The AM Forum

THE AM BULLETIN BOARD => Technical Forum => Topic started by: WU2D on January 11, 2011, 07:00:03 AM

Title: TCS VFO Fixed
Post by: WU2D on January 11, 2011, 07:00:03 AM
I have a TCS pair that is a lot of fun to use. The transmitter developed an instability about a year ago where the frequency would shift around randomly by about +/- 500 Hz. This sounds funny on CW and can even be heard on AM as a crackling noise. First I gave it a good cleaning on all contacts and relays. Then put a meter on the screen and on the plate. They both showed normal. I pulled all of the tubes but the VFO next.

The problem was still there so I knew it was likely isolated in the oscillator section. The VFO is a modern electron coupled oscillator type with a 12A6 Beam Power tube in a Hartley circuit with the output always doubling, a very stable circuit. This is quite advanced from a technology standpoint, especially compared to an ARC-5 that gets stability out of a simpler direct coupled triode circuit from physical design alone.

Next I checked the capacitors for leakage current and resoldered everything I could get to around the tube socket. I sort of gave up on it at this point.

Well last night I dug it out again and popped it out of the case and mount and flipped it over. I wanted to fix it once and for all and was desperate enough to attempt to open the VFO compartment which was under a harness and a shaft etc...

This looked really bad, but those clever Collins engineers made it easy. After removing the front panel nuts the mic and ON/OFF Sw harness folded back and the shaft was cleverly secured by an oversized front knob assembly and it simply pulled out the front. After that it was only 7 screws to remove and I was looking into a VFO compartment with a BIG ceramic coil (the TCS VFO operates in the Broadcast Band and doubles), a variable bandset cap, a 20pF fixed temp comp cap, a 3W 1 Meg grid leak resistor and 50pF El-Menco Silver Mica capacitor.

It turned ot to be the 50 pF El-Menco grid leak cap. 15 minutes later - Happy TCS.

Mike WU2D

Title: Re: TCS VFO Fixed
Post by: AJ1G on January 11, 2011, 07:50:16 AM
My TCS MO was a bad drifter when I first got the sets from Steve WA1HUD waaaay back in the day- swapped in a new 12A6 and its been rock stable ever since.  Doesn't even chirp - what fun is there in that?

Now to figure out why the RX BFO craps out a few minutes after I turn the set on, after it dies, cycling the BFO switch sometimes will bring it back few a few seconds, but then it dies again....suspecting a leaky cap

Title: Re: TCS VFO Fixed
Post by: WU2D on January 12, 2011, 11:19:29 AM

The components used in the TCS are top grade heavy duty and the soldering and point to point wiring and harnessing is fantastic but every 75 years or so one part will fail! Ha. The caps in the receiver are more plentiful and the screen bypass caps are always possible issues in old receivers.

The BFO circuit in the TCS RX is a reflex type, that is, the BFO does double duty as an audio amplifier and a 455 kHz Meissner oscillator. This puts special stress on the quality of the old caps.  By the way, the 12SQ7 internal diodes are the detector and AGC. That is a lot of functionality from one 12SQ7!

If the BFO is not oscillating but you still have audio...

1. Change the tube with a known good one and listen for BFO.

2. Rock and spray Z204, the pitch control cap. The can may need to be removed at some later point because there is a possibility the fixed internal cap is leaky.

3. Check the plate voltage Pin 6 (V206) 12QS7. It should be well over 150 VDC. I have found that in the TCS and in the ARC-5 receivers, starvation of voltage is the main culprit on the BFO. This is due to feed resistors value going high, capacitors getting leaky and most of the time both at the same time. The leaking cap causes more current to flow heating the resistor, raising the resistor value over time by aging.

Pull the 12SQ7 and Measure Pin 6 again.

Measure R222 22K - if this goes high it can starve the oscillator.

If still low - Check C230, the 50 pF main bypass for the RF oscillator (remember my 50 pf went bad?). By the way this is right across the 22K plate resistor but is "invisible" to audio.

C230 0.01 uF the audio coupling cap could also be leaky.

Next check C229, 0.002 uF, the grid leak capacitor for the oscillator. If this is bad , the oscillator will not be biased correctly.

Next Check C238 (0.1 uF) Cathode bypass capacitor. This can effect gain and thus stop oscillation.

Check R226 the cathode resistor 2.2K.

That is about all it could be except for a bad coil.

Good luck,

Mike WU2D
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands