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Old 9th Apr 2021, 1:26 pm   #1
telstar
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Default Bush VHF61 FM band dead.

Hi I have a Bush VHF61 radio that works on long and medium waves but not on VHF. In this case I have tried replacing ECC85 & EABC80 valves but to no avail. All that I can hear is a background hiss but no stations. Wonder if I should do a modification using a ECC81 valve (changing heater connections) since good ECC85 valves are nearly impossible to obtain (due to bulk buying)? However when switching from a.m band to f.m, I can hear weak stations, but these soon disappear?
Cheers
Mike
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Old 9th Apr 2021, 1:50 pm   #2
Leon Crampin
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Default Re: Bush VHF61 FM band dead.

Mike, I wouldn't recommend making this change. The '81 lacks the internal screen between triode sections which is fitted to the '88, has a lower gm and differing internal capacitances. This will play havoc with a neutralised stage and the performance (which is good on FM, less so on AM) will suffer.

ECC85s are not impossible to source, but a possible alternative is the ECC88 which is pin-compatible.

At risk of stating the obvious, make sure the string drive to the FM permeability tuning is functional...

A Hunts eviction cruisade is usually necessary on these sets, switch slider contacts can give trouble and, unfortunately, leakage from contacts carrying HT to other switch contacts via the very close spacing can cause crackles and odd results. For some reason, the ECH81 seems to be prone to poor operation due to changed resistor values around that stage.

Best of luck - make a few measurements on FM and report back. Can you use a signal generator to check the gain at various points?

Leon.
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Old 9th Apr 2021, 4:00 pm   #3
westcliff
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Default Re: Bush VHF61 FM band dead.

When I restored one of these a few years ago, the ECH81 required replacement before FM would work.

Gus.

Last edited by westcliff; 9th Apr 2021 at 4:03 pm. Reason: Can't spell ECH81!
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Old 11th Apr 2021, 8:57 am   #4
Aubade65
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Default Re: Bush VHF61 FM band dead.

I have repaired several of these sets and I have found that unless all of the hunts capacitors are replaced (about a dozen or so) then the performance will be very poor.

Also it will need a reasonable FM aerial to receive FM transmissions-the internal aerial will be OK if you are local to transmitter otherwise you will need a proper aerial.

An ECC88 is an excellent replacement for the ECC85 in my opinion. It provides a noticeable improvement in gain.

Lastly check the HT rail as the EZ80 is run close to its limits in these sets and is often found to be tired and in need of replacement.

Regards
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Old 11th Apr 2021, 3:09 pm   #5
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Default Re: Bush VHF61 FM band dead.

I have been doing some experiments using different valves, I have had very good success using 6n1p (6bq7) pin compatible as a cheap replacement for the ECC85 and have been working well for over a year, I would not fit a ECC81 as messing with heater connections can cause problems to the FM tuner head, a PCC189 can also be used to test with again pin compatible, always one or two about in spares box.
as mentioned change all hunts and paper capacitors first that may be all that is wrong with the set.
John
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Old 11th Apr 2021, 3:27 pm   #6
cathoderay57
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Default Re: Bush VHF61 FM band dead.

I'm dubious about replacing the ECC85 with an ECC88. The latter valve has a max spec anode voltage of only 90v whereas the former goes up to 250v. Yes, the difference in gain isn't surprising since the latter has a gm of 12.5mA/v whereas the former is only 5.9mA/v. I would be concerned on 2 counts: the higher anode voltage would require alteration of the HT line dropper resistor(s) and, whilst the increase in gain could be seen as beneficial, it could also lead to instability. Jerry
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Old 11th Apr 2021, 10:07 pm   #7
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Default Re: Bush VHF61 FM band dead.

Quote:
Originally Posted by telstar View Post
good ECC85 valves are nearly impossible to obtain (due to bulk buying)?
I agree with you 100% on that one - and it's not the first time I've said it!

Going forward, it would be good to experiment with different valves to see if there's something that at this moment in time is more readily available than the ECC85 at the right price. The ECC85 only had a relatively short window of usage, so although it's not desirable for audio like the ECC83, it's actually becoming harder to find good new ones available and it's only going to get more difficult over time.

