Ampli R3680 A ( Ami MM1 )

Hey Guy's
Now I must waithing for spare-parts of the Canteen electronic jukebox , will I try to fixed a reserve ampli R3680A that came out
a Ami Rowe MM1 jukebox.
There is one channel that not works ( Left ) all fuses are good , also the power transistors are good.
When I used the input channels without the pre-ampli it's the same fault. So I try the figure out what's wrong.
But I find nothing ? All the voltages are good . I guess it's maybe the driver print , but all the transistors are good ?
I don't know where to looking now , maybe here someone try to help me. I have a wiring diagram .So I have two speakers connected where it must be. When I measure (with a analoge multimeter) the big transfo then you hear the speaker scraching. When I do this with the little print on the side ( where the resistors and the diodes stays) then the speaker scraching.



Kind Regards

I measure out somethings ? I placed a picture of this ampli here.
When I put a signal on the Right Channel , then both speakers works , when I pull the fuse out , then no sound.
When I connect a speaker in the middle of the connector see picture , then the speaker works , when I pull out the other fuse
then the volume increase half ? So I guess that this ampli is connected for mono ??
So who can help me futher ?

Kind Regards - Luc

Hello Luc,

This amplifier was the first transistor amplifier made by Rowe Ami, there are known faults on it with regards to the AVC (automatic volume control) I have not done many repairs on these amplifiers, they were only used in the MM1 and coursed that many problems for Rowe Ami that they went back to the valve amp on the standard package for their 200 selection jukeboxes right through the MM series and were also used on the TI. That 50 watt vave amp using the separate transistor pre was a great combination.

The amplifier was a stereo amplifier but the bass speakers would have been connected in mono across both channels which was the normal arrangement with most of the Rowe Ami amplifiers.

I still have a few of these chassis if you require a full amp or parts.

Regards
Alan

Alan Hood
ami-man
UK

Hello Luc

As Alan said earlier, the bass speaker is operated monaural. The bass speaker is therefore connected to both channels in a bridge circuit. To make this work, one channel must be connected at the cartridge out of phase (red and black wire exchanged on one channel).

The bass speaker is not connected to pins 2 and 5, as in your picture but to pins 1 and 4 (labeled in your picture with "Right Channel"). The left midrange speaker is connected to the pins 2 and 5 (labeled in your picture with bass speaker), the right to the pins 3 and 6 (labeled in your picture with Left Channel). Both mid-range speakers are connected via a bipolar capacitor (32MFD / 50 volts) in series

As shown in your picture, the switch "PHONO SPEAKER WATTS" is switched in position 1.5 watts. This switch is used to reduce the volume of the internal speakers if external speakers are connected. If no external speakers are connected, the switch should be switched to position 24 watts. This switch also caused sometimes contact problems. You should therefore spray some electronics - contact spray into the switch and switch back and forth several times. This cleans the contacts.

The circuit is shown in the service manual. I think you can get one here in the shop (even if it is not listed), if you do not have any. You can also only order the schematic for the amplifier [ssl.kundenserver.de] .

Best regards - charly49

Hey Charly49 and Alan,

I understand what you say , and I know that the speakers where not correct on the picture , but It was to let see what I found
in the measures. and thanks for both answers.
But the question whas , why one channel works and the other not works. When I see the wiring diagram then you have 2 channels
Right and Left , But left channel not works .
So when I see this , and plug a signal on the left , then you must only sound hear in the left speaker.
when I plug a signal on the right , then only Right speaker must work , except de Bass speaker they must works if one
signal is given in L or R input
Is this right what I say ?

Hello Luc

Have you checked that the speakers are connected in the manner described? Have you also checked whether the cartridge is connected correctly, out of phase with one channel. The balance control must be set in the middle position. The volume control must be properly connected.

If everything is as described, the following should happen. If you have one of the input - plugs plugged in, the music would come from the corresponding mid-range speaker. To exclude errors in cartridge or audio cable, you use every time the same input - plug. Through the bridge circuit must also come music from the bass speaker at reduced volume and a very small amount of music from the other mid-range speaker. If both input connectors are plugged in, the music must come from both mid-range - speakers. The bass speaker should sound at full volume. If the bass speker is too quiet, the cartridge is not connected correctly.

Optionally you can also use a pair of external speakers (internal speakers unplugged) for testing. Each of them is connected to terminals E2 and E6. In this case the two channels are completely separate, there is no crosstalk between the two channels.

If the checks show that there is a fault in the amplifier, it must be repaired. I do not know if you have the necessary knowledge and experience. If not, you should find someone who has it. Without the necessary knowledge you should not try to repair the amp.

Best regards - charly49

Hey Charly49,
Thanks , for your answer .
What I all did is first checkout the input with preampli , so I put first one cable of the cartridge in the right input ( with pre-ampli)
I hear sound (both speakers and bass , is good . I take the other cable of the cartridge in the right input ( with pre-ampli) I hear sound ( both speakers and bass speaker) So the cartridge works fine . When I do this all with the 2 cables of the cartridge in the left input ( with pre-ampli) I hear nothing.
So I guess maybe is the fault in the pre-ampli . So I do all the results again , but with a radio signal in the right input ( on driver right) (whithout pre-ampli ) I have sound in both speakers and bass speaker. When I do this with the left input (on driver ), no sound nothing.
When I do the test , with the terminals then only the right terminal works , the left nothing.
Then a figure out what's wrong , but I find nothing ?? And this is not my first jukebox that I fixed , but sometimes I see the trees no more in the forest :)

Hello Luc

If you have only one input - plug plugged in, the music should come mostly from one of the midrange - Speakers. The second speaker must play very softly. If the volume of both speakers is almost the same, something is wrong with the connection. The fault may also be located in the wiring inside the amplifier. First, it must be ensured that the speaker connection is correct.

