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Old 4th Mar 2016, 6:54 pm   #101
sobell1980
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Default Re: HMV 653 Broadcaster all wave.

RE screened cable replacement.
I purchased some minature single core screened cable.please see link, if anyone can advise if this suitable? I bought two metres. There are only two screened cables I need to replace. They are located between C18 and R10 and then the other side of C18 to the pick up socket. I'm expecting that these signals and voltage here are minimal? So cable voltage rating is not critical? The gauge wire on the new screened cable is tiny and much smaller than single core I snipped out. Please see photos. Probably me being over cautious. Many thanks.
Dave
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Old 4th Mar 2016, 7:00 pm   #102
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Default Re: HMV 653 Broadcaster all wave.

That looks fine to me, no voltage worries.

Lawrence.
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Old 4th Mar 2016, 7:02 pm   #103
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Default Re: HMV 653 Broadcaster all wave.

Screen shots of new screened audio cable specs. Unable to post a direct link from my phone.
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Old 4th Mar 2016, 9:04 pm   #104
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Default Re: HMV 653 Broadcaster all wave.

The capacitance is 320pf per metre, the capacitance of the one in the link I posted in post#92 is 75pf per metre.

Will it make any difference? Only on the higher frequencies but if it's just a run of a few inches probably not a great deal.

Just for reference: If you know the capacitance per metre of a cable, the source impedance, and need to work out the roll off frequency for a particular length of that cable then this handy calculator can save some brain work:

http://www.sengpielaudio.com/calculator-cable.htm

Lawrence.
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Old 6th Mar 2016, 1:10 pm   #105
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Default Re: HMV 653 Broadcaster all wave.

RE my post #101 and the screened cable I purchased. Could anyone tell me approximately what voltages I should be seeing on the two screened cables. I guess the one cable from the pick up socket to C18 I won't expect to see anything unless I have something plugged in! But from the other side of C18 to volume control R10 ? If it's attatched to the pick up socket I'm thinking millivolt? Such as the output voltage from a record player cartridge? Before I posted my post #101, I emailed maplins for working voltage of the cable I'd bought. Please see screenshot. I accept what you are saying but I wanted to get an Idea in my head so I don't have to keep asking what voltages to expect where. It's not like I can measure on this set yet as I'm not currently able to switch it on. It will just help my learning and confidence in future repairs. I'm thinking it's just audio signal the same as put out from a record player cartridge before being amplified. Many thanks.
Dave.
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Old 6th Mar 2016, 2:39 pm   #106
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Default Re: HMV 653 Broadcaster all wave.

Will be getting some time on the set this afternoon and hopefully get this screened audio cable soldered in. If anyone could help to put my mind at rest on this I would be most grateful. Sorry if I'm being a bug bear on this .
Many thanks.
Dave.
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Old 6th Mar 2016, 3:26 pm   #107
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Default Re: HMV 653 Broadcaster all wave.

The insulation in the screened cable(s) will only be subjected to low signal voltages and a low level of DC via the volume control and the bias network, it'll be ok.

Just a tip, don't change too many components without testing the receiver in between as it can often lead to prolonged diagnostic problems if a fault has been inadvertently introduced, just change the usual suspects that might cause damage if left in circuit.

Lawrence.
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Old 6th Mar 2016, 3:42 pm   #108
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Default Re: HMV 653 Broadcaster all wave.

Thankyou Lawrence for your help once again. Sadly the set was in a very bad way. I rescued it from a farm and was stored in a barn along side other artefacts. The wiring was falling apart and the insulation had literally fallen off the wires all over the set. What looked good when handled broke up and crumbled away. So I didn't dare just change the usual caps and switch on. It had also been got at with the electrolytic main block removed and substitutes botched in the wrong places and exposed shorted wiring. I've had this set a few years. I didn't feel confident to tackle it till now. It has remained in my dry warm workshop since, hopefully that will have done it some good too. So unfortunately, it has had to have a rewire. Taking my time. It will be a beautiful set when it's finished. I'm going to refit a new block, housing the new electrolytics where it originally would have been on top of the chassis running the wires through and underneath. The old ones were just hanging on their wires and had been poked in under the chassis when it had been refitted back to the cabinet. This set deserves to live again.
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Old 6th Mar 2016, 3:47 pm   #109
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Default Re: HMV 653 Broadcaster all wave.

