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 HMV little nipper 61-51 distortion
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 Return to top of page · Post #: 1 · Written at 4:44:57 PM on 24 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Hi guys,

Well i've moved on from my Kriesler for now...deciding that it works well enough for me!

I've returned to my little nipper....which I recapped and rolled a few tubes through but have a persisting distortion listening to music.

Of course there's really only one am channel left here that plays music....1116....and rest are talk back (which seems alright though it's harder to know).

I don't have the equipment but I'm fast realising how much time it would save investing in at least a signal generator....

Anyway apparently coupling caps can cause this but I've changed c18....I got a bit carried away and slightly adjusted the If's (carefully marking where they were) and I think I've tuned in that station better but it's not really changed the distortion.

I've tried the antenna trimmer and again it's improved reception but not removed the distortion.

I have also slightly moved the padder and oscillator trimmer....nothing affects the distortion.

The audio signal wire to the volume control looked quite perished...so replaced that but no change.

I don't have the equipment to test much else....was thinking of putting in a mp3 player connection to volume control to rule out output stage but can't find a spare connector to cut up I'll have to purchase one to try that.

I've heard mica's can go bad so as a last ditch random hope for some success I tried swapping c19, c16 & c12....from my old spare chassis here just because I really was running on frustration....but to no avail (they aren't readily available off the shelf here - I'll order some new ones next I think). It hasn't even affected the alignment I don't think (was expecting it to!).

So basically after any tips (other than purchase the necessary equipment just yet lol) - there's the chance it's just interference too - won't know until I add a different source - unless theres some good ideas on this forum Smile


 
 Return to top of page · Post #: 2 · Written at 6:42:22 PM on 24 August 2023.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 2161

Two things you can check. The rectifier if not doing OK can cause distortion or check all resistors, especially any 50k types.


 
 Return to top of page · Post #: 3 · Written at 8:19:44 PM on 24 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

I’ve swapped the rectifier valve but that didn’t help.

I also checked the resistors - although lazily just compare to my spare chassis - I should go over them per the schematic.

I really thought swapping a few micas may have nailed it as it’s just that muddy sound. Of course the ‘spare’ micas I used may be no good also - another thing I need to do properly…there’s no other old caps left on it now other than micas.

Most the voltages seem reasonable too (some a bit high but supply seems to measure 250v here so that probably ups the rest a bit).


 
 Return to top of page · Post #: 4 · Written at 8:58:26 PM on 24 August 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

That run of models use pretty much the same circuit and I have refurbished over a dozen as they are seriously popular: Even have one.

Distortion is rare. Failed mica's in them are rare. One particular group had two 22K resistors in parallel on the front end screens and the bean counters most likely saw it changed to 10K: I squared R prevails and quite often it cooks due to the excess current.

If it has any paper caps left in it you will never fix it. That also applies to electrolytic types as well, which should have been replaced. some of those 450V caps are crap & I have seen several fail. Do check voltages if they are low it can distort.

6AV6 can be a horror and an out of spec 10M resistor on the grid is never unusual. Has the Output transformer wiring been messed with secondary side? The speaker polling can also produce distortion; As does an overloaded transformer, albeit hum is more common with them.

That is a shielded transformer, and it is best to ground the chassis, with them.

I have been fixing these things for decades and a signal generator is a must. Silicon Chip has a Kit one. I have found in the case of distortion a Signal Generator and Oscilloscope to be a formidable combination. Oscilloscope is also used in Calibration as I find it more sensitive. If you see distortion, it becomes the signal tracer.

Some meters have them built in (Arwon). Never forget: A lot of modern test equipment has issues with RF especially if its riding on DC and tube radio voltages.

https://www.kevinchant.com/uploads/7/1/0/8/7108231/chassis_type_61.pdf


 
 Return to top of page · Post #: 5 · Written at 8:01:46 AM on 25 August 2023.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 2161

When I said check the resistors I meant identify them and use a multimeter to see if they are more than 20% high.


 
 Return to top of page · Post #: 6 · Written at 8:41:33 AM on 25 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Thanks so much for the tips!

What colour is your HMV? I really wanted a green one but so far just have cream and burgundy....but keeping an eye out!


I have replaced every single cap except a handful of micas - all electros & paper even before I fired it up.

This one has the pair of 22k's - I'll strip it down and measure them & the 10M properly to check those.

I had done the voltages - plate voltage a bit high on 6be6 and 6ba6 & 6m, screen also a bit high on 6be6 & 6m5...but supply is 250ac too so that may impact those.

I have a spare set of valves but they didn't make a difference.

I think my hesitation to buy more equipment is because I don't know how to use it! But the amount of time this one has taken would have made it worthwhile now....maybe for my next build unless I can't seem to fix this one properly....


 
 Return to top of page · Post #: 7 · Written at 9:02:53 AM on 25 August 2023.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 2039

Have you tried a replacement speaker?


 
 Return to top of page · Post #: 8 · Written at 9:16:42 AM on 25 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

I checked the 10m but it is within spec....I swapped it out anyway in case perhaps that changes with voltage etc but no difference when I turn it on.

The 2 x 22k's measure ok too.

The output transformer did have extra wiring running out the back on the secondary side which I removed (perhaps had a 2nd speaker connected before?) but original wiring to speaker looks all fine. I have tried a different speaker which was same issue.......I have a spare output transformer would you suggest experimenting swapping that over?

Whats the best way to ground the chassis?

I've pulled this thing apart and put back in the case so many times I may strip the screw threads soon!

Thanks!


