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Old 13th Aug 2017, 10:31 am   #1
ValvoStef
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Default HMV 1122 Capacitor Confusion

Hi guys,

With regards to an earlier question, it was confirmed the short capacitor is 100 pf. In this case it is C12 according to the shematics it should be 180 pf. C13 located below should be 100pf but it has only 38000VE printed on it. See the attached photo. The schematics have also a component layout which confirms the top capacitor is C12 180pf and the capacitor below is C13 100 pf. The RF section looks pretty original. Both capacitors are in the oscilator circuit, the values are therefore pretty important. Any ideas?

Thanks,

Stef.
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Old 13th Aug 2017, 10:38 am   #2
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Default Re: HMV 1122 Capacitor Confusion

Maybe this photo is a bit better ...
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Old 13th Aug 2017, 11:00 am   #3
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Default Re: HMV 1122 Capacitor Confusion

Are you planning on changing these? Although covered in wax they are not wax paper capacitors and are generally reliable. Changing them will upset the set's alignment.
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Old 13th Aug 2017, 11:03 am   #4
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Default Re: HMV 1122 Capacitor Confusion

It appears the top one is short, that is why I noticed it has a different value than stated in the schematics.

Well, I should mention that the radio has no reception at all, only static noise on all three wave bands. The AF section and the power supply working fine.
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Old 13th Aug 2017, 11:12 am   #5
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Default Re: HMV 1122 Capacitor Confusion

Fair enough but these caps are only in circuit on LW and won't affect all three wavebands.
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Old 13th Aug 2017, 11:20 am   #6
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Default Re: HMV 1122 Capacitor Confusion

Did you disconnect the cap before you measured it as short circuit?
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Old 13th Aug 2017, 11:25 am   #7
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Default Re: HMV 1122 Capacitor Confusion

Quote:
Originally Posted by ValvoStef View Post
With regards to an earlier question, it was confirmed the short capacitor is 100 pf. In this case it is C12 according to the shematics it should be 180 pf. C13 located below should be 100pf but it has only 38000VE printed on it. See the attached photo. The schematics have also a component layout which confirms the top capacitor is C12 180pf and the capacitor below is C13 100 pf. The RF section looks pretty original. Both capacitors are in the oscilator circuit, the values are therefore pretty important. Any ideas?
I've looked at two separate service sheets, Trader (1025) and another one whos origin I can't identify but it looks like the one in this link:

http://www.service-data.com/product....88/3678/e12688

There appears to be a layout difference between the two...

In the Trader sheet the LW padder is C12 (180pF) and the LW trim is C13 (100pF) and in the layout the LW padder (C12) is furthest from the switch and the LW trim (C13) is nearest to the switch.

On the other service sheet the LW padder is C11 (180pF) and the LW trim is C12 (100pF) but in the layout the LW padder (C11) is nearest the switch and the LW trim (C12) is furthest from the switch.

Lawrence.
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Old 13th Aug 2017, 11:45 am   #8
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Default Re: HMV 1122 Capacitor Confusion

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Did you disconnect the cap before you measured it as short circuit?

This is good advice generally and in this case particularly as C13 is shorted out on MW and SW by S15.

Also as has been pointed out mica capacitors are very reliable and although it is not unknown for one to go short circuit it is very unusual.
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Old 13th Aug 2017, 11:47 am   #9
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Default Re: HMV 1122 Capacitor Confusion

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Did you disconnect the cap before you measured it as short circuit?
I was reluctant to ask that question!
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Old 13th Aug 2017, 4:37 pm   #10
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Default Re: HMV 1122 Capacitor Confusion

Quote:
Originally Posted by peter_sol View Post
Did you disconnect the cap before you measured it as short circuit?
No, my mistake, should have done it first. The cap is good.
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Old 13th Aug 2017, 4:39 pm   #11
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Default Re: HMV 1122 Capacitor Confusion

Quote:
Originally Posted by ms660 View Post
I've looked at two separate service sheets, Trader (1025) and another one whos origin I can't identify but it looks like the one in this link:

http://www.service-data.com/product....88/3678/e12688

There appears to be a layout difference between the two...

In the Trader sheet the LW padder is C12 (180pF) and the LW trim is C13 (100pF) and in the layout the LW padder (C12) is furthest from the switch and the LW trim (C13) is nearest to the switch.

