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Old 8th May 2009, 2:44 pm   #121
stonehopper
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Default Re: Roberts RT7 - problem

The AC128's have a TO-5 layout of 'pin out' as shown in http://www.vintageradio.me.uk/info/germanium.htm#cases. The OC81M's I had suspected, as was conformed by another poster, as having a grey line on the jacket in place of the paint mark against the collector pin. I still think this may have been correct, and believe I did the install in the correct order.

However, I have contacted Cricklewood for confirmation of the 'pin out' on the AC128, and as both Tim and Graham have stated, the metal tag protrudes nearest the emitter.

Prior to testing, battery Voltage at 9.6V
Results of checks:
Bias:
Followed instructions exactly, with TR6 & TR7 NOT installed, meter set to 0 - 20V DC Red lead to chassis and black to junction of R26 & R27 = 0.2V

Voltage check on TR5;
Actual/Service sheet
Base: 1.87V/1.69V
Collector: 9.15V/8.7V
Emitter: 1.75V/1.6V

Current:
Meter set at 20mA range (3 o'clock on my meter) = 0.00
Meter set at 10ADC (with plug moved to correct socket) = 0.01
(The latter being about the same as current passed through my body between fingers held on probes).

After testing, battery Voltage = 9.48V.

How are we doing?
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Old 8th May 2009, 3:14 pm   #122
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Default Re: Roberts RT7 - problem

That all looks OK apart from the last test. I'm surprised that you didn't get a reading on the 20mA range. 0.01 amps is 10mA, so I would have expected to see a reading on the 20mA range. I assume the set was turned on? Are the normal sockets the ones to be used on the 10mA range?

I guess the next thing to do is fit the output transistors. Then switch on whilst monitoring the current drawn from the battery. If OK check the voltages on the output transistors.
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Old 8th May 2009, 5:57 pm   #123
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Default Re: Roberts RT7 - problem

Quote:
Originally Posted by Station X View Post
That all looks OK apart from the last test. I'm surprised that you didn't get a reading on the 20mA range. 0.01 amps is 10mA, so I would have expected to see a reading on the 20mA range. I assume the set was turned on? Are the normal sockets the ones to be used on the 10mA range?

I guess the next thing to do is fit the output transistors. Then switch on whilst monitoring the current drawn from the battery. If OK check the voltages on the output transistors.
Yes - set was switched on, then off, then on again. This was with both plugs in the lower two sockets and the dial at 20mA range.
I switched off and then tried the bottom and top sockets with the dial at 10ADC range = 0.01

I'll run another check, and if I find anything changed I'll post again. If not, I'll put the AC128's in and see what happens.

Derek

PS Picture of meter in Post 93 Page 5.
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Old 8th May 2009, 7:36 pm   #124
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Default Re: Roberts RT7 - problem

OK The TR6 &TR7 have been installed.
Battery Voltage 9.6V
Switched on with meter in 20mA range and set up between battery + and radio, same result as before = 0.01

Switched off, removed meter, and connected radio to battery.
Switched on, no sound from speaker, on either Long or Medium, went up and down the bands with no result - no sounds. No sound from speaker when removing battery clip and replacing.

Checked Voltage at TR6 & TR7 between chassis;
TR6
Emitter 0.18V
Base 0.00V
Collector 8.95V

TR7
Emitter .19V
Base 0.2V
Collector 9.48V

Switched off, battery reading 9.57V.

None of the components seemed to get warmed up, the new TR's were just about body temp, no more. I must be missing something.

Derek

PS I'm thinking if the base from TR6 & 7 connect to the T1 transformer, and one shows 0.2V and the other 0.00, might there be a problem with the T1?

Last edited by stonehopper; 8th May 2009 at 7:44 pm.
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Old 8th May 2009, 7:47 pm   #125
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Default Re: Roberts RT7 - problem

The bases of TR6 and TR7 are connected together through the seondary winding of T1, so they should both have the same voltage (0.2V) on them. Check your soldering of T1 and TR6 base carefully.

The readings on the collectors and emitters of TR6 and TR7 indicate that the primary winding and one of the secondary windings of T2 is OK.
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Old 8th May 2009, 7:55 pm   #126
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Default Re: Roberts RT7 - problem

To quote from post #66:-

Quote:
TR6 Emitter 1.9v; coll. 4.87v; base 1.76v
TR7 Emitter 3.35v; coll. 2.13v; base 2.10v
T1 secondary was obviously OK then so it should be so now.
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Old 8th May 2009, 8:40 pm   #127
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Default Re: Roberts RT7 - problem

I have gone back and rechecked the soldering. It seemed OK. So with power on I began to gently tap the TR's and C's on top with a plastic tool. Then went under the board to do the same on the joints. Still no squeaks or pops, so went to check the Voltage readings on the TR6 & TR7 again. Felt around and all cool to touch.

TR7 E 0.08V; B 0.23V; C 9.24V
TR6 E 0.18V; B 2.78V - at which point I stopped, because that read zero before. I then went back to check TR7's Base again, as I thought I might have got the decimal point in the wrong place. It read 4.6V and rising. I then put my finger on the TR6 & TR7, and now have a circular burn for it! Immediately unclipped the battery and left it alone. battery now at 8.73V. T2 was definitely warm to touch now. (Finger OK, not stinging, just a shiny circle!)