 
 Return to top of page · Post #: 9 · Written at 9:49:07 AM on 25 August 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

There should not be stray wires coming off the speaker. There should be four. Two (from) to the secondary and one wire to ground; then the other takes the 6M5cathode bypass to ground through the speaker & secondary. That is on the circuit from Kevin Chant that I posted.

I would be suspicious that if fed out of phase it may cause an issue. Agree that speaker may be polling: Not all that common, but if the cone is damaged that does not apply. Distortion will be common.

+/- 20% is acceptable variance in voltage. New caps will cause a slight increase as will a tube commonly Output not drawing enough current.

Voltage chart is in the info I posted. R7 can be checked "in circuit". C22 will be destroyed if R13 is open, or seriously high. R13 cannot be measured in circuit with the cap connected.

Marc


 
 Return to top of page · Post #: 10 · Written at 9:53:50 AM on 25 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Well I swapped over the output transformer already and guess what? No difference ahhh!!

So I guess it basically must be a resistor that appears in spec when off....or that I have 2 faulty tubes (i actually have 3 of all but the 6av6 which I only have 2 of) or that there's a mica somewhere causing the issue.....

Any other tips welcome! Otherwise I suppose next move is to obtain some new mica's to replace the re-used ones I swapped in....


 
 Return to top of page · Post #: 11 · Written at 10:17:13 AM on 25 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Yes I can only figure the previous owner connected a 2nd speaker - it was a pair of wires soldered to the beginning of the current speaker wires where they attach to the chassis bracket.

I cut c22 & measured R13 - it was about 300ohm's but when I measure a new one it measures at 300 too so I figure my multi must be a little out.

I put in a new c22 for good measure but it's same sound....the station tunes in so perfectly on my old Kriesler although it would be unfair to compare the two!


 
 Return to top of page · Post #: 12 · Written at 2:53:38 PM on 25 August 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

The shotgun approach to fixing is like our current bureaucracy and has rarely proven successful.

Mica's when they go are more commonly hiss or static in fail mode. Not convinced that they are the problem. Did you disconnect the speaker & power it with something else, or disconnect it and sub it?

You are likely to not be happy with my comment. However, I resorted to fixing them close to 50 years ago & have equipment.

Firstly: My method of picking up on distortion starts from the instant its re-powered from overhaul, unless that is what I started out too trace. The best distortion testers I have, are oscilloscopes and signal generators RF / AF & Function Generator. This lot has proven to be above all else in calibrating an finding distortion in the circuits and often speakers themselves.

Calibrating the IF with a Generator that puts out a clean signal is ideal. An Oscilloscope will see any distortion of the signal. That saves a huge amount of time & stress. Time is important with a commercial fix. With audio circuits the Function Generator supplies a clean signal over a range of frequencies. This enables delivering the whole audio spectrum to the speaker. if its dodgy it is liable to rattle at the lower end. That can be done using a separate amp to drive it.

On overdriving: Ensure that there are no coupling caps with a decimal error.


 
 Return to top of page · Post #: 13 · Written at 3:26:03 PM on 25 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Ah yep shotgun is me I can’t help it! I tried….but reverted back to old ways!

It’s such a simple radio I figured a bit or trial & error must work eventually….but alas the radio is 90% swapped & yet same issue.

I really need a seperate signal to test it with as you say - when I can get to Jaycar I’ll see what they offer.

I was also going to get a mp3 cable and connect to volume control with a switch just to feed that seperate signal through it & see whether there is any distortion as an mp3 connection would be handy anyway.

I have 3 speakers & all exhibit same issue with this amp.

And both output transformers too….


 
 Return to top of page · Post #: 14 · Written at 9:23:40 PM on 25 August 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

Now things were almost going well until it wasn't. And there was a crash. The Radio Designers handbooks RCA do have a whole chapter on distortion, as is can even be caused by a stage oscillating. 6AV6 area is one that causes grief and I have seen several innovations, including more shielding. Modern caps inducting can be an issue.

There is a convention where the output transformers have red and blue primary wires and it might be worth a try to reverse them as there may be a phase issue due to the cathode bypass being channeled through the secondary.

The issue can start at the front end, with over modulation and that can be a wrong part. One does not tamper with Mica, or Ceramic caps on coils that are not in the parts list. The RF coils and gang are a matched set. They come factory adjusted and interfering with those caps, can be catastrophic.

I do think that the only way you are going to sort this is with an oscilloscope, to determine the true source area. Do also be aware that those speakers are not "full range" they are usually "mid range" and speakers with out baffling can sound horrible.

The Oscilloscope despite its looks, can be a problem, as you need to be able to interpret what it is showing you.

Marc


 
 Return to top of page · Post #: 15 · Written at 7:05:47 AM on 26 August 2023.
Monaro's Gravatar
 Location: Brisbane, QLD
 Member since 24 October 2013
 Member #: 1439
 Postcount: 131

Thanks for such a detailed reply Marc, it’s great to get so much help & tips from an expert!

I’ll try the output transformer input change - although this is the ‘spare’ output transformer - the original sounded the same too. When swapping the inputs do the outputs need to be swapped too or just leave those?

The shielding of the 6av6 is one I never would have thought of - maybe just a tin sleeve for it screwed to chassis?

The speaker has the baffle on it - and I tried my spare too but it has same issue -
Didn’t know they were only mid range I wonder if there’s merit in trying a full range? Might not be easy matching impedance though I guess.

Yes the oscilloscope looks the only way - and trying to work out how to use it…!


 
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