On the other service sheet the LW padder is C11 (180pF) and the LW trim is C12 (100pF) but in the layout the LW padder (C11) is nearest the switch and the LW trim (C12) is furthest from the switch.
Thanks Lawrence, great help.
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Old 13th Aug 2017, 4:53 pm   #12
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Default Re: HMV 1122 Capacitor Confusion

No problem.

With the receiver switched to LW you can measure the resistance across the capacitors with them in circuit to verify which is which, but as has been said by others neither of those capacitors would cause failure of all three bands.

Lawrence.
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Old 13th Aug 2017, 4:54 pm   #13
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Default Re: HMV 1122 Capacitor Confusion

Quote:
Originally Posted by ValvoStef
No, my mistake, should have done it first. The cap is good.

Quote:
Originally Posted by ValvoStef View Post
....the radio has no reception at all, only static noise on all three wave bands. The AF section and the power supply working fine.
Hi Stef,

If you haven't already done so replace C21 and C24 (Trader sheet 1025). This is to protect the output stage from possible damage.

I assume you have connected a good aerial to the set.

Measure the voltages on V1 and V2 with respect to chassis, stating the valve number and pin number in each case. This may give us a clue and enable us to point you in the right direction.
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Old 14th Aug 2017, 9:19 pm   #14
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Default Re: HMV 1122 Capacitor Confusion

Hi Colin,

I will do your suggestions coming Sunday morning.

The aerial I am using is a long wire in the attic.

Stef.
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Old 20th Aug 2017, 10:16 am   #15
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Default Re: HMV 1122 Capacitor Confusion

Morning peeps,

I carry on with the fault diagnosis. As suggested, I measured the voltages on valves V1 and V2. On the service sheet the anode voltage should be 168V, I measure 233V. The anode voltage on the triode section should be 90V, I measure 81V. The anode voltage on valve V2 should be 168V, I measure 233V. The voltage before the HT 2K smoothing resistor is 293V and after the resistor it is 259V. As both valves have no cathode resistors I can only think that both valves are faulty. Any other suggestions?
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Old 20th Aug 2017, 11:28 am   #16
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Default Re: HMV 1122 Capacitor Confusion

Assuming the valve heaters are alight, check the potential divider resistors R2 and R3 it would appear the local oscillator has stalled as the valves are not conducting.
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Old 20th Aug 2017, 12:40 pm   #17
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Default Re: HMV 1122 Capacitor Confusion

Hi Stef

If I understand your figures correctly you have a 26 volt drop across the primary of both IF transformers (259v-233v) which is high, but it certainly tells us that both V1 and V2 are drawing current. The Trader sheet doesn't give DC resistance values for inductances but the ERT sheet gives the resistance of the first IF transformer primary as "very low" and that of the second as 3.25 ohms. From that you can see that a volts drop of 26 volts suggests both transformers are faulty which is unlikely but is possible if they have suffered a high current fault or corrosion. It may be prudent to recheck those measurements or simply measure the resistance of those windings with the power off.

You don't give any screen voltages for V1 and V2 which would tell us what the actual screen currents are.

Measuring voltages in the local oscillator is difficult because the meter can change things dramatically.

If you put a transistor radio near your HMV and tune it up and down the band (the HMV that is) you should hear beat notes on the transistor set if the HMV oscillator is working.

BTW do you have a signal generator?

Last edited by ukcol; 20th Aug 2017 at 12:45 pm.
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Old 20th Aug 2017, 2:44 pm   #18
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Default Re: HMV 1122 Capacitor Confusion

HT voltages seem too high, voltage on the RF side of R16 should be about 200 Volts.
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Old 20th Aug 2017, 4:16 pm   #19
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Default Re: HMV 1122 Capacitor Confusion

The usual cause of HT voltage too high is the output valve is drawing too little current. This can be confirmed by measuring the output valve V4 cathode voltage which according to trader 1025 should be 4.7V.
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Old 27th Aug 2017, 9:33 am   #20
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Default Re: HMV 1122 Capacitor Confusion

Morning all,

I carry on with the diagnosis. The voltage across R18 is 4.8 V, so the output pentode is good. Like I said before, the AF section is ok, I connected an MP3 player to the record player input and it works.
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