The ECC81 has been mentioned, but as said, the lack of that internal screening and other factors tends to in theory rule it out, but sometimes theory doesn't always happen in practice, so might be worth playing with one just to see what happens.

I would fully agree with what's been said regarding the ECH81, these can be responsible for all kinds of faults that you wouldn't think likely, so certainly try a replacement if you have a known good one to hand.
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Old 12th Apr 2021, 1:14 pm   #8
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Default Re: Bush VHF61 FM band dead.

Hi everyone for your feedback. I will try another ECH81 in the radio (well it is an I.F amplifier when switched to VHF). However, I also will try measuring the supply voltages to the two anodes of the ECC85 when it is switched to VHF (when the valve is removed).
I have loads of un-boxed N.O.S valves that I can go through.
Cheers
Mike
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Old 12th Apr 2021, 1:32 pm   #9
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Default Re: Bush VHF61 FM band dead.

Quote:
Originally Posted by telstar View Post
However, I also will try measuring the supply voltages to the two anodes of the ECC85 when it is switched to VHF (when the valve is removed).
?? that's not going to tell you anything! The valve has to be in place to measure the correct anode voltages....with no valve, there is no current being drawn (other than the miniscule current drawn by your meter) so the voltage will be high (at or near HT). All it will tell you is that supplies are present which I suppose is ok if you just want to check if the FM switch is actually working.
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Old 12th Apr 2021, 9:28 pm   #10
Aubade65
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Default Re: Bush VHF61 FM band dead.

I found the comment made by Cathoderay57 in post#6 regarding the maximum anode voltage of the ECC88 being only 90v interesting as I have replaced several ECC85s in Bush VHF61 sets with no obvious problem.
Looking up the valve data online the 90v anode voltage is actually listed as a 'typical characteristic' presumably it is a recommended value. The typical use for this valve is quoted as a cascode amplifier so it could be used from a 180v HT supply.
The maximum anode voltage is listed as 130v but the characteristic curves shown are plotted with an anode voltages up to 300v - if they were created in practice or if the curves are a theoretical exercise is difficult to say but it would be curious if data with anode voltages up to 300v is provided if the valve was not capable of it.
The Bush service sheet for this set shows the typical anode voltages being 150v and 127v so I would not have thought that an ECC88 would be too stressed in this position particularly if the increased gain produces a higher anode current and a lower anode voltage - something to check when I get another one on the bench.

In the spirit of experimentation perhaps just try it and see - I did and it works.

Regards
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Old 13th Apr 2021, 3:19 am   #11
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Default Re: Bush VHF61 FM band dead.

Bush sets from this period with VHF seem to be notoriously problematic, I've had about 3 sets that I've never been able to get that waveband working on. Even with replacing the hunts and changing valves they just continue to do very little but hiss, but perfect on MW.
I'll be following this thread with interest to see if there's a common fault that's rendering VHF useless on these sets.
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Old 13th Apr 2021, 8:32 am   #12
cathoderay57
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Default Re: Bush VHF61 FM band dead.

Points in Post #10 are well considered and well made. In my Graetz Sinfonia one anode of the ECC85 is running at 255v 8mA so I wouldn't risk it. Maybe a spectacular if rather short life. Cheers, Jerry
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Old 13th Apr 2021, 10:22 am   #13
Leon Crampin
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Default Re: Bush VHF61 FM band dead.

Looking at the ECC88 data in the Brimar data manual No 9, although the max stipulated anode voltage is 130V, the Ia Vg curves are plotted for 3 different anode voltages up to Va = 200V. I can't see any problems using this device in a Bush set, although I've always used an ECC85. Tuning drift is much reduced if a blackened screening can is fitted, which allows the valave to run cooler.

Having restored three VHF61 receivers, I would judge the FM performance to be excellent, with more than the average gain due to the set having an extra IF stage. The DC level feed to the tuning indicator for the VHF band really needs to be attenuated, otherwise the eye closes on almost any FM signal. The AM performance of these sets is a disgrace...

Leon.
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Old 13th Apr 2021, 11:33 am   #14
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Default Re: Bush VHF61 FM band dead.

