Racal RA7915 main receiver board schematic needed

"John Crighton" wrote I have had another look again at my filter.
Last Friday that S looked like an 8 to me. Under a magnifier it is S
So Brian, the filter in my transciever TRA7948 is
TOYO 10MLS. Freq 10.7MHz. Mfg no U 121. Date 11-72

The Mfg no R052 and U 121 are different. I don't know what
this means but I would be happy to use a filter from a transciever
that can be picked up fairly cheaply.

**** John,many thanks for your efforts.A large dollop of luck saw me
pick up a cheap single channel 7915 over the weekend,and,yes,it is
fitted with the USB filter!!!

Regards,Brian Goldsmith.
 
On Mon, 24 May 2004 08:55:16 +0800, budgie <me@privacy.net> wrote:

On Thu, 20 May 2004 23:46:43 GMT, john_c@tpg.com.au (John Crighton) wrote:

while we were discussing the shitty resistors, I just remembered
the shitty oven oscillator resistors and lousy workmanship
on the oven casting.
(snip)

I want to warn you about the "later model" oven oscillator unit.
The heater transistors, are screwed to the side of the oven
casting. Fair enough, but the casting is very rough and uneven,
the lazy bastards in production would not file it flat. So the
contact between the casting surface, mica washer and flat
pack power transistor BD??? is bad. To make matters
worse, the little brass threaded insert that is pushed into the
casting to take the screw of the power transistor sometimes
stands proud of the casting. Meaning there is a gap between
the power transistor and casting when screwed up. The
thin mica washer can get damged by the proud insert and
not insulate the transistor from the casting. Out the door they
went just like that for other people to fix in the field when
they failed later on.

File your casting surface flat where the heating transistor goes.

Also have a look at the clearance between the casting and
solder pads of the heater transistor. It is close, sometimes the
casting touches the solder pads. File a small bevel on the bottom
of the casting where the transistor is fitted just for clearance.
You probably wont have this problem with the early ovens
but you will with the later ovens.

Next, have a look at the emitter resistors for the heater transistor,
I bet they are way out of tolerance like you found elsewhere.
Instead of 1.2 or 1.5 ohms, I cant remember exactly, they will have
gone very high in value and the oven will take ages to heat up.
They should have fitted nice little wire wound types in this spot.

Here is the part that everybody falls foul of. After all the screws
are removed from the oven casting, it still does not come off.
A little force is used and then we have a broken thermistor.
The thermistor is cemented into the base of the casting. The
thermistor leads are soldered to the tracks on the PCB. A little
warning label could have fitted to warn technicians to unsolder
thermistor before removing the casting. Words of warning
etched on PCB would have been fine.
A label would have cost money, but heaps of time and money
were spent drilling out the thermistor with the snapped leads.
Having the thermistor not glued in to the casting is fine provided
heaps of that white heatsink compound is used to fill the hole
where the thermistor goes. Cemented in is better but the risk of
damage is great. So Jeff, watch out for the thermistor. :)

Does that apply to the TRA7928/38 transceivers as well? It's a long time since
I've looked inside them - although I have a working '38A stowed in the garage.
Click to expand...
Hello Budgie,
yes all those bad things apply to your oven oscillators too.
Write yourself a little note saying
"Warning unsolder oven thermistor before removing casting"
and slip it inside the set. If your clarifier switch/pot has an
orange coloured plastic switch then change it.
10K ( A linear) with dual switch.
I changed so many pots, I still remember the value. Heh heh heh....
The switches on the back of the pot are unreliable.
Maybe they go bad because they don't switch much
current. Don't know. Anyway, they give problems by not
switching from AM to SSB reliably.

Budgie, you have your finger on the pulse regarding
spare parts and where to get them.
I am looking for a couple of ICs
SL1621C and NE571N (SA571N)
Farnell have the SA571N but not the SL1621C
I would like to buy them at the same place to save
postage. No luck at RS for either of them.
Any ideas?

Regards,
John Crighton
Sydney
 
On Mon, 24 May 2004 02:07:54 GMT, "Brian Goldsmith"
<brian.goldsmith@nospamecho1.com.au> wrote:


**** John,many thanks for your efforts.A large dollop of luck saw me
pick up a cheap single channel 7915 over the weekend,and,yes,it is
fitted with the USB filter!!!

Regards,Brian Goldsmith.


Lucky You! Great stuff!
Click to expand...
If you have an ordinary RF sig gen, you can disconnect the
channel oscillator coax at say the main board and use your
sig gen as a channel oscillator (10.7 Mhz above your desired
channel) and have a play around as though the set had
variable tuning, 2 to 12 Mhz ( was it 10 or 12? I can't remember.)

A posh sig gen with digital readout is great for this purpose.
That is what I liked about these old Racal transceivers and
receivers such as yours. Was it an ebay purchase?

