As always measure in reference to ground.
Once connected back into the main board we can measure at the following points when turned on and see the following
Solder point T1 on the main board. This is where the B+ line runs into the tube card.
solder point T2 on the main board. This is where the heaters(DC) run into the tube card.
On card 5600-7373 measure the top of R38. It's the +34VDC for the class A circuits.
On card 5600-7373 measure the bottom of R43 as that is the -34VDC for the class A circuits.

There are acceptable variances on the class A stuff usually with in a volt or two. the B+ has an acceptable swing of around 5V. so while it is expected to be around 185V, anything with in 5V is considered acceptable, so like 180VDC is is often found.
From there the 12V for the relays can be measured at the lamps as it also feeds the lamps and the leds. If they are working then your 12V is o.k.

At no point should you have 12VDC at the tube heaters. The tube heaters have their own supply in the PSU. the 12VDC you are seeing would be for the relays and leds.

So lets tests first. You can test the PSU without a load and using VDC probe the molex and confirm voltages. YOu can then reconnect the molex and measure at the following.
Measuring DC voltage in reference to ground, have the black lead of the DMM on the ground and probe T1 solder point on the main circuit board. This will be the B+/high voltage. it should be around 185 but I have seen 190 before. Then using the same setup probe solder point T2. This is the heaters. in a working unit I expect to see 6.3/6.2 volts with all the tubes installed.
On card 5600-7373 measure the top of R38. It's the +34VDC for the class A circuits.
On card 5600-7373 measure the bottom of R43 as that is the -34VDC for the class A circuits.
Clearly you have the +/-15VDC for the servos and meter driver.
If the leds are on you have your 12VDC too which also drives the relays.

If your heaters are dropping voltage on the 737 you most likely have a faulty capacitor and or rectifier in the PSU for the heater supply.
The easiest thing to do is replace the rectifier and cap.
For the heaters you have a kbu6d. The easiest way to remove it, is clip the part out using diagonal cutters to remove each pin and then suck out the solder to have fresh through hole pads to solder to. Any attempt at trying to desolder and remove the rectifier intact requires way more heat. In turn, you can easily damage the PCB and pull a solder pad and trace.
then you will want to replace C23(I think it is c23) on the circuit board. IIRC it's a 16V cap. I forget the value of the cap but 2200uF or something like that comes to mind. it's common for it to short out over time. This cap and rectifier combo is a common fault in older units but one of very few faults that happen in those things.

 

[studiotech]

Thanks for the hints so far everyone.

All my measurements are referenced to chassis ground. Tubes are the factory originals. Electroharmonix 6922.

As mentioned above, power supply voltages test as expected from the molex connector when NOT connected to the main PCB. I get 190, +-34, 6.3 and 12VCD. relays are firing, so I know 12V is getting distributed properly and all Opamps have +-15VDC. Board with Class A devices is getting +-34.

As far as PCB test points, I do get 190 at T1 and 6.3 at T2 without tubes in their sockets. As each tube is inserted, the 6.3 drops some, until all 4 tubes are inserted and the 6.3 drops to 2.9VDC.

Looks like I'll go for the regulator and that cap, but I find it odd to measure the 190 on pin 4 and pin 9 of all sockets.

Thanks again.

Greg

 

[Brian Roth]

190V on the heater pins...perhaps they used an elevation voltage to prevent heater to cathode breakdown. However, 190 seems waaay high.

Bri

 

[pucho812]

190V on the heater pins...perhaps they used an elevation voltage to prevent heater to cathode breakdown. However, 190 seems waaay high.

Bri

They don't do that here.

Thanks for the hints so far everyone.

All my measurements are referenced to chassis ground. Tubes are the factory originals. Electroharmonix 6922.

As mentioned above, power supply voltages test as expected from the molex connector when NOT connected to the main PCB. I get 190, +-34, 6.3 and 12VCD. relays are firing, so I know 12V is getting distributed properly and all Opamps have +-15VDC. Board with Class A devices is getting +-34.

As far as PCB test points, I do get 190 at T1 and 6.3 at T2 without tubes in their sockets. As each tube is inserted, the 6.3 drops some, until all 4 tubes are inserted and the 6.3 drops to 2.9VDC.

Looks like I'll go for the regulator and that cap, but I find it odd to measure the 190 on pin 4 and pin 9 of all sockets.

Thanks again.

Greg

you're measuring 190 on pin 4 and pin 9? Are you certain you are measuring the right pins. in a 6922 pin 9 is a ground/internal shield.
It has me thinking you are measuring pin 1 and pin 6 which would have the B+

 

[mjrippe]

you're measuring 190 on pin 4 and pin 9? Are you certain you are measuring the right pins. in a 6922 pin 9 is a ground/internal shield.
It has me thinking you are measuring pin 1 and pin 6 which would have the B+

Viewing the tube from the bottom, pins are numbered counterclockwise from the gap. Indeed going the wrong way would result in 4&9 being confused with 6&1.

Edit: Obviously I AM confused.

 

[Brian Roth]

????? Counterclockwise as seen from the bottom?

https://frank.pocnet.net/sheets/049/6/6922.pdf
Bri

 

[mjrippe]

????? Counterclockwise as seen from the bottom?

https://frank.pocnet.net/sheets/049/6/6922.pdf
Bri

Thank you Brian! I also turn screws the wrong way the first time if I can't see them