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Old 6th Jul 2018, 3:26 pm   #161
FERNSEH
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Default Re: HMV 1804 Television.

Hi Sam,
The screen grid of the KT61 output valve is connected to a decoupled HT supply shared with the anode resistor of the DH63 audio amplifier triode.

The oscillations are of a very low frequency, about 2 to 3 Hz and are of such a high amplitude as to cause the focus of the picture to vary at the same rate.

Corrections to the circuit diagram. The output valve is a KT61 and not KT63. A 100pF capacitor is connected between pin 3 of the D63 double-diode and chassis. An essential component for the correct operation of the sound noise limiter.

DFWB.
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Old 6th Jul 2018, 3:32 pm   #162
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Default Re: HMV 1804 Television.

Does altering the AGC TC alter the frequency?

Lawrence.
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Old 6th Jul 2018, 4:19 pm   #163
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Default Re: HMV 1804 Television.

Could some of the sound IF be finding its way to the output valve?
It looks as though the 50pF cap in the left-hand cathode circuit of the double diode is intended to prevent this, but perhaps it is faulty.

Given the chance, the output valve would happily amplify high frequencies and modulate the HT rail, the electrolytic decouplers would not do a great job of bypassing such high frequencies to ground and thus the modulation would be fed back to earlier stages.

John
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Old 6th Jul 2018, 4:48 pm   #164
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Default Re: HMV 1804 Television.

Hi John,
The AVC coupling capacitor is OK, it's a good quality mica component. The cathode bias resistor of the DH63 DDT is unbypassed so there will an audio component across it. The resistor also provides the AVC delay voltage. There was no such refinement in the original HMV 1804 sound circuits, the two diode sections of the DH63 were returned to the cathode.
I reckon that the cathode resistor should be bypassed with a 50microfarad capacitor. The AVC delay voltage should be dead smooth.
I experienced a strange fault in a Murphy A36 radio, all sorts of strangenessness going on. The fault turned to be the cathode bypass capacitor of the AC/HL/DD detector and AVC valve. Used an oscilloscope to find the fault.

"Does altering the AGC TC alter the frequency?"
Hi Lawrence, I reckon the fault might well lie inn the AVC circuit and that unbypassed bias resistor. The volume control is turned up to certain point and the oscillation starts, the frequency hardly alters at all but the amplitude increases as the control is turned up further.

DFWB.

Last edited by FERNSEH; 6th Jul 2018 at 5:09 pm.
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Old 6th Jul 2018, 8:58 pm   #165
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Default Re: HMV 1804 Television.

Connected a 50microfarad capacitor across the triode amplifier's cathode bias resistor. No improvement. The 100Kohm anode load resistor has gone high value, replaced it, no improvement. I'll continue to muddle my way though the sound circuits. Not giving up on it yet.


DFWB.
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Old 6th Jul 2018, 9:43 pm   #166
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Default Re: HMV 1804 Television.

Could you try connecting the 100 ohm KT61 screen-grid resistor directly to the junction of the 820 ohm and 10k resistors, to see if that eliminates the feedback causing motor-boating at high volume levels?

Ron
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Old 6th Jul 2018, 9:49 pm   #167
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Default Re: HMV 1804 Television.

AGC positive feedback David? Can you isolate the audio amp completely and eliminate it from our inquires? John.
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Old 6th Jul 2018, 10:14 pm   #168
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Default Re: HMV 1804 Television.

Hi John,
Because the demodulator signal is below the AVC delay voltage no negative control voltage is present on the AVC line. With the Aurora connected to the receiver and the contrast control set for a well contrasted picture the AF signal across the detector load resistor is only 600mV P - P.

DFWB.
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Old 6th Jul 2018, 10:45 pm   #169
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Default Re: HMV 1804 Television.

"Could you try connecting the 100 ohm KT61 screen-grid resistor directly to the junction of the 820 ohm and 10k resistors, to see if that eliminates the feedback causing motor-boating at high volume levels?"
Hi Ron, that has been considered but it is very difficult to access the wiring around the KT61 and DH63 valveholders. Actually, it's an awful set to work on. Disconnected the grid lead to the top cap of the DH63/6Q7G and connected a 1Megohm resistor between the top cap and chassis. Result, the low frequency oscillation has ceased.
So it follows the source of the amplifier problems must be in the AF signal path between the grid of the triode and the noise limiter.

DFWB.

Last edited by FERNSEH; 6th Jul 2018 at 10:50 pm.
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Old 7th Jul 2018, 10:11 am   #170
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Default Re: HMV 1804 Television.

Hi David,
I remember having motor boating on my gec bt2147 after replacing one of the ht decoupling electrolytics. Moving the physical position of the cap helped, but I think in the end a grid stopper was the cure there too.

