detrola: 190 Series; T3 (T 3)
detrola: 190 Series; T3 (T 3)
Electrically, it is identical to a Detrola 190.
6A7, 6D6, 75, 76, 42, 42, 6G5 and 80 rectifier.
http://www.nostalgiaair.org/PagesByModel/919/M0003919.pdf
The problem:
Lots of inter station white noise. Like what you'd expect from
a "hot" receiver, except... it's deaf as a post only able to
hear the local flamethrower stations.
I've checked the B+, and the AVC voltages. They are as one would
expect. But occasionally, the radio will go up and back down in
volume. Very annoying.
Some of the test equipment I'm using on this train wreck.
HP 3325A synthesized signal generator. (Makes a very nice sweeper.)
Tektronix 422 dual channel scope. (Run of the mill, but nice.)
Motorola R-2018 Service Monitor. (Using it as a spectrum analyzer.)
Using the sweeper at 436 KHz to 476 KHz I get about a 15 KHz wide
peak at 456KHz through the IF transformers. I have to scope connected
to the top of the volume control so I can set the sweep rate at
0.05 seconds per sweep.
Using the sweep generator set at both 1500 KHz and 600 KHz, all of
the trimmer capacitors (and main tuning caps) appear to be working
properly as far at physically and electrically.
But... The radio is deaf as a post. I can only hear a couple of the
local flame thrower stations and only just barely above the noise.
Using the spectrum analyzer with a short wire pickup probe inside
either of the IF transformer cans, I'm seeing a pass band of noise
that is 30 dB above the noise floor. This is apparently enough to
fully close the eye tube with the detected noise and subsequent
AVC voltage.
Now, I've pulled the grid cap of of the 6D6 IF amplifier and the
wide band high level noise is still there in the 1st IF can.
Grounding the 6A7 mixer input grid cap with a .01 uF capacitor
eliminates the noise. (along with everything else)
This radio is the one with the motorized tuning. The push buttons
go to settable commutator stops to position the main tuning cap.
They do not go to additional antenna and local oscillator coils.
As yet another attempt in solving this problem, I took a 7 pin tube
base and an octal socket and made a "6A8 for 6A7" tube adaptor. It
actually works, shifted the receive frequency down a little bit, but
is still deaf as a post.
The basic B+ as measured off of the rectifier (and supplying the
plates of various tubes is about 220 VDC.
At this stage, I'm about ready to gut the mixer and IF section and
install a 6BE6 and 6AU6 with a couple of K-tran IF transformers and
call it day.
(Yes, I've been fighting this one for 3 weeks now.)
Jeff
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Dear Jeff,
may be a stupid suggestion, but did you check the screen grid bias of the 6D6 IF amplifier (pin 3)?
Regards, Emilio
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I have 217 VDC on the plate(s) (both the 6A7 and the 6D6)
I have 93 VDC on the screen grid(s)
And I have about 0.9 VDC bias on the cathode(s)
AVC voltage measures, with the overwhelming amount of noise, roughly -16 VDC.
Jeff
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Dear Jeff,
it looks like the AVC detector is actually rectifying the RF of the local oscillator, in the 6A7. A path can be the cathode connection between the 6A7 and the 6D6 IF amplifier, when the decoupling capacitor is open. Try to put a new capacitor in parallel to the existing one, from the cathodes to chassis ground. Unfortunately the diagram is unreadable, but any value from 0.01 to 0.1uF should work fine.
Regards, Emilio
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Capacitor
Dear Emilio and Jeff,
yes the schematic is hard to read. In Rider's Vol. 9 the respective capacitor is labelled either C16 or C19. The component listing gives C16 = 0.2 µF/200V and C19 = 0.05 µF/200V. The combined screen bypass capacitor (most likely) C29 has 0.2 µF/400V.
Regards Götz Linß (Goetz Linss)
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Is it possible to inject an AM frequency , say around 1000 and attempt
to follow the signal through the AM broadcast coils, through the
bandswitch contacts , etc. with your scope?
If I overload the front end so I have a usable signal, I can
stuff an AM Broadcast Band signal through the antenna coils
and to the grid of the 6A7 with very little loss "on frequency".
This is as it should be.
If you switch to a different band, can you inject a signal through the
antenna and hear it ?
Yes, and again, the results are the same, Signal in a is little
more than the signal at the 6A7 input grid.
you were able to peak the IF transformers right?
Well, by right, I have a distinct peak on all four trimmers at
about mid range. So yes, I'd say the IF is aligned correctly.
I have used the synthesized sweep generator to verify bandwidth
and center frequency of the IF.
Maybe we could compare the voltage we get at the cathode or grid of
the 6A7 to my set?
The cathode of the 6A7 (and common with the 6D6 cathode)is
approximately 1 VDC.
maybe try to use a volmeter to try each connection to the bandswitch
to see if you could have a bad contact , or something getting grounded
that shouldn't be?
I have gone trough the bandswitch at least a dozen times. I did
create a drawing in AutoCAD to help with identifying which contact
goes where and to what it connects to as it rotates. It has always
been "correct" and made contact.
I think what we are lacking is the next sequential step in logical
troubleshooting.. obvoiusly there is some problem we just haven't
found yet. I am thinking it might be an issue with the wires to the
bandswitch because of the intermittent you mention.
