Bush VHF61 repair.

[sobell1980]

Hi all

Finally getting some well deserved time on this set.
I've replaced all the wax caps and Hunts mouldseals, every one of them! Tested the set in between every couple replaced.

The set is receiving on FM, LW and MW. It sounds very quiet even on full volume with a coax cable dangled out the window. I would say MW and FM sound about the same volume as if the set were set at half volume. LW is barely audible.
I've run through all the valve voltages, please see photos of findings. The set had mains hum so the triple section smoother/ reservoir cap has been replaced. I don't think the alignment is out. The set looked like it had come from somewhere like a fish and chip shop. The whole chassis and valves were coated in a thick greasy furry substance that I've spent hours cleaning so I think the set is untouched in regards to alignment.

I can only think its a valve or a couple of valves. I've tried another 3 ohm speaker and was the same.
I know from previous reports that mw and lw on these are considered mediocre but even FM is low. I'd like to chase it with my sig gen but it's not completely deaf and if it's a couple of low emission valves then this could be hard to decide on where the fault lies as the valves are all giving some sort of gain.
I've put a post on the wanted section for some valves but are some more likely to be suspect than others?

Many thanks

Dave

[Aubade65]

I think you may want to split the problem in half - try using an audio signal applied to the gram sockets (with the gram switch selected) and see if you can obtain a decent output. If the problem is the same then there is a problem with the audio output. If the volume is OK then you are looking at an RF\IF issue.
The voltages you have provided seem to be on the low side particularly the HT which could well cause low volume - you might well have a tired EZ80. With a modern DVM, voltage readings close to or exceeding those in the service sheet should be expected.

I think the criticism of the AM performance is sometimes a bit overstated as there are 2 IF stages on AM where some recommend that the majority of the signal gain should be concentrated. I have renovated several of these sets and I have found the volume to be more than adequate on all bands.

[sobell1980]

Many thanks for your reply. I did feel the HT was down a little. With the mains hum from the reservoir and smoothing caps I thought replacing these would bring it up somewhat but there was no change. I have 208volts on the EZ80 cathode. The service sheet states 250volts but reading other repairs on the forum, plus 200 volts would seem adequate? I have 240 volts ac at the anode so it's seeing full mains. I thought possible EZ80 but wanted to discuss this here. If all HT is slightly down this maybe the issue.

[Mr 1936]

Hi

I tested my VHF61 recently by bringing it up slowly on a variac. Surprisingly, it seemed to work on FM with decent audio volume even with only 200 volts mains input (Tap set for 240).

By all means get the voltages right, but I have a feeling your low audio problem lies elsewhere. The check with audio fed into the Gram input is the logical next step, followed by a check of relevant resistor values.

[Aubade65]

The HT current draw for this set is quoted at 84mA which is very close to the EZ80 maximum rating of 90mA. I have had several sets where the EZ80 had gone low emmission giving low HT. The remaining valves have mainly been usable but I think the EZ80 may not be capable of providing long term reliability because it is run close to its limits - but it is over 50 years old.
I would fix the obvious faults first which would be low HT; I would certainly replace the EZ80 as a first attempt. An EZ81 would be a substitute for a quick test but would not be suitable for any length of time as the heater current is nearly twice that of the EZ80.

[sobell1980]

I have a full set of valves on their way hopefully arriving for the weekend. I have lots of valves but none for FM sets. So it will be good to have the ones I don't use this time as spares. I will try the EZ80 first as its the quickest and easiest thing to eliminate. The service data quotes 250v at the Ez80 cathode. If this is correct I'm a way off at 208v. Also it takes a few minutes to get to the 208v climbing very slowly from approx 160v and seems to peak at 208v. I could understand it being a resistor if only one valve HT was low. As I say, if the data quotes 250v and im running at 208v with new reservoir/smoothing caps surely it has to be the rectifier? I will do the gram socket test also. Could I do this with my sig gen?

