Mullard HSVT E7600/3 - UK Vintage Radio Repair and Restoration Discussion Forum

milairuk · 7th Aug 2006, 9:42 pm

Further to my previous restoration posts, I am sorry to advise that the Mains Tx is no more - it is a dead parrot. All was going well - got the -85volts up ok, so decided to move onto the EY51 area, replaced the (leaky) 4uf caps, and initially up came a (focussed) spot on the CRT. After approx 5minutes a nice smell of insulation wafts past the nostrils and the spot vanishes. A check on the AC output from the Tx shows 220v initially rising to approx 430v when the fault develops. Even with the gate switch open there is a slowly rising AC voltage on the unswitched lead to the EY51's so I assume that the Tx is leaking to earth.
After leaving overnight the spot initially returns, again followed by the smell and lack of spot. Tonight you could almost fry an egg on the Tx after 5 minutes.

PS - all the other gate selection pins were insulated by thick plastic sheet - so I did not short the HT windings.

PJL · 7th Aug 2006, 11:17 pm

It's probably worth checking it out further as even my 'working' transformer leaks enough to raise a pretty high voltage on the EHT capacitors when the gate is open and it does increase slowly BUT the transformer does stay cool and I have left it on for 24hrs without problems. Have a look around to check it is not an external short somewhere.

If it's been in a shed for years it might possibly recover if you dry it out. Someone recommended a hot air gun to me but I must admit I didn't try it. Maybe others can recommend something they have tried succcessfully....Peter

Sean Williams · 7th Aug 2006, 11:45 pm

I would disconnect the transformer, and megger the windings out - it may reveal a fault due to damp - then bung it in the airing cupboard (or very low oven) for a few houre and try again - if the readings improve, you might be able to save this unit......

Cheers
Sean

milairuk · 8th Aug 2006, 8:53 am

Thanks guys, I'll give it some more work - but don't fancy removing it with all the connections involved !
Does anyone know the correct AC output to the EY51's ? I would assume that the 400+ is nearer the mark or else why use EY51's ? However the CRT spot only seems visible when the initial 200+ is there

?? I have a circuit but no voltage tables (if such exists) so I am just working on assumptions. It does seem strange that with the gate switch open the Tx is quite happy powering the heaters and the -ve supply (Still waiting for the GZ rectifier valve). If the insulation was breaking down I would have thought it would happen all the while.

PJL · 8th Aug 2006, 11:17 am

I think the 440V AC is about right. The spot problem sounds like you have something wrong with the CRT amplifier. I suggest you put a 100W light bulb in series with the AC input to give you some time to look around for problems without blowing the transformer.

As I recall the heaters and reference supply are on without a card in place (hence why the valves end up with low emissions!) but the EHT/HT requires a card. There is a switch at the bottom of the card slot which cuts the AC input if you attempt to close the gate without a card. Check this switch before going any further as it is prone to sticking due to debris in the card slot and the consequences of failure are VERY BAD!

The rising leakage is identical to mine and I spent some time investigating without success. I carefully desoldered the leads from the windings to the connections on the transformer and could still measure the leakage current to earth. Damp is one possibility but I suspect it is capacitive coupling to an adjacent winding. You might well be chasing the wrong problem.

milairuk · 8th Aug 2006, 11:50 am

Thanks for that Peter, you could be right - I'll disconnect the HT feeds to the CRT amp and substitute with a load resistor on the +ve & -ve rails to pass a few milliamps. I've replaced all the paper caps and the EF91's are both brand new.

The gate switch is fine, As I have no original cards at all, I have made a blank 1mm thick HIP plastic card up to actuate it without allowing any of the other pins to short out.

John

GMB · 8th Aug 2006, 12:13 pm

It's curious that none of the manuals says what the EHT voltage should be for the versions with the doubler so perhaps it was a bit variable (?).

The original E7600 is documented as having a 475V EHT, and I have heard it suggested that they didn't change the transformer for the later models.

PJL · 8th Aug 2006, 2:11 pm

I was thinking you should put the lamp on the primary of the transformer. The EHT problem could be a red herring so you may have a problem elsewhere.

milairuk · 8th Aug 2006, 2:43 pm

Fair comment but it behaves itself as long as the gate switch is open. I am missing the GZ33 and EL37 as I have yet to buy these. So unless I am wrong I cannot see anything else that will be "on" with the gate switch open ?

