WB4TUR's GE MASTR Pro: Page  and MPro Tips

GE MASTR Pro UHF Transmitter strip -

Curing a 'dead'/bad UHF strip due to: A) dirty screen-ring contacts and B) Mica cap failures in the IPA stage

 
I spent the better part of a Friday chasing something that has hounded me for over a decade - I discovered the reason why certain General Electric MASTR Pro UHF transmitter strips (like the 4ET59, 4ET60 series) 'work' for a few years in repeater service - then slowly die - sometimes in as short a period as a month as one of mine did. It comes down to a couple of silvered-mica capacitors that 'go bad'. Replacing them restores the transmitter's ability to generate 50 watts of UHF RF energy with the GE model # 4EP38 Power Supply (PS) strapped to deliver a B+ of 450 VDC to the PA stage. (I don't punish the PAs in these xmtr strips - I want a little useful life out of them!)

The reason 'why' these caps go bad is another story - as is the way I 'discovered' they were the culprits for these strips going bad.

The tip-off that there was a problem was when I was returning back into the city and as I was coming back into range of the UHF repeater this transmitter strip is part of - the repeater's signal seemed anemic, VERY anemic as a matter of fact. There was a subtle amount of 'picket fence' while in the class A service area - this shouldn't be happening! But it was ...

To make a long story short, RF power output from the duplexer was about 2 watts where normally I should see thirty watts into the Termaline watt-meter. From that point is was a matter of about a day's troubleshooting, measurements, tweaking, head-scratching and finally replacement of one cap followed by the other - more tweaking and tuning and - 50 watts RF out of the strip easy! GE Mastr Pro UHF transmitter strip back in service!

Finding the 'bad caps'
Shown to the left at about the 2 and 7 O'clock positions around the 12-pin tube socket are the two offending Mica Capacitors.

I 'found' the bad caps by running the transmitter strip with the back cover off (while the transmitter was swung out on it's hinge while attached to the 4EP38 pwr supply) for about 20 seconds - then putting my finger on each capacitor to see which ones had warmed-up while transmitting. As the GE Mastr Pro series uses tube-type drivers and PA - I made sure to be back in *receive* mode so as not to contact the +300 or +450 VDC power supply voltages present. I am *not* afraid of these kinds of voltages - I just practice the proper procedures while troubleshooting!

Schematic of the affected circuit

 
The affected circuitry is shown in the schematic diagram to the left. This is the "IPA" (Intermediate Power Amplifier) stage which precedes the Varactor diode Tripler stage (also shown).

The two caps are shown in red; C107 12 pF and C111 220 pF. 

C107 is from V102's grid to ground.

C111 is from V102's plate to L107/L108; C111 is part of a Pi-network matching circuit between V102 and the Tripler stage. I think the power level in this stage is on the order of 5 to 10 Watts and perhaps as much as an amp of 150 MHz RF current circulates through this capacitor. Unless a low ESR cap is used, self-heating will take its toll after some numbers of hours of operation.  

Of C107 and C111, C111  is by far the more critical, more failure prone capacitor; the final change I made to effect a cure was replace the single 220 pF dipped Silvered-Mica with two caps in parallel; a 100 pF and a 120 pF.

 

GE MASTR Pro UHF Strip Troubleshooting and Repair - Symptoms and Cures

Here are a couple of problems and the cures for the GE MASTR Pro UHF Transmitter strip.

Symptom #1: Transmitter's output power starts 'going away' and the strip starts to output garbage on the band. The PA stage also tunes as if it is unstable (tunes funny).

The two steps to the cure detailed below worked to 'clean up' the strip and also brought the power back up. I found in this strip that one of the fingers was not even touching the tube - and all had become dark-colored and tarnished (as they are made of silver)!

Cure #1:

1) Make sure the silver "fingers" that contact the PA tube are very clean. In fact, I applied silver polish ("Wright's Silver Cream" bought at a local  local grocery store) using a Q-tip and followed this up by wiping the contact fingers with a damp rag.

2) Adjust the silver "fingers" so that they stick out further and will make good, solid contact the PA tube's screen ring. Pull them out while sticking your finger down into the PA socket area. Then practice a few times putting the tube in - I also put a swab of WD-40 lubricant on the tube pins to make sure they slide in easy. I also lightly sanded (with crocus cloth) the PA tube's "screen ring" then swabbed that and the contact fingers with WD-40.

Symptom #2: UHF Transmitter strip dies - sometimes over time. Looking at all the stages with the GE service panel hooked up - it seemed like all readings past position "D" were weak and didn't really 'peak' when tuning the various associated tuning adjustments (the various C's and L's).

I have had around three of four UHF transmitter strips in repeater service that died in-service over the last 10 years. I think I may have found the problem with them - there are several Silvered Mica caps that go bad!

The one I had just fixed for Symptom #1 died again about four months later after I cleaned up the PA tube fingers (as per symptom #1's cure above) and *after* I had recently done some continuous-duty tests on this transmitter. This last fact is the key  - I'm sure this *really* heated up these caps and hastened their failure. I had never done any continuous tests on this particular xmtr strip before - it's almost like these caps will 'run' for just so many hours before they need to be changed! Kind of like tubes! (I have yet to see an actual tube in one of these UHF xmtr strips fail yet. I'm chalking up the 'fails' I've seen on previous xmtr strip failures to failed caps.)

Another sign that these this is the caps failing: Re-tuning C115 fully closed *seemed* to help. Maximum power out of the strip was about 15 Watts whereas normally with 450 volts B+ it would do over 50 Watts (when it was good). C115 *was* only about 2/3 to maybe 3/4 meshed before - but it took fully meshing it to work with the *bad* caps.

Cure #2:

1) Replace C107, a 12 pF capacitor on V102 (the IPA stage), pin 10 to ground on pin 8

2) and C111, a 220 pF capacitor from V102 pin 3 to L103/L102.

UPDATE! I had the replacement cap go bad about a year later - I have now replaced the 220 cap with a parallel combination of a 120 pF and a 100 pF. We will see how this combo works out in time.
*Mine* were both the usual brown-bodied dipped SM (Silvered Mica) style caps and *both* were getting hot! (Be careful when testing/checking these yourself! You can die if you contact B+ (plate supply) voltage!

When I removed the old caps I  found that the leads sticking out of the caps could actually be "turned" very easily - something was very wrong with these caps internally. With the new caps in place the Transmitter output power is now back up to snuff (over 50 Watts out with the PA stage B+ set for +450 VDC).
 

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