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Old 8th May 2016, 8:29 pm   #141
sobell1980
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Default Re: HMV 653 Broadcaster all wave.

Ok here are my findings for V4 anode .
-Tone control open circuit, no temporary resistor fitted, and C22 connected the anode current draw was variable with volume control. Max vol, anode current measured 8mA, valve was high pitch squealing. min vol, anode current draw was 33mA and the valve high pich squealing had stopped.

-Anode current draw with 240K ohm temporary resistor across tone control wires 39 mA regardless of volume control position and no high pitched squeal from the valve in any volume position.

-Anode current draw with C22 disconnected but 240K ohm temp resistor fitted on tone control wires 39 mA . No difference in current draw with volume control adjusted and no valve squealing .

-Anode current draw C22 disconnected, no temp resistor fitted for tone control 27 mA, no high pitched valve squeal and the anode current did not vary with volume control, connection of c22 back up with insulated pliers and the valve starts to squeal again and the anode current becomes immediately variable with volume control, disconnect c22 with my pliers and the noise stops and the anode current becomes stable/ non variable.

-CG voltage with C22 disconnected is -11.20 volts dc and non variable with volume control.
-CG voltage with C22 connected is then variable from -11.20 volts dc to -17.00 volts dc with the volume control . This is with the tone control open circuit.

Voltage tappings from resistor network are as follows again with tone control open circuit and c22 disconnected.
Tapping between R17 and R18 -950 mV dc
Tapping between R18 and R19 -1.88 V dc
Tapping between R19 and R20 -11.25 V dc
Tapping between R20 and centre tap -12.30 V dc

Connect C22 back up and all the above readings from the resistors become variable by 1 volt or so with volume control. As soon as C22 is connected i get a high pitched squeal from V4 which can be switched or turned down using the volume control. If i connect a resistor across the tone control wires, the squeal vanishes also .
Ive checked all my wiring back from c22 through the volume control through to R5 and R6 screen grid resistors and all associating caps and made sure wire termination is correct including the cap values etc. C19 is also reversed polarity as per the diagram. All the valves are supplied and tested from Ed Dinning recently and i have two V4's to try both doing the same. However i have fitted the low emission v4 that Ed supplied simply for testing. Can't get my head around this one. Most strange. Thanks for all the replys.
Dave.
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Old 8th May 2016, 8:36 pm   #142
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Default Re: HMV 653 Broadcaster all wave.

Lawrence, forgot to say i self calibrated the scope and set up as said. I could not get a trace whatsoever, it just flatlined. I tried different voltage settings on the scope on a x10 probe. Because it flatlined and didnt move or show any sign of a trace i also tried it on x1 also. The anode was still connected through my meter measuring anode current at the same time. Would this have effected the outcome of the scope? Many thanks,
Dave.
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Old 9th May 2016, 2:35 am   #143
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Default Re: HMV 653 Broadcaster all wave.

Are you saying that the set is stable with the tone control and capacitor disconnected?
Because with it connected it gives positive feedback so it will squeal. If you need a tone control perhaps connect it from V4 anode to chassis.
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Old 9th May 2016, 7:14 am   #144
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Default Re: HMV 653 Broadcaster all wave.

Yes the set is stable with the tone control and c22 disconnected. But even then I'm getting approx 11.00 volts negative feedback.

With c22 connected and a dummy resistor fitted across the tone control wires I end up with approx 2 volts positive on the CG.

With the dummy resistor across the tone control wires removed and c22 connected the valve squeals. Connect the dummy resistor it stops squealing with c22 still connected. Disconnecting c22 with the dummy resistor connected it stops squealing also.
When I say squealing I mean that the noise is coming from within v4 itself and not the speaker.
I'm not sure what you mean by if I want a tone control connect the anode to chassis? Won't this create a short?
Please see post #141 for what voltages I had where with regards to c22 disconnected and the dummy resistor removed and installed and the neg feedback resistor readings. Many thanks,
Dave.
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Old 9th May 2016, 7:55 am   #145
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Default Re: HMV 653 Broadcaster all wave.

I meant the tone control and capacitor as is but bottom end to chassis instead of grid.
What do you mean by "negative feedback"? 11 volts where?
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Old 9th May 2016, 8:30 am   #146
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Default Re: HMV 653 Broadcaster all wave.

These voltage and current readings seem to vary somewhat in different posts, not sure what's going on with that.

It was I who suggested disconnecting the tone control, that was based on the resistance readings obtained when that control was operated as reported in an earlier post, the idea of the fixed resistor connected in the controls place was to simulate a typical tone control setting so that fault finding could be continued.

