E.A.R. Autobat capacitor question

[tracypaper]

Hi all.
Working on this rare Autobat which actually still works but has the sea shell ocean sound through the amp. I have changed out the 3 x 100uf electrolytics as they were the TCC Elkomoulds, there are also two other electrolytics which are 16uf, but when looking at the circuit diagram it states 8uf. If I change these out to 10uf will it be ok or is it better leaving the 16 ufs in place?
Cheers.
Gaz.

['LIVEWIRE?']

Given the age of those 16uf Caps (if they're original they must be well over 50 years old by now), I'd be inclined to change them, maybe fitting 22uf Capacitors. What part of the circuit are the 8 (16 or 22)uf) caps. in?

[tracypaper]

Hi.
one of the 16ufs is fitted between an OC81D transistor and an OC71 transistor the other is also connected to the OC71 and a 330K resistor (not shown on circuit diagram) and the 1meg ohm tone pot.
Gaz.

['LIVEWIRE?']

The first sounds like an A.F. coupling capacitor, though 8 or 16uf seems a bit large, whilst the second appears to be part of the tone control, so changing it's value by too much will affect the control's frequency response. I do have the circuit diagram somewhere, but not immediately to hand.

[tracypaper]

Hi.
Here is the circuit for the Autobat.
Will changing the 2 x 8uf to 10uf be ok or will the suggested 22ufs be better?
Gaz.

[Techman]

That circuit image won't enlarge for me.

I wouldn't have thought the capacitors were the cause of the noise - have you tested to confirm they're faulty?

More likely a dodgy old transistor, or even a carbon resistor.

['LIVEWIRE?']

That circuit diagram won't enlarge for me, either. As for the cause(s) of the noise, you may be right, Techman.

[tracypaper]

The image won't enlarge for me, copied it from another thread?
Changed quite a few resistors as they were out of spec. Not tested yet, would prefer to change caps first as they may be wrong size and over 50 year old.
Gaz.

[tracypaper]

Circuit diagram at post 15 on this thread below


https://www.vintage-radio.net/forum/...ht=ear+autobat

[julie_m]

That isn't a thumbnail, the actual image is that small. The circuit is available Up Top, so that might have something to do with it. I don't expect the landlord of my local would be entirely thrilled if I turned up with a crate of beer and started selling bottles to the other regular customers .....

[tracypaper]

I have bought the diagram from above as I have bought many of them, I could have probably bought the CD twice with the amount I have purchased, I just have had difficulty in loading an image from the one I purchased from above.
Gaz.

[Biggles]

Any rustling or noise could be caused by a carbon resistor in the base bias network of the first stages of the amplifier. A bit of noise isn't unusual in a germanium based audio amp anyway. It all depends how loud the background noise is I suppose.
Alan.

[tracypaper]

What about the caps? diagram shows 2 x 8uf, I have two 16uf fitted, do I change out for 10uf or 22uf? I know from what people have said the noise is more than likely resistors (most already changed) or the transistors to be checked later.
Gaz.

[julie_m]

The values of these capacitors aren't so critical. They are just there for DC blocking. 10µF will be fine.

However, rustling noises aren't usually caused by capacitors. More likely are carbon resistors. They can be noisy even if they read within tolerance on a meter. The transistor nearest the input is the next suspect.

['LIVEWIRE?']

Having looked again at the circuit diagram, IMHO, any of the three values mentioned would be O.K., though both caps. should be the same value. As you no doubt know, doubling the value of a capacitor results in half the impedance at any given frequency, and vice-versa, but given the wide tolerance in value of electrolytic capacitors, this isn't important. It's unlikely that any difference in frequency response will be audible through the full-range speaker used in the Autobat. (Crossed with Julie M.)

['LIVEWIRE?']

Another look at the R007 Amp.'s diagram reveals what, to me, is a circuit oddity (nothing to do with the value of the capacitors), which is that to 25 ohm speaker is connected in parallel to one half of the o/p transformer winding. I don't know what DC resistance the o/p Tx winding is, but the load seen by the 'upper' o/p transistor will be lower than that seen by the 'lower' one due to the shunting effect of the speaker. Anyone know the reason for this? From a diagram I drew out at the time, some Autobat amplifiers (Type R007/A) were produced which used a different o/p transformer with a conventional isolated secondary intended, presumably for a lower-Z speaker, but this was short-circuited and a 25 ohm speaker(with the parallel180 ohm resistor) wired as per the R007 circuit. According, also, to my notes, made at the time, the tone control was a conventional 'top-cut' one, using a 2Meg Lin pot, wired as a variable resistor, across the Volume control (1meg. log.), with an 0.1uf cap. in series with the tone pot's wiper.

[Herald1360]

I don't think each half sees a different load- the transformer is acting as an autotransformer. Using the hi Z speaker this way saves a few pence on the transformer.

[Nickthedentist]

Direct link to big version of circuit diagram mentioned in post 9 by Tracypaper: https://www.vintage-radio.net/forum/...0&d=1180560406

[ms660]

https://www.vintage-radio.net/forum/...3&d=1180630380

Lawrence.

[tracypaper]

After waiting weeks for back order resistors, it turns out the background rustle was nothing to do with resistors but the tags on the end of the pick up wires which had gone white and dusty with corrosion, after a good clean all is great, can not believe I did not check them first as this is the first thing I usually do with noisy players (guess old age is catching up with me).
After reading posts about the Autobat I have loaded some pictures of it as it is very rare and may be the only one left (unless some one knows any different).
It was also fitted with a stereo cart as the Autobat was available with an add on amp. I have hot wired a 9v mains adaptor and socket to the battery terminals through the rear battery compartment but did not mod the cabinet in any way so as to keep originality and to still be able to use batteries.
Feel free to send pics to the Radio Museum as the one listed on the site as an Autobat is wrong.
Cheers everybody.
Gaz.