Avalon 737 Repair - 6922 Plate voltage...

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stitch-o

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Hey Forum!
I have a 737 that came around with some issues:
Thin distorted input with very very low output signal.
Joe at Avalon has been as helpful as he can be, though (Im sure peeps here know)
I can not get the schematic or any part thereof whatsover...bit of a PITA for servicing.

The Power Supply had given up several transistors and a resistor.
Eventually, I had sent a new one from Joe at Avalon.

With the new PS in, I'm looking at a similar issue:
low output (never above -60dB in PT with all gain up fully), distorted if fully pushed.
Though the low signal is no longer thin sounding..
Issues between mic, line and inst inputs:
with mic disconnected, the noise level is unnoticeable, but when mic connects,
there is a loud hum/buzz. I even had some crosstalk by playing with the preamp gain pot
while line input a 1k tone but selected on mic input.
Instrument input is the cleanest, with very little hum/buzz, but
still very very low output.

At first I thought it was V4 output tube issue,
but now I suspect a power rail issue across the tube daughterboard (3200-7375)

I removed the EQ and comp cards, double checked all tubes with a spare set...still have issue.

I was looking at the 6922 data sheet and noticed that Plate as spec'd is 90-100V (max 220V)
When I tested pin 1 & 6 on each of the 4 tubes, they are at 180V.
Would anyone here know what  the plate voltages for tubes in the 737
should be?

Cheers and thanks!

-s

EDIT 1: Hope you all dont mind I use this thread as a talking it out tool...

Found a failed pot: the Preamp Gain dual 25kΩ.
One side seems to have failed and there is a weird pull on the pot shaft when it goes to the
end of the throw. I was chatting with my friend who just repaired an Avalon compressor
and confirmed the pots are not great quality and have failed on his Avalon units.
I'll replace and then diagnose next issue...
 

stitch-o

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Bingo!

On V3 (I think) cathode (or grid leak?) resistor had failed.
10k 3W replaced. Output volume is now full.

Last and seemingly final issue: still have bad
buzz/hum.
Something is still leaking into the tubes I think...

EDIT: or maybe not...
Further testing reveals (using SM57)
-When mic is plugged in and input selected to mic, full input signal.. buzzy signal
When mic plugged in and selected to either line or inst - buzz is gone
When selected input for mic input and mic cable is unplugged, buzz goes away.

-Preamp gain knob (and high gain switch) raises the buzzing noise at corresponding level

-Interesting part of issue: with nothing plugged into XLR/.25" and gain cranked,
very quiet. When I touch the preamp gain pot - Noise appears (not loud, but its there and same buzz with input signal)!
If I touch chassis screw while touching preamp pot...noise disappears..
Something with ground near input still is wrong it seems

I thought it was possibly rooted in the IO buffer card (5600-7372), but now I dunno..

EDIT 2: Actually...SO I recapped all the 10 and 100uf on the IO buffer board (7372)
With nothing plugged into XLR/.25" Inputs, line input is quiet regardless.
Both inst and mic input still show same issues though now I can distinctly
hear the two different noises: theres a hum (now only on inst and mic input) but also at times
there is a buzz (when touching preamp pot)  and of course when a mic is plugged in.

Ill replace all the BD139 transistors and the smaller C546B and C556B transistors.
At least I can eliminate the buffer card which Ive had a feeling about all along...

Cheers for letting me talk out my repair!
 

stitch-o

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Final Edit:
Replaced all BD139 and 140s on the IO buffer card, replaced a relay that was suspect on the tube socket daughterboard...
Laughed as I noticed I had ground and pin 2 swapped at the PCB input (FWIW ground are the square pads)
fixed and fired up - Works great!!

I dont think Ill ever accept an Avalon to fix again though.
lol
 

stitch-o

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pucho812 said:
they are not so bad, just have to go slow... there is a lot of kit in there.

Thanks Puch!
I actually fixed that bugger. It was some resistors sitting under the tube socket board (as well as some transistors in the IO buffer PCB)...took two gos through to find them.
I did a lil dance when that sucker fired up right!

Def a confidence builder of a repair!

Cheers!!
 

pucho812

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stitch-o said:
Thanks Puch!
I actually fixed that bugger. It was some resistors sitting under the tube socket board (as well as some transistors in the IO buffer PCB)...took two gos through to find them.
I did a lil dance when that sucker fired up right!

Def a confidence builder of a repair!

Cheers!!

yes they are. but in the end you got it. which is the best part.  ;)
 

reza

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Thanks Puch!
I actually fixed that bugger. It was some resistors sitting under the tube socket board (as well as some transistors in the IO buffer PCB)...took two gos through to find them.
I did a lil dance when that sucker fired up right!

Def a confidence builder of a repair!

Cheers!!
Did you replace the gain pot too?
 

Ike Zimbel

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I haven't worked on one of these in years, but I recall that there was an issue with pots on the original ones. If the pots have white bodies, they are the ones with issues. The replacement pots were blue (but not Bourns). I'm not going to name either brand until I can confirm who made what.
 

Zenzen

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Hello , yes i know but without service manual impossible what IS the right tension value on pin 1nd 6 of each tube of this SP 737 Avalon ...I think it s 180 v but i m not Shure...
 

pucho812

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Hello , yes i know but without service manual impossible what IS the right tension value on pin 1nd 6 of each tube of this SP 737 Avalon ...I think it s 180 v but i m not Shure...
There is another thread where I spell out what should be happening. I am going to quote myself here.
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 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.
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.
So ultimately tension as you call it, should be about 180-185VDC. That is what is expected. off the B+ line the 737 derives it's phantom power. A neat Avalon design trick.
 

Zenzen

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Thanks a lot for the détail .
All voltage are ok . AMP op , relay etc...
The high tension IS a circuit soft power , i have to find why there is only 111volts and sometimes 0 🙄 ... Thanks a lot again , now i m sûre it s must be 180v cause it s a Avalon engineering choice 😁
 

Zenzen

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I think the problème is on the command few transistor of high votage .i ve already check the diode bridge , condensator ....
 

pucho812

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If you are not getting the correct voltage check the electrolytics capacitors in the PSU.
Check the PSU disconnected from the circuit to see if the voltage is correct.
Reconnect the PSU and test the voltage without tubes installed
then add a tube and see if it changes.
 

Zenzen

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Hello
Yes all this work have been already doing cause its my work to repair audio équipement .i wasn t Shure of the voltage needed on tube . An other technicien has work and no find the problèm... Cause it s a discret activ or passive composant somwhere wich IS dead or work a Litle like a Bad résister or Bad transistor junction . I have to find it without schématic 😊
Thanks again ans have a nice day
 
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