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Post subject: Help needed: Info on an old Bassman
Posted: Mon Oct 29, 2012 11:36 pm
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Hi - First post around here so I hope this is the right place to ask:

I am helping a friend rebuild what appears to be a 1979 Bassman Ten. However, the switches on the faceplate do not match the pictures that I see on (Ampwares.com), and I have found in my initial review of the components that the wiring diagrams on that same site do not match the circuit that I am looking at.

For example, the two leftmost capacitors in the picture linked below are 70uF/100V and the closest that I see on the diagram in value to that is 70uF/350V, but based the fact that they link into a diode on the other side of the tray, they would have to be the 50uf/70v capacitors from the diagram. However, if that is true then the resistor between them (2.2kOhm) is wrong as the diagram shows 3.8kOhm.)

Can anyone point me to the correct wiring diagram for this amplifier or provide any advice on re-building this amp?

I have identified it a 1979 based on its serial: A957023
This album has pictures of the tray and the faceplate: Bassman Pics

The scale of rebuild necessary is still unknown. I need to replace some components in the amp, but I have yet to identify which. The symptom is that it works decently (at low volume at least) for a short time then blows its fuse. I was figuring I would start with the capacitors, but if I can't get a good wiring diagram I may be out of luck.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 2:23 am
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Maelstrom88 wrote:


The symptom is that it works decently (at low volume at least) for a short time then blows its fuse. I was figuring I would start with the capacitors, but if I can't get a good wiring diagram I may be out of luck.


IMO exact schematic for this amp , a late CBS is not easy to find.

You are not doing trouble shouting the right way. The amp working before with same resistor value. Most of the time , tube amps's trouble are ...tubes. Output tubes or rectifier tube. Your amp did not have rectifer tube so check output tubes this way ;

1- Remove output tubes , put new fuse and Power ON : Fuse ok = output tubes defect.
2- Fuse blow ; you have to check B+ from power supply by isolate it . You'll see where is the defective component.

Don't change any cap or other parts until you are sure you must replace it.

Be careful ; LETHAL VOLTAGE IN TUBES AMP


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 7:05 am
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Sometimes there were big changes year to year. I'd try to get the right schematic from Fender. Might be available and it won't hurt to ask.

Start by contacting Fender's Consumer Relations Department.

Fender Musical Instruments Corporation
Attn: Consumer Relations Dept.
17600 N. Perimeter Drive, Suite 100
Scottsdale, AZ 85255

Telephone: (480) 596-7195
Fax: (480) 367-5262
E-mail: consumerrelations@fender.com

Include that model and serial and model year and any other info that will help. Often there is a tube diagram with a number on it also, but not sure about this model having the tube diagram number. You might get an answer faster by phone. Good luck.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 7:59 am
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stratele52 wrote:


1- Remove output tubes , put new fuse and Power ON : Fuse ok = output tubes defect.
2- Fuse blow ; you have to check B+ from power supply by isolate it . You'll see where is the defective component.



That's the strange part - The amp works for a variable amount of time, then sometimes it will work again when you replace the fuse and other times it will blow through a load of fuses all at once. In any case, I will try these steps. I need a guide for how to check the B+ by isolating it. Do you know of any good ones offhand? If not I'll just hit google.

brotherdave wrote:
Include that model and serial and model year and any other info that will help. Often there is a tube diagram with a number on it also, but not sure about this model having the tube diagram number. You might get an answer faster by phone.


Unfortunately, I just have the serial. The tube chart is no longer intact below the pictures of the tubes themselves (all the numbering at the bottom dried up and broke off). Still, I'll give it a try.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 10:49 am
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The amp illustrated on the Ampwares website is the earlier 50-watt version (yours is the later 70-watt ultra-linear platform). That's why the pic does not match your front panel -- the 70-watt amp had a middle control added to the bass-instrument channel.

I'm seeing substantial physical damage to one of the reservoir caps under the doghouse -- I would investigate that issue first.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 11:44 am
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So, here's some interesting developments-

First, I called Fender and got the schematic for the 70 Watt amp, so at least I have somewhere to work from now.

Second, I went out to the store and acquired some of the recommended fuses (LittelFuse 31302.5 - 2.5a/250V slo-blo)

Third, and most importantly - with the new fuse, I cannot seem to make the fuse blow the way my friend did. Granted, I am using my Casio Privia in Bass mode, it isn't blowing out. I looked at the old fuse he was using - it was an "ADL" 2.5A/250V fuse. There are no other markings. Could he simply have been using the wrong fuses? It also may be that I wasn't running it for long enough to blow out the fuse.

