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Old 13th May 2021, 5:16 pm   #1
Spencervs
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Default Bush BAC31 mains unit

Hi there, I had a post about this elsewhere in the forum but it was posted in the wanted section and became a bit of a beg for help, so figured I would write a new post in a more appropriate section now that I have a bit more knowledge.

So I have recently restored this radio and changed all the wax capacitors and reformed most of the electrolytics. I tried it with battery and I get a good powerful performance from this radio however with the mains unit, the radio requires quite a bit of volume to be a reasonable output. I find if I change the transformer tap to 220v-230v then I get a good performance.

Valve voltages were tested with the batteries (90v and 7.5v) and I found they all matched the service sheet with the exception of DK92 which measured 88v on the anode and 40v on Screen. The mains unit output is 5.5v and 79v (when on 240/250v tapping). Is this normal?

Thanks in advance
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Old 13th May 2021, 8:50 pm   #2
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Default Re: Bush BAC31 mains unit

So have you measured your actual mains voltage? It might be low. In any case I would think the 220-230V setting was appropriate but it's worth checking your actual mains voltage and use the best setting. Don't assume that the mains is 240. Typically here it's usually around 230 - 236. Don't forget that the average owner of these sets (or more likely the supplier) would have set the mains to the appropriate mains voltage for the area and it would have been left at that. The radio would have been designed to cope with drops in mains (except very big drops) and continue to work. The original owner would not have been expected to check the mains each time he used the set and then change the setting! So a mains variation of + or -5% would be easily handled in normal use. I'm pretty sure mine is on the 240-250V tap.

I've not actually measured the voltages from the power supply on my BAC31 since it gives plenty of audio output on mains but I can do so when I next get in to the workshop.
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Old 13th May 2021, 9:00 pm   #3
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Default Re: Bush BAC31 mains unit

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Originally Posted by Spencervs View Post
The mains unit output is 5.5v and 79v (when on 240/250v tapping). Is this normal?
What values are given on the circuit diagram? They would be classed as 'normal'. Think about the valve filaments in series. Three of them are 1.4V, the output valve is 2.8V (it has two filaments in series). Add those together and you get 6.4 volts so you would expect to see at least 6.4V (in practice it might be slightly less) from the power supply. 5.5 volts I would say is borderline.

These little valves generally will run down to about 1.1 volts on the filaments but if they are original and slightly low emission, they might struggle. This could be your problem if the output from the power supply is slightly low.

Check your actual mains voltage first.
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Old 13th May 2021, 9:29 pm   #4
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Default Re: Bush BAC31 mains unit

I'll dig out my BAC31 and measure the voltages tomorrow.

Keith
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Old 13th May 2021, 11:10 pm   #5
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Default Re: Bush BAC31 mains unit

Thank you for the replies! I have checked my mains voltage coming in and measuring exactly 240v AC.

Just took the chassis out and had a check on the valve voltages. According to the service sheet, I should expect the following:

(V1) DK92 = 90 - 42v anode / 62v screen
(V2) DF91 = 90v anode / 62v screen
(V3) DAF91 = 42v anode / 22v screen
(V4) DL94 = 85v anode / 90v screen

These are my actual readings on the 240v tap:

(V1) DK92 = 78v anode / 35v screen
(V2) DF91 = 80v anode / 60v screen
(V3) DAF91 = 56v anode / 49v screen
(V4) DL94 = 76v anode / 78v screen

So keeping in mind that I have checked the resistors and noted that all are in tolerance and the wax capacitors have been replaced, I can't help but feel V1 is quite low on the screen and V3 is high on the screen. Is this an issue?

Also finally the EZ41 shows these readings:

Anodes =131 V AC (125V recommended)
Cathode = 45v AC (106V recommended)
output = 135v DC

Perhaps I'm being fussy and should leave it on the 220/230v tap?
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Old 14th May 2021, 7:59 am   #6
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Default Re: Bush BAC31 mains unit

According to the manufacturers service manual early versions had a single 33k resistor feeding the screen grids of both the DK92 and DF91 giving a voltage of 62V on both valves. Later versions separated the screen grid feeds with 180k for the DK92 and 56k for the DF91. The fact that you have different voltages on the screen grids indicates that your radio has the two separate resistors. Using the screen grid currents in the manual and your anode voltage measurements gives screen grid voltages of approximately 42V for the DK92 and 58V for the DF91 which are not far from your measurements. So nothing to worry about.

I suspect the manual was not updated following the change to the separate resistors.

Keith
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Old 14th May 2021, 2:03 pm   #7
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Default Re: Bush BAC31 mains unit

Quote:
Originally Posted by Spencervs View Post
Also finally the EZ41 shows these readings:

Anodes =131 V AC (125V recommended)
Cathode = 45v AC (106V recommended)
output = 135v DC

Perhaps I'm being fussy and should leave it on the 220/230v tap?
Where do you get your AC reading on the cathode?? It's a rectifier so by definition it should only have DC present. The output of 135VDC would be entirely expected.
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Old 16th May 2021, 2:51 pm   #8
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Default Re: Bush BAC31 mains unit

Hi there, according to the manual, I should be recieving 125v AC on the anodes (it specifies AC for anodes) and 106v on the cathode. It doesn't actually specify AC or DC for cathode but I assume they are the brown wires from the transformer and it only measures in AC. So the connections I tested were the red lines from the transformer (assuming anodes) which measure at about 135v and the brown cables I am assuming are the cathode (based on the diagram) but I only measure 40-50v on the AC range and nothing DC on both of those pins. The DC voltage comes from the red cable leading to the capacitor.