The tone capacitor is shown as C22 on the Broadcaster sheet and C24 on the Trader sheet.

It could still be a feedback problem, the valve squealing....need to be sure that it's not the output transformer squealing, if it's the latter then that would suggest that the loudspeaker circuit needs investigating.

Just some observations.

Lawrence.
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Old 9th May 2016, 8:48 am   #147
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Default Re: HMV 653 Broadcaster all wave.

Ok I believe by feedback you mean grid bias
If the resistance measured from grid to chassis is 600ohms and according to the valve data the Anode and screen current combined is 40MA then the GB should be 24 volts and you have 11 perhaps V4 is faulty?
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Old 9th May 2016, 8:58 am   #148
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Default Re: HMV 653 Broadcaster all wave.

I'd have thought the last thing the OP needs to do at this stage is to modify the tone control circuitry. There are enough problems to sort as it is.

I'd leave it disconnected as suggested by pwdrive.

Which service sheet is the OP using?
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Old 9th May 2016, 9:49 am   #149
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Default Re: HMV 653 Broadcaster all wave.

You should (of course) have a -ve voltage on V4's control grid.

If connecting the tone control affects the control grid voltage with C22 (Trader Sheet 732 reference) - the grid coupling capacitor, then by far the most likely problem is that C24 (in series with the tone control) is leaky.

You say that C24 has been replaced. What sort of capacitor did you use, where did you get it?

One worry is that if C24 is defective then other capacitors you've fitted might be too.
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Old 9th May 2016, 10:30 am   #150
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Default Re: HMV 653 Broadcaster all wave.

Hi all
Just a thought, I had a set that had been visited by the phantom fiddler and I had a similar fault. Turned out to be that the speaker field coil was wired in series with the primary of the output transformer! This produced all kinds of noises, almost like electronic music.
If it were my set I would disconnect everything except the heater and cathode ground from V4 Then reconnect the anode, then the grid that go’s direct to HT+ At this point it should be possible to carefully touch the control grid of V4 and get a nice buzz from the speaker. If its weeping and wailing at this point then the only fault can be either the output transformer & its wiring or the HT has some kind of ripple on it (field coil wired up wrong)?
If it works ok then the fault lies in what remains disconnected.
Paul.
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Old 9th May 2016, 9:16 pm   #151
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Default Re: HMV 653 Broadcaster all wave.

Many thanks for everyones input to my ongoing issues with this set. I have a very busy working week ahead, so I will have very little time to spend on it. I will try and grab 30mins here and there when I can and get back to your replies accordingly. Please don't think I'm being rude or have lost interest, it's purely lack of time. I will keep reading your valued posts and running through the scenarios in my head. I also have the circuit saved to my phone for analysis on my lunch at work so I can relate to what you mention. I will be in touch as soon as I have some available time. Many thanks.
Dave.

Last edited by sobell1980; 9th May 2016 at 9:28 pm.
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Old 9th May 2016, 9:22 pm   #152
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Default Re: HMV 653 Broadcaster all wave.

I look forward to hearing more. I want to know what the fault is, as it is really puzzling me.

As I see it, if connecting a capacitor between 2 points in the circuit (which is what the tone control does, effectively) alters the DC conditions (such as the grid bias) then either

The capacitor is leaking and allowing some DC current to flow through it

or

There are some AC voltages in the circuit, perhaps the output valve is oscillating.

The first is the common problem and worth eliminating first. The second could prove interesting especally if that well-known amateur M0RON has been amongst it.
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Old 11th May 2016, 8:34 am   #153
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Default Re: HMV 653 Broadcaster all wave.

Just been reading through the posts again, the OP (Dave) expressed his concern about the connections of the audio output transformer, specifically the secondary circuit (Post#134)

It might be worth re visiting that area, a quick test would be for him to connect a meter in series with one of the feed connections from the transformer secondary to the loudspeaker, it should measure no more than a few ohms.

Lawrence.
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Old 17th May 2016, 8:36 pm   #154
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Default Re: HMV 653 Broadcaster all wave.

