Aerotron MPAC UHF Repeater Transmitter Rejuvenation
This webpage describes a 'cure' for a soft Aerotron UHF transmitter, but first, a tour of the UHF Aerotron MPAC 80BR1, 80BT90 (and 80BT40) series will be given. If you need a full copy of the manual for this radio and matching power supply, contact me and for twenty-five bucks I'll make you a copy and mail it to you.
Above is a picture of:
- Aerotron MPAC UHF Base Station Receiver model number 80BR1 and an
- Aerotron MPAC UHF Base Station Transmitter model number 80BT90 (actually, 80BT90R, with the 80BT40 being similar but only capable of 40 Watts output power as it doesn't have the dual PA transistor board the 80BT90 has).
These units are 19" relay-rack mountable, and the 'ears' for rack mounting may be moved close to the front for "flush mounting" the chassis via pem nuts permanently installed for just this purpose.
Service doors open on receiver (top) and transmitter (bottom).
Side view - note the large heatsink on the rear of the transmitter.
Also note the alternate mounting holes for the rack mounting 'ears' allowing the receiver and transmitter to be mounted flush in a rack.
View of an Aerotron Model 1093 Power Supply, the matching rack-mounted Power Supply.
This is an unusual Power Supply. It does not simply supply 12 Volts, it supplies switched 12 Volts to the transmitter that is supplied to the transmitter only when commanded to do so by the PTT line from the receiver.
Side rear view of Power Supply - the large heatsink, the high current power transformer and filter caps are clearly visable.
Inside the Power Supply is a second, smaller power transformer, what I'll call the low current power transformer. This smaller, low current transformer has it's own fuse which is accessable inside the Power Supply. The primaries of both the high current and low current transformers are run through the breaker on the front panel.
View inside the Power Supply with the service door open.
Note the power transformer on the left; this is the low-current power transformer. On the right is the regulator circuitry.
There are separate outputs for the receiver and transmitter, and the outputs for the transmitter are actually only brought 'active' during transmit!
This means that "+12 V Transmit" output from the Power Supply is "off" until the unit is actually put into "transmit" mode. This is done by actually leaving the high current regulator "off" until needed for transmit.
View of transmitter - angled view.
The exciter and modulator stages are on the right, the power amplifier 'chain' (driver and PA stages) is on the left.
View of transmitter Driver stage and dual transistor PA stage.
The driver stage is in the background, the dual transistor 90 Watt PA (Power Amplifier) is in the foreground.
Transmitter Driver Amplifier Stage
200 mW is input on the green Teflon coax on the left and amplified to maximally 40 Watts which is then output on the green coax on the right.
The trained RF eye can see three 'devices' (transistors) interconnected via 'stripline'; one stud-mounted package and two "J-Zero" packages. These stripline segments are actually "L" and "Pi" matching networks, matching the low input and output "Z" of each transistor stage to a higher impedance (normally 50 Ohms or some other interstage value). Small 'sandwich' Mica capacitors are seen between ground and the stripline 'inductors' in places and also as DC blocks between stages; the placement of these capacitors and their values are the result of a design process recommended by the transistor manufacturer, followed up by tweaking in the engineering lab during equipment design and followed up by any 'changes' that the transmitter manufacturing line at Aerotron saw fit to make during mass production of these amplifier boards.
The trained RF eye can also recognize the base and collector 'bias' circuits; the small, narrow back and forth 'loops' that zig-zag either side of each transistor except for the last stage's collector. The last stage's collector uses a more traditional inductor versus something 'etched' onto the board. It is likely that the DC current value is too great (this is the 40 W PA in the Model 80BT40 transmitter) to allow fabrication of an inductor directly on the board so a traditional coil was used instead.
Examination of the first stage showed it to be 'biased' Class A; this doesn't match the printed factory documentation so it was most likely a 'field mod'. The following two stages are operated with no bias - and this results in Class C operation with bipolar (Silicon heterojunction) RF devices (NPN RF power transistors in this case).
The first stage is also the 'target' of the RF ALC (Automatic Level Circuit) developed by the Power Control Board with collector voltage (and base-bias voltage) being controlled though a PNP 'series pass' transistor on the Power Control Board. The Power Control Board views a DC voltage that is developed by an RF sampling circuit (rectified RF output) that samples the 90 Watt (or so) RF output signal from the final PA stage.
The problem with this old amplifier design is - a succession of Class C biased stages is extremely sensitive to drive level below a certain point; if each stage is not 'driven' sufficiently it may fail to amplify sufficiently and not deliver enough output power to drive the next Class C biased stage! THAT was ocurring with this old amplifier chain; until the devices in the amplifier chain are quite literally 'warmed up' a little, the output power would only be 10 to 20 Watts (at a 70 Degree F. room temperature) from the nominally-rated 90 Watt transmitter!
Fortunately, there is a cure. Someone had already modified the first stage in the driver stage to be 'biased'; adding bias to the second stage allowed the output power to be 90 Watts with no warm-up needed!
