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Old 13th May 2018, 10:15 pm   #1
sobell1980
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Default HMV 653 broadcaster

Hi all,
Some of you may remember about 2 years ago I did a long full overhaul of this set that many of you were involved with.
It was given as a Christmas gift in 2016. Today it was returned to me in a non working condition. When I say not working, I plugged it in, stations on all wavebands can be heard and I can tune easily into stations and make out what they are. The problem is that all stations on all wavebands sound off alignment. It sounds like when you just tune off a station and I can't quite just tune it in then you tune past it to silence. The voices sound distorted and it also lacks volume and depth. To me this sounds alignment but would have to be just on the intermediate frequency to sound the same across all wavebands. I will first check all the valve voltages against the trader sheet first, that's a first and then possibly check frequency on my oscillascope and alignment with my dig gen. It's the distortion making me think this. If it were just low volume but could still tune in clearly id say a low output valve or caps . Haven't been on here for a while but I'm still working on various sets, currently this one and a Bush VHF61 that is also a basket case. Hopefully I will get the chassis out on the HMV this week and take some valve voltages. Start with the basics and then work through checking alignment. Any thoughts as ever gratefully received.
Dave
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Old 13th May 2018, 10:18 pm   #2
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Default Re: Hmv 653 broadcaster

I forgot to say that this set has moved house. Could a vigorous journey and being boxed up etc cause a misalignment? Just seems strange. Unless someone has changed anything or fiddled with the alignment it should still be spot on.
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Old 13th May 2018, 10:52 pm   #3
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Default Re: Hmv 653 broadcaster

Could be IF instability. Something oscillating when it shouldn't be. Check decoupling capacitors around the RF and IF stages. I'm not familiar with the set but if it uses an EF39 or similar sometimes the metallic coating connection fails and causes IF stability problems. Or change the valve for a different one?
Alan.
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Old 14th May 2018, 8:57 am   #4
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Default Re: HMV 653 broadcaster

Make sure the loudspeaker's ok.

Lawrence.
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Old 14th May 2018, 9:44 am   #5
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Default Re: HMV 653 broadcaster

No, a house move would not affect alignment otherwise all mine would be off after coming 300 miles.
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Old 18th May 2018, 4:16 pm   #6
sobell1980
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Default Re: HMV 653 broadcaster

Sorry for the delay in replying. Thanks for the replies. Life's very hectic at the minute and I managed to literally grab a few moments on the set. I did think about the speaker as soon as I heard it but because the distortion didn't get any worse as the volume was turned up I kind of dismissed it. Also the low volume also made me think it was other things.
I have however tried another speaker. I left the field coil on the speaker connected to the HT. I snipped the two wires from the output transformer at the speaker end and connected a permanent magnet type speaker. The distortion is still there and the same with the field coil still connected to the original speaker and the speaker wires connected to a separate known good speaker. I guess it's coming apart and run through the valve voltages.
Dave.
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Old 18th May 2018, 8:39 pm   #7
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Default Re: HMV 653 broadcaster

Hi Dave

I recall a similar symptom found on an RGD upright console radiogram many years ago. As I was checking voltages around the output valve with my (then) Avo 8, the sound level suddenly increased and the distortion cleared. Removal of test prods reverted to the fault. A '100k' grid bias resistor had gone much higher than its stated value and the Avo's particular voltage setting at the time just happened to put the right amount of 'ohms per volt' in parallel with said resistor, thereby restoring the valve bias. Having replaced the resistor, all was well

Cheers
Guy
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Old 18th May 2018, 9:02 pm   #8
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Default Re: HMV 653 broadcaster

There are many things it could be. Guessing might get lucky, but then again lots of guesses could get you nowhere.

You don't know what's wrong with it
None of us know what's wrong with it

Currently, only it knows what's wrong with it.

What is needed is a way to get it to tell us.

If quick fixes don't do the deed, then it's the slow way. But the slow way is certain. Thee are two ways: Follow a signal down the path in the receiver, looking at the signal and assessing its quality and strength, OR work backwards towards the aerial socket injecting suitable and good signals while listening to the result.

With limited test equipment, the signal injection process is easier. And a sig gen is needed to do a good realignment if that's what it is.

There is a possibility that whatever anchors one IF slug got misplaced last time and a slug has shifted under vibration, but that's just guessing.

Stick some good audio in at the volume control and listen to see if the quality is OK. If it is, inject a modulated signal at IF into the detector and listen. Then work backwards up the IF strip, reducing level as you go. Listen at each step. Going to the front of the mixer, switch the generator to the RF frequency and keep moving along.

This should get you down to the faulty stage.

A general check over the voltages on valve electrodes against the service data may unearth a clue.

The slow way isn't fast enough to be profitable in the commercial repair shops of yore, but it's what everyone has to fall back on with an unfamiliar fault.

You'll get there.

