HMV 653 Broadcaster all wave.

[PJL]

HT voltage too high and V4 anode current too high resulting in unstable output stage that could be responsible for capacitor damage from flyback.

You need to get the HT voltage and current right before the set can be left powered up to do any other diagnosis.

There are several possibilities:
1. The readings are wrong
2. The field coil is not wired in correctly
3. The field coil is the wrong value (1710 ohms) or is arching internally

As the set has been rewired, I suggest you start by checking your wiring on the speaker assembly. Then checking the resistance of the field coil by measuring between the electrolytics C25+ and C28+ terminals (Broadcaster).

If this is all OK, connect your meter between C25+ and C28+ and power up (be careful as the meter will be at 300+V). The voltage should start rising after a minute to around 100V. If it fails to start rising after a couple of minutes then switch off.

[PJL]

The hum-bucking coil referred to in the trader sheet is a winding on the speaker voice coil and has nothing to do with the output transformer.

The field coil serves two purposes:
1. The DC current creates the magnetic force for the speaker to work
2. The inductance forms a choke used to reduce ripple on the smoothing capacitor

The problem is that the 100Hz AC current component in the field coil also modulates the magnetic force which induces current in the voice coil and deflects the speaker cone resulting in hum. To overcome this speaker designers added a few turns on the speaker voice coil wired in series with the field coil so the effect of the modulated magnetic force could be balanced out.

[sobell1980]

Thankyou PJL for your reply. Your input is greatly appreciated. I still have some tidying up to do on the set before I can power the set back up and take some sensible readings again. I have the tone control and C24 to replace/ refit from the previous testing scenarios. Life is busy at the moment with work etc so I'm only getting 10 mins here and there which is very frustrating. I'm also needing to tidy my bench and kit up. I get in a frenzy with it trying to fault find. Nothing got put away and I really dislike working in chaos. The speaker field coil resistance has previously been measured in post #36 and the lengthy thread is full of various measurements and how the field coil / HT connections have been made . It's a long read but may be worth while having a read of what I haven't and have done. I have done this myself too so I'm not going round in circles. This set was very badly got at. I think I have read your valued posts on anot her thread involving a murphy with hum. I found the that thread useful being a similar set up to my set with the valve biasing. I don't think we can go any further till I've put the set back to as is and power up again.
Regarding the OPT. The HMV sheet definately shows four wiring connections coming from the secondary of the OPT. The trader sheet does not show this but only two connections. Please see earlier in the thread about the opt resistance measurements I made out of circuit regarding the two secondaries I have. Hopefully I can get the set tidied up and put back as is this coming week and move on . Any further help, thoughts or comments greatly appreciated.
Kind regards.
DAVE.

[sobell1980]

Picture of hmv sheet OPT connections. I think I need to definately connect both secondaries up.
Many thanks.
Dave.

[peter_sol]

There are only 2 connections shown to the secondary of the output transformer.

[sobell1980]

What I'm probably trying to get across badly is that on the picture in post #164 is that there is a connection shown across each end of the secondary winding to earth. This is not shown on the trader sheet and probably this is how I should be connecting my extra pair of wires on the secondary of the OPT. This extra winding / pair of wires is currently disconnected out of circuit as advised previously in this thread. The 0.5 ohm winding is connected to the loudspeaker.
Kind regards,
Dave.

[ms660]

The secondary winding in that schematic has a tap, only one end of the winding and the tap is shown connected to anything in that schematic, one end of that winding, the top end, connects to chassis, one of the external loudspeaker sockets and to one side of the loudspeakers voice coil via a hum cancelling coil.

The tap is connected to the other side of the loudspeakers voice coil and the other external loudspeaker socket.

The bottom end of the winding isn't connected to anything according to that schematic.

Lawrence.

[PJL]

The circuit diagram is not very helpful, may be inaccurate, and does not relate to your loudspeaker. I have drawn around the output transformer and loudspeaker in the attached.

The voltage and current readings you have given suggest that the field coil is not in circuit. You should not power this radio up until you have checked the wiring. If you want us to check the wiring then we night be able to do this if you post pictures here.

[Station X]

In my opinion things are shown much more clearly in The Trader sheet for this radio,

[sobell1980]

Yes I agree, Graham . That's what I have been using for the repair. The only reason I have used the HMV sheet is because it shows the secondary of the OPT transformer connected to earth, whereas the tradar sheet shows no such connection, I thought thus may explain the 4 wires coming from the secondary of the OPT. However testing continuity of the secondary shows that there is only continuity on two pairs. There is no resistance reading between each pair or combination of pairs to show,indicate where the secondary will then be earthed to chassis and connections to speaker. I hope I've explained myself clearly. I think my previous measurements for the OPT are in post #36. If you read further on also earlier in the thread my speaker and speaker field connections do not corrolate to either service sheet and it was agreed at that time to leave the lowest value secondary disconnected. Many thanks for your reply Lawrence and PJL. And for labelling the drawing for me. I will get some proper time on this Sunday and Monday so will post my HT measurements through the field coil then.
Many thanks,
Dave.

