[gregb]

Its working perfectly Ron, it has filtered out both the highs and the lows, and maybe everything in between as well!

Gregb

[morzh]

Ron


BTW I've seen these dark brown dip caps drifting in value in some things I restored from 50s Ithink. Keep in mind.
Yours says 472 (4.7nF).

[Ron Ramirez]

Well,

I have a left channel again. Sort of. And I have managed to localize the trouble - and it had nothing to do with the low-high filter mod.

Using the signal tracer, I had volume in the left channel all the way to the input terminal of the volume control, but lost it at the left channel wiper.

After looking things over...and over again...something told me to look at the left channel 7199 tube, V20.

I discovered that the wire where R134, 820K, connects to pin 3 (pentode section screen grid) was broken! So the pentode section of V20 was getting no screen grid voltage. R134 is highlighted in the partial schematic below.

[Image: http://www.philcoradio.com/images/phorum...00_033.jpg]

I went ahead and re-replaced R134 as the alternative was to lengthen its lead, I had several on hand, and they were cheap, so I just replaced it.

Then I fired up the TA-600 again.

At first I only had hissing in the left channel - which was an improvement over before, when I had nothing.

Some snapping, crackling, and popping followed - at which point the left channel came back to life!

But it is coming and going. I then discovered by poking and tapping components around V20 that I could make the left channel come and go.

So some very close inspections of solder joints are in order, and I may have to replace a few more components.

At least it is easier to work around the 7199 than it was in the front section of the chassis.

I'm getting there...slowly but surely...

[Ron Ramirez]

At the workbench.

I think the main problem was the balance control.

Just now, I fired it up again and after a few coughs and sputters, the left channel came to life. Wiggling/tapping components around V20 had no efect.

Then I twisted the balance control.

The left channel went away, only to return a short time later.

A-HA! The balance control is made in such a fashion that the rear portion of the control (the left channel) has the terminals at the bottom of the chassis, so it cannot be reached with DeoxIT or any other spray chemical unless one (a) disconnects and removes the control in order to clean both sections; or (b) gets the rear cover just open enough to get some chemicals inside.

I had forgotten that I cleaned every control except that section of the balance control.

I went with option B. Carefully, I managed to get the rear cover open just enough to give that section a brief blast of DeoxIT, followed a short time later by FaderLube.

I tried it again - and the right channel came up, followed by the left channel a few seconds later. I'm hoping that is because I'm testing the unit with its original (and extremely weak) Fisher-branded 7189 tubes.

I twisted the balance control left and right, and there was no snap, crackle, or pop.

I can live with a few seconds' delay from right channel to both. Although, this unit wasn't doing that before my latest round of replacing coupling caps and making the low-high filter mods.

I think I will move on to the IBAM board now.

[Ron Ramirez]

IBAM Preparation

First, I put together a modified schematic of the output circuit showing the necessary changes for the IBAM mod:

[Image: http://www.philcoradio.com/images/phorum...bam_sm.jpg]

Click here for a larger version of this partial schematic.

And then I built the actual IBAM board, then called it a night last night.

Tonight after work, I started preparatory work on the chassis to get things ready for the IBAM circuit.

[Image: http://www.philcoradio.com/images/phorum...00_035.jpg]

Since I had to remove some resistors, I just went ahead and replaced all of the resistors in the area of the output tube sockets.

I will next install the new 10 ohm cathode resistors, then I will hook up the IBAM wires and we'll see what happens after that.

[Ron Ramirez]

Well...I have good news and bad news.

Good news: The IBAM board is hooked up, as are the 10 ohm resistors.

[Image: http://www.philcoradio.com/images/phorum...00_036.jpg]

Bad news: I found this warning in another AudioKarma thread, so after performing the test called for in that thread, I put the original weak Fisher 7189 tubes in place, and set the Variac to 110 volts.

My IBAM doesn't work.

The voltage across every 10 ohm resistor is fluctuating between 57 and 65 mV. And the trimpots have no effect. The sound is now distorted (probably, I suppose, because of the bias now being wacky?). D**n the luck.

I shall have to set this aside for awhile, and will return to it sometime next month. In the meantime, I would certainly appreciate any comments, suggestions, etc.

Edit: Forgot to mention...last night I cleaned the socket contacts of the V20 (7199) tube socket with DeoxIT and one of those tiny brushes meant to be used between one's teeth. The left channel seems to be OK now (no longer suddenly popping in after a few seconds' delay).

[Mondial]

Ron, it seems like you have no negative bias at all on the output tubes.

First, check the voltage on the cathodes of the output tubes. You should have +20 to +24 V, measured to chassis.

Then, check the voltage on the points marked A,B, C, D on your schematic. The voltage should vary from about +5 to +15 V to chassis, as the respective bias pot is turned from end to end.