Diagrammatically, here was the original Bias situation (with only the first driver modified field modified) in the amplifier chain as I had received the Aerotron transmitter:
- 3 transistor Driver Stage - | | - PA Stage -
| |
| | |-- PA #1 --|
| | | Class C |
Driver #1 --- Driver #2 --- Driver #3 --> >--| |-- Output
Bias: Class A Class C Class C | | |-- PA #2 --|
| | Class C
During testing with base bias voltage applied to the second driver, full bias (.7 volts or so) wasn't needed; just a few tenths of a volt (.2 volts) from a variable bench power supply moved up the 'turn on' point to allow the second driver stage to deliver sufficient output power to the third driver to 'drive' it and the final PA stage transistors "into conduction".
Diagrammatically, here is the modified situation:
- 3 transistor Driver Stage - | | - PA Stage -
| |
| | |-- PA #1 --|
| | | Class C |
Driver #1 --- Driver #2 --- Driver #3 --> >--| |-- Output
Bias: Class A Class B/AB Class C | | |-- PA #2 --|
| | Class C
Transmitter Final Amplifier Stage
30 Watts to approximately 90 Watts.
With the ALC/Output Power Control set for a REASONABLE output power (for repeater/continuous duty) at a power level of say, 60 Watts, the input power is probably on the order of only 25 Watts.
Full size schematic diagram of Driver stage here.
Note that the schematic has been marked up detailing a bias circuit for the first amplifier stage, Q01 (Q301 on Parts Location drawing).
The second transistor, Q02 (Q302 on Parts Location drawing), is the amplifier stage this web page addresses and where a 'bias' circuit was added to the base circuit to 'bring back' or rejuvenate the old Aerotron transmitter I received.
Full size schematic diagram of PA stage here.
Full size Parts Location for the transmitter amplifier chain here.
Notice this factory documentation does not detail bias circuit components seen in the first stage of the driver circuit.
Driver Stage Transistor Complement
As-built devices (devices actually in amplifier, several possibly as a result of repair):- 2SC1808
- CD2772
- MRF648
Printed Documentation (schematic) devices:
- CD3276
- CD2772
- CD2773
Final Stage Transistor Complement
As-built devices (devices actually in amplifier, several possibly as a result of repair):- SRF2926K
Printed Documentation (schematic) devices:
- CD3286
Driver Stage Transistor Specifications
First Transistor
The First transistor, Q01 (Q301 on Parts Location drawing).
Closest Motorola devices (circa 1983):
Motorola Case styles:
Comparing Transistor Specifications
| Driver Board | Driver design gain
total Ap= 23 dB |
As built
devices, specs |
Schematic specified
devices |
Other devices
(margin notes) |
| Q01 stage | Ap= 10 dB
Pin = 200 mW Pout = 5 W |
2SC1808 (Mitsubishi)
Pin = ? Pout = 10W |
CD3276
? |
MRF652 (Mot)
.5 W / 5 W / 10 dB SD1433 (STM) 1.5 W / 10W / 8 dB |
| Q02 stage | Ap= 8.2 dB
Pin = 5 W Pout = 13.8 W |
CD2772
? |
CD2772
? |
SD-1488 (STM)
10 W / 38 W / 5.8 dB |
| Q03 stage | Ap= 4.8 dB
Pin = 13.3 W Pout = 40 W |
MRF648 (Mot)
Apsat= 4.4 dB Pin = 22 W Pout = 60 W |
CD2773
? |
SRF-2926
? |
Conclusion
After digesting all the above and piecing together the history of this transmitter:A. The replacement Q03/Q303 device would appear to be "over sized" for the application as a 40 Watt output amplifier stage. This replacement would have less gain and therefore require a bit more drive in order to 'conduct' in non-biased Class-C operation.
Simply 'dropping in' a larger power transistor in this application doesn't address factors such as *each* of these transistors has "internal impedance matching" circuitry withing the transistor package - simply 'dropping in' one device for another can substantially alter the RF voltages and currents that are seen in that circuit; matching to a lower input impedance would result in lower RF voltages seen at the actual base of the transistor (within the package) and therefore require more drive for a Class-C stage to function. More drive means the base will be "forward biased" - a condition required for amplification; there is no getting around this fact.
B. This is an old (at this moment, approximately 23 years old to be exact) amplifier. The manufacturing date stamps show a date of "10/81". It is not unusual for semiconductor devices to 'drop off' in gain due to age, especially power devices/power transistor that have operated as elevated temperatures. The manual states that "the 80BT90 is rated at 50% duty cycle when the optional temperature controlled fan box is not installed." I don't think a fan box was ever installed and the PA stages show the the result of "cooked PAs" (although not fatally) as a result.
C. Paperwork accompanying this amplifier shows that repairs were affected to the Driver Board in 1986; it looks as though the Q01 and Q03 stages were 'changed out'. This could have been a failure due to high PA temperatures and early transistor failure - I don't know.
D. With the Q02 stage biased, the amplifier is capable of once again meeting output power specifications. It looks like a few more productive years may be obtained from this Transmitter without replacing costly PA transistors after all!