I agree about the 1804, it has such a deep and busy chassis. The hmv 904/905 and the 907 are also like this. Of course the later 1807 didn't have this flaw as there were no components in it

Cheers.
Andy

Last edited by beery; 7th Jul 2018 at 10:13 am. Reason: Spelling
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Old 7th Jul 2018, 10:49 am   #171
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Default Re: HMV 1804 Television.

Hi Andy,
Agreed, these old EMI sets are not very nice to work on, and this is supposed to be a hobby! 1807 is a much simpler design, got less parts and is more unreliable. How does that work out?

Some progress this morning. In my previous post I suggested that the motorboating might be caused by something in the detector and noise limiter circuits and sure enough it is.
Removed the temporary 1meg grid resistor and reconnected the top cap connector to the DDT valve. Sure enough the motorboating noise is present again. Disconnect the 0.02mfd coupling capacitor, motorboating noise gone. Reconnect the capacitor and move on to the noise limiter circuit, not much to go wrong there.

Not much left to check. Disconnected the 2.2meg resistor between pin three of the double diode and the sub HT line. Noise gone.

Standby for more news.

DFWB.
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Old 7th Jul 2018, 11:19 am   #172
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Default Re: HMV 1804 Television.

Introduced a 1.5megohm resistor between the "top" of the 2.2meg resistor and the sub HT line. Connected a 1microfarad capacitor between the junction of the two resistors and chassis. Motorboating completely gone.

As an alternative to fitting extra components the top of the 2meg resistor could be connected to the main 325 HT line. Let's try this.

DFWB.
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Old 7th Jul 2018, 11:38 am   #173
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Default Re: HMV 1804 Television.

That didn't work so it'll have to be the extra decoupling components.

DFWB.
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Old 7th Jul 2018, 5:34 pm   #174
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Default Re: HMV 1804 Television.

Finalised the modification to the noise limiter:

A 1meg resistor is inserted between the lower end of the 2meg resistor and the anode of the diode. A 1microfarad capacitor connected between the junction of the two resistors and chassis.
A 50mfd electrolytic capacitor is connected across the 1Kohm cathode bias resistor of the DH63 DDT. This gives a useful 3db audio gain and provides a steady AVC delay voltage.

An all glass D63 double-diode replaces the metal 6H6, actually marked as D63.

DFWB.
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Old 8th Jul 2018, 4:10 pm   #175
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Default Re: HMV 1804 Television.

The revised circuit diagram of the sound stages of the HMV 1804.

The vari-mu pentode type W62 IF amplifier is replaced by a Z66 high slope pentode. The Z66 replaces the Mazda SP61 used in the original circuit.
Z66 and SP61 have similar electrical characteristics.

The anode load resistor of the noise limiter diode is now 1Megohm and the original 2Meg resistor is now along with the 1mfd capacitor decoupling components. The diode anode is now supplied from the main 325 volt HT line. The motorboating fault has been completely solved by fitting these parts.

DFWB.
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Old 8th Jul 2018, 4:49 pm   #176
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Default Re: HMV 1804 Television.

Excellent work!

Jac
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Old 8th Jul 2018, 6:43 pm   #177
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Default Re: HMV 1804 Television.

Good evening Jac,
we have no idea what the reception conditions were like in the early fifties to motivate the unknown engineer to create this incredible receiver. In terms of sensitivity the receiver is as good as any mid sixties 405 line TV receiver equipped with frame grid valves.
Picture quality is good for 3Mhz.
Surely an easier solution would have been a simple two stage aerial pre-amplifier and a frequency changer to supply the set with Channel B1.

DFWB.
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Old 8th Jul 2018, 10:09 pm   #178
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Default Re: HMV 1804 Television.

Hi David
I`ve been following this thread with great interest.
I knew you would crack that sound fault in the end, well done.

Robin
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Old 9th Jul 2018, 6:31 am   #179
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Default Re: HMV 1804 Television.

Just a thought:

One thing that encourages LF oscillation is the unnecessarily low cut off frequencies of the RC coupling between stages, such as 0.02uF and 1Meg, or 0.02uF into 500k, where the roll off is below 10Hz and 20Hz.

You would probably find if the coupling cap values were altered to 0.0022 to 0.01uF the audio low frequency response would still be fine and the oscillations stop too. When the LF response in the amp is too low, then it requires much more heavy bypassing/decoupling in the HT supply to suppress the feedback and stop LF oscillations.
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Old 9th Jul 2018, 7:36 am   #180
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Default Re: HMV 1804 Television.

Respect, sir! A fascinating exercise brought to fruition.
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