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Old 24th Feb 2018, 12:51 pm   #21
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Default Re: Bush VHF 81 Restoration

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Originally Posted by OldTechFan96 View Post
...When the radio is tuned the dial cord looks to bunch up on the control spindle making the radio difficult to tune...
I'm not familiar with the VHF81, but does the cursor just "float" on the cord or does it slide along a guide (like my VHF54)? If the latter, then you may need to thoroughly clean & lubricate the guide.

Without the drive cord, is the tuning drum easy to turn, without any tough spots? If not, you need to address this first.

You need 3 & a half turns around the spindle for a VHF81.

With my Bush I was able to photograph the spring before removing the old cord, so I had some idea how much "stretch" I needed when setting up the new one. I also exercised the tuning cord by running up and down the band quite a few times, then left it for a few days to settle before re-tensioning the spring.
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Old 24th Feb 2018, 9:55 pm   #22
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Default Re: Bush VHF 81 Restoration

I decided to remove the dial cord and inspect the control spindle. It was covered in patches of surface rust so I fired up my rotary tool and removed the rust with a wire wheel. Once the control spindle was smooth I restrung the radio and the bunching stopped. It looks like the rust was causing excessive friction on the control spindle, resulting in bunching. The cursor 'floats' on this radio.

I also restrung the FM tuner properly.

The radio is working well without any hum. HT is sitting at 220V. I'll check all of the valve voltages later on. I'll also look at realigning the radio.
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Old 25th Feb 2018, 3:19 am   #23
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Default Re: Bush VHF 81 Restoration

Don't try re-aligning if it is working well. This model is not too bad but if you try twiddling the IFs of many radios you risk the cores breaking or breaking free of the brass threads.
That can be the end of the set.
Its a case of if it isn't broke, leave it.
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Old 12th Mar 2018, 4:06 pm   #24
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Default Re: Bush VHF 81 Restoration

Before I measured the voltages on the valves I decided to learn about how valves work and how they are drawn on a circuit diagram. All interesting stuff!

For the most part the voltage readings were low when compared to the trader sheet. Usually by about 20V to 60V or more. HT is 220V

I would like them to be as close to specification as possible.

When presented with low voltages what would be your diagnosis? Out of spec. resistors would be my first guess. Would a weak valve also cause low voltages?
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Old 12th Mar 2018, 4:33 pm   #25
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Default Re: Bush VHF 81 Restoration

Voltages on anode and screen grid would tend to go higher with low emmision valves, cathode voltages would tend to go low with low emission.
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Old 12th Mar 2018, 9:25 pm   #26
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Default Re: Bush VHF 81 Restoration

Another thing you need to know...the voltages given in the original circuit were made using equipment o the time...meters typically were 1000 ohms to 20,000 ohms per volt...it will usually say which meter was used. Unless you have a similar vintage meter it is likely you will be using a much more modern digital meter which has a much higher impedance (or internal resistance if you prefer). Therefore expect voltages to be different, sometimes by significant amount. This doesn't indicate a fault, it simply means the meter presents a much lower load on the circuit making some voltages read significantly higher.

Back in 'the day' using meters of the time, we expected voltages to be around 10% - 15% within published figures...experience also told us if there was anything really wrong. Digital meters will tend to read higher voltages in high impedance circuits....the anode voltage of the UABC80 for example may be quoted on the circuit as 80V but your digital meter may indicate 120V. No it's not a fault, your meter is just loading the circuit less and reading a higher voltage. However measure the main HT rail which is a much lower impedance and you'll probably find it measures close to the published voltage (assuming the rectifier is up to scratch).

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Old 13th Mar 2018, 11:57 am   #27
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Default Re: Bush VHF 81 Restoration

Thanks for the advice Nuvistor and Sideband.

I was using an Avo8 to measure the valve voltages. That was a fortnight or so ago. I'm going to retake the measurements and enter them into a spreadsheet.
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Old 13th Mar 2018, 1:35 pm   #28
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Default Re: Bush VHF 81 Restoration

An AVO 8 is fine. I think these were 20,000 ohms per volt so you should expect voltages to be somewhere near their specified figures.
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Old 13th Mar 2018, 2:43 pm   #29
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Default Re: Bush VHF 81 Restoration

I'm very sceptical about meter loading affecting voltage measurements in typical valve circuits, especially for commonly quoted test points like supply rails, anode and cathode Voltages.

The reason old service sheets specified a 20,000 Ohm per Volt meter back in the day, was because some people had very low resistance volt meters back then. So this would have made a difference to readings, but modern digital meters are high impedance.

