Hi,
I'm trying to convert a broken vintage Telefunken 302TS tape recorder into a simple Deacy-style guitar amp according to Mullard's reference schematic (or close to it). Schematic of the tape recorder can be found here d_telefunken_m302ts_sch1b.pdf - Google Drive and the radio museum page with more details here Magnetophon 302TS M302TS R-Player Telefunken Deutschland TFK
I reused all transistors and transformers from the recorder (AC150 at input, AC122 driver and two AC117P), but used new capacitors and resistors. Now, the issue is that it doesn't run. The output is dead quiet. If I slam the guitar strings hard you can hear a buzzing noise very briefly and then back to quiet. When I power it up, even without the speaker hooked up, the circuit itself makes a quiet but noticeable hum. Here's how it sounds 20170923_222157_Custom.flac - Google Drive
Not sure which component could cause it, maybe the transformer?
I traced circuit and listened what comes out of certain nodes in the circuit through another amplifier and everywhere in the circuit except for the input I get the same noise, just louder. Here's a recording at collector of the input transistor 20170923_222230_Custom.flac - Google Drive
I'm not sure that I wired the driver transformer correctly, although I tried two combinations with similar results. Telefunken transformer has more windings, so maybe I misunderstood how it's supposed to work. I'm pretty sure that terminals 1, 2, 3 are the secondary. I wired 5 and 6 as the primary, although I've tried 7 and 8 as well. 5-6 measure 150ohms, 7-8 measure 650ohms.
Any ideas would be appreciated. It's possible that transistors are bad, although I converted a few vintage radios into amplifiers without any issues, just by replacing electrolytic capacitors.
Below are the voltages and deacy schematic
(all voltages are negative, omitting the minus sign)
B-: 7.7
Q1
E:0.7
B:0.85
C:2.75
Q2:
E:1.35
B:1.25
C:7.7
Q3
E:0
B:0.09
C:7.7
Q4
E:0
B:0.12
C:7.7
I'm trying to convert a broken vintage Telefunken 302TS tape recorder into a simple Deacy-style guitar amp according to Mullard's reference schematic (or close to it). Schematic of the tape recorder can be found here d_telefunken_m302ts_sch1b.pdf - Google Drive and the radio museum page with more details here Magnetophon 302TS M302TS R-Player Telefunken Deutschland TFK
I reused all transistors and transformers from the recorder (AC150 at input, AC122 driver and two AC117P), but used new capacitors and resistors. Now, the issue is that it doesn't run. The output is dead quiet. If I slam the guitar strings hard you can hear a buzzing noise very briefly and then back to quiet. When I power it up, even without the speaker hooked up, the circuit itself makes a quiet but noticeable hum. Here's how it sounds 20170923_222157_Custom.flac - Google Drive
Not sure which component could cause it, maybe the transformer?
I traced circuit and listened what comes out of certain nodes in the circuit through another amplifier and everywhere in the circuit except for the input I get the same noise, just louder. Here's a recording at collector of the input transistor 20170923_222230_Custom.flac - Google Drive
I'm not sure that I wired the driver transformer correctly, although I tried two combinations with similar results. Telefunken transformer has more windings, so maybe I misunderstood how it's supposed to work. I'm pretty sure that terminals 1, 2, 3 are the secondary. I wired 5 and 6 as the primary, although I've tried 7 and 8 as well. 5-6 measure 150ohms, 7-8 measure 650ohms.
Any ideas would be appreciated. It's possible that transistors are bad, although I converted a few vintage radios into amplifiers without any issues, just by replacing electrolytic capacitors.
Below are the voltages and deacy schematic
(all voltages are negative, omitting the minus sign)
B-: 7.7
Q1
E:0.7
B:0.85
C:2.75
Q2:
E:1.35
B:1.25
C:7.7
Q3
E:0
B:0.09
C:7.7
Q4
E:0
B:0.12
C:7.7
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There is some feedback network in this circuit that must be tuned to the transformers. I really doubt that your self wound transformers come close to the originals - these audio transformers might contain some hundreds or even some thousands turns of very thin wire.
Next problem are the output transistors - AC128 is not what I call a power horse - I suppose you might get something like 50mW of output power, if erverything is layed out correctly.