Ok, I see.

If you keep going careful then all should be alright, it's good it's had lot's of time to dry out, sometimes damp can penetrate into IF/RF coils etc and cause tuning/performance problems.

Lawrence.
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Old 30th Apr 2016, 6:57 pm   #110
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Default Re: HMV 653 Broadcaster all wave.

Hi all,
well believe it or not progress has been made slow but sure over the last few weeks. I have been carefully and methodically carrying out the rewire on this set followed by a recap. The only bits i have not yet done is the aerial tuning circuit side of things such as the caps in and around the wavechange switch and tuning coils. However this area looks unmolested and the wiring safe, so for now im leaving this side of things alone as it is now at a point i can apply power safely now the wiring has been done.

I applied power this morning, panel bulb lit up, all the valve heaters/ filaments could seen to be glowing but no sound. Due to the state this set was in i didn't expect it to work straight off like when carrying out a standard recap. I was pleased with the progress i've made at the very least.

So, now to find out why the set was getting power but no sound of any description. Not even a pop from the speaker when the wavechange switch was rotated or the usual rustle from noisy volume and tone pots. First thing i did was to carry out the basic HT and LT checks to the valves. All the valves are receiving approx 6.6 volts to their filaments which is i know a little high as it should be approx 6.3 volts ac. I'm not sure if this difference is because i'm using a DMM and not an analogue Avo meter. But this should be ok for now to test the set briefly to see how things are working. I could later on fit a resistor perhaps to bring this voltage into spec, but I may find this will come down by itself once the set is fully functional.

This is where things get intersting. I carried out my voltage checks of all the valves anodes. These were again checked with my DMM with negative to chassis as quoted in the service data. Here are my findings.
Valve 1 anode 375 volts DC
Valve 2 anode 375 volts DC
valve 3 anode 127 volts DC
Valve 4 anode 375 volts DC
V5 anodes x2 350 volts ac

The only sensible reading is valves 3 and 5.

I also noticed that the output valve 4 was extremely hot, too hot to touch. This is my first dealings with these types of valve so not sure if this is normal. This was after a couple of minutes switch on for testing i noticed this.

After this i tested to see if Valve 1 is oscillating . I checked this with my scope at where R1 and R2 meets the valve pin connection. I had a good strong trace that shortened and lengthened as i adjusted the tuning (c31) on MW and LW. I couldn't get a trace on SW as i don't think my scopes parameters can check this unless someone can tell me otherwise? But a definate strong trace on two wave bands.

I then checked the speaker with my signal generator. Connected the positive and negative leads of the sig gen to my respective speaker connections and a quiet tone can be heard through the speaker with the set switched on. I'm assuming it is only a quiet tone i can hear due to the fact the attenuation is millivolts and I am directly connected across the speaker with no valves amplifying the sig gens tone, although the set is switched on to energise the speaker field coil.
I then moved my sig gens connections back to the anode of valve 4 to test through the OPT. Nothing can be heard whatsoever. This is where i have always had my concerns with this project and this mystery extra second set of secondary from the OPT that is not connected to anywhere. Please read posts under discussion of this very problem i had identifying the OPT connections on pages 3 and 4 of this thread. I am sure this is where the main problem lies and why not even touching the speaker terminals with the set switched on can i get a pop out the speaker. Please remember testing the speaker with my sig gen and the set switched on shows it is working with the faint tone that is heard.
Do you think the high anode voltages are related to this too? The only anode valve reading that makes sense is V3 which is run through it's own resistor R11. All the other HT voltages for the anodes are achieved through transformers. All the windings of the OPT have been tested with my DMM for resistance readings as discussed previously in pages 3 and 4. Is it possible i have the primary of the OPT connected the wrong way round, such as the each side of the primary reversed or would this not matter? I'm worried i'm going to destroy the valves if left switched on too long with this HT being too high. Any advice or help greatly received. Perhaps i need to try connecting the extra secondary up to the speaker instead? For reference i'm using the Marconiphone trader sheet, i find it much easier to follow. It is trader sheet 732. I hope i'm looking in the right area. Im sure the experts here will give me something to go on.
Many thanks,
Dave.

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Old 30th Apr 2016, 7:42 pm   #111
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Default Re: HMV 653 Broadcaster all wave.