Will leave it for tonight.

Derek
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Old 8th May 2009, 10:10 pm   #128
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Default Re: Roberts RT7 - problem

Sounds like we're back to square one. Best to check whether T1 secondary is faulty.

Remove TR6 and TR7.
Meter on 200 ohm range.
One lead of meter connected to junction of R26 and R27.
Other meter lead connected to point to which TR6 base was connected. Note reading.
Other meter lead connected to point to which TR7 base was connected. Note reading.
If you get an over range indication switch to the 2000 ohm range.
Report readings here.
Both halves of the winding should show continuity.

Meter on 2000mV (2 volt) range.
Red lead of meter connected to chassis
Turn on the set.
Black meter lead connected to point to which TR6 base was connected. Note reading.
Black meter lead connected to point to which TR7 base was connected. Note reading.
If necessary switch up or down a range for a more accurate reading.
Report readings here.
You should see about 0.2V (200mV) on each base track.
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Old 9th May 2009, 11:27 am   #129
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Default Re: Roberts RT7 - problem

OK, chores done, back on the set.

Exactly as instructed;
200Ohm range:
R26 & R27 - where;
TR6 base was connected = 34.5
TR7 base was connected = 37.5

2000mV;
TR6 base was connected =194
TR7 base was connetced =195

Did not try any other range - are these accurate enough?

Derek
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Old 9th May 2009, 1:21 pm   #130
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Default Re: Roberts RT7 - problem

Quote:
200Ohm range:
R26 & R27 - where;
TR6 base was connected = 34.5
TR7 base was connected = 37.5
That's OK. Approximately 35 ohms in each half of the winding.

Quote:
2000mV;
TR6 base was connected =194
TR7 base was connetced =195
That's OK too. Approcimately 200mV or 0.2V on each point. If there were a short circuit between primary and secondary (ie a fault condition) There would have been a higher reading.

I am at a loss to explain why you saw first 0V and later 4.6V on TR6 base. Visually check for breaks in the track going to TR6 base and contacts with other tracks.

-----------------------------------------------------------------

While the output transistors are out check the windings on T2:-

Resistance readings are always measured with the set turned off.

T2 primary. Measure resistance between chassis and the points where TR6 and TR7 emitters were connected.

T2 upper secondary. Battery removed. Measure resistance between the negative battery lead going into the set and the points where TR6 and TR7 collectors were connected.

Measure the resistance of R30 and tell us what its colour code is.

T2 lower secondary. Disconnect one lead of the speaker. Disconnect one end of R28. Measure the resistance between the speaker wires going into the set. While R28 is disconnected you might as well check its value. Reconnect speaker and R28.

------------------------------------------------

Now for some voltage readings. Voltage readings are always taken with the set turned on.

T2 primary. Measure the voltage between chassis and the point where the TR6/7 emitters were connected. Expect a low or zero reading.

T2 upper secondary. Measure the voltage between chassis and the point where the TR6/7 collectors were connected. Expect to see the full battery voltage.

---------------------------------------------

It would be a good idea to check the operation of your meter's 20mA range.

Battery out of set. 2.2K resistor (a spare one, not the one in the set) to positive of battery. Positive lead of meter to other end of resistor. Negative lead of meter to negative side of battery. Expect a reading of about 4 to 5mA.
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Old 9th May 2009, 3:00 pm   #131
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Default Re: Roberts RT7 - problem

The AC128 connections are as the picture viewed from underneath.

A temporary way to find out if it is the transformer would be to disconnect the two emitters from a, b, windings and use just one of the AC128ís as it will need an NPN such as an AC176 as well as the AC128 to make a push-pull output. It does work as I have modified a radio with a duff transformer and it still gave acceptable volume.

Fit two resistors (quarter Watt will do, low value 2.2 Ohm minimum as shown or even up to 5 Ohm each (this is to limit the current through the transistors)

Disconnect the speaker from the transformer secondary winding , but leave the feedback resistors in circuit

Connect the emitter resistor common junction to the negative end of an electrolytic capacitor (value not critical 100uf to 1000uF 12Volt) The capacitor is to block the DC.

Connect the positive end to the speaker,

This will produce a mismatched output (that is why the output-matching transformer is there) but if there is a signal, it will show up.

It is shown with the negative at the top of the diagram

The approximate circuit is in the diagram.

Geof
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Old 9th May 2009, 3:19 pm   #132
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Default Re: Roberts RT7 - problem

Firstly, the explanation to why my reading of 0.00V on the base initially might well have been down to not making sufficient contact to the chassis with the probe. On the second attempt, I had to jiggle the tip into the metal and voila! Down to me I'm afraid.

Results:T2 primary measured on 20k Ohm range
TR7 - 0.01
TR6 - 1 .00 i.e. open circuit

T2 upper secondary
TR7 - 0.01
TR6 - 0.01

R30 code: Red Violet Brown Silver = .37 (20k Ohm range)

Voltage: T2 primary; chassis to emitters -
TR7 = 0V
TR6 = 0V
Upper secondary;
TR7 = 9.23V
TR6 = 9.23V

Meter check revealed a zero reading using the resistor connected as instructed.
Duff meter range?

