Bush DAC90A long awaited repair. - UK Vintage Radio Repair and Restoration Discussion Forum

sobell1980 · 5th Jun 2019, 7:34 pm

Hello everyone

It's been quite some time since I last posted about my repairs on the forum. That doesn't mean I've not been busy with the world of wireless, I've just been trying to figure these sets out a bit more for myself although I ought to perhaps post something in the success stories.
I've tackled the DAC90A, but have become a bit stuck. Let me tell you a bit about the repair. I started by replacing all the usual waxies. This brought the set to life but only just. I was faces with two broken valve pins giving intermittent connections. Only MW could be heard. I noticed someone before had replaced the wave change switch. I investigated as I found S2 Was not being made across the wave change switch. I found someone had replaced the wave change switch with an incorrect type. It was a 2pole 6 way switch that could be adjusted to 2 right up to 6 way. The problem being though that on LW both S2 and S3 have to be made at the same time, so two sets of contacts. This being a 2 pole switch only one set of contacts were being made on LW. Hence why LW was inoperative but MW worked as S1 shorts L1 out and S1 being the only switch needed to be made for MW . I replaced the wave change switch for a 4pole 6 way switch and set the position of the switch with the adjusting washer to 2 way and LW was restored, result.
Now I had got stable operation I ran through the valve voltages and found the HT to be low. I checked the various HT resistors and found they had gone high so I thought winner! I replaced R15, R16 and R18. 're checked my readings and achieved a slight rise in HT but nothing substantial. I then swooped out valve UY41 for a known good one, new old stock. Still the HT not much better. So here are my readings

V1 anodes 78.5 volts and 50 volts
V1 screen 50.00 volts
V1 cathode 0.8 volts

V2 anode 78.5 volts
V2 screen 51.00 volts
V2 cathode 0.8 volts

V3 anode 35.00 volts
V3 Cathode 0.8 volts

V4 anode 160 volts
V4 screen 84 volts
V4 cathode 4.50 volts

V5 anode 185 volts ac
V5 cathode 178 volts dc

R17 is set at the correct tapping and measure 305 ohms across its connections and 1.249 k ohms end to end, so within spec. If anyone can comment on this is be very grateful.

Also LW volume still feels low. I put my finger on L1 aerial and it really boosts it up. I'm hoping If I can get the HT up this will help it. The aerial coils seem in good health and don't appear to be shorting.
Many thanks for reading.

Dave.

Sideband · 5th Jun 2019, 8:22 pm

The HT on these is not particularly high but I seem to recall the main HT is around 200 and the decoupled HT is about 90. The only other thing that could be loading the HT is the UL41....probably the usual leakage between the pins. These are somewhat rare now and if you can find a new one it'll probably set you back around £20. If you have a scout around the forum you will find suggestions on how to clear the leakage in the valve......this can be quite successful. Also you can try disconnecting anything that is connected to pin 4 of the UL41. Pin 4 should be an unused pin and it's often used as an 'anchor' point for other components. Just isolate the connections from pin 4. That alone could make a big difference to the HT reading.

It's also possible that some realignment is required if touching the aerial coils significantly improves reception. You don't know if it's been twiddled by the person who changed the wavechange switch.

Actually looking at your voltage readings, I would say the HT is acceptable.

sobell1980 · 5th Jun 2019, 8:58 pm

I will have a look at what you mention tomorrow night. I'm hoping not alignment. I have a sig gen but I always do it as a last resort. Radio 4 LW seems about the right place and can be heard through a fairly large section of the tuning scale as on all my other sets. I've worked on a fair few different sets now to know this should perform better than it is currently. It may do me good to attempt an alignment on the set. I will do the other things you mention first though to try and raise the HT.
Many thanks
Dave.

AD360 Rob · 6th Jun 2019, 8:25 pm

If R4 on long wave is in about the right position on the scale it may be as simple as a tweak of the LW aerial trimmer but do bear in mind that the internal aerial on these sets is not perfect and will always respond to a bit of hand capacitance. I would say it's worth trying that first before getting the sig gen out.

sobell1980 · 6th Jun 2019, 9:40 pm

I only managed to get 20 mins on the set tonight. I isolated pin 4 on valve 4 which did not make any difference to either LW or the HT.
I then thought best to check L1 and L2 aerial coils. The service data states that they should approx be 3.2 ohms. With the wave change switch set to MW both L1 and L2 read 3.2 ohms. However with the wave change switch set to LW L1 reads 7 ohms and L2 reads 3.2 ohms. Switch it back to MW and L1 changes from 3.2 to 7 ohms dependant on wave change position. I'm measuring each coil from end to centre tap.

