Avalon VT 737 sp

[JD26]

I have a dead Avalon vet 737so, I understand it’s difficult to get schematics for this unit. Anyone have any insights where to begin on these fairly common devices? It powers up, no output. Was working fine, then one day nada.
nice environment, nothing wacky. Just kaput.

 

[a soBer Newt]

I work a one of the contract houses that built 737s; When you first turn it on is it in bypass? Then after a delay it should come out and then wont pass audio? There is a bipolar 40V rail 12V for relays 190V for plates, and 6.3V have you checked those?

 

[JD26]

I work a one of the contract houses that built 737s; When you first turn it on is it in bypass? Then after a delay it should come out and then wont pass audio? There is a bipolar 40V rail 12V for relays 190V for plates, and 6.3V have you checked those?

Thanks Will do…

 

[pucho812]

Someone call the Avalon whisperer?

I’ll send you some notes.

 

[pucho812]

To say it’s not working is vague. Not working maybe be bad tubes, bad class A circuit or anything in between.

I always start by measuring the psu.
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 R34. 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. I have seen as low as 175 and been fine.
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. Unless a lamp is burned out, your 12vdc could be fine but the lamp is out. Replace lamp.

If the voltages are acceptable, we can look for visible damage and check the tubes,

Most common reason for lack of audio is a failed 6.3vdc circuit in the psu or bad tubes.
For the 6.3 circuit you probably have a failed rectifier and shorted cap C23. I forget it’s value off the top of my head but it is a lytic and easy to get. Go for a 105 temp version.
Bad tubes, can test and replace or swap all four at once like most blokes.

 

[JD26]

Thanks!! I really appreciate the advice…

 

[pucho812]

Thanks!! I really appreciate the advice…

For my next trick we can discuss the psu chassis and how to access the psu properly.
1. Four screws on the right side of the unit if you are looking at it from the front
2. Heatsink bolts, there are 3.
3. Internal lid on the psu
4. Slide the psu chassis forward. This gives you access to get the pcb out to do any necessary soldering work.
But before we go that far let’s confirm your voltage measurements.

 

[JD26]

My friend is the smart tech doing the work, I’ve been forwarding your advice to him. I am a mere producer/idiot.
hopefully he’ll get to it soon…he’s also fixing my tube tech CL1B…thanks again, I forwarded your last post as well…I wish I was smart enough to fix this stuff myself…

 

[gyraf]

..your tech should probably join here..?

(and thanks, Pucho - this is good stuff)

/Jakob E.

 

[pucho812]

But wait there’s more.
I forgot to mention the rectifier for the 6.3v circuit is the big honking one, a KBU6D(iirc). It’s the big one in the column of rectifiers, the one at the bottom of the column. Often times you will see it with brown to black marks due to thermal issues. If you have to replace it, chop it off and then desolder each individual rectifier pin. As a whole unit, it will absorb all the heat preventing proper solder flow. Once each pin is removed can clean the solder pads to install a new one.

 

[djowel]

Great info Pucho! I have a 737SP on the workbench for a client. He told me he turned the unit on and smoke came from the inside. Turned it of immediately. I found the large bridge rectifier (6.3v) burned and cracked, so last posts info will help alot!

Can you maybe shed some light why it burned? Maybe underlaying defect, faulty tube? I can't see any visible damage further in circuit on psu.

 

[pucho812]

Great info Pucho! I have a 737SP on the workbench for a client. He told me he turned the unit on and smoke came from the inside. Turned it of immediately. I found the large bridge rectifier (6.3v) burned and cracked, so last posts info will help alot!

Can you maybe shed some light why it burned? Maybe underlaying defect, faulty tube? I can't see any visible damage further in circuit on psu.

Well when it comes to the 6.3vdc there are two main parts that fail, the rectifier and C23 on the psu board. I forget the value off the top of my head. Normally when c23 shorts or the rectifier fails you just lose the 6volts for the tubes.
Never seen the rectifier crack and smoke. thought that the fuse would have poped by then. Seen that enough times. As always bring up on a variac and monitor current draw. A working unit will do big in rush current then idle low. But if it keeps going up and staying up, then that’s an issue.
When you replace the rectifier and the c23 cap. Run the unit without tubes and take measurements as I have listed before. Then can either shotgun the tubes replacing all 4 or try them one at a time until you get the bad one.

Also do a visual inspection on the tube board sub-assembly. I had one causing issues once and the main culprit was a failed 10uf film capacitor.

 

[NoisyIndividual]

"Never seen the rectifier crack and smoke. thought that the fuse would have poped by then."


Is that when the fuse dies through extreme old age?

And then there is a puff of white smoke before the new one is fitted?

 

[djowel]

Again great and useful info. Thank you. Will do on the variac and no tubes after replacing rectifier. C23 measured ok in circuit with Peak ESR meter. Fuse is still intact. So, will check current for suspicious idle behaviour.

 

[oldskool1]

I have one of these in my studio, and it stopped working one day about 15 years ago. The time delay relay wouldn't switch on like it should. I traced it to one of the Molex connectors. I think it was a 6 pin, or 8 pin connector. The wire wasn't making good contact with the crimped connector pin, I would press down on the wire and the unit would then turn on. It took a while to find, and I only found it by tracing the voltages through the unit. Hope this helps!