Bush SRG138 Radiogram left side Hum - UK Vintage Radio Repair and Restoration Discussion Forum

louington · 29th Sep 2020, 12:18 am

Good evening all,

I've recently inherited my grandfather's Bush SRG138 Radiogram and I'm trying to get it back to working order.

I need to source a new cartridge for it (9TAHCG), which if anyone knows where I could find one that'd be great! However, the main issue right now is when you turn it on there is a constant hum from the left side speakers. The right side is fine and doesn't have the same hum. If you turn up the bass dial the sound gets louder, treble makes no difference.

I'm assuming it's a component issue but any advice would be appreciated. I'm confident taking it apart and doing any soldering needed as I've already had to reset a loose component.

I'll upload any pictures required to help diagnose/point me in the right direction.

Thanks in advance

Nuvistor · 29th Sep 2020, 5:40 am

The cartridge will probably be difficult to source and any NOS items could well suffer from the rubber mounts deteriorating. Is it definitely faulty?
The hum, is it there on radio as well as records or when switched to gram? If there just on records it could be just a bad connection on the plug and socket on the cartridge. Careful with the cartridge wiring both the wiring and cartridge are fragile.

Service info available from.

https://www.service-data.com/product...92/1033/a10692

louington · 29th Sep 2020, 6:46 am

I don't have a cartridge with it so it's definitely not coming from the turntable. Also, it is only on the left side speakers, not the right. It's there as soon as you turn it on.

I've been testing output using the Tape input and it comes through fine, just the hum on the left side output.

I have the service data, was a great help getting it apart correctly but I think I need to brush up on reading my circuit diagrams!

Edward Huggins · 29th Sep 2020, 12:12 pm

It might be worthwhile you checking the pick-up wiring to the amplifier by testing the LH and RH channels via the Sonotone push-on cartridge plug. Replacement Sonotone cartridges are available, but they are priced at c.£45-00 and will be at half-a-century old.

Nuvistor · 29th Sep 2020, 6:06 pm

Quote:

Originally Posted by louington

I've been testing output using the Tape input and it comes through fine, just the hum on the left side output.

Does that mean the tape has no hum when you say that’s fine just the gram with the hum?

louington · 29th Sep 2020, 7:05 pm

No there is always a hum on the left side speakers. I just mean that the input comes out as expected but with the hum.

Edward Huggins · 29th Sep 2020, 8:49 pm

Ah, so you are getting a good and clear signal from your source, whether radio or pick up (even if just a Buzz from the original Sonotone connector plug), through both channels - but with hum just on the left hand speaker. Just to be clear about your Bush model, there should be just the one speaker each side, not "speakers". Please advise.

Nuvistor · 29th Sep 2020, 8:56 pm

Does the volume control affect the hum, trying to decide if the fault is in the LH audio amp or being introduced before the volume control.
If the volume control has no effect on the hum check the electrolytic capacitors in the LH amp. Both stages are supplied from the main reservoir capacitors so are unlikely to be the cause.
If the volume control removes the hum at it minimum setting then check wiring and chassis connections for the LH amp before the volume control.

louington · 29th Sep 2020, 9:30 pm

Hum seems to stay at a pretty consistent level and if I increase volume (with input) you can hardly hear it unless you're really listening for it.

I believe this is the section I need to tinker with as its for the left hand speakers (I Know that because I had to re-seat one of the components which removed one of the issues on the left side speakers).

Nuvistor · 29th Sep 2020, 11:33 pm

What i think are R72 and 73 look damaged. R80 and 81 are under the CCT board these could also be damaged. Damage to these components usually is due to excessive current through the output transistors so checks in that area are required.

louington · 30th Sep 2020, 9:51 am

Quote:

Originally Posted by Nuvistor

What i think are R72 and 73 look damaged. R80 and 81 are under the CCT board these could also be damaged. Damage to these components usually is due to excessive current through the output transistors so checks in that area are required.

Thanks for the input Frank.

Looking at the RH Board R52 & R53 look the same. FYI, R73 is what needed to be resoldered as it had actually come off the board. I'll get it back open later and get more pictures of as much as I can so I can keep looking back at them. I'll also look at R80 and R81.

Where do you advise to look to check the output transistors? I have the Service Data so feel free to shout Componant reference values and I should be able to find them.

Many Thanks for your time!

Nuvistor · 30th Sep 2020, 12:02 pm

The only service data I have is the R&TVS information. It gives a circuit and values for the components but no voltage or current values, or I can’t see any.
The burnt resistors indicate excessive current through the output transistors, the amp is D.C. coupled so the fault could be in any component. It may be as simple as the adjustment (RV11) faulty. Take some voltage and current reading and try and find some anomaly , you have a good amp to check against.