Just an observation, the max. voltage of the DC component of Vkf is also the same as Va max. that's quoted, that is to say 130 volts.....2nd page note 3):

http://www.r-type.org/pdfs/ecc88.pdf

Same for the ECC84, another valve also designed for cascode operation, in this case it's 180 volts:

https://frank.pocnet.net/sheets/155/e/ECC84.pdf

I wonder if Va max was quoted in that context even though it's quoted for each section.

Lawrence.

Last edited by ms660; 13th Apr 2021 at 12:03 pm. Reason: extra info and link
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Old 13th Apr 2021, 12:06 pm   #15
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Default Re: Bush VHF61 FM band dead.

Same thing for the ECC189/PCC189, another valve designed for cascode operation:

https://frank.pocnet.net/sheets/030/p/PCC189.pdf

Lawrence.
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Old 13th Apr 2021, 1:08 pm   #16
telstar
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Default Re: Bush VHF61 FM band dead.

Quote:
Originally Posted by Sideband View Post
Quote:
Originally Posted by telstar View Post
However, I also will try measuring the supply voltages to the two anodes of the ECC85 when it is switched to VHF (when the valve is removed).
?? that's not going to tell you anything! The valve has to be in place to measure the correct anode voltages....with no valve, there is no current being drawn (other than the miniscule current drawn by your meter) so the voltage will be high (at or near HT). All it will tell you is that supplies are present which I suppose is ok if you just want to check if the FM switch is actually working.
Actually, it did confirm that the supply to the valve is o.k, and the switches were working. Now i know to look for other things that are stopping the VHF band from working. We often hear about faulty switching on radios.
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Old 13th Apr 2021, 2:42 pm   #17
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Default Re: Bush VHF61 FM band dead.

Whilst inside the FM tuner/enclosure, it's worth checking:-

1. The screws holding the ceramic trimmers are not protruding/touching any metalwork. (Difficult to see, but a piece of paper should slide between the metal back & trimmer mounting screws, without snagging).
2. The red fibre washers for the ceramic trimmers haven't gone conductive.
3. The Hunts cap inside has been replaced.
4. Resistors are in spec.

On our VHF61, one screw was shorting to the rear of the enclosure, and one red fibre washer had gone conductive.

Since sorting these issues, FM works great, and the annoying crackle has vanished.

Mark
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Old 13th Apr 2021, 7:52 pm   #18
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Default Re: Bush VHF61 FM band dead.

I've just removed the ECC85 from a VHF61 and replaced it with an ECC88 and it's been running without any flashes or crackles for the last ten minutes - I did leave the valve 'hat' off when I first switched on to be able to see any possible flashovers from within the valve. I've now put the valve hat back on and screwed up the back cover, so we'll run the radio for a few days (not overnight, obviously) and see what happens. I think there was a slight tuning difference between the two valves, but it was only very slight.

It might be worth putting an ECC88 on the bench with small crock clips on the appropriate pins while on a heater and variable HT supply and wind up the HT until it sparks, that'll prove things once and for all.

There's some very interesting things coming out regarding possible tuner faults on the VHF61, particularly the previous post.
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Old 13th Apr 2021, 8:51 pm   #19
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Default Re: Bush VHF61 FM band dead.

Still listening to the Bush VHF61 with the ECC88 fitted and it's not blown up yet, although I don't think any of us would have thought it would anyway. Now what other valves can we try while we're at it? Actually, I think most other possibilities will require alteration of base wiring, and I'm not planning to mess about with this particular radio in this way, so someone else can try that if they want to.
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Old 13th Apr 2021, 9:39 pm   #20
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Default Re: Bush VHF61 FM band dead.

The Philips data sheet for ECC88 gives 90v for its typical operating condition where Gm is spec'd It gives 130v as the limiting value. (550v cold).

It also gives 130v as the max DC rating for heater to cathode voltage.

As this valve is intended for cascode operation, it seems to be being specified in cascode terms and the heater-cathode insulation may be the limiting factor and the cause of the 90v/130v rating. The higher voltage with a 'cold' stipulation reeks of heater-cathode insulation.

Then, a few pages in, we find the anode characteristic curves.... and they go all the way to 300V so much for that 90/130V rating.

So long as the ECC88 isn't in a cascode circuit, it looks like a rather good improvement on the ECC85.

Sometimes toob rolling can be the answer!

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