Being a technician in the motor trade I've found that when I'm straying off on a tangent and you are baffled, confused and not making progress, going round in circles, one of two things have occurred. One, I have completely misdiagnosed it or two, something simpler has been missed or overlooked, assumed to be ok.
Having a week to dwell on this away from staring at the set and no time to really work on it I thought, I'm getting carried away here, something is wrong such as my wiring, something simple. It shouldn't be this hard. So I took a few moments to take a look at the hard facts of what was happening . From these facts logically a diagnosis must be able to be made. I also felt I was giving you chaps different readings each time i took measements. Strangely the longer the set was left on the more the voltage dropped on the anodes and CG. So that explains the different readings.
So, armed with what Lawrence and Tony have gone through with me I went back to basics. The HT and CG varied wildly with tone control . As you pointed out a short in the tone control or a failed C24 were the areas of suspicion. I assured you that C24 was new and of correct value, which it is and connected correctly. Ive gone back to C24 and snipped it out of circuit to test on my DMM. Sure as damn it the new C24 had failed. Testing C24 on resistance range it measured around 20K ohms, the rest of the same caps not used in the sealed bag all tested on my DMM as open circuit or as my meter displays O.L for off limits. So it would appear it is shorted internally. This explains the tone control varying the HT as it's getting through C24 as you rightly said. I haven't yet had chance to put back a new one and try it yet but I'm sure that this one testing as a steady resistance and all the other new ones test open circuit this is my main fault. There is lots to put back now from testing that I've snipped out etc so I need to concentrate on getting these bits back together before I move onto the dreaded OPT.
I need to refit the tone control and remove the dummy resistor fitted across its wiring before I move onto the speaker fault.
Regarding this and the OPT, I'm sure this extra secondary winding needs to be wired in as per the HMV 653 sheet. The extra winding is shown on this sheet and not on the trader sheet. I'm sure I remember that one speaker wire was connected to a tag strip and the lower half of the tag strip connected to chassis ground along with one of the secondary windings, which one I'm unsure. If anyone can shed some light on this extra secondary?. It is mentioned as a bug bear of mine much earlier in this thread and I'm sure if it were connected the speaker will work and will also bring the voltage readings in line as the set will the be drawing the correct current with the speaker working correctly. I shall try and get C24 changed tomorrow and tone control refitted and report back. I'm sure this will solve the HT issue varying with tone control.
Anyway thought I'd keep you up to date. Many thanks.
Dave.
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Old 17th May 2016, 9:15 pm   #155
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Default Re: HMV 653 Broadcaster all wave.

It's unusual for a capacitor to be leaking from new but not unheard of, presumably it was of a high enough working voltage, I used to use ones rated at 1,000VDC for ones in the audio output anode circuit.

Make sure you have full continuity for the output transformer secondary circuit before powering up.

Good luck.

Lawrence.
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Old 17th May 2016, 9:47 pm   #156
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Default Re: HMV 653 Broadcaster all wave.

The cap is rated at 1kv. If im honest it may have got too hot when being soldered or if I was soldering near to it perhaps. Either way it tests as failed. I remember that due to its location it was difficult to get a crock clip on it to act as a heat sink. School boy error I think Lawrence.
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Old 17th May 2016, 9:50 pm   #157
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Default Re: HMV 653 Broadcaster all wave.

I don't bother with heatsinks, modern devices can usually cope with normal soldering temps/times.

What power iron are you using, Dave?
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Old 18th May 2016, 6:27 am   #158
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Default Re: HMV 653 Broadcaster all wave.

My electric soldering iron had stopped working and I have been saving to buy a proper soldering station from maplins. Because I was keen to get the set finished I've been using my gas soldering iron that I use at work on cars. It probably wasn't the heat of the tip that caused the issue but more the heat given off by the burning red hot catalyst on it whilst I may have been soldering something else in that area. You have to make sure the catalyst is facing away from damaging other components which is easier said than done on a radio compared to the huge working free space on a vehicle. Lesson learned. So I'm sure this is what's happened. Does every one agree that the suspect cap should not be reading a steady 20 k ohms? All the other new caps read open circuit as the meter quickly climbs through its range to open circuit.
Dreaded OPT next.
Many thanks.
Dave.
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Old 18th May 2016, 8:20 am   #159
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Default Re: HMV 653 Broadcaster all wave.

Correct. A good capacitor should read open-circuit or 'infinity' on an analogue ohmmeter, or "OL" or whatever indicates infinite resistance on a DMM. It's conceivable that your gas iron has caused the damage.

I have a spare electric soldering iron, it's only a basic 25 watt job but you are welcome to have it for the cost of postage. PM me for further details.
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Old 18th May 2016, 8:21 am   #160
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Default Re: HMV 653 Broadcaster all wave.

Ah, that's interesting.

If money's tight, an Antex XS25 is a very good buy: http://www.my-tool-shed.co.uk/p10930...FdW4GwodEi0C7Q

Yes, 20K ohms indicates that the insulation in the cap has broken down.

Good luck with it, Dave,

Nick.

Edit: Crossed with Phil
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