HMV 650. Output stage bodge? - UK Vintage Radio Repair and Restoration Discussion Forum

RoundyMooney · 22nd Jul 2007, 11:35 pm

As mentioned elsewhere, I'm currently attempting to restore a 1930s HMV 650.

I had a spare hour tonight, and so stripped the audio/PSU chassis in preparation for cleaning/rewiring etc., and bunged it in the dishwasher.

It seems the phantom bodger was in there sometime in the early sixties (a dated RS electrolytic was within). Now I was aware of this, and the fact that the three wet electrolytics were u/s. I may try reforming two of them, but more of that later. While stripping down the chassis, I found a few anomalies, one of which was a circa 1950s era output transformer masquerading as a filter choke. (I may try and "age" this cosmetically and return it to service, as it's physically a perfect fit). What I also found was a relatively small, (think small 1970s mains transformer size) choke across the audio output, at the speaker terminals.

Now the radio worked, and reasonably well too. Schematic specifies a 50 ohm resistor across the output IIRC, which is present and ok, but this choke looks original.

Is it possible that it was some sort of modification that wasn't documented on the service sheet, or is there a valid reason for it being there at all? Aside from possibly loading the output stage (class A PP with switchable speaker configuration btw), a job which should be served by the resistor, I can't think of one

TIA for any thoughts on this, the first of many head scratching events I reckon.

PJL · 23rd Jul 2007, 2:39 pm

I looked at the schematic and it is a very impressive set with an RF stage, seperate OSC and 2 IF stages. The set is supposed to have a couple of chokes as well as the output transformer mounted on the PSU/output chassis.

Is it simply that the previous repairer has re-wired it up incorrectly when replacing the output transformer?

RoundyMooney · 23rd Jul 2007, 5:13 pm

Nope, the audio output trafo is present (or at least was!) and correct. Another, later audio output trafo was pressed into service as one of those filter chokes, probably some time in the sixties.

Having thought about it, perhaps it's a period manufacturer mod to reduce high frequency response? That is, at least, the only effect i can see it having. What I'm going to try at switch on is with and without, and whichever is most pleasing to the ear is what I'll run with...

Radio Fixer · 29th Jul 2007, 10:09 am

I have seen this choke on the Marconi equiv 561 but have a 650 that does not have it, but does have the drilled fixing holes. This to me confirms it is not a bodge but an unrecorded factory mod fitted to some chassis.

If you have BVWS Bulletin, for Summer 2005, on page 7 there is a picture of the finished O/P chassis for the 561 (this is part of an 8 page restoration article). In this pic, a little obscured by the output valves, the choke can be seen. It is mounted at an angle of about 45 degrees, rightwards, behind the valve nearest the rear of the chassis.

Hope this helps...

Radio Fixer · 29th Jul 2007, 6:29 pm

EMI did a number of changes to these 11 valve chassis that are not recorded.

For example on the 561 R46 was 10K and R26 was 1K against the marked values of 5K and 500 Ohm, as in the 650
Also the choke CK4 was ommited from the 561. The 561 has an extra Mica cap between S3 tag 7 and S4 tag 9. Never did figure out what this was meant to do. Note the components referenced were all original factory items and not had a soldering iron on them since the chassis were made.

The heaters are not wired exactly as the circuit either. This shows the Osc V3 returning via its own cable to the PSU chassis. In fact V3 joins up with the other heaters on a tag panel on the main chassis. Then the cable form has both return wires (P2 and P5) in parallel back to the PSU chassis.

So watch out for unrecorded differences...

RoundyMooney · 1st Aug 2007, 3:30 pm

Very informative, thank you.

I feel a little more confident now

Radio Fixer · 2nd Aug 2007, 7:48 pm

The osc on my 650 did not work on SW2. Had a subtle fault that took a little finding so this may be of interest. The coil L22 (6T and 7T) has ends that are soldered into the rivets of trimmer cap TC16. But these rivets are not soldered to the brass plates either side of the mica. It relies on the rivets making good contact which after all these years they didn't. Once scrapped clean and soldered then it was working with no more trouble.