Actually, that was the volume control a 2 Meg audio taper with a
switch section. I've sent email to Mark Oppat for a replacement.
In the mean time, I've installed a 2 Meg pot with a solid carbon
trace properly lubed and installed. The intermittent volume issue
has been resolved, and I was hoping that MAYBE it was somehow
related to the other problem. It isn't.
does the motor run ok?
The autotune system works perfectly. As long as I pick one of
three stations I can actually hear. It will swing the dial
around and stop where it's been told to (no automatic frequency
control, purely mechanical.)
one thing I never understood well was how the circut of the radio
plays upon the function of the motor. there is a cap c19
( and there is a note) perhaps this is just to silence the radio when
it is switching stations? if so , could yours be getting silenced all
the time?
Yes, that is an "interstation" noise muting system. It lets the
bias on the audio swing up and silence the radio while the motor
is turning.
Again, the primary issue is that there is enough amplified noise
in the IF passband (30 dB higher than the noise floor) that, for
the most part, completely overwhelming the detector and creates
about -16 VDC of AVC voltage.
One of the things mentioned to me over on the radiomuseum.org
forum was a possible coupling of the local oscillator into the
2nd IF via the common cathodes and screen grids. If I remove
the 6D6 IF ampliflier, the additional 30 dB rise in the noise
floor is still there. If I ground the 6A7 grid cap with a 0.01
uF capacitor, it goes away. Both would seem to indicate to me
that the excess noise is being caused by some problem with the
6A7 and not the 6D6 tube.
But to satisfy my curiosity, I removed the two common jumpers
between the screen grids and the cathodes, reconnecthem with
a 2.5 mH choke and additional 0.1 uF bypass at the other sides.
As to be expected, it made NO difference at all.
Jeff
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Dear Jeff,
the AVC voltage you find, -16V with no signal, is abnormal. With such a bias, it is normal to have a very poor gain of the RF and IF stages. This voltage must be generated somewhere. To locate the source, you can remove the 6D6 IF amplifier from its socket and measure again the AVC voltage; normal value should be around minus 1 / 1,5 volts. I am not able to read the attached schematic diagram, and I am not sure where are joints or crossings in the wiring, but I see a fixed bias coming from a resistor (value?) in the power supply section, between the filter choke and chassis ground.
If you still measure -16V with the IF tube removed, the problem could be an excessive current drain from the power supply, which gives the voltage drop on the said resistor. You can also remove the 75 to be sure that the grid bias voltage you find just comes from the power supply section. Causes should be searched elsewhere, may be a leaky filter capacitor, may be one or both output tubes saturated because of positive grid bias, due to leaky coupling capacitor(s). I am sure that you will solve the mistery.
Best regards.
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If I am reading correctly, all questionable parts were replaced, including vacuum tubes.
Being that everything is new in the radio, is there a suspect transformer, that could cause all of the above mentioned in this thread, even though everything is measured to spec?
What else could be the cause of such behavior?
.
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Phil has a Detrola model 175 on his website with a bunch of interesting pictures and comments about it.
http://members.shaw.ca/philsvintageradios/html/detrola.htm and specifically this link for a corrected schematic that you can read the part designations on:
http://members.shaw.ca/philsvintageradios/images/ed_sch.gif
For the record, I have replaced ALL of the original capacitors and a majority of the original dogbone resistors in this radio.
The -16 VDC AVC is generated by a high level of noise in the IF amplifier. However, it (a) goes away if I ground the grid cap of the 6A7 with a .01 uF cap, or if I ground pins 3 (G1) or 4 (G2) of the 6A7. It is still present (monitoring with a spectrum analyzer) even if I remove the 6D6 IF amplifier tube.
Jeff
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I think I've found the problem.And I've put out a contract on Manny Moe and Jack. The 75 detector tube apparently has a problem with one of the diodes. Replacing it with the only other 75 in the shop seems to have cleared up a substantial bit of the problems.
Re: Manny Moe and Jack, the tube in question has a paper label on it stating "This tube has been tested by Pep Boys" with a date of around 1938.
Jeff
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Translation please?!
For those that don't know about Pep Boys, here is the link, http://en.wikipedia.org/wiki/Pep_Boys
This place happens to be an auto parts supply store in the US, with more than 80 years of history, and at one time sold vacuum tubes.
I hope that Jeff is able to put a close to this thread , with a solution!
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Apparently the volume going up and down was also attributable to the 75 triode section and a flaky grid cap. With a different 75 in the radio and a 1N270 diode in place of the pin-4 diode inside the tube, the radio is performing as it should.
I realize that to some replacing all the components, much less adding a germanium diode made border on blasphemy. But the bottom line on this radio is that the customers wants to listen to it. With it deaf as a post or with varioius 70 year old components failing (leaky capacitors, dried out electrolytics etc.) leaving it "stock" was not an option.
Thank you everyone for your suggestions on resolving the problem with this radio.
Jeff
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Where are the pictures on the model please?
Dear Jeffrey
Thank you for always responding. Nice that you have now the solution.
But the model is still left without any pictures. RMorg wants to help all collectors by showing a most complete radio catalogue inclucing pictures and schematics. Pictures are for instant very important for finding a model without model name. Pcturesof the original wiring and other details including name plate are also important.
Can you please prepare and load up pictures for this radio? The model shows none at all ...
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