The bias of the output stage lacks any temperature compensation and bias current will vary over a wide range depending on supply voltage - making the worst out of this deprecated technology.
Next problem are the output transistors - AC128 is not what I call a power horse - I suppose you might get something like 50mW of output power, if erverything is layed out correctly.
The bias of the output stage lacks any temperature compensation and bias current will vary over a wide range depending on supply voltage - making the worst out of this deprecated technology.
thanks for the reply. Transformers are not wound by me, they are Telefunken originals, as I mentioned in the original post. Output transistors are AC117P, not AC128. Finally, I kept the thermistor that was already mounted on the transformer for temperature compensation (shown below).
Hmm. Are those base voltages set by the voltage divider between 2.2k and 39r? If yes then they should be close to 0.13v if my math is right.
Hmm, yes, those two should be the same. I measured resistance of the driver transformer secondary and it's perfectly symmetrical, 100ohms on each side of the center tap. Voltage divider sets the center tap to 0.14V, but one base is at 0.12-0.13V, the other around 0.09-0.1V. What could cause the diff? Is it possible that one side draws more current? Maybe bad output transistors?
Also measured output transformer, the primary measures 1.2ohm around the center tap. The secondary measures 0.4ohm.
Also measured output transformer, the primary measures 1.2ohm around the center tap. The secondary measures 0.4ohm.
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Yes, although that could not cause these issues. I also tried putting a pedal at the front goop bring the impedance down
Thanks
Thanks
Glad to (try) to help Bancika, many of us owe him a lot for his great PCB design software 🙂
1) if Deacy amp and Telefunken demand -9V supply, by all means use that voltage, not 7.7V
2) the amp is as it is (or was), don´t try to reengineer it with modern tracking biasing, input buffers, etc; in that case build a chipamp and call it a day.
The idea here is to recreate a Classic as it was, no matter how cheesy outdated it is.
3) I have a great doubt with the transformers you show: in principle the driver one should be smaller and have higher resistance windings, the output one should be the largest and windings have very low resistance, YET:
* the larger one, should be the output one, to boot has power transistors attached to it, yet is labelled "zwischenübertrager" , literally "between transformer", a good approximation to "interstage transformer" of which a "driver transformer" is a subtype . WTF???
* the smaller one, should be the driver, yet is labelled "ausgangsübertrager" , literally "output transformer" . WTF?
4) you "left the NTC thermistors" but are not showing them in the schematic, please post the real circuit you built and are trying to debug.
5) Radio Museum schematics are blocked for non members, if you can see it please cut and paste the power amp section and upload it to some image server, such as IMGURL , then use that link to show the image here as a "visible inside the post" one (jpg/gif/png).
Avoid proprietary servers such as google drive which force you to register just to download files, which to boot have to be opened "somewhere else" , not inside DIYaudio pages.
In interactive repair threads it´s better/faster to have directly clickable links, rather than having to download first and then open somewhere else.
6)
Do it on both and identify which if which.
Since labels appear to be inverted and confusing, you might call them TsL (arge) and TsS (mall)
7)
We expect some 0.25 to 0.35V forward biasing BE and BC junctions , and in theory "open" reverse way, also open CE in any direction, but Germanium transistors are lossy so you might read something ... in any case a couple kiloohms if using a resistance scale, so that alone does not mean damage.
8) once yu have identified everything, I will suggest a minimalist setup to test them stage by stage, literally.
Once every stage is normal, then the whole amp , as a system, can be made to work and tweaked .
Good luck and thanks 🙂
1) if Deacy amp and Telefunken demand -9V supply, by all means use that voltage, not 7.7V
2) the amp is as it is (or was), don´t try to reengineer it with modern tracking biasing, input buffers, etc; in that case build a chipamp and call it a day.
The idea here is to recreate a Classic as it was, no matter how cheesy outdated it is.
3) I have a great doubt with the transformers you show: in principle the driver one should be smaller and have higher resistance windings, the output one should be the largest and windings have very low resistance, YET:
* the larger one, should be the output one, to boot has power transistors attached to it, yet is labelled "zwischenübertrager" , literally "between transformer", a good approximation to "interstage transformer" of which a "driver transformer" is a subtype . WTF???
* the smaller one, should be the driver, yet is labelled "ausgangsübertrager" , literally "output transformer" . WTF?
