The new girl - a super funky Blonder Tongue 'Audio Baton' w/weird frequency dip

[Biasrocks]

Just picked up this beauty in mostly non-working condition.

Fixed the obvious things and got her working pretty well, definitely an interesting sounding piece.

I have one lingering problem with it. There's a big hole at around the 2k5 mark, which happens to correspond with one of the frequency bands offered. Tubes are good and the 2k5 control and the adjacent control boosts and cuts as it should. But even with both the 2K5 and 5K boosted to the max, this hole persists.

Here's a shot of the frequency response showing the hole.

As you can see with the other frequencies, there's slight holes around the other bands and it's anything but linear with all controls set to flat, which seems to be the part of the fun of this EQ.

But it appears the 2k5 hole is way too big.

I could shotgun every component, but I'm hoping to narrow down my troubleshooting to a specific area of the circuit.

Here's a snippet of the schematic showing the 2K5, 5K and shelving filter section.

Here's the full schematic

http://sharktankpro.com/GDIY/BT/Blonder-Tongue_Audio_Baton_B9B.pdf

Any help from more experienced eyes would be appreciated.

Mark

 

[PRR]

> I could shotgun every component

"Shotgun" the 2.5KHz channel. First step: swap that tube with another, see if the "weak band" moves with the tube.

 

[CJ]

prr used to carry a tube caddy

fixed peoples tv's while they watched in amazement,

jus sayin...

 

[mjrippe]

I know this is an old topic, but many moons ago I was looking to buy a second one of these on a newsgroup and received this email:

Subj:  Blonder Tongue Audio Baton
Date:  Wed, Feb 26, 1997 5:09 PM EDT
From:  bhtongue@***.superlink.net
X-From: bhtongue@***.superlink.net (bhtongue)
To: threemile@***.com

re your post "WTB Blonder Tongue Audio Baton":
I don't have one for sale, but since you have one I advise you to
replace the paper capacitors connected fron plate to cathode and from
cathode to ground in the seven 12AX7 half sections. They control the
center frequency of seven of the bands. The original production units
used, for the most part, metalized paper caps that drifted downward in
value a lot (after many decades of time, that is).
I know this since I am the inventor of the circuit and supervised the
design of the product
Ben H. Tongue

 

[MontanaWay]

I know this is an old topic, but many moons ago I was looking to buy a second one of these on a newsgroup and received this email:

Subj:  Blonder Tongue Audio Baton
Date:  Wed, Feb 26, 1997 5:09 PM EDT
From:  bhtongue@***.superlink.net
X-From: bhtongue@***.superlink.net (bhtongue)
To: threemile@***.com

re your post "WTB Blonder Tongue Audio Baton":
I don't have one for sale, but since you have one I advise you to
replace the paper capacitors connected fron plate to cathode and from
cathode to ground in the seven 12AX7 half sections. They control the
center frequency of seven of the bands. The original production units
used, for the most part, metalized paper caps that drifted downward in
value a lot (after many decades of time, that is).
I know this since I am the inventor of the circuit and supervised the
design of the product
Ben H. Tongue

Hi,

I am looking at perhaps building this equalizer and would appreciate some more information about this circuit, in particular your recommendation of a suitable transformer etc..
My plan is to build a stereo unit of this, your input would be very much appreciated, thank you.

 

[mjrippe]

The SAMS Photofact for the B-9 states that the power transformer has secondaries of 550vct @0.007A and 6.3v @2.3A.  If you can find 550vct @20ma and 6.3v @5A you should be good.