[sobell1980]

Success!
Now receiving all 3 wave bands loud and clear.
C24 had failed short circuit causing the tone control to vary the HT. I have since replaced this. I now have approx 11.50 volts neg on V4 control grid.
Then I removed the OPT again. Some how I'd wired the secondary in wrong. This OPT has two secondaries, so four wires and two pairs giving different readings. One pair 0.5R the other 0.1R. There were no readings when combinations of the pairs were checked. What I had done without realising was solder in to the speaker one wire from each pair. When I checked what I thought was the secondary I'd wired in correctly for a resistance reading, I was met with open circuit. It was then I'd realised I'd soldered in one wire from each pair of secondaries.
HT is still a little high such as v4 anode at 289 volts measured with my DMM. Anyway at last progress.
Would you recommend dropping the HT anymore ? Such as with a resistor after L17 ? I will repost my valve readings now the set is working and HT is stable.
Dave.

[ms660]

Good result, you're on the way now.

Lawrence.

[sobell1980]

School boy error but part of my learning curve I guess. I had the OPT remotely fitted to the set on long wires so I could play about with combinations of the two secondaries. Once it was apart I could see from my meter readings it was obvious what I had done.

[sobell1980]

Ok, here are my valve readings taken again now I'm receiving stations and the HT is stable.

V1 ANODE 290 V DC
V2 ANODE 285 V DC
V3 ANODE 134 V DC
V4 ANODE 285 V DC
V5 ANODES 340 V AC

V1 SG 90V DC
V2 SG 90V DC
V4 SG 294V DC

The trader sheet 732 quotes that these valve readings were taken on a mains voltage of 228V .
I've measure my mains voltage into the set at 240 v. Would this account for my higher than quoted HT ? And SG? Would this work out about right using 240v and not the quoted 228V. I'm also using a DMM and not an Avo.
Many thanks,
Dave.

[ms660]

The increased mains voltage will increase the HT but by what percentage I'm not sure, however, V4 according to my quick calcs is passing the correct anode current give or take a couple of mA, which is a good sign.

So, provided the capacitors connected across the HT are suitably rated I wouldn't worry too much at this stage but if you are concerned you could insert a dropper resistor in series with the rectifier's output.

Lawrence.

[kalee99]

Well done,
Most faults are simple when you get down to it. I have had one of those Output transformers with an extra secondary not used. I think its for sets that have a feedback circuit. It would just be what they had in the stores or its a replacement.
Well done

[peter_sol]

I wouldn't worry, the voltages will be high using a modern DMM

[PJL]

Only voltages from high impedance sources will be affected by the higher input resistance of a DMM and the HT voltage is low impedance.

The trader voltages were on a set powered by 228V but this would probably be with a different mains input voltage selection.

I would still want to check why the HT is so high:
1. Did you measure the DC voltage drop across the field coil?
2. Have you got the mains input selection at the highest setting?

If these both seem OK then it is probably a weak output valve and/or the use of higher value reservoir and smoothing capacitors.

[sobell1980]

I took a quick peak at ridiculous o clock before work this morning regarding the HT. Feeding L17 the HT was 364 volts. The other side of L17 was 290v. So a voltage drop of 74 volts across L17. I'm a great believer in finding the root cause rather than sticking a dropper in to obtain the correct HT. Thinking about it today, I realised for powering up the set first time I used a low emission output valve, valve 4 which was just for testing. I realised I hadn't swopped it back for the known tested good one. I popped this in and measured across L17 again. A voltage drop of 96 volts across L17 giving a HT after L17 of 246 volts instead of 290 volts after L17. Changing v4 really had made a difference. Also operation of the tone control is much more noticeable effect than when the low emission test valve 4 was in place. I noticed that I got a high pitched squeal from V4 when tone control was used and hardly any difference could be heard in tone. With a known good V4 fitted, no strange valve noises and much, much better improved tone control.
I don't think the test valve is much good now after screaming it's nut under test with no negative grid bias etc it was really under pressure. But, it has served it's purpose and saved ruining another good valve. So my HT is now 246 v at V4 anode pin. I don't think I'm going to get it much more down than this without now fitting a dropper. The mains transformer is set on its highest voltage setting. Also the trader sheet quotes that the quoted valve readings were also taken with the mains transformer on its highest voltage setting using a mains voltage of 228 v ac. My mains was 240v ac so would this now explain for my slightly higher HT than the quoted valve readings? Many thanks.
Dave.

[ms660]

Good man, the clue, in the absence of a replacement valve, was in the voltages that were measured in post#174, how's the receiver performing now?

Lawrence.