If you are reading an anode voltage of (say) 210V with a 20,000 Ohms per volt meter (i.e. one with a 50uA movement), you must be using a Volt range of 250V or higher. 20,000 x 250 = 5M Ohms. I doubt that putting a 5M Ohms resistor between anode and chassis would make any difference in most wireless circuits. Or to put it another way; anode & cathode currents are usually several mA, while an AVO 8 generally draws less than 50uA.

However, you may need to be careful with grid voltages (I think only screen grid voltages are generally quoted) as a few uA of additional load could make a big difference.

So whether you are using an AVO 8 or a high impedance digital meter should generally not make much difference (ignoring calibration error), and the difference is likely to be less than the natural spread of readings when comparing one good valve with another.
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Old 13th Mar 2018, 3:23 pm   #30
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Default Re: Bush VHF 81 Restoration

I must admit that I've been lead up the garden path more than once when taking voltage readings in valve circuits using a DMM having a high input impedance. The readings were high when compared with those in the service sheet.

The reason is that an analogue meter with a low sensitivity (ohms per voltage) can simulate say a leaking a leaking screen grid decoupling capacitor and effectively modify the circuit. This pulls down the reading to that quoted in the service sheet.

This brings me to another point. A low screen grid voltage can be down to the screen dropper resistor going high in value, or the screen grid decoupling capacitor being electrically leaky.
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Old 13th Mar 2018, 3:34 pm   #31
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Default Re: Bush VHF 81 Restoration

I have checked the Makers circuit and the Trader one, both agree on the voltages and both taken with an Avo 8, the makers circuit states the Avo 8 was used on the 1000v range and the 10 volt range, no distinction is made on the Trader circuit.

Note there are two voltage readings one for MW and one FM, the FM stages will draw extra current and give lower voltages as shown in the voltage table. The HT voltage is high at 220 volts but if that is the reading on MW it is not out of specification, correct reading is shown as 213, that difference does not matter. If its 220 on FM then that may require investigation but it is still not that far out, especially if your mains voltage is high.

Taking the reading on the voltages that are low by 20 to 60 volts, 60 volts low in this set I would investigate, 20 volts low depends on the circuit, it may well indicate a component out of tolerance but the radio perhaps would work ok

Take the readings again and post the results, at least you are using an Avo 8 so that will remove any doubt about meter sensitivity.
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Old 13th Mar 2018, 4:20 pm   #32
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Default Re: Bush VHF 81 Restoration

When I refurbished my Bush VHF54 recently I came to the conclusion that I should not worry too much about circuit voltages.

I have 2 service sheets; The Trader 1252 data sheet agrees with the voltages written on the circuit within the Bush Service Data Sheet, but the Bush data includes a table where some of the voltages are different to those on the circuit!

Also, most of the resistors have a 20% tolerance, so a wide range of circuit voltages are permissible.

Probably best to get the supply voltages close (i.e. set mains transformer taps, replace rectifier, check those big electrolytic capacitors), and worry less about anode, cathode and screen grid voltages unless the set is not running as expected.
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Old 13th Mar 2018, 4:23 pm   #33
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Default Re: Bush VHF 81 Restoration

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Originally Posted by stevedee View Post
When I refurbished my Bush VHF54 recently I came to the conclusion that I should not worry too much about circuit voltages.
Checking circuit voltages is one of the best ways I know of locating faulty components.

A set with a leaky grid coupling capacitor will run as expected for a while, then bang goes your output valve, output transformer and possibly the mains transformer if there is one. Better by far to check that there's no positive voltage on the control grid.
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Old 13th Mar 2018, 4:40 pm   #34
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Default Re: Bush VHF 81 Restoration

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...Checking circuit voltages is one of the best ways I know of locating faulty components...
I don't disagree with you, its just that OldTechFan96 said he wanted to get the voltages as close to spec as possible. But if you have checked the usual suspects and an anode reads 210V and the circuit says 200V, its probably well within spec.
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Old 13th Mar 2018, 4:45 pm   #35
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Default Re: Bush VHF 81 Restoration

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I'm very sceptical about meter loading affecting voltage measurements in typical valve circuits, especially for commonly quoted test points like supply rails, anode and cathode Voltages.
Try measuring the voltage on the triode anode of an E(U)ABC80 or any other voltage amplifier that uses a high value anode load (somewhere around 220K). The impedance here is high and you'll find that a digital meter will read a higher voltage than a 20,000 ohm per volt AVO. HT rails are a much lower impedance and wont be affected by the meter load so they will show about the same as the published voltage.