One thing I notice about this radio is that it derives its bias voltages from a chain of resistors connected in series with the HT supply. It would be a good idea to check control grid voltages, especially on V4. While you're at it check the screen grid voltages too.

Also note that C19 (Trader Sheet) has to be connected POSITIVE to chassis.
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Old 30th Apr 2016, 7:47 pm   #112
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Default Re: HMV 653 Broadcaster all wave.

Related to that, the -ve side of capacitor C25 (Trader Sheet 732 numbering) is not connected to the chassis and is not common with other capacitors like C26. If you've made a mistake and grounded the -ve side of C25 then there will be no bias. The output valve would get very hot. The HT voltage would also probably appear higher than normal as the output from the transformer/rectifier is the sum of the bias voltage and HT voltage.
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Old 30th Apr 2016, 8:11 pm   #113
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Default Re: HMV 653 Broadcaster all wave.

Just a note which might or might not help

Looking at the data sheet for the output valve the curves seem to indicate a -ve bias voltage on V4's control grid of around 17 volts -ve with respect to cathode for the given anode current shown on the Trader sheet.

Lawrence.
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Old 30th Apr 2016, 9:15 pm   #114
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Default Re: HMV 653 Broadcaster all wave.

Thankyou for the continued interest and your replies. I will take the measurements you describe and go back over my HT circuit. I know for sure C19 is wired in with positive to ground as I double checked this as I thought it was strange. C25 has its negative to the centre tap on the mains transformer for the V5 anodes. It is not wired to chassis ground. I will double go over everything again.
Could a failed R17,18,19,20 ,21 cause HT to go high? Resistors normally go high in value which I imagine would give a low HT. Getting a bit late now so i will check in the morning. Many thanks.
Dave.
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Old 1st May 2016, 12:24 pm   #115
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Default Re: HMV 653 Broadcaster all wave.

Just a note, having read through the posts again it appears that the loudspeaker is working in some sort of fashion but as to how efficiently is hard to say as it relies on the energizing coil which in turn relies on the standing current drawn by the receiver.

The anode of V4 has already been checked and although reported as high it's still a voltage that would allow V4 to function, I would check V4's screen voltage, the control grid -ve voltage and the cathode voltage at the valve base, the latter is just to be sure that the cathode is connected to the chassis, the latter should be 0 volts.

If the voltages approach anywhere something like I would then feed a signal from the AF sig gen to the control grid of V4 via a capacitor, eg: .047uf, if the AF tone is produced from the loudspeaker then try and assess it's loudness, eg: dismally feint, feint (ish), reasonable, loud etc.

If the bias voltage developed across the main bias resistor (R21 Trader sheet) is low, eg: noticeably lower than around 17 volts -ve with respect to chassis then it would be worth checking the anode current of V4 just to try and confirm what's what.

Good luck.

Lawrence.

Last edited by ms660; 1st May 2016 at 12:25 pm. Reason: duplication
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Old 1st May 2016, 2:38 pm   #116
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Default Re: HMV 653 Broadcaster all wave.

I have gone back over my measurements this time around and allowed the set to stay on longer to allow the readings to stabilise, they have all dropped down but still high.

V1 anode 300 V DC
V2 anode 300 v DC
V3 anode 110 V DC
V4 anode 275 V DC
V5 anodes 350 V AC to each anode.

I have never had to measure the control grid voltages and they are not normally on the service sheets, so not sure entirely what to expect but here they are.

V1 CG -350 MV DC
V2 CG 1.60 V DC
V3 CG -571 MV DC
V4 CG -16MV DC

V1 SG 83 V DC
V2 SG 81 V DC
V4 SG 268 V DC

I have measured the bias voltage across R21 to chassis ground. Max reading im getting is -14.00 volts. This is with max volume, it decreases with volume and tone adjustment.

C25, to clear up any confusion i wired from between the HT spur off the mains transformer and L17. I grounded it at the centre tap for V5 anode supply from the mains transformer. There is then a wire linking this centre tap and the negative of C25 to to the point between R20 and R21, (so at the point of R21 before R21 is chassis grounded).

