Sailor T128 Marine Transmitter.

[Station X]

The system uses a 12 hour clock and times in the hour after midnight are prefixed 12 rather than 00 and suffixed AM.

[Radio Wrangler]

Originally Posted by crestavega

matic at the bottom of the PDF.

[ the shack is conveniently a faraday cage

I just spotted this and thought I'd better comment.

A 'Faraday' cage is not an effective screen at all. It's a special one designed to let magnetic fields through and to stop only electric fields. This is a common confusion, but most people automatically prefix any mention of a screening cage with 'Faraday' and it completely reverses what they were probably intending to say.

Michael Faraday invented the screen named after him in order to demonstrate his invention, the transformer, and to counter the arguments that it worked by capacitive coupling. As far as keeping radio signals in, or out, Faraday cages leak like a sieve.

Faraday screens are commonly built into good quality transformers to prevent capacitive coupling of noise and interference between primary and secondaries without interfering with the magnetic operation.

David

[crestavega]

I'm trying to work out what I am looking at in the way the T128 deploys this '1596 balanced modulator ...

... with a view to seeing if any of its peripheral lines are gubbed, before trying to source a replacement IC.

it's not immediately obvious how the "supply voltage" - if any - is delivered to the IC....
could anyone please give me some pointers on what the IC is actually doing. I see that the IC is supposed to provide gain, which sort of implies that it must have a power supply but I can't immediately see where this is.

Edit - I see that L102 is choking an 18V supply line (labelled 3-orange in the serviceman) which will be the energy supply to the active devices in this area. I was reading the wiring loom wrongly.

[Station X]

In an earlier post you say that TP13 was at 0.7V with an expected reading of 3 - 4.5V.

Later you say that TP13 is fed from an 18V supply via L102 which is connected to TP13 via the primary winding of TR102.

Unless L102 and TR102 have high resistance, the readings sound unlikely to me.

There seems to be quite a lot of potential divider circuitry in both sides of the supply. Have you checked the resistors there? What about the zener diode D101? Does it have 7.5V across it?

[crestavega]

sorry. and thanks.
The supply voltage is 18V DC (at least, I assume it's DC, I only glanced at the drawing so far)
the smaller voltages I was referring to were Vpp and read from a scope. they refer to the ac signal waveforms that the IC is processing.

[Station X]

Forget my previous post. As you say you were talking of p-p signal waveform voltages, not DC.

It might be worth checking the DC conditions though.

[crestavega]

indeed - I had just stumbled on that helpful little table also.
thanks very much.

[crestavega]

something defintely crank hereabouts, pls see attached measurements.

before delving further, can I get a sanity check: the service manual states 18V DC should be present on the supply line. It is.

T101 collector is connected to this 18V line, but the table says that T101 c = 14.6V
does this just reflect the on-load volt dip that should be present in this area when all is well?



Taking the case of T102, the table shows that its collector voltage should be 4.8V.

T101's base is biased by this voltage , divided over R105 and R101... (?)
R105 = R101 = 22K therefore the T101 base should be half the voltage seen at T102's collector. = 2.4V
however the table says that T101 base should be 3.3V

am I barking?

(by the way, D101 does indeed have 7.5V across it, and this voltage is present at pins 7 & 8 of the '1596 IC so I think the supply to her is OK)

[Station X]

I too was surprised that 101's collector wasn't at 18V. I put the difference down to volt drop in L102 which supplies all three transistors and the IC. If that's not the case then I guess the table is wrong.

Your calculations for T101's base do not take into account any current flowing into the base. This will cause a volt drop in the 22k resistor.

If the table is substantially correct, then the voltage on T101's base is very low and merits further investigation.

[crestavega]

what is the purpose of T101 & T102 in this area? they appear to be "just in case" function where the main signal path is straight to T103 as a buffer before the IC.

[crestavega]

apologies, there is a relatively comprehensive explanation of these parts in the serviceman.