Sideband · 6th Jun 2019, 11:23 pm

It is VERY unlikely that you will have any problems with the aerial coils...more likely the w/c switch or the wiring....and remember that the given DC resistances of coils is only approximate.

Herald1360 · 7th Jun 2019, 9:04 am

Sounds like you're using the Trader service sheet. That's rather misleading about the wavechange switch connections showing three SPNO switches rather than the two SPCO ones actually used.

Function is very simple, on LW, the first section switch contacts are open across the LW coil, connecting both aerial coils in series and closed to connect extra capacitance across the tuning gang aerial section. The second section switch contacts are closed to connect extra capacitance across the tuning gang oscillator section. For MW, the first section switch contacts are closed across the LW coil, shorting out one section of the aerial coils and open to disconnect the extra capacitance across the tuning gang aerial section. The second section switch contacts are open to disconnect the extra capacitance across the tuning gang oscillator section.

It's a neat bit of cost saving design, though maybe a little less optimal RF gain/tracking wise than the usual much more elaborate ways of doing it.

sobell1980 · 15th Jun 2019, 3:38 pm

Success! Look what dusting off the old signal generator did! LW very loud and clear. The set is now waiting to go to it's new home. It was a good repair to get me back in the driving seat of wireless repairs. It's good sometimes to have something more than just a recap. This definitely had the wrong wave change switch fitted. Corroded valve pins, resistors gone high and the phantom twiddler had been in a majorly adjusted the the tuning coils and trimming caps. Click on the link to see the success story. Thanks again for all your replies and advice. https://youtu.be/224JWI6YJZY
Dave.

sobell1980 · 16th Jun 2019, 8:44 pm

This thread can now be closed.

5th Jun 2019, 7:34 pm	#1
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Bush DAC90A long awaited repair. Hello everyone It's been quite some time since I last posted about my repairs on the forum. That doesn't mean I've not been busy with the world of wireless, I've just been trying to figure these sets out a bit more for myself although I ought to perhaps post something in the success stories. I've tackled the DAC90A, but have become a bit stuck. Let me tell you a bit about the repair. I started by replacing all the usual waxies. This brought the set to life but only just. I was faces with two broken valve pins giving intermittent connections. Only MW could be heard. I noticed someone before had replaced the wave change switch. I investigated as I found S2 Was not being made across the wave change switch. I found someone had replaced the wave change switch with an incorrect type. It was a 2pole 6 way switch that could be adjusted to 2 right up to 6 way. The problem being though that on LW both S2 and S3 have to be made at the same time, so two sets of contacts. This being a 2 pole switch only one set of contacts were being made on LW. Hence why LW was inoperative but MW worked as S1 shorts L1 out and S1 being the only switch needed to be made for MW . I replaced the wave change switch for a 4pole 6 way switch and set the position of the switch with the adjusting washer to 2 way and LW was restored, result. Now I had got stable operation I ran through the valve voltages and found the HT to be low. I checked the various HT resistors and found they had gone high so I thought winner! I replaced R15, R16 and R18. 're checked my readings and achieved a slight rise in HT but nothing substantial. I then swooped out valve UY41 for a known good one, new old stock. Still the HT not much better. So here are my readings V1 anodes 78.5 volts and 50 volts V1 screen 50.00 volts V1 cathode 0.8 volts V2 anode 78.5 volts V2 screen 51.00 volts V2 cathode 0.8 volts V3 anode 35.00 volts V3 Cathode 0.8 volts V4 anode 160 volts V4 screen 84 volts V4 cathode 4.50 volts V5 anode 185 volts ac V5 cathode 178 volts dc R17 is set at the correct tapping and measure 305 ohms across its connections and 1.249 k ohms end to end, so within spec. If anyone can comment on this is be very grateful. Also LW volume still feels low. I put my finger on L1 aerial and it really boosts it up. I'm hoping If I can get the HT up this will help it. The aerial coils seem in good health and don't appear to be shorting. Many thanks for reading. Dave.