22nd Jul 2007, 11:35 pm	#1
RoundyMooney Tetrode Join Date: Apr 2007 Location: Cork, Ireland Posts: 56	HMV 650. Output stage bodge? As mentioned elsewhere, I'm currently attempting to restore a 1930s HMV 650. I had a spare hour tonight, and so stripped the audio/PSU chassis in preparation for cleaning/rewiring etc., and bunged it in the dishwasher. It seems the phantom bodger was in there sometime in the early sixties (a dated RS electrolytic was within). Now I was aware of this, and the fact that the three wet electrolytics were u/s. I may try reforming two of them, but more of that later. While stripping down the chassis, I found a few anomalies, one of which was a circa 1950s era output transformer masquerading as a filter choke. (I may try and "age" this cosmetically and return it to service, as it's physically a perfect fit). What I also found was a relatively small, (think small 1970s mains transformer size) choke across the audio output, at the speaker terminals. Now the radio worked, and reasonably well too. Schematic specifies a 50 ohm resistor across the output IIRC, which is present and ok, but this choke looks original. Is it possible that it was some sort of modification that wasn't documented on the service sheet, or is there a valid reason for it being there at all? Aside from possibly loading the output stage (class A PP with switchable speaker configuration btw), a job which should be served by the resistor, I can't think of one TIA for any thoughts on this, the first of many head scratching events I reckon. __________________ Putting the smoke back into antiquated junk since 1977

23rd Jul 2007, 2:39 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 650. Output stage bodge? I looked at the schematic and it is a very impressive set with an RF stage, seperate OSC and 2 IF stages. The set is supposed to have a couple of chokes as well as the output transformer mounted on the PSU/output chassis. Is it simply that the previous repairer has re-wired it up incorrectly when replacing the output transformer?

23rd Jul 2007, 5:13 pm	#3
RoundyMooney Tetrode Join Date: Apr 2007 Location: Cork, Ireland Posts: 56	Re: HMV 650. Output stage bodge? Nope, the audio output trafo is present (or at least was!) and correct. Another, later audio output trafo was pressed into service as one of those filter chokes, probably some time in the sixties. Having thought about it, perhaps it's a period manufacturer mod to reduce high frequency response? That is, at least, the only effect i can see it having. What I'm going to try at switch on is with and without, and whichever is most pleasing to the ear is what I'll run with... __________________ Putting the smoke back into antiquated junk since 1977

29th Jul 2007, 10:09 am	#4
Radio Fixer Retired Dormant Member Join Date: Jun 2006 Location: thorpe bay Posts: 25	HMV 650. Output stage bodge? I have seen this choke on the Marconi equiv 561 but have a 650 that does not have it, but does have the drilled fixing holes. This to me confirms it is not a bodge but an unrecorded factory mod fitted to some chassis. If you have BVWS Bulletin, for Summer 2005, on page 7 there is a picture of the finished O/P chassis for the 561 (this is part of an 8 page restoration article). In this pic, a little obscured by the output valves, the choke can be seen. It is mounted at an angle of about 45 degrees, rightwards, behind the valve nearest the rear of the chassis. Hope this helps...

29th Jul 2007, 6:29 pm	#5
Radio Fixer Retired Dormant Member Join Date: Jun 2006 Location: thorpe bay Posts: 25	Re: HMV 650. Output stage bodge? EMI did a number of changes to these 11 valve chassis that are not recorded. For example on the 561 R46 was 10K and R26 was 1K against the marked values of 5K and 500 Ohm, as in the 650 Also the choke CK4 was ommited from the 561. The 561 has an extra Mica cap between S3 tag 7 and S4 tag 9. Never did figure out what this was meant to do. Note the components referenced were all original factory items and not had a soldering iron on them since the chassis were made. The heaters are not wired exactly as the circuit either. This shows the Osc V3 returning via its own cable to the PSU chassis. In fact V3 joins up with the other heaters on a tag panel on the main chassis. Then the cable form has both return wires (P2 and P5) in parallel back to the PSU chassis. So watch out for unrecorded differences...

1st Aug 2007, 3:30 pm	#6
RoundyMooney Tetrode Join Date: Apr 2007 Location: Cork, Ireland Posts: 56	Re: HMV 650. Output stage bodge? Very informative, thank you. I feel a little more confident now __________________ Putting the smoke back into antiquated junk since 1977

2nd Aug 2007, 7:48 pm	#7
Radio Fixer Retired Dormant Member Join Date: Jun 2006 Location: thorpe bay Posts: 25	Re: HMV 650. Output stage bodge? The osc on my 650 did not work on SW2. Had a subtle fault that took a little finding so this may be of interest. The coil L22 (6T and 7T) has ends that are soldered into the rivets of trimmer cap TC16. But these rivets are not soldered to the brass plates either side of the mica. It relies on the rivets making good contact which after all these years they didn't. Once scrapped clean and soldered then it was working with no more trouble.