BUSH VHF61 - MW/LW woes - UK Vintage Radio Repair and Restoration Discussion Forum

awc · 18th Oct 2011, 4:55 pm

Have just completed a recap (all 'orrible Hunts removed) and general service of the above. Although VHF works fine, all I get on MW and LW is a fierce crackle of the kind that makes you switch back to VHF immediately!

Have checked my work and can't find any fault. Have cleaned all the contacts on the switchbank with a cotton bud dipped in Isopropyl. Tried removing ECH81, this reduces crackle to an odd pop or two only.

Any thoughts on where to check next?

Thanks in anticipation

Alex

vinrads · 18th Oct 2011, 5:10 pm

Hi, did it work on AM prior to the rebuild?

PJL · 18th Oct 2011, 5:14 pm

This sounds like the power to the local oscillator (ECH81) which is switched by S3b according to the Bush service instructions. IPA is not much good at switch cleaning, you need Servisol.

You can confirm this by measuring the volts on pin 8 of the ECH81.

awc · 18th Oct 2011, 6:15 pm

Thanks for your replies. Regretfully, I didn't try the set prior to the recap as the smoother/reservoir was blown and having replaced that, just carried on with the restoration. A lesson for the future perhaps.

I am working from the Trader Sheet 1301, which doesn't have a switch S3b, can you advise the switch number concerned on the Trader Sheet?

Many thanks

Alex

Sideband · 18th Oct 2011, 6:49 pm

On the Trader sheet it appears to be S13. L12 and L11 is the oscillator coil. R10 is the oscillator anode load and C31 is the decoupler.

SB

awc · 3rd Nov 2011, 12:49 pm

Update. Having double-checked the work I did and resoldering a couple of joints I inverted the set and switched to MW in darkened room, to immediately see sparks flashing around S4 S13 and S14 on the FM switch and S12 and S7 and 9a and 9b on the MW switch. Nothing immediately apparent as to why.

Is this likely do you think to be the usual paxolin tracking or a fault elsewhere causing the switches to spark? Any thoughts will be much appreciated.

Alex

Leon Crampin · 3rd Nov 2011, 1:34 pm

I'm afraid tracking through the (poor) paxolin on the Plessey switch banks used by Bush is all too common.

The worst problem is the switch in the audio feedback loop which changes the gain for FM - it tracks straight through to the HT on the reverse of the wafer. There are actually enough spare contacts on the other wafers to enable a complete work-around with a bit of extra wiring. Tracking here makes a dreadful noise.

The combination of poor materials and ridiculously close contact spacing makes the restoration of any push button Bush which has been stored in the damp a real problem. VHF90 wavechange switches (same maker) do this as well. Once they track, there is no permanent fix other than to use another switch - or undamaged spare contact.

Leon.

paulsherwin · 3rd Nov 2011, 1:52 pm

You can rewire the HT supplies so that they are no longer switched. This means that HT will be permanently on for all valves whatever band the radio is switched to. It will slightly shorten valve life compared to the original arrangement, but this shouldn't be an issue in practice.

Leon Crampin · 3rd Nov 2011, 4:31 pm

Unfortunately on the Bush, there are just too many switches at HT potential as the (multiple) IF stage transformers are switched. Any modifications here would affect the alignment and potentially cause trouble - they're switched for a reason. Keeping the AM local oscillator running on FM would be interesting too...

Leon.

paulsherwin · 3rd Nov 2011, 4:44 pm

I've actually done this on simpler sets and it did work without problems, but I didn't appreciate the complexity of the HT switching here, as I've no experience of this set. Given what you've said I'd agree that rewiring would be a last resort.

awc · 3rd Nov 2011, 5:09 pm

Hail to the wisdom of the mighty forum!

I decided to go for removing the ht leads to S4 and S5 and joining them together and moving S12 down to the unused set immediately below and "hey presto" the fault is completely cured!

Many, many thanks

Alex

awc · 8th Nov 2011, 10:22 pm

Footnote: Having got this set all nicely sorted and back in its case, I suddenly started getting interference on VHF. This took the form of a rythmic beat, one long burst followed by two short bursts, reflected also in the magic eye going in and out.

My initial thoughts were that there must be some sort of fault in the FM Tuner Head until that was that I turned off a battery charger of the switched mode power supply variety that was charging up some AA batteries nearby.

Need I say more?

Alex

18th Oct 2011, 4:55 pm	#1
awc Retired Dormant Member Join Date: Jul 2005 Location: Somerset, UK. Posts: 334	BUSH VHF61 - MW/LW woes Have just completed a recap (all 'orrible Hunts removed) and general service of the above. Although VHF works fine, all I get on MW and LW is a fierce crackle of the kind that makes you switch back to VHF immediately! Have checked my work and can't find any fault. Have cleaned all the contacts on the switchbank with a cotton bud dipped in Isopropyl. Tried removing ECH81, this reduces crackle to an odd pop or two only. Any thoughts on where to check next? Thanks in anticipation Alex

18th Oct 2011, 5:10 pm	#2
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,756	Re: BUSH VHF61 - MW/LW woes Hi, did it work on AM prior to the rebuild?