Attached AC128's with tab by emitter.
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Old 9th May 2009, 3:37 pm   #133
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Post Re: Roberts RT7 - problem

Quote:
Originally Posted by stonehopper View Post
Results:T2 primary measured on 20k Ohm range
TR7 - 0.01
TR6 - 1 .00 i.e. open circuit


Meter check revealed a zero reading using the resistor connected as instructed.
Duff meter range?

Attached AC128's with tab by emitter.
If TR6 - 1 .00 i.e. open circuit is an accurate reading this would indicate this winding is open circuit. Doesn't explain the overheating though.

Recheck the meter current range, these do sometimes have a fuse inside which can blow.

R30 = 270 Ohms Red Violet Brown Silver 10% tolerance

Geof
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Old 9th May 2009, 3:39 pm   #134
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Default Re: Roberts RT7 - problem

Quote:
Results:T2 primary measured on 20k Ohm range
TR7 - 0.01
TR6 - 1 .00 i.e. open circuit
Readings have to be multiplied by 1000, so you have 10 ohms and 1000??ohms respectively. Something wrong there. How does your meter indicate open circuit ie switched to 20K range with no leads connected? Is there a reason for your writing 1 .00 rather than 1.00? Try again using the 2000 and 200 ohm ranges.

Quote:
T2 upper secondary
TR7 - 0.01
TR6 - 0.01
10 ohms per winding. Looks OK check using the 200 ohm range

Quote:
R30 code: Red Violet Brown Silver = .37 (20k Ohm range)
Colour code indicates a 270 ohm resistor. At 370 ohms it's out of spec. Check on the 2000 ohm range.

Quote:
Voltage: T2 primary; chassis to emitters -
TR7 = 0V
TR6 = 0V
Looks OK, but an open circuit primary would still result in 0V readings.

Quote:
Upper secondary;
TR7 = 9.23V
TR6 = 9.23V
Looks OK.

Quote:
Meter check revealed a zero reading using the resistor connected as instructed.
Duff meter range?
Yes. If you're doing the test right that range is faulty. What do you get on the 200mA range? Some meters have an internal fuse for current ranges. Check it's OK.
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Old 9th May 2009, 3:49 pm   #135
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Post Re: Roberts RT7 - problem

Quote:
Originally Posted by Station X View Post

Colour code indicates a 270 ohm resistor. At 370 ohms it's out of spec. Check on the 2000 ohm range.
Makes no difference to the actual problem though as it is not doing anything critical.

Why not check out my circuit, it will prove one way or another if the transformer is faulty. And if it gets the radio working at reasonable volume you may even leave it that way, which is the equivalent of amore modern circuit.

Geof
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Old 9th May 2009, 3:50 pm   #136
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Default Re: Roberts RT7 - problem

Geof and I seem to be in agreement.

Look at the picture of the meter in post #93. It's set to 20K with the leads not touching. Reading is "1 .", not 1 .00.
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Old 9th May 2009, 3:57 pm   #137
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Post Re: Roberts RT7 - problem

Quote:
Originally Posted by Station X View Post
Geof and I seem to be in agreement.

Look at the picture of the meter in post #93. It's set to 20K with the leads not touching. Reading is "1 .", not 1 .00.
My homemade digital displayes the same thing with the leads not touching. Just the leading 1 and the dot.

But to measure 270 and such it needs to be on the lowest Ohms range of course

Had a job finding that post, so far back

Geof
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Old 9th May 2009, 4:04 pm   #138
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Default Re: Roberts RT7 - problem

If the reading was 1.00 rather than 1 .00 this would indicate a resistance of 1K which would indicate a faulty transformer. 1K is far too round a figure though. I'd expect something like 1.03K or similar.

Once we get confirmation of the readings from stonehopper we should know more about the state of the transformer.

----------------------------------------------------

EDIT. Just noticed that the service sheet gives resistance readings for the windings.

T1 secondary 35 ohms each half. (Measures OK)

T2 primary 2.5 ohms each half.

T2 upper secondary 0.5 ohms each half.

T2 lower secondary 0.2 ohms.

We need readings taken on the 200 ohm range.
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Old 9th May 2009, 4:53 pm   #139
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Default Re: Roberts RT7 - problem

Sorry chaps, getting lots of exercise back and forth to the shed.

Dammit! Fuse gone in the tester. No spare.

For some inexplicable reason I have been putting two zeros after the decimal point when showing open circuit. As you can see from that picture of the meter on Page5 post 93, open circuit with probes touching nothing = 1 . - not 1. or 1 .00 which is what I have been putting. Doh.

Switching to 2000 screen reads 1
Switching to 200 reads 1 . two spaces after the 1

My error, now I need a fuse.
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Old 9th May 2009, 5:14 pm   #140
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Default Re: Roberts RT7 - problem

That would appear to confirm that one half of T2 primary is open circuit. As a final check what are the readings for each half of the winding with the meter on the 200 ohm range?
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