5th Jun 2019, 8:22 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: Bush DAC90A long awaited repair. The HT on these is not particularly high but I seem to recall the main HT is around 200 and the decoupled HT is about 90. The only other thing that could be loading the HT is the UL41....probably the usual leakage between the pins. These are somewhat rare now and if you can find a new one it'll probably set you back around £20. If you have a scout around the forum you will find suggestions on how to clear the leakage in the valve......this can be quite successful. Also you can try disconnecting anything that is connected to pin 4 of the UL41. Pin 4 should be an unused pin and it's often used as an 'anchor' point for other components. Just isolate the connections from pin 4. That alone could make a big difference to the HT reading. It's also possible that some realignment is required if touching the aerial coils significantly improves reception. You don't know if it's been twiddled by the person who changed the wavechange switch. Actually looking at your voltage readings, I would say the HT is acceptable. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 5th Jun 2019 at 8:32 pm.

5th Jun 2019, 8:58 pm	#3
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Bush DAC90A long awaited repair. I will have a look at what you mention tomorrow night. I'm hoping not alignment. I have a sig gen but I always do it as a last resort. Radio 4 LW seems about the right place and can be heard through a fairly large section of the tuning scale as on all my other sets. I've worked on a fair few different sets now to know this should perform better than it is currently. It may do me good to attempt an alignment on the set. I will do the other things you mention first though to try and raise the HT. Many thanks Dave.

6th Jun 2019, 8:25 pm	#4
AD360 Rob Hexode Join Date: Jun 2012 Location: Chester, Cheshire, UK. Posts: 421	Re: Bush DAC90A long awaited repair. If R4 on long wave is in about the right position on the scale it may be as simple as a tweak of the LW aerial trimmer but do bear in mind that the internal aerial on these sets is not perfect and will always respond to a bit of hand capacitance. I would say it's worth trying that first before getting the sig gen out.

6th Jun 2019, 9:40 pm	#5
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Bush DAC90A long awaited repair. I only managed to get 20 mins on the set tonight. I isolated pin 4 on valve 4 which did not make any difference to either LW or the HT. I then thought best to check L1 and L2 aerial coils. The service data states that they should approx be 3.2 ohms. With the wave change switch set to MW both L1 and L2 read 3.2 ohms. However with the wave change switch set to LW L1 reads 7 ohms and L2 reads 3.2 ohms. Switch it back to MW and L1 changes from 3.2 to 7 ohms dependant on wave change position. I'm measuring each coil from end to centre tap.

6th Jun 2019, 11:23 pm	#6
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: Bush DAC90A long awaited repair. It is VERY unlikely that you will have any problems with the aerial coils...more likely the w/c switch or the wiring....and remember that the given DC resistances of coils is only approximate. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

7th Jun 2019, 9:04 am	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Bush DAC90A long awaited repair. Sounds like you're using the Trader service sheet. That's rather misleading about the wavechange switch connections showing three SPNO switches rather than the two SPCO ones actually used. Function is very simple, on LW, the first section switch contacts are open across the LW coil, connecting both aerial coils in series and closed to connect extra capacitance across the tuning gang aerial section. The second section switch contacts are closed to connect extra capacitance across the tuning gang oscillator section. For MW, the first section switch contacts are closed across the LW coil, shorting out one section of the aerial coils and open to disconnect the extra capacitance across the tuning gang aerial section. The second section switch contacts are open to disconnect the extra capacitance across the tuning gang oscillator section. It's a neat bit of cost saving design, though maybe a little less optimal RF gain/tracking wise than the usual much more elaborate ways of doing it. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O Last edited by Herald1360; 7th Jun 2019 at 9:24 am. Reason: Second thoughts!

15th Jun 2019, 3:38 pm	#8
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Bush DAC90A long awaited repair. Success! Look what dusting off the old signal generator did! LW very loud and clear. The set is now waiting to go to it's new home. It was a good repair to get me back in the driving seat of wireless repairs. It's good sometimes to have something more than just a recap. This definitely had the wrong wave change switch fitted. Corroded valve pins, resistors gone high and the phantom twiddler had been in a majorly adjusted the the tuning coils and trimming caps. Click on the link to see the success story. Thanks again for all your replies and advice. https://youtu.be/224JWI6YJZY Dave.

16th Jun 2019, 8:44 pm	#9
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: Bush DAC90A long awaited repair. This thread can now be closed.