18th Oct 2011, 5:14 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BUSH VHF61 - MW/LW woes This sounds like the power to the local oscillator (ECH81) which is switched by S3b according to the Bush service instructions. IPA is not much good at switch cleaning, you need Servisol. You can confirm this by measuring the volts on pin 8 of the ECH81.

18th Oct 2011, 6:15 pm	#4
awc Retired Dormant Member Join Date: Jul 2005 Location: Somerset, UK. Posts: 334	Re: BUSH VHF61 - MW/LW woes Thanks for your replies. Regretfully, I didn't try the set prior to the recap as the smoother/reservoir was blown and having replaced that, just carried on with the restoration. A lesson for the future perhaps. I am working from the Trader Sheet 1301, which doesn't have a switch S3b, can you advise the switch number concerned on the Trader Sheet? Many thanks Alex

18th Oct 2011, 6:49 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: BUSH VHF61 - MW/LW woes On the Trader sheet it appears to be S13. L12 and L11 is the oscillator coil. R10 is the oscillator anode load and C31 is the decoupler. SB __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

3rd Nov 2011, 12:49 pm	#6
awc Retired Dormant Member Join Date: Jul 2005 Location: Somerset, UK. Posts: 334	Re: BUSH VHF61 - MW/LW woes Update. Having double-checked the work I did and resoldering a couple of joints I inverted the set and switched to MW in darkened room, to immediately see sparks flashing around S4 S13 and S14 on the FM switch and S12 and S7 and 9a and 9b on the MW switch. Nothing immediately apparent as to why. Is this likely do you think to be the usual paxolin tracking or a fault elsewhere causing the switches to spark? Any thoughts will be much appreciated. Alex

3rd Nov 2011, 1:34 pm	#7
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: BUSH VHF61 - MW/LW woes I'm afraid tracking through the (poor) paxolin on the Plessey switch banks used by Bush is all too common. The worst problem is the switch in the audio feedback loop which changes the gain for FM - it tracks straight through to the HT on the reverse of the wafer. There are actually enough spare contacts on the other wafers to enable a complete work-around with a bit of extra wiring. Tracking here makes a dreadful noise. The combination of poor materials and ridiculously close contact spacing makes the restoration of any push button Bush which has been stored in the damp a real problem. VHF90 wavechange switches (same maker) do this as well. Once they track, there is no permanent fix other than to use another switch - or undamaged spare contact. Leon.

3rd Nov 2011, 1:52 pm	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,940	Re: BUSH VHF61 - MW/LW woes You can rewire the HT supplies so that they are no longer switched. This means that HT will be permanently on for all valves whatever band the radio is switched to. It will slightly shorten valve life compared to the original arrangement, but this shouldn't be an issue in practice.

3rd Nov 2011, 4:31 pm	#9
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: BUSH VHF61 - MW/LW woes Unfortunately on the Bush, there are just too many switches at HT potential as the (multiple) IF stage transformers are switched. Any modifications here would affect the alignment and potentially cause trouble - they're switched for a reason. Keeping the AM local oscillator running on FM would be interesting too... Leon.

3rd Nov 2011, 4:44 pm	#10
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,940	Re: BUSH VHF61 - MW/LW woes I've actually done this on simpler sets and it did work without problems, but I didn't appreciate the complexity of the HT switching here, as I've no experience of this set. Given what you've said I'd agree that rewiring would be a last resort.

3rd Nov 2011, 5:09 pm	#11
awc Retired Dormant Member Join Date: Jul 2005 Location: Somerset, UK. Posts: 334	Re: BUSH VHF61 - MW/LW woes Hail to the wisdom of the mighty forum! I decided to go for removing the ht leads to S4 and S5 and joining them together and moving S12 down to the unused set immediately below and "hey presto" the fault is completely cured! Many, many thanks Alex

8th Nov 2011, 10:22 pm	#12
awc Retired Dormant Member Join Date: Jul 2005 Location: Somerset, UK. Posts: 334	Re: BUSH VHF61 - MW/LW woes Footnote: Having got this set all nicely sorted and back in its case, I suddenly started getting interference on VHF. This took the form of a rythmic beat, one long burst followed by two short bursts, reflected also in the magic eye going in and out. My initial thoughts were that there must be some sort of fault in the FM Tuner Head until that was that I turned off a battery charger of the switched mode power supply variety that was charging up some AA batteries nearby. Need I say more? Alex