Regards,
John Crighton
Sydney
 
On Mon, 24 May 2004 13:59:15 +1200, "Jeff" <jeff_164@hotmail.com>
wrote:

Hi John,
The Oven unit is next on the list for a look at. I'll be checking all the
resistors, including the one you mention.
I'm going to do the mod to widen the freq range of the clarifier into a BFO,
I see there is a cap that changes value if clarifier or BFO mode is wanted.
Another thing, I might disable the heater, to see if the xtals are stable at
room temp. Ive read this was a common mod that was done to these sets?
JEFF
Click to expand...
Disabling the heater is not as bad as it sounds, Jeff.
For us hobbyists, quite a good idea reliability wise, just means
you will have to adjust the little trimmers and maybe change the
little fixed capacitor associated with the trimmers for each channel
xtal as well as the 10.7Mhz carrier insertion oscillator.

I was just mentioning to Brian that you can use a signal
generator 12.7Mhz to 22.7 Mhz to replace the fixed xtal
channel oscillator to make your receiver continuously
tunable from 2 to 12 Mhz. Just for fun.

Regards,
John Crighton
Hornsby
 
On Mon, 24 May 2004 04:33:56 GMT, john_c@tpg.com.au (John Crighton) wrote:

On Mon, 24 May 2004 08:55:16 +0800, budgie <me@privacy.net> wrote:

On Thu, 20 May 2004 23:46:43 GMT, john_c@tpg.com.au (John Crighton) wrote:

while we were discussing the shitty resistors, I just remembered
the shitty oven oscillator resistors and lousy workmanship
on the oven casting.
(snip)

I want to warn you about the "later model" oven oscillator unit.
The heater transistors, are screwed to the side of the oven
casting. Fair enough, but the casting is very rough and uneven,
the lazy bastards in production would not file it flat. So the
contact between the casting surface, mica washer and flat
pack power transistor BD??? is bad. To make matters
worse, the little brass threaded insert that is pushed into the
casting to take the screw of the power transistor sometimes
stands proud of the casting. Meaning there is a gap between
the power transistor and casting when screwed up. The
thin mica washer can get damged by the proud insert and
not insulate the transistor from the casting. Out the door they
went just like that for other people to fix in the field when
they failed later on.

File your casting surface flat where the heating transistor goes.

Also have a look at the clearance between the casting and
solder pads of the heater transistor. It is close, sometimes the
casting touches the solder pads. File a small bevel on the bottom
of the casting where the transistor is fitted just for clearance.
You probably wont have this problem with the early ovens
but you will with the later ovens.

Next, have a look at the emitter resistors for the heater transistor,
I bet they are way out of tolerance like you found elsewhere.
Instead of 1.2 or 1.5 ohms, I cant remember exactly, they will have
gone very high in value and the oven will take ages to heat up.
They should have fitted nice little wire wound types in this spot.

Here is the part that everybody falls foul of. After all the screws
are removed from the oven casting, it still does not come off.
A little force is used and then we have a broken thermistor.
The thermistor is cemented into the base of the casting. The
thermistor leads are soldered to the tracks on the PCB. A little
warning label could have fitted to warn technicians to unsolder
thermistor before removing the casting. Words of warning
etched on PCB would have been fine.
A label would have cost money, but heaps of time and money
were spent drilling out the thermistor with the snapped leads.
Having the thermistor not glued in to the casting is fine provided
heaps of that white heatsink compound is used to fill the hole
where the thermistor goes. Cemented in is better but the risk of
damage is great. So Jeff, watch out for the thermistor. :)

Does that apply to the TRA7928/38 transceivers as well? It's a long time since
I've looked inside them - although I have a working '38A stowed in the garage.

Hello Budgie,
yes all those bad things apply to your oven oscillators too.
Write yourself a little note saying
"Warning unsolder oven thermistor before removing casting"
and slip it inside the set. If your clarifier switch/pot has an
orange coloured plastic switch then change it.
10K ( A linear) with dual switch.
I changed so many pots, I still remember the value. Heh heh heh....
The switches on the back of the pot are unreliable.
Maybe they go bad because they don't switch much
current. Don't know. Anyway, they give problems by not
switching from AM to SSB reliably.
Click to expand...
OK John noted all above for the next time I get THAT deep into the garage ...

Budgie, you have your finger on the pulse regarding
spare parts and where to get them.
I am looking for a couple of ICs
SL1621C and NE571N (SA571N)
Farnell have the SA571N but not the SL1621C
I would like to buy them at the same place to save
postage. No luck at RS for either of them.
Any ideas?
Click to expand...
Thanks for the glowing compliment, but I don't have any leads on the 1621. Is
that a Plessey chup?
 
"budgie" wrote

Is that a Plessey chup?