Try it with a battery valve like a DAF96 where the anode load can be several megohms.
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Old 13th Mar 2018, 4:49 pm   #36
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Default Re: Bush VHF 81 Restoration

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Quote:
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...Checking circuit voltages is one of the best ways I know of locating faulty components...
I don't disagree with you, its just that OldTechFan96 said he wanted to get the voltages as close to spec as possible. But if you have checked the usual suspects and an anode reads 210V and the circuit says 200V, its probably well within spec.
The OP said that the HT supply reads 220V against a specified figure of 206V or 213V depending on whether MW or FM was selected. That's fine.

He also said that some voltages were up to 60V low. Given that the highest voltage in the set is around 220V, that's not OK and needs further investigation.

Let's wait for the OP to post some actual voltage readings, then we can suggest what might be wrong.
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Old 13th Mar 2018, 5:34 pm   #37
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Default Re: Bush VHF 81 Restoration

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Originally Posted by Sideband View Post
Try measuring the voltage on the triode anode of an E(U)ABC80 or any other voltage amplifier that uses a high value anode load (somewhere around 220K). The impedance here is high and you'll find that a digital meter will read a higher voltage than a 20,000 ohm per volt AVO. HT rails are a much lower impedance and wont be affected by the meter load so they will show about the same as the published voltage.

Try it with a battery valve like a DAF96 where the anode load can be several megohms.
An interesting point..... AVO 8 on 1000V range = 20M, cheap DMM = 1M, decent DMM = 10M.

All will upset the operating voltage, but the AVO (on that range) will have the least effect. It's not necessarily so that the DMM will be nearest to actual operating volts!
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Old 14th Mar 2018, 11:52 am   #38
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Default Re: Bush VHF 81 Restoration

Yes, you are absolutely right. Different instrument types will give different results due to the way they load the circuit under test. But what is the magnitude of the differences compared to the 'within tolerance' variations of the circuit components?

To be honest, I didn't know. So I have just worked through an example using the circuit data for my Bush VHF54.

For the EABC80 in my Bush circuit, it shows a 180k anode resistor fed from 215V, with an anode voltage of 85V. So the calculated anode current is 0.72mA. I think this gives an equivalent anode to chassis 'resistance' of approximately 118k, which naturally includes the parallel AVO 7 on 1kV range (i.e. 20M Ohm).

- Substitute a 1M Ohm DVM and the calculated anode voltage drops to: 79.6V
- Substitute a 10M Ohm DVM and the calculated anode voltage drops to: 84.7V

So it looks (from this example only) like a 10M input resistance DVM would be a suitable alternative to an AVO 7. Otherwise (if it is important to you) either avoid the 1M variety or do the maths (I'm just plugging values into my spreadsheet). Generally, a circuit with higher resistance values may be more sensitive to the 'shunt' resistance due to the application of test equipment.

Now if we leave the 20M AVO in place and change the 180k anode resistor to its limits (in this case its a +/-10% resistor):-
- With 180k -10% the calculated anode voltage rises to: 90.5V
- With 180k +10% the calculated anode voltage falls to: 80.2V

This is approximately +/-5Volt, but I don't know what the additional variation (range) could be by comparing one 'good' valve to another, or the variation due to cathode resistor tolerance.

There is also the calibration of your meter to consider.

As my service sheet says, in relation to test point voltage readings: "The figures quoted are approximate and variations may occur without impairing the performance of the receiver".

I don't know whether there is a useful 'circuit voltage' guideline we can give newcomers to wireless servicing (+/-5%, +/-10% ?) because it mostly comes down to detective work.
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Old 14th Mar 2018, 12:04 pm   #39
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Default Re: Bush VHF 81 Restoration

All very interesting, but as I said earlier the OP is reporting readings which are up to 60V low in a circuit where the maximum voltage is 220V. That's an error of 27% minimum and is clearly indicative of a fault.

Until we get actual readings from the OP stating which electrode/s are 60V low, it's difficult to give further advice.
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Old 14th Mar 2018, 12:37 pm   #40
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Default Re: Bush VHF 81 Restoration

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Originally Posted by stevedee View Post
For the EABC80 in my Bush circuit, it shows a 180k anode resistor fed from 215V, with an anode voltage of 85V. So the calculated anode current is 0.72mA. I think this gives an equivalent anode to chassis 'resistance' of approximately 118k, which naturally includes the parallel AVO 7 on 1kV range (i.e. 20M Ohm).
So far as I know the Avo 7 has an OPV of 500 ohms, and an OPV of 1,000 ohms when the divide by 2 function is used.

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