Techman · 30th Sep 2020, 1:50 pm

I'm not familiar with this particular gram, but I agree with all that's been said. I think your best course of action would probably be to test the junctions of each transistor with a meter on an ohms range with the power off - an analogue meter is better, but a DMM on the diode range can be used. Regard each transistor as a two diodes - you should be able to read up on how to do it on-line. Depending on the circuit (which I haven't looked at) you may need to remove a transistor from the circuit if you get strange readings due to other components around it. Those burnt resistors will need replacing, unfortunately they indicate that the output transistors could be burnt out. Faults like this are usually caused by the wiper on that adjusting pot not making proper electrical contact with the carbon track, or a short circuit on the speaker output. So you've got two choices of either taking voltage readings or testing all the transistors, particularly the two output transistors. You may find that the two burnt resistors have been 'sacrificial' and replacing them and powering up could possibly cause more damage, so static checks of components could be the best course of action first.

I wouldn't worry about getting a replacement cartridge at this stage and I certainly wouldn't be looking for an original type, as any you find are likely to have problems by now. You're lucky in that there's reasonably cheap replacements to be had due to this gram having a multistage amplifier, but they won't be a direct fit and you'll have to get a tracking weight gauge to set the pickup head weight with a new and different cartridge in place.

I shouldn't say this, but if you wanted to do a bodge to get it working for the time being, you could disconnect the power supply completely from the faulty amplifier and rewire the audio input so that both channels feed the one good amplifier and just run it in permanent mono. You would only be able to have the one speaker in operation, so best to get it working properly if you can.

Edward Huggins · 30th Sep 2020, 8:04 pm

Or you could wire the 2 speakers (in series) so that there is Mono sound from each side.

But I'm still confused by the OP's continual reference to there being "speakers" on each side. I'm wondering if (non-standard) additional drivers have been added?.

AC/HL · 1st Oct 2020, 12:20 am

Each channel has a 10" bass speaker and a 4" tweeter with a simple series capacitor.

Edward Huggins · 1st Oct 2020, 8:58 am

Thanks for the clarification as to the speakers, I must have been reading the wrong Bush SRP specification. However, I am aware that the younger generation will often refer to a single loudspeaker as "speakers", although I have never quite understood why they need to pluralise it?

louington · 5th Oct 2020, 4:03 pm

Thanks for your help so far all. I'm trying to locate the dodgy components with a meter and will let you know how I progress however it's worth noting that since I had it open to do some investigation the other day (Mainly just looking and getting pics and readings) the hum volume/intensity has decreased...

In the meantime, Can anyone tell me what type of resistor this is? I know the resistance value from the circuit diagram however I want to know if you can still find this type or if I can replace it with a carbon film one with the same resistance value?

Click image for larger version

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It's one of the R72/73 resistors that looks burnt out on the LH output board.

Thanks in advance.

29th Sep 2020, 12:18 am	#1
louington Triode Join Date: Aug 2009 Location: Hereford, Herefordshire, UK. Posts: 12	Bush SRG138 Radiogram left side Hum Good evening all, I've recently inherited my grandfather's Bush SRG138 Radiogram and I'm trying to get it back to working order. I need to source a new cartridge for it (9TAHCG), which if anyone knows where I could find one that'd be great! However, the main issue right now is when you turn it on there is a constant hum from the left side speakers. The right side is fine and doesn't have the same hum. If you turn up the bass dial the sound gets louder, treble makes no difference. I'm assuming it's a component issue but any advice would be appreciated. I'm confident taking it apart and doing any soldering needed as I've already had to reset a loose component. I'll upload any pictures required to help diagnose/point me in the right direction. Thanks in advance

29th Sep 2020, 5:40 am	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Bush SRG138 Radiogram left side Hum The cartridge will probably be difficult to source and any NOS items could well suffer from the rubber mounts deteriorating. Is it definitely faulty? The hum, is it there on radio as well as records or when switched to gram? If there just on records it could be just a bad connection on the plug and socket on the cartridge. Careful with the cartridge wiring both the wiring and cartridge are fragile. Service info available from. https://www.service-data.com/product...92/1033/a10692 __________________ Frank

29th Sep 2020, 6:46 am	#3
louington Triode Join Date: Aug 2009 Location: Hereford, Herefordshire, UK. Posts: 12	Re: Bush SRG138 Radiogram left side Hum I don't have a cartridge with it so it's definitely not coming from the turntable. Also, it is only on the left side speakers, not the right. It's there as soon as you turn it on. I've been testing output using the Tape input and it comes through fine, just the hum on the left side output. I have the service data, was a great help getting it apart correctly but I think I need to brush up on reading my circuit diagrams!

29th Sep 2020, 12:12 pm	#4
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,340	Re: Bush SRG138 Radiogram left side Hum It might be worthwhile you checking the pick-up wiring to the amplifier by testing the LH and RH channels via the Sonotone push-on cartridge plug. Replacement Sonotone cartridges are available, but they are priced at c.£45-00 and will be at half-a-century old. __________________ Edward.

29th Sep 2020, 7:05 pm	#6
louington Triode Join Date: Aug 2009 Location: Hereford, Herefordshire, UK. Posts: 12	Re: Bush SRG138 Radiogram left side Hum No there is always a hum on the left side speakers. I just mean that the input comes out as expected but with the hum.