Here's a video of the Bassman in action with the correct fuse: Bass Test 1

Any advice based on the video and my continuing work is appreciated. Thank you.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 12:50 pm
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Could be the wrong fuse the one you had before.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Oct 30, 2012 12:59 pm
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Agree with Arjay, I'd check that beat up cap up and down.

There is a reason the original fuse blew and the wrong fuse got used. I'd look for that reason and start with the power supply caps and rectifier.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Wed Oct 31, 2012 10:26 am
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brotherdave wrote:
There is a reason the original fuse blew and the wrong fuse got used. I'd look for that reason and start with the power supply caps and rectifier.


+1!

Don't neglect the bias supply either -- without that, nothing else will matter.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Tue Nov 13, 2012 8:38 pm
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Update time:

I replaced the capacitors with F&T caps (with some different ratings based on capacitor availability)

70uFd/100V -> 100uF/100V

20uFd/500V -> 22uFd/500V

220uFd/285V -> 220uFd/300V

I also replaced the resistors in the doghouse and the two 100k resistors by the transformer with wirewound resistors (or metal film where wirewound was not available).

Overall, my thoughts are - it's a LOT more powerful than before I recapped it.

Here's a video of the amp at volume 5 (or maybe it was 3) (I turn it on about 30 secs in or so) with the piano it's hooked to about 1/3 of the way up it's volume setting. I will note that my cellphone could only capture so much sound. Standing there filming, I could feel pressure on my eardrums. With the volume any louder, I could not make it past the loudest part of the song without having to move based on discomfort.

Tell me what you think: http://www.youtube.com/watch?v=YJbJaB1C9bU

Here's the questions I have left at this point:

1. Still a fairly significant hum which increases and decreases with volume on the Bass circuit. (The guitar circuit doesn't seem to have this problem). The hum balance makes the hum softer when the dial is turned to either extreme and really bad when the balance is centered. Any advice on what could be causing this? I did check the continuity of the ground in the plug, and it was connected.

2. In the doghouse there were two white wires that went to nowhere (they stopped inside one of the through channels in the doghouse). Any idea what they are for?

3. Any other general observations? I have yet to blow a fuse.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Wed Nov 14, 2012 3:04 am
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Maelstrom88 wrote:

1. Still a fairly significant hum which increases and decreases with volume on the Bass circuit. (The guitar circuit doesn't seem to have this problem). The hum balance makes the hum softer when the dial is turned to either extreme and really bad when the balance is centered. Any advice on what could be causing this? I did check the continuity of the ground in the plug, and it was connected.

2. In the doghouse there were two white wires that went to nowhere (they stopped inside one of the through channels in the doghouse). Any idea what they are for?

3. Any other general observations? I have yet to blow a fuse.


Answers to;
2- Photo will help . Channel ( ?? ) in the dog house ; more explanation and photo
Is are those wire make a short and blow fuse ? You should fix that BEFORE everything else

1- Problem is in the stage BEFORE volume control . Check wiring dress ( do you know how ? ) Photos of the input jack to tubes. Check input shorting jacks if contact to ground is good ; Normal channel, clean it.

3- Fuse blow after all those work ; check the 2 white wires in dog house. Read my answer october 30


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Wed Nov 14, 2012 5:38 am
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stratele52 wrote:

Answers to;
2- Photo will help . Channel ( ?? ) in the dog house ; more explanation and photo
Is are those wire make a short and blow fuse ? You should fix that BEFORE everything else

1- Problem is in the stage BEFORE volume control . Check wiring dress ( do you know how ? ) Photos of the input jack to tubes. Check input shorting jacks if contact to ground is good ; Normal channel, clean it.

3- Fuse blow after all those work ; check the 2 white wires in dog house. Read my answer october 30


Okay, i will check the plug cleanliness. What i meant by channel was "one of the through paths from the doghouse to the other side of the casing". The wires terminated in the plastic through channel and were not stripped so it is highly unlikely they were doing anything, if you look at my old picture of the caps, the two wires run from the upper right to the through channel on the lower left and appear to be white and tan in the pic.

Also, it may have been unclear from how I said it, but I should make clear that I have NOT blown a single fuse during my work on this thing. I think the main fuse issue was the crap radio shack fuses the owner was using.


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Post subject: Re: Help needed: Info on an old Bassman
Posted: Wed Nov 14, 2012 6:04 am
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Ok I understand now , thank's


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