amstrad 901 faults low TX

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Ur5by9
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amstrad 901 faults low TX

Post by Ur5by9 » 12 Apr 2019, 19:16

I picked up some scrap radios to learn from and use as parts.

In the batch was an Amstrad 901 which was complete and in okay shape.

The purpose of this was to learn and using schematics with my test meter etc

The Amstrad powered up with no audio in or out and only the slightest of TX from the beep could be picked up on a near by radio.

Firstly i checked the board for any obvious signs of damage or component failure.
The audio chip was getting very hot so i removed this chip LC4422 which turned out to have hole through the other side.
I replaced both the LC4422 and the LA1230

This returned the audio back to the radio the squelch works and i can use the PA function fine.
Receive is fine through from channels 1 to 40

So i turned to the TX stage and although I could pick up the faintest of signals on a nearby radio I checked the vco voltages and only needed to make slight adjustments but still had no output showing on my dummy load power meter.

I replaced all transistors
Q5 2sc536
Q6 2sc536
Q7 2sc2086
Q8 2sc2166
the voltages around these transistors seem okay and this has brought the radio into TX givings some power out

So the only problem i seem to have is the TX output is at a push giving out 2.5 watts peaked
This drops back to around 2 watts after a few minutes TX and Q7 gets fairly warm - I replaced Q7 a second time and the results are the same.
Low power is slightly above 250mw

The large green resistor at the back of the radio is intact and reading the correct value.
I have cleaned the pots with switch cleaner but seem to have hit a stumbling block resolving the power out issue.

Seems to be quite a common chassis radio and Im looking for some advice on troubleshooting this final hurdle.

Many thanks all help is greatly appreciated.

schematic can be found at http://www.cbtricks.com/radios/amstrad/ ... 901sch.pdf

:wave:

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The Collector
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Re: amstrad 901 faults low TX

Post by The Collector » 12 Apr 2019, 19:50

Removing that green "caterpillar" will boost tx power, but it shouldn't be done if the radio's not operating properly as it'll probably fry something.

That's all I know ;)

ChrisMagpie
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Re: amstrad 901 faults low TX

Post by ChrisMagpie » 14 Apr 2019, 06:06

Ur5by9 wrote:
12 Apr 2019, 19:16

The audio chip was getting very hot so i removed this chip LC4422 which turned out to have hole through the other side.
Sounds like it's had reverse polarity :|
Some good videos here:
https://www.youtube.com/watch?v=LfIIwQL1GU8

https://www.youtube.com/watch?v=cGLTPFxlP9Y

Maybe try cleaning the high/low power switch, too?

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astradyne
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Re: amstrad 901 faults low TX

Post by astradyne » 14 Apr 2019, 11:49

Has someone been twiddling with the tuning cans?
Never, ever, ever....bloody anything ever!

Far too many radio's to list here. But I do own a legal one...

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Buick Mackane
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Re: amstrad 901 faults low TX

Post by Buick Mackane » 14 Apr 2019, 12:30

Align the TX chain, By the way out of curiosity where was it made ? Best models were made in japan
I Was asked to QSL Via the Bureau, But the FBI Said i'll be prosecuted if i phone them again, :shock:

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Re: amstrad 901 faults low TX

Post by Buick Mackane » 14 Apr 2019, 13:36

The Collector wrote:
12 Apr 2019, 19:50
Removing that green "caterpillar" will boost tx power,
Aye dave, By doing that you'll get 10 watts for 10 seconds :)
I Was asked to QSL Via the Bureau, But the FBI Said i'll be prosecuted if i phone them again, :shock:

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Ur5by9
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Re: amstrad 901 faults low TX

Post by Ur5by9 » 14 Apr 2019, 20:02

its not the japan model - i'm only spending the time for the experience and learning.

I would take it for granted every can in it has been twiddled.

The receive from channels 1 through to 40 is fine
The transmit audio is fine its only the output power i cant tweak it any more than 2.5 watts. low is around 250mw
I have replaced all the transistors in the output stage with new.
Could the final be a cheap clone that is unable to make the power ? 2sc2166 which i got from ebay ?
I think i have a 1969 output from a cybernet radio can this be used as a direct replacement?

All pots and switches have had some cleaner.

I feel i'm also there with this radio just need some final help.

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1968neil
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Re: amstrad 901 faults low TX

Post by 1968neil » 03 May 2019, 13:10

Low tx is usually caused by the little red trimmers having wax in them or just old and frail.
I would go thru the alignment first with proper test gear.
Otherwise you are shooting in the dark, im sure you can find some alignment details via a google search :)
Regards
Neil

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