Best regards - charly49

Hey Charly49,
I use only one input for the measure , so I use the input R channel then I have connected one speaker to pin 6-3 (yel-white)
the other speaker on pin 2-5 ( Red-GRN) and the 3 speaker (bass) on pin 4-1 (Blu-orn) then the both midranges speakers works , I thought that only the right speaker must work ?? There I use only the right input ?
When I do this with the left input , no speakers works.
Now I found something else , when I remove both fuses F3 and F4 then I have only a voltage over fuse 3 about 18 V
On Fuse 4 left channel , I have nothing ???

Kind Regards - Luc

Hello Luc

Your thoughts are partially correct, as long as only the two midrange speakers are connected. Then the music must come from only one speaker when the music signal is applied to only one input. If the bass speaker is connected in addition, a small portion also comes over these to the second output transformer, and thus also to the second midrange. The bass speaker should therefore sound with reduced volume, the second midrange very soft. If the two midrange have nearly the same volume, then probably the circuit has been modified within the amplifier. May be that it was made, because one channel is defective.

Over the two fuses should only run the low frequency - power, no DC current. Therefore, should occur no DC voltage if the fuses are removed. The 18 volts at the fuse F3 indicates that the corresponding output stage is defective. I think in your amplifier should be several errors present. If you do not have the necessary knowledge, you should give the amplifier to a professional. These amplifiers are pretty difficult to repair and I recommend you not to try it yourself.

Best regards - charly49

Hey Charly49,
You have right , I guess that the early owner modified the ampli ( He whas a Jukebox repair man ) how more that I search
I find everything that not be right, when I look to the wiring diagram. When I remove fuse F2 near the rectificier , then I have a voltage of 18 V over Fuse 4, when I put back the fuse F2 , I have nothing ???
So I guess that he modified the amp for one Channel . I try to lookout for another amp .

Kind Regards - Luc

Hello Luc

To buy another amplifier is just a good idea if the seller is trustworthy. Otherwise it may be that the new amplifier is in a worse condition than your old. I don't know where you live, but there should be in your area a professional who can repair your amplifier.

Best regards - charly49

Lello Luc,

I agree with charly49 you need to get this amplifier to someone who can repair it correctly, it is a very involved circuit and a lot of the wiring is linked via tag strips if my memory is correct, in addition the pre-amp board is unique to this model of ampfifier only.

Regards
Alan

Alan Hood
ami-man
UK

Hey Ami-man,
You are right , but I have first all recapping the old capacitors on the pre-ampli.
And it works fine , both channels that I tested with another ampli.
And this pre-ampli only use this ampli R3680A . With the later ampli there are the drivers on two prints and not many wires like
this ampli.. So I try to search another ampli then I can see what's wrong white the wires.

Kind Regards - Luc

Hello luc,

I am not a great believer in changing caps on the pre amps unless needed, the power amp and supply rails is another matter.

You may great difficulty getting a chassis of the R3680A in any condition and if you can they will be expensive, I would estimate over £100.00
If you can not source one in your park of Europe please get in touch and I will get one out of stock.

Regards
Alan

Alan Hood
ami-man
UK
alan-hood@datex.co.uk

Hey Alan,
Thanks , for your replay , It's difficult to find one , there Rowe Ami the amp used in one model the MM1.
But I let You know when I no amp found.
Recapping , I do this with the most jukeboxes , Sound is better and sometimes , you see nothing on the caps but they are have a closed circuit or open . So I had this with a NSM jukebox Festival ES160 , the tantal caps are a closed circuit. I thougt first it whas the voltage regulator , but now it whas the tantal cap. So I learn my lesson.
Of the end of the week I go try to soldered out the wires input of the little transfo that connected with the driver print , maybe lay the fault there ? So I can look of the driver print works.

Kind Regards - Luc

Hey Guy's,
I try to understand the wiring diagram , but I have a question : from where come the power for the driver print ?
Come they of the little input or output transfo that's connected with the End Power transistors ?
I pull here a wiring diagram in colors , so you can see what I mean.

Kind Regards - Luc

Hello Luc

The power for the driver print comes over the red marked wires.

If you want to repair amplifiers yourself, it is absolutely necessary to learn first the basics of electronics. You must know how the individual components (resistors, capacitors, diodes, transistors, etc.) work. Also you have to know how the components interact and how the individual amplifier stages work. If you have learned all that, you know what the measured voltage value means and where the error is to look for if the values are not as expected. Without this knowledge, a reasonable troubleshooting is nearly impossible.

I still think that it would be better to give the amplifier to a specialist.

Best regards - charly49

Hey Charly49,
Thanks for your replay , I have seen it now , and I know where the fault is.
It's not my first ampli that I repair , I built several ones , but like you early says , with the normal electronic basics from the 70 and 80's . Now the fault sit him in de driver-print , the transistor Q50 2N2925 is bad ,
So I may this replace with a BC548 like my older books('74 ) say's.
Tomorrow I let You know if it's works.

Kind Regards - Luc

Hey guy's.

The ampli works fine now , many thanks for Charly49 and Alan Hood.
Hi changed the transistor Q50 - 2N2925 (NPN) with a BC548 now both channels are working fine.
So I have now a good reserve ampli.

Kind Regards - Luc