I have also tried wiring up the speaker to the mystery extra spare secondary on the OPT but still nothing. With the set switched on and my sig gen connected to the anode of V4, no tone can be heard through the primary of the OPT . If i connect my sig gen across the speaker it is dismally faint. I wish i could get to the bottom of this extra secondary winding of the OPT. I can try feeding in my sig gen through the CG of V4 but i dont think i will hear a thing as i've tried this from the anode of V4 through the OPT and the signal is lost. Just to be sure my connections are correct on my sig gen, neg was to chassis and my positive sig gen lead was to V4 anode.

Perhaps like you say we need to see what L17 is drawing?
Many thanks for your help.
Dave.
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Old 1st May 2016, 2:51 pm   #117
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Default Re: HMV 653 Broadcaster all wave.

As I understand the circuit, V4's control grid should be several volts -ve with respect to the chassis. The fact that it isn't would explain why V4 is gettng very hot.

I also find it curious that the bias voltage is affected by the setting of the tone control. I can think of 2 possible causes off the top of my head (there could well be others), the likely one being that C24 is leaking. The other is that the output stage (V4) is oscillating above audio frequency (so you can't here it), but that is not likely.

I would start by working out why there is no -ve bias on V4. Are the resistors R16-R21 good? Is there a short to chassis in the V4 control grid circuit, perhaps C23?
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Old 1st May 2016, 3:12 pm   #118
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Default Re: HMV 653 Broadcaster all wave.

Check out what TD say's, if no joy then measure the anode current of V4 and report so various aspects can be confirmed/eliminated, Trader sheet say's 34ma.

Just a word of note/precaution, don't power up the receiver with the loudspeakers voice coil disconnected.

-ve 14 volts across R21 is in the ball park I would say.

Lawrence.
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Old 1st May 2016, 9:12 pm   #119
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Default Re: HMV 653 Broadcaster all wave.

Many thanks Tony and lawrence for your guidance on this. I will go check all the bias resistors shortly and report back. Other than experience, how do you know that V4 bias should be negative by several volts? or what it should be on any other valve for that matter? Just so i can learn and add it to part of my learning curve to aid my future diagnosis. I'm here to learn about vintage radio, not for a quick fix. I fully understand why valves are biased but not sure of what voltages to expect other than what is qoted in servie data. I know what to expect on the high and low tension side of the valves but not control or screen grids. I guess this is another new bit for me to learn on how the negative voltage bias is developed and where on valve circuits so it becomes routine for me to know what to measure and where. I've never experienced a fault really with resistors or biasing arrangements. If i could ask you to have the patience to talk me through it that would be great. I will also take the time to read my reference books, but, i find you can beat the knowledge and how things are explained by yourselves on here.
Right, off to take some readings.
Many thanks.
Dave.
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Old 1st May 2016, 10:04 pm   #120
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Default Re: HMV 653 Broadcaster all wave.

Ok ive snipped one end of each resistor to make the measurements on each resistor for developing the bias. They are all higher then quoted by the trader sheet but bare in mind I am using a DMM and not an Avo.

R16 314.00 k ohms should be 230 k ohms
R17 122.20 K ohms should be 100 k ohms
R18 116.10 K ohms should be 100 k ohms
R19 1.284 M ohms should be 1 M ohm
R20 120.4 K ohms should be 100 k ohms
R21 270 ohms, Should be 270 ohms.

C23 has been replaced for a new cap and ive double checked it's value all ok and wired in correctly from R16 and then to chassis ground, a link then taken from c23 (not the ground side) to between R19 and R20.
C24 is also new and not shorted.

On my first power up i could hear a very, very high pitched tone as things warmed up. You mentioned V4 going into inaudible oscillation? this tone could be made to dissapear or be brought back with the tone control . However the set only did this once on first power up. It was ver faint, not loud and as high pitched as say tinnitus is may be higher. Just thought this worth a mention but now no longer does this.

I have seen sets work before with resistors that have gone high and i have left them be, as they did have very large tolerances in the day. I guess though however this depends on their application in the circuit. Would these being high as they are for the bias arrangement cause an issue? Some of them have gone well over 10% tolerance especially R16. What sort of effect would these high resistors have on the bias arrangement? Would it be the symptoms i'm seeing now?

Surely as well if the OPT is not working as i suspect it isn't then this could also cause V4 to overheat? This is why you shouldnt run a set with a speaker etc disconnected. Anyway, these are my findings this evening. Many thanks as always,
Dave.

Last edited by sobell1980; 1st May 2016 at 10:27 pm.
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