7th Aug 2006, 9:42 pm	#1
milairuk Pentode Join Date: May 2005 Location: Chelmsford, Essex, UK. Posts: 150	Mullard HSVT E7600/3 Further to my previous restoration posts, I am sorry to advise that the Mains Tx is no more - it is a dead parrot. All was going well - got the -85volts up ok, so decided to move onto the EY51 area, replaced the (leaky) 4uf caps, and initially up came a (focussed) spot on the CRT. After approx 5minutes a nice smell of insulation wafts past the nostrils and the spot vanishes. A check on the AC output from the Tx shows 220v initially rising to approx 430v when the fault develops. Even with the gate switch open there is a slowly rising AC voltage on the unswitched lead to the EY51's so I assume that the Tx is leaking to earth. After leaving overnight the spot initially returns, again followed by the smell and lack of spot. Tonight you could almost fry an egg on the Tx after 5 minutes. PS - all the other gate selection pins were insulated by thick plastic sheet - so I did not short the HT windings.

7th Aug 2006, 11:17 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Alas poor E7600/3 - I knew it well It's probably worth checking it out further as even my 'working' transformer leaks enough to raise a pretty high voltage on the EHT capacitors when the gate is open and it does increase slowly BUT the transformer does stay cool and I have left it on for 24hrs without problems. Have a look around to check it is not an external short somewhere. If it's been in a shed for years it might possibly recover if you dry it out. Someone recommended a hot air gun to me but I must admit I didn't try it. Maybe others can recommend something they have tried succcessfully....Peter

7th Aug 2006, 11:45 pm	#3
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Alas poor E7600/3 - I knew it well I would disconnect the transformer, and megger the windings out - it may reveal a fault due to damp - then bung it in the airing cupboard (or very low oven) for a few houre and try again - if the readings improve, you might be able to save this unit...... Cheers Sean __________________ Engineers make things work and have spare bits when finished

8th Aug 2006, 8:53 am	#4
milairuk Pentode Join Date: May 2005 Location: Chelmsford, Essex, UK. Posts: 150	Re: Alas poor E7600/3 - I knew it well Thanks guys, I'll give it some more work - but don't fancy removing it with all the connections involved ! Does anyone know the correct AC output to the EY51's ? I would assume that the 400+ is nearer the mark or else why use EY51's ? However the CRT spot only seems visible when the initial 200+ is there ?? I have a circuit but no voltage tables (if such exists) so I am just working on assumptions. It does seem strange that with the gate switch open the Tx is quite happy powering the heaters and the -ve supply (Still waiting for the GZ rectifier valve). If the insulation was breaking down I would have thought it would happen all the while.

8th Aug 2006, 11:17 am	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Alas poor E7600/3 - I knew it well I think the 440V AC is about right. The spot problem sounds like you have something wrong with the CRT amplifier. I suggest you put a 100W light bulb in series with the AC input to give you some time to look around for problems without blowing the transformer. As I recall the heaters and reference supply are on without a card in place (hence why the valves end up with low emissions!) but the EHT/HT requires a card. There is a switch at the bottom of the card slot which cuts the AC input if you attempt to close the gate without a card. Check this switch before going any further as it is prone to sticking due to debris in the card slot and the consequences of failure are VERY BAD! The rising leakage is identical to mine and I spent some time investigating without success. I carefully desoldered the leads from the windings to the connections on the transformer and could still measure the leakage current to earth. Damp is one possibility but I suspect it is capacitive coupling to an adjacent winding. You might well be chasing the wrong problem.

8th Aug 2006, 11:50 am	#6
milairuk Pentode Join Date: May 2005 Location: Chelmsford, Essex, UK. Posts: 150	Re: Alas poor E7600/3 - I knew it well Thanks for that Peter, you could be right - I'll disconnect the HT feeds to the CRT amp and substitute with a load resistor on the +ve & -ve rails to pass a few milliamps. I've replaced all the paper caps and the EF91's are both brand new. The gate switch is fine, As I have no original cards at all, I have made a blank 1mm thick HIP plastic card up to actuate it without allowing any of the other pins to short out. John

8th Aug 2006, 12:13 pm	#7
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Alas poor E7600/3 - I knew it well It's curious that none of the manuals says what the EHT voltage should be for the versions with the doubler so perhaps it was a bit variable (?). The original E7600 is documented as having a 475V EHT, and I have heard it suggested that they didn't change the transformer for the later models.

8th Aug 2006, 2:11 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Alas poor E7600/3 - I knew it well I was thinking you should put the lamp on the primary of the transformer. The EHT problem could be a red herring so you may have a problem elsewhere.

8th Aug 2006, 2:43 pm	#9
milairuk Pentode Join Date: May 2005 Location: Chelmsford, Essex, UK. Posts: 150	Re: Alas poor E7600/3 - I knew it well Fair comment but it behaves itself as long as the gate switch is open. I am missing the GZ33 and EL37 as I have yet to buy these. So unless I am wrong I cannot see anything else that will be "on" with the gate switch open ?