Dave

[yestertech]

Dave
Looking at your tabuated voltage readings you're not showing anything for the EABC80 ?
You need to check the anode voltage in case the anode resistor has gone high which would reduce the AF gain. If the set is sensitive but the audio is quiet, then either low AF gain or low IF gain ( phantom twiddler ? ).
A 'finger test' at the volume control or pickup sockets should make a decent noise. This will indicate whether the problem is in the audio stages or further back.
I don't think the lack of HT will be the culprit nor probably will replacing valves make any difference. Most sets are able to work across very large tolerances as regards voltages /valve emissions.

Andy

[Aubade65]

Any type of audio signal will do - if your signal generator goes down to audio frequencies then it should be fine. If not any device that outputs audio such as a mobile phone headphone socket will do. Some members of the forum use a Vellman signal tracer built from a kit to carry out this type of work. The idea is to work out if the RF stages are causing the problem or if the audio (and power supply) sections are not working correctly

[sobell1980]

Many thanks for everyone's input, interesting and worthwhile replies.
V5, the EABC80. The service data does not quote a a figure for anodes a,b, and c. Only for anode d, which is quoted at 70v and I measured 65v at anode d. I can measure anodes a,b and c but have nothing to reference it against.

Dave

[Gabe001]

I don't have a signal generator so I use a cheap bluetooth receiver. You can use either of the channels or sum them up via 2 resistors

[sobell1980]

I've gone over v5 anodes again. That's the EABC80.
Although the service data doesn't specify a value for a,b and c here is what I found.
Anodes a and b are the FM side, the set was switched to FM for these readings and switched to AM for c and d anodes.

Anodes a- minus 290mv
b- minus 174mv both readings taken on FM

C- minus 0.400 volts
d- 65 volts both readings taken on AM.

The mv readings are low as they are in respect to chassis negative I'm assuming which is why the service data doesn't quote anything .

[Aubade65]

The diodes in the EABC80 are used as signal demodulators - 2 for FM and 1 for AM. Any signals that are present here are likely to confuse a DVM so the readings cannot be relied upon. The anode with 65v on it will be the preamplifer triode which will have a high value resistor for the anode load. This resistor often goes quite high in value.

[sobell1980]

I've spent some time on the set. I've substituted all the valves and it remains the same. FM is now much louder than AM. Radio 4 on LW can only just be heard on full volume. MW slightly better but nowhere near where it needs to be.

I tried the sig gen through the gram socket with the gram switch engaged. I kept the attenuation down on the sig gen and I was nearly blown away by the 465khz signal on full volume. So we know the fault is before the gram socket.

I then worked back testing through C59, all OK. R14 and R24 seemed to make the sig gen quieter, loud one side of the resistor and quieter the other. I snipped R14 and R24 out and tested. Both had gone high in value so I replaced them. However, no difference. This was purely tracing back from c59. Perhaps the sig gen was quieter across the resistors due to normal action of them. But as they measured high I replaced. So that's where I am. I did notice that one of the IF can cores on L21 is screwed quite a way in compared to the others. But with the service data, this is the core you adjust in till you get the 2nd peak on alignment. Also, when I got the set it looked completely unmolested and covered in greasy thick dust. So I can't see the phantom twiddler has been in there. I will get some more testing done today. But if anyone has anymore thoughts in the meantime?

Many thanks

Dave

[ms660]

Feeding RF into a gram socket? Did you fix the low HT?

Lawrence.

[sobell1980]

I've obviously not done this correctly then Lawrence. I was going by posts 9 and 10 but I guess I should have fed a line level AF signal in?

I've now replaced R29 Anode resistor for valve 5 anode (d). This has now increased the voltage from 65 to 78 volts . Slight improvement but nothing that convinces me its as it should be.

[ms660]

Yes, AF to test the AF stages and IF/RF to test the IF/RF stages, the anode and screen voltages in the manual were taken with an Avo 7 meter on the 1000 volts range, that means the resistance as presented by that meter to the measuring points would be 500k, all circuits are potential dividers so the usual potential divider rules apply, measuring across the same points with a modern DMM will display a higher voltage reading than would be displayed using an Avo 7.

Did you sort out the low HT problem?

Lawrence.