If you don't have these voltages, check your wiring of the bias board and the connections to the output tubes and circuitry.

[TV MAN]

Sometimes weak output tubes can do goofy things, like the plates turn red. Did you try the new tubes before the mod? Also, remember, a tube (or set) can test good on a tester, but when the real current and voltages hit it, it cant keep up. I'm sure you know this but I thought I would mention it anyway. Food for thought...

[Ron Ramirez]

Tim - Thanks, but you missed the main point which is the fact that the trimpots are having no effect on bias. It's as if I didn't connect the IBAM board (which I did).

I want to try IBAM out using the original Fisher 7189s in case something goes wrong. If something goes wrong and I damage the original weak outputs, then there is no loss. Once I'm satisfied that everything is OK and that there is no danger of redplating/damaging the output tubes, then I will install the NOS-NIB 6П14П-EB tubes and readjust the IBAM trimpots to suit the new tubes.

Mondial - Thanks, will check...

[Ron Ramirez]

Okay, this is really getting weird...

I doublechecked my connections and all are correct. I replaced the 8.2K 1/4 watt resistor with an 8.2K 1/2 watt resistor just for fun.

I then hooked everything back up and, after letting the TA-600 run for a few minutes, I found that the millivolt readings across each 10 ohm cathode resistor are changing with rotation of each respective trimpot. However, I'm not getting readings anywhere near 34 mV. More in the range of 330-370 mV on V15-V16, and 190-230 mV on V21-V22.

These readings were made with the original, old, weak 7189s still installed. I don't want to put the new 6П14П-EB tubes in until I know this IBAM circuit is going to work correctly.

I'm beginning to suspect my meter is not giving accurate readings in the millivolt range.

I'm going to ask a friend to bring over a known good meter this weekend, and we will try it again.

Otherwise...I found that the distortion went away after it played for a few minutes, and the left channel difficulties have returned but now seem associated with either the Selector switch, the Mono-Stereo switch, or both. I'll clean them again.

I'm going to recheck the voltages Mondial asked for and report back later...

[Ron Ramirez]

I guess it's time for me to consider purchasing a Fluke multimeter. My digital multimeter is proving itself to be very unreliable, giving voltage readings that keep fluctuating around.

Well, it's an Extech (Radio Shack) unit that I've had for about eight or ten years, so I guess I managed to get my money's worth out of it and it's time to retire it.

So I pulled out my cheapie Harbor Freight analog multimeter.

Mondial, here's what I'm getting with it. Consider the cheapie meter used as you look over these results.

Cathodes - all around 14 VDC

A,B,C,D - all varying between 2.8 and 5 VDC

I've checked and doublechecked my wiring and it is correct.

???

[Mondial]

Hey Ron, if you are measuring 200 to 300 mV across the 10 ohm resistors, then you are ok. That's exactly what you should be seeing.

The cathode current of each output should be around 37 mA or .037A. .037A times 10 ohms gives .37 V or 370 MV. I think you are confusing mA and mV.

[Ron Ramirez]

Mondial

Thanks, that's just the info I needed.

I just went back and re-read the AK info and you are exactly correct.

So...why was I getting only 190-230 mV on two of the output tube cathode resistors, when the readings are all about the same when measured from cathode to ground? Or from each respective point (A, B, C, D) to ground?

[Mondial]

The reason you are only seeing 200 mV is because the tubes are very weak. They cannot deliver the full cathode current, regardless of the bias.

That's also why you are only seeing 14 V on the cathodes. That voltage is dependent on the total cathode current of all four output tubes. You should get 20 to 24 V there, but because the tubes are so weak they cannot supply the amount of current necessary to get the full voltage across the 12AX7 filaments which serve as the cathode resistor.

I think if you install the new tubes you will be fine. Preset all four bias pots to the setting which gives the lowest voltage from A,B,C and D to ground. Then power up slowly with your variac while monitoring the voltage across each 10 ohm cathode resistor.

[Ron Ramirez]

Bingo. The new 6П14П-EB tubes are in.

I set my Variac to 125 volts and was able to adjust three of the four to 340 mV. I can't get V16 below 352 or so mV. Close enough, I suppose. All four of the cathodes are now measuring 20.1 to 20.3 volts to ground.

I think that when I run this unit, I'm going to run it through a bucking transformer to cut the line voltage to around 110-112 volts. My line voltage here is usually 125 volts, with occasional 126 volt spikes.

For now, though, those new 6П14П-EB tubes do sound good, even on AM and through my tiny Minimus-7 speakers. I can't wait to get this thing hooked up to my Bozak E-300s.

But I still need to install an inrush current limiter and re-clean the Selector and Mono-Stereo switches before I can button it up and call it done.