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HMV 1421

PostPosted: Thu Jul 07, 2016 5:54 pm
by 6.3volts
Hello all after a long absence in the restoration wilderness.
I hope this my be of some interest- I acquired this in spite of its 9 transistors as it struck me as an interesting example of the changeover in thinking seemingly forced on designers by solid state technology.
As a boy I remember my first transistor radio being an example of the change with its transformer coupled audio stages and output section. This model has all that and a hard wired metal chassis and standard wooden cabinet taken directly from the 1960s valve radios. I'm guessing that the chassis might have been a redesign of one carrying B7A valves in a previous life. It's interesting to note a B9A valve base in the VHF section which is used as a tag-strip for the components underneath. How's that for economy in redesign !! The cabinet is date stamped 8th Sept 1960
One picture shows the radio as purchased; the others are of the chassis out and working on FM.
When first connected to a power supply with a cautious 6v it was silent as it consumed over 1amp of current. This was not surprisingly due to failure of the OC81s. Having disconnected the feed to them I listened with phones for any sign of life from the rest of the set running on 9v; The audio Trs 6 and 7 responded to mains hum when touched so I suspected the lack of any other sounds was further back
A bit of research came up with the likely problem-tin whiskers growing inside the OC170s. This was news to me as my voluntary involvement with these new-fangled things has always been reluctant and minimal. Anyway by the time I'd discharged a capacitor through the three OC170s and resoldered them in place I had a working receiver including FM, the two OC171s in the FM section not requiring any attention.
The sound quality through phones taken from the driver transformer seems very good,however using a pair of nos OC81s in the output stage I get reasonable sound at low levels but distortion as I increase the volume. The two OC81s give hfe readings of 5 and 81 which suggests wild mis-matching. In use the base and collector waveforms show little variation in amplitude and I'm hoping that someone with a better grasp of these things will be able to offer some advice on how to improve the sound quality.Could this be crossover related? So far I've done nothing else to the chassis as it seems to work just fine. I include a circuit for reference.
Regards to all, Jonathan

Re: HMV 1421

PostPosted: Thu Jul 07, 2016 8:57 pm
by merseyswimmer
That OC81 with a gain of just 5 is obviously faulty. The other transistor's gain at 81 isn't too far from expected. Should you have no more OC81s then a pair of AC128s would do for a test. The lead configuration is: e b c reading clockwise. The output pair should be fitted with slide-on heatsinks - those two holes below the terminal strip are for the bolts to secure them to the chassis. In fact a pair of AC128K can be fitted as these come in a heatsink block that just needs bolting to the chassis.

Take care when following the circuit - it's upside down with the common +ve line at the top. In PNP circuits the negative rail is usually the "HT" but in your radio the chassis is used for the negative. In case of excessive quiescent current draw check those diodes marked as X5a and X5b which are contained in a combined selenium diode pair. That upside-down circuit was obviously designed by those more accustomed to valves.

MS

Re: HMV 1421

PostPosted: Thu Jul 07, 2016 9:12 pm
by PYE625
Hi, please forgive my ignorance, but am I right in thinking that a valve is used in the FM front end? ECC85?

DERRRRR....read the above properly lol :ccb :ccb

A very interesting set indeed :aad

Re: HMV 1421

PostPosted: Sat Jul 09, 2016 7:27 pm
by crackle
A nice example of a crossover set from valve to transistor.

