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FDP Forum / Amp Mods, Repairs, and Projects / Old Blackface Showman / no sound except knob crackle

Iron Broadsword

Canada

Apr 20th, 2018 07:06 AM   Edit   Profile  

First post, cool forum. Alright, so last time I fired this thing up it worked fine but it's been sitting for about a year and now it makes no sound when you play through it. It does however give off some volume knob crackle when you rotate those, so I'm assuming the problem is somewhere early on in the preamp. This is all through the normal channel.. in the vibrato channel it seems completely dead (or maybe it's just that that volume knob doesn't have a crackle issue)

What I've done so far:
- Tried all the tubes in another amp, and they work fine there
- Cleaned all jacks (cleaning all pots is next on the todos)
- Cleaned all tube sockets
- Tried obvious stuff like different guitar / cables

It's never had a full cap job.. so yeah I'm thinking it might be time for that but the big electrolytics were replaced 2 years ago. It seems like V2 and V3 don't get very warm, especially V3. I can hold it with my bare fingers and it's only just pretty warm so it could be that the heater supply voltage is the issue. Tried a different tube in here first and same issue. I'm pretty handy with a soldering iron, built a bunch of pedals and have modified a handful of amps to great success, but I'm crap with a schematic.

Where do I go from here? It's a '65. Thanks in advance


(This message was last edited by Iron Broadsword at 09:09 AM, Apr 20th, 2018)

Iron Broadsword

Canada

Apr 20th, 2018 09:00 AM   Edit   Profile  

Checked the preamp heater voltages at pins 4/5, and yep it's about 5mV so that's definitely the problem. Plate voltage was on point though at 469v so the thing isn't completely dead. Going to try to hunt down a schematic

Peegoo
Contributing Member
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Get ready for the

Buena Buena
Apr 20th, 2018 09:11 AM   Edit   Profile  

Here's the

AA769

Peegoo
Contributing Member
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Get ready for the

Buena Buena
Apr 20th, 2018 09:11 AM   Edit   Profile  

Here's the AA768

schematic

Hammond101
Contributing Member
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So. Cal. USA

Apr 20th, 2018 09:23 AM   Edit   Profile  

5mV? With a voltage that low you would see no glow of heater filaments and feel no heat. I can't imagine it would make even a crackle but I've been wrong before. Meter should be set on AC volts. Start at the pilot lamp socket and test across the two green wires and work your way across the tube sockets checking voltage at each socket. You are looking for 6.3 VAC.

Measure all DC voltages on the preamp tubes pins 6, 8 & 1 and compare to the linked schematic.

This sounds like a signal issue to me but I like to start with voltages first. You are on the right track.

Edit to add:

I see Peegoo posted while I was typing. Your amp being a '65 should follow the AB763 schem. The AA768 is for silverface models that were released mid/late '68.

AB763 Showman Schem.

(This message was last edited by Hammond101 at 11:30 AM, Apr 20th, 2018)

Peegoo
Contributing Member
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Get ready for the

Buena Buena
Apr 20th, 2018 09:49 AM   Edit   Profile  

Bam!

You're too fast for me!

Iron Broadsword

Canada

Apr 20th, 2018 10:07 AM   Edit   Profile  

Nice catch, you're right I was measuring DC. Pins 4/5 are all at 3.1VAC without tubes installed.

Thanks for the schem, here are some gut shots:
https://pix.sfly.com/A8eWq4
https://pix.sfly.com/rO1Pjv

I'll grab those other voltages asap

(This message was last edited by Iron Broadsword at 12:09 PM, Apr 20th, 2018)

Iron Broadsword

Canada

Apr 20th, 2018 10:34 AM   Edit   Profile  

Alright, got some pretty wild readings. (Btw my V1-V4 is assuming V1 is furthest away from the power tubes). Tubes were installed while grabbing these.

I got 6.3V across green wires at pilot light.


V1:
6 - 615mV
8 - 18.1mV
1 - 622mV

V2:
6 -618mV
8 - 12mV
1 - 621mV

V3:
6 - 367V
8 - 33mV
1 - 373V

V4:
6 - 299V
8 - 73V
1 - 301V

I got 2.9V on Pins 4/5 on all 4.


So V1 and V2 are both clearly not getting enough voltage, V3 is very high on pin 1 and very low on pin 8, and V4 is very high on pins 6 and 1 and a little low on 8. And the heater voltage is very low on all.

(This message was last edited by Iron Broadsword at 12:38 PM, Apr 20th, 2018)

willie
Contributing Member
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Too Near Atlanta GA

Amp Tech Emeritus
Apr 20th, 2018 10:50 AM   Edit   Profile  

I'd check that 4.7K dropping resistor as well as the cap for the "D" voltage (plates for V1/V2).

w

Hammond101
Contributing Member
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So. Cal. USA

Apr 20th, 2018 05:43 PM   Edit   Profile  

I've been working the day gig all day. Willie as always has you pointed in the right direction. You might check all the voltages on the power rail under the dog house too.

Iron Broadsword

Canada

Apr 21st, 2018 03:53 PM   Edit   Profile  

Alright, so the wire that connects D to the 4.7k had come loose. Fired it up and it's ALIVE again, but there are still some issues. Gonna grab voltages again tomorrow.

Hammond101
Contributing Member
**********
**

So. Cal. USA

Apr 23rd, 2018 10:28 AM   Edit   Profile  

Yay!

FDP Forum / Amp Mods, Repairs, and Projects / Old Blackface Showman / no sound except knob crackle




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