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Old 26th Aug 2020, 4:14 pm   #1
Gavman68
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Default Bush SRG-91 hot resistor

Having collected a somewhat sad and neglected Bush SRG-91 from a kind donor on this forum I am in the process of restoring it. I have recapped it (where needed) and also replaced a few out of tolerance resistors and started to bench test it but have a resistor getting hot and burning out. It is a 150ohm resistor running from pin 3 to ground on one of the EL84 valves. Also running from pin 3 to ground is a 50uf cap. Any ideas what may be causing this resistor to heat up and fail, I presume too much voltage or current running through it can't figure it out?
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Old 26th Aug 2020, 4:39 pm   #2
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Default Re: Bush SRG-91 hot resistor

Possible grid 1 coupling capacitor or the EL84, other possibility but those are worth checking. Also worth checking your work in case you have made a mistake.
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Old 26th Aug 2020, 5:15 pm   #3
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Default Re: Bush SRG-91 hot resistor

Pin 3 is the cathode, so it looks as if this 150R resistor is one of the cathode bias resistors. It'll be getting hot because the valve is conducting too hard, possibly because one of the interstage coupling caps is leaky, allowing high voltage dc on to the grid of the EL84 turning it hard on.

The first thing to check is the coupling caps, these being C44 and C45. If you keep running it in this condition you stand a chance of frying the valve (bad) or the primary of the output transformer (very bad!)

You'll want a circuit diagram - if you don't have one, you can download from the link at the top of the page.

Cheers,

Frank
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Old 27th Aug 2020, 5:01 pm   #4
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Default Re: Bush SRG-91 hot resistor

Thanks Frank, C44 & 45 appear ok and were replaced and I've swapped the EL84s over but am having the same issue? Voltage across pin 3 to ground is around 9.3v but I don't know if that's too high?
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Old 27th Aug 2020, 5:08 pm   #5
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Default Re: Bush SRG-91 hot resistor

I would expect about 5.5 volts across the 150 ohm so the 9.3 is too high. Have you measured the value of the resistor?
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Old 27th Aug 2020, 6:06 pm   #6
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Default Re: Bush SRG-91 hot resistor

As said above, don't leave it powered up for more than a few minutes or it will damage the transformers or valve.

This set has has 2 separate channels with independent cathode bias resistors. Are you saying that when you swapped the valves over the same resistor overheated?
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Old 27th Aug 2020, 6:20 pm   #7
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Default Re: Bush SRG-91 hot resistor

Also, check the grid-to-ground resistor on the valve which is running-hot: this resistor will be a high value [330KOhm or higher] one - and such high-value resistors are prone to drifting-high-with-age (I've seen a 470KOhm turn into a 2.2MOhm!).

If this has happened, it only needs the slightest bit of leakage-current within the valve - or across the insulation between adjacent valve-socket contacts - to cause problems.

Try connecting the problem-valve's control-grid directly to ground with a croc-clip-lead then see what voltage you then get across the 150Ohm resistor.
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Old 31st Aug 2020, 9:46 am   #8
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Default Re: Bush SRG-91 hot resistor

Thanks for all the suggestions so far, I've been working through them but can't seem to find the root cause as all caps and resistors around that part of the circuit have been replaced and checked but I still can't fix it. I'm beginning to suspect the main power transformer and luckily I have managed to source another complete SRG91 unit over the weekend for just £20! This one on first glance actually looks in better condition, well certainly the cabinet does so I'm going to remove the electrical gubbins inside and compare against the one I have been working on. I'm sure I should be able to get one of them working! I now have 2 of these taking up space in my garage and all I've shelled out is £20, I've spent more on Caps, Resistors and Valves!!

One thing I did notice that on switching on power on the unit I've been restoring, the small EB91 valve momentarily glowed very brightly before settling down to normal, would this indicate an issue? I did swap it out for a NOS EB91 with the same result.

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Old 31st Aug 2020, 11:01 am   #9
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Default Re: Bush SRG-91 hot resistor

Don't worry, EB91s just do that!

