Gyraf Pultec - Can't pass audio, why?

[thinktank]

I just got done building one of the gyraf pultecs. The only
difference is that I'm using the oep a262a2e instead of the
lundahls. I can't seem to pass audio through the unit.
Any idea where to check first. I've went throught it and
it all looks ok. The tube lights up. Not that that really means
a whole lot. I'll take some photos later and post them.

Thanks for any help on troubleshooting this unit.

Nate

 

[chrissugar]

First check only the tube amplifier circuit without the passive filter.
If it pass audio then you should troubleshoot the filter.

chrissugar

 

[Svart]

as with any device, start with the simple stuff and work your way to the complex stuff. It's usually something like a wrong value somewhere that can screw you up totally. make sure you are getting proper voltages to all parts, then check from the input onward. you're bound to run into the cause of the problem if you just work systematically.

good luck! :thumb:

 

[thinktank]

I just got the time to look at the g-pultec. I disconnected
the filter section and jumped it together but still don't have
any audio passing. I'll test the board to see what kind of
voltages I'm getting.

 

[thinktank]

The wiring looks correct. I took readings at the
power transformers and at the 220 side, I
have 120 or so at each leg. At the remaining
4 posts that need the 12v and 15v, I have
7.4 per leg. Is that right?

Another question, should there be continuity
at between certain points of the input/output
transformers. Meaning from pin 1 to pin 6?

Thanks.

 

[PRR]

> I'm using the oep a262a2e instead of the lundahls.

Be sure the transformers are connected properly. The OEP pinouts are surely different from the Lundahls. For a bare-bones test: wire XLRs to (what you think are) the transformer input and output: it must pass signal.

Also the amplifier without transformers should pass signal. I don't have the schematic handy, but if you can connect the output transformerless (you may need a blocking-cap?) and put your finger on the input grid, it should buzz and the tone of the buzz should be affected by the EQ controls.

 

[thinktank]

I went to the oep site and didn't find the winding schematic.
Any idea where I'd look for this? If they have different
pinouts, that's surely the problem as I'm using the same
lundahl configuration on the circuit board..

 

[thinktank]

So here's what I did. I changed the pin connections
according to the oep winding schematic at the canford
website and still no clear audio, just a loud humming.
Better than nothing I guess. I disconnected the filter
portion and am just applying audio to the input. The
hum will actually start when the power is turned on and
the output cable is connected. The input cable has no
bearing on the hum starting wether it's plugged in or not.
All of the xlr's are grounded together at the mains.

From the 5402 layout on the gyraf board, I put the oep
transformer in. On the input trannie, I took the secondary
and wired s2 and f1 together, s1 to ground, and f2 to
the pad where the lundahl pin 10 would be. On the primary
side, I wired s1 and s2 together and f1 and f2 together.
The screen went to ground. At this trannie, at the primary,
I have continuity between all 4 pins. At the secondary,
I only have it at the s2 and f1 pins rightfully. Is the primary
suppose to be this way?

At the output trannei, I took the secondary and wired s2 and
f1 together, s1 to the pad where the lundahl pin 7 would be
and f2 to where the lundahl pin 10 would be. On the primary,
I wired s2 and f1 together, f2 to ground and s1 to where the
lundahl pin 2 would be. At this trannie, at the primary, I have
the same continuity issue between all 4 pins and the primary
only has it at s2 and f1. Is this ok?

Any idea if this is normal or am I missing something?
Thanks again,

Nate

 

[thinktank]

Are these transformer connections for the OEP right
as a substitute for the lundahl?

 

[chrissugar]

Hi thinktank

When you disconnect the filter section do you make the link between the in and out pins from the filter connector. If not you will have no audio, probably only humm.
Look at the main PCB, locate the 3 pin connector called "TO FILTER PCB" and make a link from the pin "IN" to the pin "OUT". If everything is ok it should pass audio.

chrissugar

 

[thinktank]

I jumped those together, but I still have hum.
I think that the oep's are giving me the problem.
It wasn't till I tried to wire them according to the
schematic from the canford website that I got
any type of noise. Before, they were wired
the same way as the lundahl's. Now, I think
I wired them right, but I'm still getting a hum,
even with the filter section disconnected.

Not sure what's going on here. It must be my
wiring of the oep's. Still not sure about the
continuity issue with the primaries of both
the input and output. If that's right, then there
is something else happening. Maybe a grounding
issue. Still, I can't pass audio.

Hmmm...

 

[Hulk]

hhmm

This is very interesting because I'm just about to begin work on the Gyraf Pultec also with OEP trafo's :?

Has anyone here succesfully made a Gyraf Pultec with OEP trafo's??

 

[alk509]

OK, just to make sure, here's how you should be connecting the input transformer:

PRIMARIES: F1 connects to S2. Signal gets injected between pins S1 (+) and F2 (-).
SECONDARIES: S1 connects to S2, F1 connects to F2. Signal is taken between S1/S2 junction and F1/F2 junction.

And here's how the outpt transformer should go:

PRIMARIES: F1 connected to S2. Signal is taken between S1 (+) and F2 (-). Yes, you read that right... this transformer goesin backwards.
SECONDARIES: F1 connected to S2. Signal from output of SRPP amp goes to S1, F2 goes to ground.

If you have everything hooked up like that, and it's still not passing signal you either A) have a wiring mistake you're not seeing, or B) have dead transformers (unlikely).

Hook them up like I describe above, put some signal at the input and scope the secondaries of the input transformer, then the output of the filter board, then the output of the amp board and finally the output of the transformer. Wherever you stop getting signal, there's your problem.

Peace,
Al.

 

[Hulk]

I'm also working on the Gyraf Pultec..

Can I use this cap for the 2.2uF cap on the front PCB or should it be a polyester/polypropylene caps?? The reason I ask is that I'm having a difficult time finding a poly cap that fits the PCB


P.S. Have you gotten your OEP's to work thinktank??