There is a theory that doing a first check halfway down the set's signal path and answering the question 'Is it before or after here" and then doing the same to the half that the answer points at may be the most efficient game plan like a "successive approximation" ADC. But the slow series of steps way is less confusing.

David
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Old 25th May 2018, 8:36 pm   #9
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Default Re: HMV 653 broadcaster

Good evening all and thanks for your replies. Sorry I've not been on here, I've been away on holiday so please don't think I've lost interest or not replied.
I've only got a signal generator and an oscillascope so I'm hoping to find out what the problem is with that. I can compare what's coming out the sig gen on the oscillascope, then check I'm getting the same through the set obviously plus the IF frequency. Hopefully I will be able to hear the sig gen is distorted enough to hear what is good and isn't. It does it on all wave bands so that's a starting point. Anyway, just thought I'd catch up and hopefully make some progress over the weekend. I will run through my valve voltages off the service data first then run through alignment and go from there. I don't think for a minute it's alignment but something may or may not show up. Hopefully I will find a section where it's good and then bad.
Dave.
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Old 28th May 2018, 9:47 pm   #10
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Default Re: HMV 653 broadcaster

I managed to get an afternoon on the set yesterday and resolved the issue. I found two areas of concern after running through the working voltages. I found R12,13,14 For the AVC had all gone massively high in resistance. They had gone into 3+mega ohms. The final fault was valve 3 anode load resistor had also gone high. Valve 3 anode was receiving 3 volts only. I checked out R11 and had gone so high it wouldn't register on my meter. I replaced R11 and normal service resumed.
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Old 9th Jun 2018, 1:49 pm   #11
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Default Re: HMV 653 broadcaster

This set is now back to working order and waiting for collection under my bench. Thank you for those that left replies. Luckily a working voltage check revealed the issue.
This thread can now be closed.
Dave.
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Old 9th Jun 2018, 4:48 pm   #12
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Default Re: HMV 653 broadcaster

Quote:
Originally Posted by sobell1980 View Post
I managed to get an afternoon on the set yesterday and resolved the issue. I found two areas of concern after running through the working voltages. I found R12,13,14 For the AVC had all gone massively high in resistance. They had gone into 3+mega ohms. The final fault was valve 3 anode load resistor had also gone high. Valve 3 anode was receiving 3 volts only. I checked out R11 and had gone so high it wouldn't register on my meter. I replaced R11 and normal service resumed.
Glad you got it sorted. Interesting that 3 resistors had all gone high. Have you seen this sort of thing before? Being new to restoring these old sets, I wondered if this is typical of how old resistors go faulty.
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Old 9th Jun 2018, 5:17 pm   #13
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Default Re: HMV 653 broadcaster

Yes, most old carbon resistors that go faulty go high or sometimes open circuit, some of the lower values can sometimes go low in value if they have been subjected to excessive power dissipation. Wire wound types usually fail open circuit.

Lawrence.
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Old 11th Jun 2018, 8:55 am   #14
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Default Re: HMV 653 broadcaster

And it's not just a UK issue, I am working on a late 30's USA midget, 500K risen to 1M, 2M risen to 6M. To avoid removing the old I am using parallel 1/8W resistors from the early 80's and these are carbon film and have not drifted.
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Old 11th Jun 2018, 10:13 am   #15
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Default Re: HMV 653 broadcaster

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Originally Posted by PJL View Post
To avoid removing the old I am using parallel 1/8W resistors from the early 80's and these are carbon film and have not drifted.
Be aware of voltage ratings with diddly resistors in valve sets!

Eighth watt CFs may only be 100V or so...... maybe not too problematic on a 110v mains set.
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Old 11th Jun 2018, 11:25 am   #16
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Default Re: HMV 653 broadcaster

Quote:
Originally Posted by Herald1360 View Post
Be aware of voltage ratings with diddly resistors in valve sets!

Eighth watt CFs may only be 100V or so...... maybe not too problematic on a 110v mains set.
Am I correct in thinking the voltage rating is the voltage dropped across the resistor and not the ultimate voltage it is connected to.

Thanks
Mike

Last edited by crackle; 11th Jun 2018 at 11:33 am.
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Old 11th Jun 2018, 1:34 pm   #17
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Default Re: HMV 653 broadcaster

Yes, as long as it's not touching anything else.
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Old 11th Jun 2018, 5:50 pm   #18
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Default Re: HMV 653 broadcaster

Quote:
Originally Posted by Herald1360 View Post
Be aware of voltage ratings with diddly resistors in valve sets!
Eighth watt CFs may only be 100V or so...... maybe not too problematic on a 110v mains set.
Good point as I would not try this on a high HT set, in this case HT is only 100V.
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Old 14th Jun 2018, 1:32 pm   #19
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Default Re: HMV 653 broadcaster

I have found that carbon resistors seem more likely to go high if they have a high PD across them( so anode and grid resistors typically) and it seems to be more prevalent with higher values (>or=10K).
Lower values do sometimes go LOW, particularly those in output valve cathode bias.
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