[sobell1980]

Good morning. Spent some time on the set yesterday. Turns out the EZ80 was causing the low HT issue alongside some resistors that had gone high. Not massively but about 10% out of spec so I decided to change them.
I refitted the old EZ80 to confirm my original low readings. The HT took forever to get up to 160v and slowly struggled to climb up to about 200v and stopped there. Putting the new EZ80 in I watched the HT rocket to 260 v at the EZ80 cathode.
Here are now my current readings.

V2 (a) anode 93v
(b) anode 126v
Screen 115v
Cathode 1.73v

V3 anode 180v
Screen 115v
Cathode 1.75v

V4 anode 174v
Screen 160v
Cathode 1.75

V5 anode (d) 79v
No other readings available for V5

V6 anode 242v
Screen 220v
Cathode 6.85v

V7 both anodes 240v ac
Cathode 260v dc

Tuning indicator anodes 170v and 25v anode fed after resistor.

I have two queries on these readings.

V6 anode is still slightly low at 242v compared to the quoted 245v and using a DMM should read higher anyway. ?

Valve 4 cathode is 1.75v compared to the quoted 2.3v ?

The set peforms loud on FM and very selective.
MW is 'okay' on full volume and LW is still poor. I've tried the set in the kitchen to see if it's a signal issue and with a long length of coax to see if MW and LW will improve but it didn't make a vast amount of difference. I tried my transistor set in the radio room I work in and it picks up MW and LW loud and clear with its internal aerial.
Perhaps I now need to try the alignment of AM circuit's and if no further improvement can be made perhaps this is as good as the set is, which in my eyes is disappointing for such a nice looking set.

[Sideband]

Don't worry about a few volts in a valve set. Your digital meter won't give greatly higher readings, if at all in the output stages or rectifier. It's only low current stages where it will (might) give higher readings. Low current stages being the RF and IF stages and the audio pre-amp in this case the EABC80 where it might give higher readings for the triode anode. If the anode of the output valve is 242V instead of 245V....that is pretty much spot-on...nothing to worry about. Most voltages will be within 10 to 20% and you will find even with very low HT, the set might still work quite well with possibly lower volume/reduced sensitivity. Mains variations will affect the HT and most sets were designed to cope with that.

I might question the 25V reading for the Tuning indicator anode although that will have no effect whatsoever on the performance of the radio.

Without looking at the circuit, I'd say those readings are OK.

[ms660]

Regarding the measurements in the manual:

If you do the Ohms Law with the resistance values given for the output valve circuit you'll see that either the anode voltage or the HT voltage given topside of the primary winding in the manual pertaining to that valve is wrong anyway.....17.6mA screen grid current anyone.

Similar for V4.

Probably to do with the range the meter was set to but little to do with it's input resistance.

Lawrence.

[Aubade65]

Glad you have been able to pinpoint the source of the trouble.

The AM performance of the set if not stellar should be more than adequate - do not forget that when this set was introduced MW and LW were the main frequencies in use for general broadcasting. I would have thought that from your location you would be able to pick up Droitwich on a piece of wet string!

I think there are a couple of things you could try -

1 To state the really obvious do not forget that the aerial is directional - the front of the set should be pointed towards the transmitter (sorry for that)

2 The aerial coils would have been handled when the chassis was removed from the set. Make sure that none of the wires to the coils have broken. Also check that the position of the coils on the ferrite rod has not been disturbed.

3 The ECH81 frequency changer may be low emission. The triode section used in the local oscillator has to work a lot harder on LW (counter intuitive) so any weakness will show on LW first - I have had several sets where you can hear a sudden reduction in noise from the speaker as the LO stops due to low emission when you tune across the LW band towards the LF end. In addition the HT is removed from the triode anode when in FM mode which can cause effects such as cathode poisoning (which can stop it from working,) if the set is used for long periods of time on FM.

4 Make sure the wavechange switches are really clean

5 A bit of a long shot this. You mentined that the chassis was covered in some sort of grease. Some of this may have got onto the aerial coils which may have lowered the Q and the sensitivity. Not sure how this could be removed - I presume with care using some type of solvent.

I would tend to look at points 2,3 and 4 first - carefully examine the aerial connections and the coil positions first and then go on to substituting the ECH81.

If the IFT adjustments have not been changed then it would best to leave them alone unless there is strong evidence that they need to be realigned