A1134A intercom amplifier

[Keith]

Does anyone have the pin connections of the 10 pin flying plug for the above - preferably with the corresponding tag numbers in the 1134a amplifier. I'm in the process of rewiring these and am very getting confused!

Thanks in anticipation

[GMB]

You can download a PDF of the manual, whose circuit seems to contain the pin numbers, from my web site....

http://home.btconnect.com/gmb/airequip.htm#A1134

[Keith]

Thanks for replying, George. In fact I think I supplied you with the manual!

Unfortunately, the numbers on the circuit refer to the internal tag board. The 10-pin connector pin-out is not given. I tried to figure out the connections as I stripped out the old wiring but as the rubber insulation had disintegrated, it was all one big short circuit. I've got most of it OK but I'm still confused on three of the pins.

On powering it up I'm getting a continuous oscillation of around 1-2kHz so I've still got a way to go before I connect it to the T1154.

Regards,

Keith
G3XGW

[GMB]

If it was you then you must have been working on it for a while. So, you didn't recently buy it on eBay - in which case you might try asking the person who did (item 180023628010) because they obviously have one too!

I have checked my photos and other scraps of info but nothing shows what you need I'm sorry to say.

[John H.]

Hi Keith,
The 10 way Mic-Tel panel plug pins are connected as follows. Looking into the plug with the centre spiggot to the left, the first pin below the gap is `A` following round anti clockwise to `K` (with no letter I).

Pin Tag
A 7
B 20
C 6
D * see below
E 5
F 17
G 18
H 11
J 12
K 15
*power supply plug 2V+

Looking at Amp unit from front, Tag 1 is top at right and Tag 10 top at left. Tag 11 is bottom row right and Tag 20 is bottom row left.

It might be worth mentioning (if not separated yet) that the Bakelite pin holder is an interference fit in the cover and is particularly difficult to get out. I have found that an overnight soaking in Plus Gas with the pins facing upwards helps when pressing it out.
John.

[Keith]

Hi John,

Thank you very much for the information. It should sort out the pins that I was unsure about. I'm actually only going to use the T1154 M+ and M- outputs but I would like to have all the other connections correct in case I ever have to plug it into a full installation!

I'm still finding the amplifier bordering on instability. This seems to be irrespective of loading. All supply voltages are OK and bias conditions correct but there is a low level oscillation at around 1kHz. This chirps up loudly as the supplies are removed (as mentioned in the manual). I suppose this wasn't a problem in service - it probably wouldn't have been heard over the din in the aircraft.

There seem to be quite a few differences compared with the circuit (although the latter is for the A1134, not the A1134A). R10 is a 150K resistor as opposed to the 2M on the drawing. I wondered whether this was the cause of the problem as it provides the negative feedback, but changing it to 2M only makes things worse (as you might expect). Also T4 is stated as being 1:1 in the manual but in fact is a considerable step down - consequently the W/Op's phones output is very low compared with the massive output to the Crew's phones. Maybe the headphone impedances were changed at some point.

Anyway I shall carry on investigating whilst I make myself a replica GB battery and try to find a suitable socket for the power connector (B5 almost fits but not quite).

Thanks again for your help,

Keith

PS thanks also to you George. No I didn't get the pristine one on eBAy - WAY out of my price range!

[John H.]

Hi Keith,
Yes, they do seem to be a bit `volatile`. Even with new valves and all components ok, mine also tends towards being unstable and microphonic.
Maybe its a combination of the VR21/210LF`s high 14:1 amplification factor and the 150 volt HT.
The manufacturers original characteristics data indicates a normal anode working voltage of 100 - 120v for this valve (although data books give 150v). I need to try this also as this one is working from its original 150 volt power unit Type173. Seems a good place to start anyway.

Re T4. The A1134 was for high impedance phones output. Primary 310 ohms. Sec 440 ohms. R10 is correct at 2.2 meg.

The A1134A was designed for low impedance phones. T4 on this is; primary 2 ohms. Sec 200 ohms and R10 is correct at 150K.

Sounds like you have an early issue of the A1134 Dgm which is not quite the same, if you PM your address I will onpass a copy of the correct A1134A cct diagram.
John.

[GMB]

Power unit 173 should be delivering 120V so if you have 150V then that's too high, at least that's what the manual says. It has a neon stabiliser across the output (CV45). In fact the neon even has a special striking electrode because on load the smoothed output might not even reach the required initial 135V striking voltage.

I have the manual for the power unit if you need it.

[John H.]

Hi George,
You are right, the A.P. clearly states 120V at 35 M/a. I`m now off to get some new glasses.