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Old 21st Dec 2009, 10:24 pm   #181
oldticktock
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Default Re: Newb Stumped on a Derwent

updated sketch with OP TX to pin 2, OP TX wire to ext speaker wire and main electolytics values
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Old 21st Dec 2009, 10:27 pm   #182
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Default Re: Newb Stumped on a Derwent

Graham, don't forget the 75 is biased off - this is why the set works when the cathodes are earthed. We really need the wiring from pins 3 and 4 and the TC of the 75.

The set might work if the 75 was auto-biased but without looking at the circuit I can't tell what the AGC is doing or if the detector is referenced to chassis or cathode. Try disconnecting the wire at pin5 of the 75 and running a 1.5K resistor from pin 5 to chassis with a .1uF in parallel.

OTT can you measure the voltage on the top cap of the 75.
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Old 21st Dec 2009, 10:36 pm   #183
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Default Re: Newb Stumped on a Derwent

PJL. I'm working from the speaker backwards hoping to get as far as the volume control which I guess is on the IF side of the triode section of the 75. By all means talk OTT through the auto biasing of the 75. It'll give me a break.

OTT. Does the set still have an energised speaker or has it been replaced with a permanent magnet one?
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Old 21st Dec 2009, 10:59 pm   #184
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Default Re: Newb Stumped on a Derwent

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PJL. I'm working from the speaker backwards hoping to get as far as the volume control which I guess is on the IF side of the triode section of the 75. By all means talk OTT through the auto biasing of the 75. It'll give me a break.

OTT. Does the set still have an energised speaker or has it been replaced with a permanent magnet one?
I think energised see picture attached
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Old 21st Dec 2009, 11:06 pm   #185
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Default Re: Newb Stumped on a Derwent

75
pin 3 & 4 go into a silver can which contains a coil TC goes to on/off/vol pot
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Old 21st Dec 2009, 11:11 pm   #186
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Default Re: Newb Stumped on a Derwent

That's energised all right.

Next question. The large picture you sent me shows a wire that runs from the centre tag of the volume control, across the chassis and then disappears through a grommet to the top of the chassis. Where does this wire go to? I suspect it goes to the top cap (grid)of the 75. If this isn't the case can you tell me where the wire goes go to and what components are connected to the top cap.
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Old 21st Dec 2009, 11:11 pm   #187
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Default Re: Newb Stumped on a Derwent

top view of chassis
view of can that the 75 pin 3,4 go to
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Old 21st Dec 2009, 11:12 pm   #188
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Default Re: Newb Stumped on a Derwent

I think you've already answered my question.
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Old 21st Dec 2009, 11:13 pm   #189
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Default Re: Newb Stumped on a Derwent

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That's energised all right.

Next question. The large picture you sent me shows a wire that runs from the centre tag of the volume control, across the chassis and then disappears through a grommet to the top of the chassis. Where does this wire go to? I suspect it goes to the top cap (grid)of the 75. If this isn't the case can you tell me where the wire goes go to and what components are connected to the top cap.
That's correct regarding the wire
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Old 21st Dec 2009, 11:53 pm   #190
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Default Re: Newb Stumped on a Derwent

I'll leave you to it Graham. Looks like the wavechange switch is involved too! There seems to be a wire running from the 75 cathode into the IF can which goes to the diode load 500K with parallel IF bypass cap. A 100K then runs off to the wavechange switch with the audio. Auto-bias should work I think but I can;t see the AGC.

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Old 22nd Dec 2009, 12:03 am   #191
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Default Re: Newb Stumped on a Derwent

Here's what I've got so far based on OTT's sketches and pictures of the chassis. Sorry about the poor quality.

Some of it doesn't look right to me.

OTT is the third terminal on the volume pot earthed or unconnected?

Splltting the bias is probably a good idea, as it will enable us to determine whether the 75 is drawing any current. We can talk about that tomorrow.