They also, like EF80s, breed like rabbits if left in dark corners so are readily available, generally for £0.00 maybe +P&P
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Old 31st Aug 2020, 11:12 am   #10
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Default Re: Bush SRG-91 hot resistor

One other thing, I have the circuit schematics for the SRG91 (from here) but can't seem to locate a service manual, any ideas?
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Old 31st Aug 2020, 11:20 am   #11
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Default Re: Bush SRG-91 hot resistor

Have you checked that there's no positive voltage wrt chassis on the control grids of the output valves? I know the coupling capacitors have been changed, but there's a remote chance one of them is faulty.
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Old 1st Sep 2020, 9:55 am   #12
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Default Re: Bush SRG-91 hot resistor

If you put positive probe to cathode negative probe to grid 1 you should have a a negative voltage, if you have a positive voltage you either have a faulty valve leaky coupling capacitor or tracking valve base, easily checked by either removing the valve check voltages cathode to grid 1, removing coupling capacitor check voltages this should point you in the right direction.
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Old 1st Sep 2020, 10:30 am   #13
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Default Re: Bush SRG-91 hot resistor

Quote:
Originally Posted by Gavman68 View Post
One other thing, I have the circuit schematics for the SRG91 (from here) but can't seem to locate a service manual, any ideas?
The service manual won’t help with this fault, it will cover alignment, disassembly etc, the circuit will give you all you need.

Follow the instructions from Graham and John.
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Old 1st Sep 2020, 11:25 am   #14
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Default Re: Bush SRG-91 hot resistor

Quote:
Originally Posted by jonnybear View Post
If you put positive probe to cathode negative probe to grid 1 you should have a a negative voltage, if you have a positive voltage you either have a faulty valve leaky coupling capacitor or tracking valve base, easily checked by either removing the valve check voltages cathode to grid 1, removing coupling capacitor check voltages this should point you in the right direction.
John
Not exactly.....

With positive probe to cathode and negative probe to grid the meter will show a positive voltage, indicating that the grid is negative with respect to the cathode.
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Old 1st Sep 2020, 2:28 pm   #15
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Default Re: Bush SRG-91 hot resistor

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by jonnybear View Post
If you put positive probe to cathode negative probe to grid 1 you should have a a negative voltage, if you have a positive voltage you either have a faulty valve leaky coupling capacitor or tracking valve base, easily checked by either removing the valve check voltages cathode to grid 1, removing coupling capacitor check voltages this should point you in the right direction.
John
Not exactly.....

With positive probe to cathode and negative probe to grid the meter will show a positive voltage, indicating that the grid is negative with respect to the cathode.
Sorry should have mentioned when using analogue meter, my mistake
John
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Old 1st Sep 2020, 2:53 pm   #16
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Default Re: Bush SRG-91 hot resistor

Quote:
Originally Posted by Gavman68 View Post
I'm beginning to suspect the main power transformer
Why on earth would you suspect that?

If you follow the advice already given by others you should soon nail down the fault as to why the valve is conducting hard.

Blanket re-capping like it seems you've done is NEVER a good way to go about things!
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Old 5th Sep 2020, 4:12 pm   #17
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Default Re: Bush SRG-91 hot resistor

When I compared the SRG91 chassis I got last weekend with the problematic one it was clearly in much better condition, in fact apart from being a bit dusty it was in very good condition. I decided to work on the better of the two and use the other one for parts/scrap. Proved a good decision, the newer chassis had clearly been kept in good dry conditions and still worked ok without any work on it at all! I replaced all the hunts & electrolytic caps anyway and it's now working perfectly in all bands with a good strong magic eye too. When I started disassembling the old chassis it was clear there was corrosion everywhere in comparison, not major but enough to suggest it had spent several years in damp conditions so I suspect that may have had something to do with it not working. Now to tidy up the record deck and get that working along with some work on the cabinet, back on track! Thanks for all the advice chaps.
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Old 5th Sep 2020, 4:18 pm   #18
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Default Re: Bush SRG-91 hot resistor

So far so good.
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Old 18th Sep 2020, 12:01 pm   #19
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Default Re: Bush SRG-91 hot resistor

Restoring record deck next
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