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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 4th Jul 2018, 3:41 pm   #1
G4_Pete
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Default HRO Dog House PSU restore

I have an original pitch potted HRO doghouse power supply that I want to restore. I know these have a habit of failing with lots of hot pitch so before going any further I was trying to determine if the transformer was suitable for 240-250 mains as it was marked 230 Volt and I was making the assumption that it should be ok on 50 Hz.

It meggers between 50 and 100 meg between the windings

I ran it up injecting a variable low voltage into the heater and all looked good with voltages appearing on the other windings.

I then tried to set up a B/H curve trace to see if I could find the core saturation point but this was more tricky as I am not sure how to fully interpret the results. I can see the current increasing more rapidly as the voltage injected is increased but it is quite "soft" with no definite saturation that I could see. I appreciate all the source impedances of my test set up will affect the curve but still not sure so I have attached the results.

To keep it safe I injected and measured on the 6.3 volt winding but I am also not sure how valid that is for this test.

The whole object was to scientifically prove the transformer would be ok rather than just try it and see ( shorted turns excluded!)
Pete
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File Type: pdf Dog House psu.pdf (686.4 KB, 163 views)
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Old 4th Jul 2018, 6:53 pm   #2
G6Tanuki
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Default Re: HRO Dog House PSU restore

The one I had back in my student days powering a HRO-MX didn't seem to mind regular use of several hours a day on whatever voltage/frequency MANWEB was delivering to cottages in the hills around Aberystwyth at the time.

I did give it an easier life though by pulling-out the 6V6 as there was no need for it to be there wasting power when I was listening on 'phones.

For some time I've been an advocate of - for "communications-" receivers - fitting a higher value of cathode-bias resistor in the output stage if it's something like a 6V6 or 6F6 - so the anode current's down to 10-15mA rather than the more-usual 45mA for a 6V6. it reduces heat dissipation, gives the valve/output-transformer/PSU an easier life, and - let's face it - in a comms receiver do you _really_ need the 4.5 Watts of audio the likes of a 6V6 run with maximum-rated anode-current can deliver?
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Old 5th Jul 2018, 11:35 am   #3
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Default Re: HRO Dog House PSU restore

Hi,
Congratulations on your very scientific approach to transformer testing which puts many of the rest of us to shame, I think. I never did understand B-H curves properly.
By way of an alternative approach, would it not be informative to monitor temperature rise of the transformer core under load, as an indicator of the losses in the transformer and so the possible prescence of problems - or not. I have never had an HRO PSU but understand that the transformers are pitch potted which might make it difficult to make contact with the core material with a temperature probe, but a measurement of the surface temperature will still provide the information required - just delayed a bit. I would think it should be fairly obvious if excessive losses are present.
Good luck
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Old 5th Jul 2018, 4:08 pm   #4
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Default Re: HRO Dog House PSU restore

Hi all, Since posting I have now put the potted transformer on the mains to soak test open circuit and it runs at 3 +/- 0.5 Watts consumption (14 VA). According to my handy plug in the wall cheap wattmeter. Stayed cold for hours so no problem with Iron loss or saturation.
Re offloading by pulling or re-biasing the audio stage (G6_Tanuki), funny you should mention that as one time I was going to stabilise the HRO supply voltage by using the bias on the audio stage to work against the soft psu regulation, just to take out the small mains variations, but then I guess a clamped mosfet/transistor in series with the cathode bias is not in the spirit of things!
Pete
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Old 5th Jul 2018, 5:09 pm   #5
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Default Re: HRO Dog House PSU restore

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Originally Posted by G6Tanuki View Post
The one I had back in my student days powering a HRO-MX didn't seem to mind regular use of several hours a day on whatever voltage/frequency MANWEB was delivering to cottages in the hills around Aberystwyth at the time.
Yes, my dog kennel ran for many hours a day for several years on whatever voltage the Yorkshire Electricity Board was supplying in those days.

In my "on-going" mid-life-update of my HRO, the 42 valve audio stage has been ripped out and a little sub-chassis with an AF stage using an EF91 is waiting for evaluation. That should save quite a few mA.

B
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Old 6th Jul 2018, 12:05 am   #6
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Default Re: HRO Dog House PSU restore

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Originally Posted by G8BBZ View Post
Congratulations on your very scientific approach to transformer testing which puts many of the rest of us to shame, I think. I never did understand B-H curves properly.
I remember struggling with the concept when we did magnetic recording principles.

If only the teacher had used the word "backlash" as an analogy for hysteresis I think I would have "got it", as that was something I was familiar with from the vernier drives I used to tune my homebrew receivers and transmitters.
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Old 6th Jul 2018, 9:47 am   #7
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Default Re: HRO Dog House PSU restore

Hi,
Your test with mains applied and no load is undoubtedly useful and it's good to hear that the temperature rise was negligible. I am not sure that it indicates no problems with core saturation, however, as the primary current - and therefore core flux - would be at a minimum with no load. Nor does a no load test tell you much about the condition of the secondary windings - though you have done other tests which would seem to indicate that they are OK. Would it not be worthwhile doing a full load test to check temperature rise? I think this would give final confirmation that the transformer is OK - or not.