Mike

Re: HMV 1421

PostPosted: Wed Jul 20, 2016 4:59 pm
by 6.3volts
Just to tidy up this job here are the details of the final stages.
Thanks for the advice about the Oc81s.
I obtained a closely matched pair and the sound quality is now very good.
I think I have found the reason for the original failures- After a period of working the set starts to emit a deafening crackling/roar which is at full volume in spite of the volume setting. I found the fault in the form of a loosely riveted connection to the on/off switch and having fixed it the set is now working perfectly.
I imagine the fault would have rapidly led to thermal runaway in the OC81s and I assume this factory fault led to its demise.
The original damaged cloth could not be repaired as it crumbled away when I tried to reinforce the holes from behind. I had some material rescued from a 1960s set so took the liberty of using it as I hoped it looked right for the era of this set.
Other work was cleaning the knobs and glass and re painting the 'white' areas at the front which I did with off-white to try to give a more aged look. The cabinet top was poor and has been re finished leaving the HMV sticker in place.
I made a mains power supply which lives inside the case and will save a fortune in batteries as the set is in daily use.
The only other point of interest is I have set up an FM transmitter for the house and workshops and this set is giving splendid sound quality on radio2 re-broadcast from a sky box. My nearest neighbours are half a mile away so I'm not spoiling anything for them (I hope).

Re: HMV 1421

PostPosted: Wed Jul 20, 2016 5:59 pm
by nuvistor
Looks great to me, the speaker cloth is fine.

Frank

Re: HMV 1421

PostPosted: Wed Jul 20, 2016 9:20 pm
by crackle
It certainly looks like the speaker cloth was made for it.
Nice looking set and I expect it sounds good too.
Mike

Re: HMV 1421

PostPosted: Thu Jul 21, 2016 5:53 am
by 6.3volts
Thanks for your votes on the speaker cloth.
Finally does anyone have one of these sets with a back?
This one is missing - where do they go?????? I would like to make one for it and want it to look something like right. Any help such as a photo much appreciated.
Regards to all Jonathan

Re: HMV 1421

PostPosted: Thu Jul 21, 2016 7:47 am
by nuvistor
Plenty photos of the front of the set around but no backs that I can find.

Re: HMV 1421

PostPosted: Thu Jul 21, 2016 1:38 pm
by merseyswimmer
6.3volts wrote:
I imagine the fault would have rapidly led to thermal runaway in the OC81s and I assume this factory fault led to its demise.


When you acquired the set were the OC81s fitted with clip-on heatsinks? If not overheating could be the cause of thermal runaway leaving one transistor damaged.

There's really no need for matched pairs nowadays given the current scarcity of germanium devices - in the early days matched output transistors were basically a sales gimmick but in reality any difference in gain of the output devices which could result in audible distortion is countered by negative feedback*. I've fitted thousands of pairs of output transistors over the past five decades and I've never heard any improvement in quality using matched pairs in yer actual bog-standard transistor set. You need test equipment to tell the difference in audio quality between matched and non-matched output transistors. If you can't actually hear it, it don't matter.

MS

*Note for pendants - I'm speaking generally, not just in this case.

Re: HMV 1421

PostPosted: Thu Jul 21, 2016 1:48 pm
by Rallycoordinator
I have a plastic cased version of this set using the same chassis with possibly the same fiberboard back design (which includes a large PP10 battery compartment).

I will post a few pictures this Sunday, when I gain access to this set.

Mike

Re: HMV 1421

PostPosted: Thu Jul 21, 2016 10:46 pm
by Paul_RK
Hello Jonathan,

A few photos of the back of mine, showing the PP10 compartment with its pair of snap fasteners.

1421-1.jpg

1421-2.jpg

1421-3.jpg

1421-4.jpg


Regards,
Paul

Re: HMV 1421

PostPosted: Fri Jul 22, 2016 9:45 am
by 6.3volts
Paul, thank you so much for the photos. This explains where the battery was housed. I will make an attempt at a replacement when I (hopefully) have finished the work on my Marconi sig gen (elsewhere on this forum).
Thanks again for your help.
Jonathan

Re: HMV 1421

PostPosted: Tue Jul 26, 2016 3:32 pm
by Rallycoordinator
Just unearthed my plastic cased Marconiphone T84BT which uses the same chassis as the HMV 1421 ( the aluminium heat sink has been temporarily removed).

This back cover used on this set appears to be a slightly later design manufactured from hardboard, rather than the more traditional fibreboard.

Mike

Re: HMV 1421

PostPosted: Wed Aug 17, 2016 10:56 am
by 6.3volts
Thank you both for your help with the back. I just need to find time to make one now.
Regards Jonathan