**** Nah,you wont find those chups with your fush down at Brudge Road
,Ruchmond at half past sux!!!

:)

Brian Goldsmith
 
John, both are listed here... are you rich?
ne571
http://www.jabdog.com/semi-lin5.htm
sl1621
http://www.jabdog.com/semi-lin6.htm

JEFF



"John Crighton" <john_c@tpg.com.au> wrote in message
news:40b17144.25754795@News.individual.net...
On Mon, 24 May 2004 08:55:16 +0800, budgie <me@privacy.net> wrote:

On Thu, 20 May 2004 23:46:43 GMT, john_c@tpg.com.au (John Crighton)
wrote:

while we were discussing the shitty resistors, I just remembered
the shitty oven oscillator resistors and lousy workmanship
on the oven casting.
(snip)

I want to warn you about the "later model" oven oscillator unit.
The heater transistors, are screwed to the side of the oven
casting. Fair enough, but the casting is very rough and uneven,
the lazy bastards in production would not file it flat. So the
contact between the casting surface, mica washer and flat
pack power transistor BD??? is bad. To make matters
worse, the little brass threaded insert that is pushed into the
casting to take the screw of the power transistor sometimes
stands proud of the casting. Meaning there is a gap between
the power transistor and casting when screwed up. The
thin mica washer can get damged by the proud insert and
not insulate the transistor from the casting. Out the door they
went just like that for other people to fix in the field when
they failed later on.

File your casting surface flat where the heating transistor goes.

Also have a look at the clearance between the casting and
solder pads of the heater transistor. It is close, sometimes the
casting touches the solder pads. File a small bevel on the bottom
of the casting where the transistor is fitted just for clearance.
You probably wont have this problem with the early ovens
but you will with the later ovens.

Next, have a look at the emitter resistors for the heater transistor,
I bet they are way out of tolerance like you found elsewhere.
Instead of 1.2 or 1.5 ohms, I cant remember exactly, they will have
gone very high in value and the oven will take ages to heat up.
They should have fitted nice little wire wound types in this spot.

Here is the part that everybody falls foul of. After all the screws
are removed from the oven casting, it still does not come off.
A little force is used and then we have a broken thermistor.
The thermistor is cemented into the base of the casting. The
thermistor leads are soldered to the tracks on the PCB. A little
warning label could have fitted to warn technicians to unsolder
thermistor before removing the casting. Words of warning
etched on PCB would have been fine.
A label would have cost money, but heaps of time and money
were spent drilling out the thermistor with the snapped leads.
Having the thermistor not glued in to the casting is fine provided
heaps of that white heatsink compound is used to fill the hole
where the thermistor goes. Cemented in is better but the risk of
damage is great. So Jeff, watch out for the thermistor. :)

Does that apply to the TRA7928/38 transceivers as well? It's a long time
since
I've looked inside them - although I have a working '38A stowed in the
garage.

Hello Budgie,
yes all those bad things apply to your oven oscillators too.
Write yourself a little note saying
"Warning unsolder oven thermistor before removing casting"
and slip it inside the set. If your clarifier switch/pot has an
orange coloured plastic switch then change it.
10K ( A linear) with dual switch.
I changed so many pots, I still remember the value. Heh heh heh....
The switches on the back of the pot are unreliable.
Maybe they go bad because they don't switch much
current. Don't know. Anyway, they give problems by not
switching from AM to SSB reliably.

Budgie, you have your finger on the pulse regarding
spare parts and where to get them.
I am looking for a couple of ICs
SL1621C and NE571N (SA571N)
Farnell have the SA571N but not the SL1621C
I would like to buy them at the same place to save
postage. No luck at RS for either of them.
Any ideas?

Regards,
John Crighton
Sydney
Click to expand...
 
On Mon, 24 May 2004 12:46:31 GMT, "Brian Goldsmith"
<brian.goldsmith@nospamecho1.com.au> wrote:

"budgie" wrote

Is that a Plessey chup?

**** Nah,you wont find those chups with your fush down at Brudge Road
,Ruchmond at half past sux!!!
Click to expand...
FWIW we have a real live actual working F&C shop in Subiaco (WA, not Italy)
whose sign says "Fush & Chups".
 
On Mon, 24 May 2004 19:51:44 +0800, budgie <me@privacy.net> wrote:


Budgie, you have your finger on the pulse regarding
spare parts and where to get them.
I am looking for a couple of ICs
SL1621C and NE571N (SA571N)
Farnell have the SA571N but not the SL1621C
I would like to buy them at the same place to save
postage. No luck at RS for either of them.
Any ideas?

Thanks for the glowing compliment, but I don't have any leads on the 1621. Is
that a Plessey chup?
Click to expand...
Yes it is Plessey chup. Heh heh heh.....
Thanks anyway,
John Crighton
Hornsby
 
On Tue, 25 May 2004 10:37:54 +1200, "Jeff" <jeff_164@hotmail.com>
wrote:

John, both are listed here... are you rich?
ne571
http://www.jabdog.com/semi-lin5.htm
sl1621
http://www.jabdog.com/semi-lin6.htm

JEFF
Click to expand...
Cripes that SL 1621 is expensive.
Thanks for your efforts there Jeff.
Regards,
John Crighton
Sydney
 
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