Having said this now I know the other valves are ok, I think it's fine. The volume is good and I have it on the 220-230 v tap as it gives best results. Only puzzle for me is that rectifier valve and why it's so different to what the manual says... Also I'd like some assurance that having the 220-230v setting is ok when my home voltage is about 238-242v.
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Old 16th May 2021, 3:08 pm   #9
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Default Re: Bush BAC31 mains unit

The brown wires aren't the cathode they're the heater, the cathode is the Red wire that's connected to the 2.7k resistor on the reservoir capacitor.

Lawrence.
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Old 16th May 2021, 9:39 pm   #10
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Default Re: Bush BAC31 mains unit

Ah thanks for the clarification! That makes a.lot more sense and I'm happy to let it be now. Only final issue, should I keep it on the 220-230v tapping? I get good sound from it but don't want to wear parts out if I can help it.

Thanks again! This is my first set that includes mains power as well as batteries!
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Old 17th May 2021, 7:50 am   #11
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Default Re: Bush BAC31 mains unit

I would measure the voltages on the 220-230 tap, including that of the rectifier, as you did above for the 240v tap

As long as the valve voltages do not exceed the maximum recommended, I think don't think you'd be causing any harm.
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Old 17th May 2021, 10:40 am   #12
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Default Re: Bush BAC31 mains unit

The most critical voltage is the filament voltage. These little battery valves won't take kindly to any overload on the filament. They are designed to run at 1.4 volts (2.8 for the output valve) and anything much above 1.5 for any length of time will cause short life. As has been stated before, check the mains voltage coming in. If it measures much above 230V, then you should use the higher voltage tap. If that causes low volume or poor operation (it shouldn't), then as alluded to before, you should change the rectifier.

Further back, you said your mains voltage was exactly 240V. If that is the case, you don't want the tap on 220 - 230V. As I said earlier, the rectifier is probably slightly below par and all you are doing by using a lower mains tap is slightly overrunning the rectifier (which is already running at or near its limit) and increasing the output from it.

I think a new EZ40 would solve your problems and you could set the voltage selector correctly.
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Old 17th May 2021, 11:25 am   #13
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Default Re: Bush BAC31 mains unit

Hi Spencer,
First I'm pleased you have the Bush going.

Then, if you read the Trader sheet it says, ''the voltages were taken using 'new batteries'. Those taken using the mains unit were 10% lower.''
Don't forget the batteries only delivered their nominal output for a short while, slowly becoming less and less as they were consumed. So the set is able to operate on a quite wide range of voltages.

The statement on the sheet 'Cathode voltage 106V' makes no sense, I would assume it is an error or part of an incomplete measurement / statement.

As has been mentioned, you really want to run the filaments at as low a voltage as possible that give reasonable results. I think about 6 volts would be ideal (5 x 1.2 volts). Over running them shortens the life of the valves especially the DK92 frequency changer.

One other thing, you say you reformed the electrolytic caps, but did you check them for actual capacity? If the value has dropped substantially then the HT voltage will measure lower than you might expect. If you have a 10uF or 22uF cap of 200 volts or more in your spares, you could clip it across C27 and see if the HT voltage increases.

I too would reset the selector to 240 volts.

Alan
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Old 17th May 2021, 11:54 am   #14
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Default Re: Bush BAC31 mains unit

106 volts at the cathode wouldn't be that far out for 80 volts out of the filter.

Lawrence.
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Old 17th May 2021, 12:03 pm   #15
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Default Re: Bush BAC31 mains unit

Indeed,
but a figure of 135 volts DC was quoted further up the thread?
I assume there is a more detailed description of how the 106V is derived in the Bush service manual. I only have a Trader Sheet for reference and that has already caused some confusion.
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Old 17th May 2021, 12:11 pm   #16
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Default Re: Bush BAC31 mains unit

Quote:
Originally Posted by snowman_al View Post
Indeed,
but a figure of 135 volts DC was quoted further up the thread?
I assume there is a more detailed description of how the 106V is derived in the Bush service manual. I only have a Trader Sheet for reference and that has already caused some confusion.
In the Radio Servicing data the HT out from the filter is given as 80 volts so that would fit with the 106 volts at the cathode (approx 9 -10mA HT load current)

The same info can be derived from the Trader sheet.

Lawrence.

Last edited by ms660; 17th May 2021 at 12:16 pm. Reason: extra info
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Old 17th May 2021, 12:35 pm   #17
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Default Re: Bush BAC31 mains unit

Cannot argue with that,
once you establish you are expecting 80 volts HT+ when switched to mains, from the Trader Sheet statements.
Wonder where the 135 volt figure came from.

Guess we are all agreeing it is actually working as specified...

Alan
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Old 18th May 2021, 11:03 am   #18
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Default Re: Bush BAC31 mains unit

So I know what the problem was. Sorry for being so slow everyone! I realised that the mains unit has to be connected via the bolt on top to the radio chassis in order to work properly. When I connected a wire to both bolt holes, it connected both neutral lines/earth I suppose and the voltage for the rectifier cathode dropped to 102v and all the other voltages seemed to even out. Regarding volume, it's ok volume on the 240v tap on some stations but just not enough on stations like radio 4, so I think it's more of a signal problem and/or possibly realignment? (which I don't have the equipment to do sadly). So I think it's ok for now. On the 230v tap, cathode reads 110v.

Also regarding batteries, the original is 7.5v for the LT however when I attach 5 D cells to make 7.5-8v you get that awful interference which sounds like someone tightening a guitar string and a lot of odd sci-fi sound effects. Well this problem is eliminated when you just use 4 batteries. And I feel better about that as I noticed originally one of the valves glowing with the 5 batteries so they clearly have a bit to much oomph.

Thanks again Alan for this fun project, learnt a lot about power supplies! Also thank you all for your patience and input.

Last edited by Spencervs; 18th May 2021 at 11:16 am.
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