29th Sep 2020, 8:49 pm	#7
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,340	Re: Bush SRG138 Radiogram left side Hum Ah, so you are getting a good and clear signal from your source, whether radio or pick up (even if just a Buzz from the original Sonotone connector plug), through both channels - but with hum just on the left hand speaker. Just to be clear about your Bush model, there should be just the one speaker each side, not "speakers". Please advise. __________________ Edward.

29th Sep 2020, 8:56 pm	#8
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Bush SRG138 Radiogram left side Hum Does the volume control affect the hum, trying to decide if the fault is in the LH audio amp or being introduced before the volume control. If the volume control has no effect on the hum check the electrolytic capacitors in the LH amp. Both stages are supplied from the main reservoir capacitors so are unlikely to be the cause. If the volume control removes the hum at it minimum setting then check wiring and chassis connections for the LH amp before the volume control. __________________ Frank

29th Sep 2020, 9:30 pm	#9
louington Triode Join Date: Aug 2009 Location: Hereford, Herefordshire, UK. Posts: 12	Re: Bush SRG138 Radiogram left side Hum Hum seems to stay at a pretty consistent level and if I increase volume (with input) you can hardly hear it unless you're really listening for it. I believe this is the section I need to tinker with as its for the left hand speakers (I Know that because I had to re-seat one of the components which removed one of the issues on the left side speakers). Attached Thumbnails

29th Sep 2020, 11:33 pm	#10
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Bush SRG138 Radiogram left side Hum What i think are R72 and 73 look damaged. R80 and 81 are under the CCT board these could also be damaged. Damage to these components usually is due to excessive current through the output transistors so checks in that area are required. __________________ Frank

30th Sep 2020, 12:02 pm	#12
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Bush SRG138 Radiogram left side Hum The only service data I have is the R&TVS information. It gives a circuit and values for the components but no voltage or current values, or I can’t see any. The burnt resistors indicate excessive current through the output transistors, the amp is D.C. coupled so the fault could be in any component. It may be as simple as the adjustment (RV11) faulty. Take some voltage and current reading and try and find some anomaly , you have a good amp to check against. __________________ Frank

30th Sep 2020, 1:50 pm	#13
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: Bush SRG138 Radiogram left side Hum I'm not familiar with this particular gram, but I agree with all that's been said. I think your best course of action would probably be to test the junctions of each transistor with a meter on an ohms range with the power off - an analogue meter is better, but a DMM on the diode range can be used. Regard each transistor as a two diodes - you should be able to read up on how to do it on-line. Depending on the circuit (which I haven't looked at) you may need to remove a transistor from the circuit if you get strange readings due to other components around it. Those burnt resistors will need replacing, unfortunately they indicate that the output transistors could be burnt out. Faults like this are usually caused by the wiper on that adjusting pot not making proper electrical contact with the carbon track, or a short circuit on the speaker output. So you've got two choices of either taking voltage readings or testing all the transistors, particularly the two output transistors. You may find that the two burnt resistors have been 'sacrificial' and replacing them and powering up could possibly cause more damage, so static checks of components could be the best course of action first. I wouldn't worry about getting a replacement cartridge at this stage and I certainly wouldn't be looking for an original type, as any you find are likely to have problems by now. You're lucky in that there's reasonably cheap replacements to be had due to this gram having a multistage amplifier, but they won't be a direct fit and you'll have to get a tracking weight gauge to set the pickup head weight with a new and different cartridge in place. I shouldn't say this, but if you wanted to do a bodge to get it working for the time being, you could disconnect the power supply completely from the faulty amplifier and rewire the audio input so that both channels feed the one good amplifier and just run it in permanent mono. You would only be able to have the one speaker in operation, so best to get it working properly if you can.

30th Sep 2020, 8:04 pm	#14
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,340	Re: Bush SRG138 Radiogram left side Hum Or you could wire the 2 speakers (in series) so that there is Mono sound from each side. But I'm still confused by the OP's continual reference to there being "speakers" on each side. I'm wondering if (non-standard) additional drivers have been added?. __________________ Edward.

1st Oct 2020, 12:20 am	#15
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,643	Re: Bush SRG138 Radiogram left side Hum Each channel has a 10" bass speaker and a 4" tweeter with a simple series capacitor.

1st Oct 2020, 8:58 am	#16
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,340	Re: Bush SRG138 Radiogram left side Hum Thanks for the clarification as to the speakers, I must have been reading the wrong Bush SRP specification. However, I am aware that the younger generation will often refer to a single loudspeaker as "speakers", although I have never quite understood why they need to pluralise it? __________________ Edward.

5th Oct 2020, 4:03 pm	#17
louington Triode Join Date: Aug 2009 Location: Hereford, Herefordshire, UK. Posts: 12	Re: Bush SRG138 Radiogram left side Hum Thanks for your help so far all. I'm trying to locate the dodgy components with a meter and will let you know how I progress however it's worth noting that since I had it open to do some investigation the other day (Mainly just looking and getting pics and readings) the hum volume/intensity has decreased... In the meantime, Can anyone tell me what type of resistor this is? I know the resistance value from the circuit diagram however I want to know if you can still find this type or if I can replace it with a carbon film one with the same resistance value? It's one of the R72/73 resistors that looks burnt out on the LH output board. Thanks in advance.