CM Howes CVF80 kit zener diode voltage? - UK Vintage Radio Repair and Restoration Discussion Forum

MrBungle · 21st Mar 2018, 11:33 pm

I'm spending a couple of hours repairing a very badly built Howes kit for a friend of mine who picked it up from a silent key. It is an 80M DC receiver. I've had this in the cupboard for about 3 months.

Looks like one of the wires has come off from some shoddy soldering and shorted the CVF80 board rail to ground. This has destroyed the pass transistor, zener and one of the resistors and toasted a trace.

Now I did some googling and found the instructions for the kit but it unhelpfully claims that the zener "is a BZY88" and that's it. No voltage or anything useful.

Can anyone shed any light as to what the voltage of this zener might be?

I can trial some values but the traces are already toasted so I'm not sure I want to risk too much soldering around that area in case anything else lifts.

Terry_VK5TM · 22nd Mar 2018, 4:30 am

Based on a supply voltage of 10-15V and VFO output levels of 3V, I would hazard a guess and say it is either 6.2 or 6.8 v.

I would also probably plunk for a 78L06 and scrap the pass transistor arrangement if the damage is that bad re tracks (unless originality is desired).

MrBungle · 22nd Mar 2018, 8:31 am

Yeah I was going to try a 6.2V one if I didn’t get an answer.

Actually that’s a really good idea with the 78L06. I have a bag of them. Thanks for the suggestion. I will do that. I’m not too worried about originality here as the whole thing is a mess.

Nuvistor · 22nd Mar 2018, 9:16 am

The G-QRP club have circuit diagrams and instruction info.
http://www.gqrp.com/howes.htm

MrBungle · 22nd Mar 2018, 10:01 am

I've already got the instructions for it. They merely state this as the part. No indication of zener voltage:

Nuvistor · 22nd Mar 2018, 10:50 am

The thought comes to my mind that perhaps it was any value between 5.6 and 7.2, whatever was available.
The 78L06 looks like a good idea.

MrBungle · 22nd Mar 2018, 11:14 am

Well I just tried a 6.2V zener in it and it appears to be as dead as a dodo still. Think something else has gone phut. I will persist further this evening.

I am getting to the point I will strip the board bare and rework it from scratch with new parts. I have all of them in stock.

There is no outcome desired from this process, merely not being defeated

Nuvistor · 22nd Mar 2018, 11:50 am

I built a few Howes kits, they all worked well, mainly test equipment and I think there was a TX and RX, thirty years ago or so ago.

MrBungle · 22nd Mar 2018, 11:57 am

Yeah I'm quite impressed with the design so far. I did some looking around at the other ones they did and I genuinely wish they were still around. I'd probably spend a lot of money

Only problem with this one is the guy who assembled it. Seen worse things done to Heathkits though!

MrBungle · 26th Mar 2018, 11:09 pm

Got it working. Disassembled the entire VFO apart from the inductor, repaired the board, cleaned and washed it and replaced all of the components with new ones. Sprang straight into action and is now quite happily tuning on a long wire antenna out of the window. VFO is quite stable. Impressed.

Used a 6.2v Zener in the end to keep it original plus it was difficult to get a 78L06 in without modifying everything.

Bit that took the longest to fix was working out why the cap wasn’t affecting VFO frequency. Turned out the wire was broken inside the insulation

Returning to original owner now in exchange for beer

21st Mar 2018, 11:33 pm	#1
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	CM Howes CVF80 kit zener diode voltage? I'm spending a couple of hours repairing a very badly built Howes kit for a friend of mine who picked it up from a silent key. It is an 80M DC receiver. I've had this in the cupboard for about 3 months. Looks like one of the wires has come off from some shoddy soldering and shorted the CVF80 board rail to ground. This has destroyed the pass transistor, zener and one of the resistors and toasted a trace. Now I did some googling and found the instructions for the kit but it unhelpfully claims that the zener "is a BZY88" and that's it. No voltage or anything useful. Can anyone shed any light as to what the voltage of this zener might be? I can trial some values but the traces are already toasted so I'm not sure I want to risk too much soldering around that area in case anything else lifts.

22nd Mar 2018, 4:30 am	#2
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: CM Howes CVF80 kit zener diode voltage? Based on a supply voltage of 10-15V and VFO output levels of 3V, I would hazard a guess and say it is either 6.2 or 6.8 v. I would also probably plunk for a 78L06 and scrap the pass transistor arrangement if the damage is that bad re tracks (unless originality is desired). __________________ Terry VK5TM https://www.vk5tm.com/

22nd Mar 2018, 8:31 am	#3
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: CM Howes CVF80 kit zener diode voltage? Yeah I was going to try a 6.2V one if I didn’t get an answer. Actually that’s a really good idea with the 78L06. I have a bag of them. Thanks for the suggestion. I will do that. I’m not too worried about originality here as the whole thing is a mess.

22nd Mar 2018, 9:16 am	#4
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: CM Howes CVF80 kit zener diode voltage? The G-QRP club have circuit diagrams and instruction info. http://www.gqrp.com/howes.htm __________________ Frank

22nd Mar 2018, 10:01 am	#5
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: CM Howes CVF80 kit zener diode voltage? I've already got the instructions for it. They merely state this as the part. No indication of zener voltage: Attached Thumbnails

22nd Mar 2018, 10:50 am	#6
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: CM Howes CVF80 kit zener diode voltage? The thought comes to my mind that perhaps it was any value between 5.6 and 7.2, whatever was available. The 78L06 looks like a good idea. __________________ Frank

22nd Mar 2018, 11:14 am	#7
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: CM Howes CVF80 kit zener diode voltage? Well I just tried a 6.2V zener in it and it appears to be as dead as a dodo still. Think something else has gone phut. I will persist further this evening. I am getting to the point I will strip the board bare and rework it from scratch with new parts. I have all of them in stock. There is no outcome desired from this process, merely not being defeated

22nd Mar 2018, 11:50 am	#8
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: CM Howes CVF80 kit zener diode voltage? I built a few Howes kits, they all worked well, mainly test equipment and I think there was a TX and RX, thirty years ago or so ago. __________________ Frank

22nd Mar 2018, 11:57 am	#9
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: CM Howes CVF80 kit zener diode voltage? Yeah I'm quite impressed with the design so far. I did some looking around at the other ones they did and I genuinely wish they were still around. I'd probably spend a lot of money Only problem with this one is the guy who assembled it. Seen worse things done to Heathkits though!

26th Mar 2018, 11:09 pm	#10
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: CM Howes CVF80 kit zener diode voltage? Got it working. Disassembled the entire VFO apart from the inductor, repaired the board, cleaned and washed it and replaced all of the components with new ones. Sprang straight into action and is now quite happily tuning on a long wire antenna out of the window. VFO is quite stable. Impressed. Used a 6.2v Zener in the end to keep it original plus it was difficult to get a 78L06 in without modifying everything. Bit that took the longest to fix was working out why the cap wasn’t affecting VFO frequency. Turned out the wire was broken inside the insulation Returning to original owner now in exchange for beer