I also find the cheapy AC mains power monitors useful - enables you to switch the AC mains off quickly before the smoke appears!
cheers
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Old 6th Jul 2018, 6:49 pm   #8
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Default Re: HRO Dog House PSU restore

Hi
Yes I apreciate there is nothing like a full-blown test to sort out the problems, but I did not have the load resistors and to test it on the radio requires a full rebuild so chicken and egg situation. So I dug out my collage Edward Hughes Electrical Technology and worked through the transformer testing procedures as an exercise. I also tested the inductor by series resonating it against a 470n capacitor , resonates perfectly at approx 50Hz and which I calculated to around 22H. Of course the other driver is that these things are potted in black pitch and in is a messy smelly job involving heating the whole psu to melt it out so I did not want to do that after a rebuild or worse still have it go into meltdown in the house. I was not popular the last time a psu went up in smoke and I had to evacuate the house!!

I am not pleased that the supposedly balanced secondaries are at 337 + 322 volts and when I run the psu calculations on the excellent Duncan Amps psu simulator the voltage is going to be a bit high at around 440 Volts when the B+ switch is opened for transmit or coil changing and of course that voltage is put straight on the supply rails when the switch is closed.

Perhaps I look at things too deeply these radios and psu’s have been around for a long long time!!
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Old 6th Jul 2018, 6:59 pm   #9
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Default Re: HRO Dog House PSU restore

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Originally Posted by G4_Pete View Post
I am not pleased that the supposedly balanced secondaries are at 337 + 322 volts and when I run the psu calculations on the excellent Duncan Amps psu simulator the voltage is going to be a bit high at around 440 Volts when the B+ switch is opened for transmit or coil changing and of course that voltage is put straight on the supply rails when the switch is closed.
You could always add a series resistor in the 'over-voltage' rectifier-anode feed to even things up a bit: such resistors [a couple of hundred Ohms] are quite a common feature in the rectifier-anode-feeds of well-designed kit to prevent the rectifier being troubled by short-duration-high-current pulses.

And your worry about the B+ going way-high is very valid: the RCA AR88-series receivers have a similar issue when the B+ supply floats unloaded if the mode-switch is turned to "Transmit", and then it dumps the high-voltage into all circuits when you switch back to 'Receive". I've shorted-out the relevant contacts on the mode-switch of my 88 so the power-supply never floats unloaded.
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Old 7th Jul 2018, 3:22 pm   #10
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Default Re: HRO Dog House PSU restore

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I am not sure that it indicates no problems with core saturation, however, as the primary current - and therefore core flux - would be at a minimum with no load.
Surely the core flux remains essentially constant. Any increase caused by increased primary current on load is backed off by an equal and opposite flux from the loaded secondaries. ISTR.

Also isn't most of the extra loss resulting from core saturation simply the increased resistive loss in the primary from the higher rms current therein?
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Old 8th Jul 2018, 4:32 pm   #11
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Default Re: HRO Dog House PSU restore

The core flux will drop when a load is applied due to the lower effective primary voltage.
The core flux will rise if the secondaries are not matched and a DC component is present.
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Old 8th Jul 2018, 4:37 pm   #12
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Default Re: HRO Dog House PSU restore

I had a doghouse go bubbly doo, all things considered I came to the conclusion that it didn't like 50Hz.

Lawrence.
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Old 9th Jul 2018, 2:00 pm   #13
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Default Re: HRO Dog House PSU restore

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The core flux will drop when a load is applied due to the lower effective primary voltage.
The core flux will rise if the secondaries are not matched and a DC component is present.
Ahh, so.

Remember "It's the volts that sets up the flux" being one of Eric Laithwaites's mantras from heavy electrical lectures at uni!

Is the lower effective voltage a symptom of the primary itself or line drop?
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Old 14th Jul 2018, 10:19 pm   #14
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Default Re: HRO Dog House PSU restore

PSU all up and running now , its been on all afternoon with an HRO load running 60 mA at 210 volts, just warm to touch.

I wired it as choke input to the reservoir with RC filtering on the 2nd half of the dual can, this reduces the volts a bit , reduces the current pulses and should reduce the I Squared R heat in the transformer and diode. Over the top but I had a new 50uF +50uF high quality 500 Volt type capacitor and used that. As it is choke input that capacity should not kill the rectifier Type 80, and it has enough head room for surges if the psu is operated on no HT load. Comments noted on wiring out B+ switches.

Although there is an imbalance in the secondary voltages 327/341 there was no detectable imbalance via current probes on the mains input so although the ripple is slightly imbalanced there does not seem to be enough DC to upset the transformer so left without a balancing resistor at the moment. I am presuming the 5/6.3 heater currents have swamped the visable imbalance.

The next question on preserving rare old things is that in the guitar amp forums they are advocating fitting suitably rated series diodes in the transformer wires to the rectifier valve so that short circuit type failure of the valve won't take out the transformer. With just a junction drop the diodes do not affect the amp performance at all.

Does anybody here do that?

Pete
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Old 15th Jul 2018, 5:15 pm   #15
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Default Re: HRO Dog House PSU restore

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The next question on preserving rare old things is that in the guitar amp forums they are advocating fitting suitably rated series diodes in the transformer wires to the rectifier valve so that short circuit type failure of the valve won't take out the transformer. With just a junction drop the diodes do not affect the amp performance at all.

Does anybody here do that?

Pete
I prefer to fit fuses: in times-past it was quite common to fit a small low-voltage/current pilot-lamp bulb in the transformer-secondary-centre-tap-to-earth lead; the usual rating being something likie 6v 100mA - or whatever caused the bulb to glow faintly under normal conditions. A HT short to earth blew the bulb and (hopefully) saved the rectifier/transformer.

What this doesn't save you from is a catastrophic meltdown in the rectifier that causes an anode-to-anode arc: to protect against that you need fuses in the rectifier-anode leads.
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