Given that OOT is mesuring voltages with a valve voltmeter with an earth probe he can only measure voltages with respect to chassis, so cathode to grid measurements are out.
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Old 22nd Dec 2009, 12:43 am   #192
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Default Re: Newb Stumped on a Derwent

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Originally Posted by Station X View Post
Here's what I've got so far based on OTT's sketches and pictures of the chassis. Sorry about the poor quality.

Some of it doesn't look right to me.

OTT is the third terminal on the volume pot earthed or unconnected?

Splltting the bias is probably a good idea, as it will enable us to determine whether the 75 is drawing any current. We can talk about that tomorrow.

Given that OOT is mesuring voltages with a valve voltmeter with an earth probe he can only measure voltages with respect to chassis, so cathode to grid measurements are out.
Vol pot
1 connected to earth
1 connected to 75 TC
1 connected to wave change

My avo 7 is working don't know what was going on with it lastnight but its registering on DC readings again.

Need to get my head down, lastnight I was dreaming this as well .......
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Old 22nd Dec 2009, 10:03 am   #193
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Default Re: Newb Stumped on a Derwent

I've posted a slightly better sketch of the circuit in post #191.

OTT can you confirm that the circuit is correct, particularly the part round the 75 and the volume control.

Does this set have a Pick Up (PU) socket and a GRAM position on the wave change switch. If so this would be a useful point to inject a test signal.
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Old 22nd Dec 2009, 10:26 am   #194
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Default Re: Newb Stumped on a Derwent

Hi Graham,

Not wanting to distract you on your excellent work on this but is the capacitor connected around the anode to cathode of the triode correct. Surely this should be an rf decoupler going from anode to ground.

Mike
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Old 22nd Dec 2009, 10:29 am   #195
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Default Re: Newb Stumped on a Derwent

Indeed it should be, but at present it's connected anode to cathode. Probably a hangover from the time when the cathode was earthed and the valves had a separate bias supply to the grids.

I intend to get OOT to split the bias and the cap will then be taken to chassis.
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Old 22nd Dec 2009, 10:43 am   #196
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Default Re: Newb Stumped on a Derwent

Morning Chaps,

Once more into the breach......................

Station X

I fed a signal into the PU sound was quite loud and clear but also suffers fade off.

Fed same signal into VOL pot less loud and same fade off.

Switched off tried again same scenario, I can also hear whats sound like a kettle whistling in the background this goes after a while. (sorry for the lame descriptions)

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Old 22nd Dec 2009, 10:51 am   #197
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Default Re: Newb Stumped on a Derwent

Given that the 75 requires about -2V of bias and it has about -15V of bias it's not surprising there are problems.

I'll work out how to split the bias, but can you confirm that the wiring round the 75 is as per my latest diagram.
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Old 22nd Dec 2009, 11:00 am   #198
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Default Re: Newb Stumped on a Derwent

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Given that the 75 requires about -2V of bias and it has about -15V of bias it's not surprising there are problems.

I'll work out how to split the bias, but can you confirm that the wiring round the 75 is as per my latest diagram.
Diagram is correct although the 0.033 you show reads on the actual component as .0033/1500vdc
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Old 22nd Dec 2009, 11:01 am   #199
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Default Re: Newb Stumped on a Derwent

The fade off is caused by the 42 output valve. On start, there would be 0V on the cathode but as the output valve warms up it will start conducting and the voltage will rise. As the other valves in the set are smaller they heat up more quickly and whilst the voltage is < 5V say, the 75 is conducting and the set works (of a sort). However, the 42 continues to warm up and conduct more current eventually stopping any current flow in the 75, stabilising at 15V on the cathode.
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Old 22nd Dec 2009, 11:07 am   #200
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Default Re: Newb Stumped on a Derwent

Agreed. I'm working out details for splitting the bias now. The circuit is easy, but modifying the existing wiring is a bit more difficult.
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