4) you "left the NTC thermistors" but are not showing them in the schematic, please post the real circuit you built and are trying to debug.
5) Radio Museum schematics are blocked for non members, if you can see it please cut and paste the power amp section and upload it to some image server, such as IMGURL , then use that link to show the image here as a "visible inside the post" one (jpg/gif/png).
Avoid proprietary servers such as google drive which force you to register just to download files, which to boot have to be opened "somewhere else" , not inside DIYaudio pages.
In interactive repair threads it´s better/faster to have directly clickable links, rather than having to download first and then open somewhere else.
6)
your terminal numbers tell us nothing, please draw the transformer windings and show terminal to terminal resistance.
Do it on both and identify which if which.
Since labels appear to be inverted and confusing, you might call them TsL (arge) and TsS (mall)
7)
Yes. Please measure them the standard way, using the multimeter diode scale.
We expect some 0.25 to 0.35V forward biasing BE and BC junctions , and in theory "open" reverse way, also open CE in any direction, but Germanium transistors are lossy so you might read something ... in any case a couple kiloohms if using a resistance scale, so that alone does not mean damage.
8) once yu have identified everything, I will suggest a minimalist setup to test them stage by stage, literally.
Once every stage is normal, then the whole amp , as a system, can be made to work and tweaked .
Good luck and thanks 🙂
Thank you so much for the response and kind words. To try to respond to your bullets.
1) I started with a full battery, but I spent so much time debugging the amp that it went down to 7.7V 🙂 It behaves the same with 9V though
2) Yes, I agree.
3) I was also confused by the transformers, but like you I tried to translate from German and confirmed their position. Also confirmed using part # from the schematic. The interstage transformer is particularly strange because it has an extra winding. Maybe someone will know what it does by looking at the schematic, could be something related to recording to tape, or headphone output or something else.
4) The circuit is pretty much as shown. I left the stock 350ohm thermistor in parallel with a 56ohm resistor instead of a single 39ohm resistor shown on the Deacy schematic. That arrangement was used in the Telefunken, so I kept it instead of a single resistor.
5) I posted a link to my google drive in the original post, but here it is again below, uploaded as jpeg
6) pin numbers should make sense once you see the schematic.
7 and 8) I'll take the output stage of the amp apart and try to measure everything separately.
Thanks again for the help!
1) I started with a full battery, but I spent so much time debugging the amp that it went down to 7.7V 🙂 It behaves the same with 9V though
2) Yes, I agree.
3) I was also confused by the transformers, but like you I tried to translate from German and confirmed their position. Also confirmed using part # from the schematic. The interstage transformer is particularly strange because it has an extra winding. Maybe someone will know what it does by looking at the schematic, could be something related to recording to tape, or headphone output or something else.
4) The circuit is pretty much as shown. I left the stock 350ohm thermistor in parallel with a 56ohm resistor instead of a single 39ohm resistor shown on the Deacy schematic. That arrangement was used in the Telefunken, so I kept it instead of a single resistor.
5) I posted a link to my google drive in the original post, but here it is again below, uploaded as jpeg
6) pin numbers should make sense once you see the schematic.
7 and 8) I'll take the output stage of the amp apart and try to measure everything separately.
Thanks again for the help!
It does appear that the "driver" also feeds the record head and is *larger* than the speaker transformer. OT is P-P and probably short bass.
You "could" have left the recorder unmolested. Put the guitar to the Mike jack. Cobble-up the switch contacts so "mike" comes out the speaker (not a normal mode). It is a MUCH better circuit than what the Darcy used; but SLAMMED it would be about the same.
You "could" have left the recorder unmolested. Put the guitar to the Mike jack. Cobble-up the switch contacts so "mike" comes out the speaker (not a normal mode). It is a MUCH better circuit than what the Darcy used; but SLAMMED it would be about the same.
Attachments
Yeah, looking back, I could've saved the original circuit. The thing is, I did that for half a dozen other germanium radios with success. But I wanted to build the exact Deacy circuit, so I butchered it. It was broken, but someone who knows more could probably fix the whole thing. Few belts and electrolytic capacitors would probably get it back running.
To anyone interested, I documented most of the journey Amps >
Radio Conversion : DIY Fever – Building my own guitars, amps and pedals
good catch, I'll re-check everything around Q2
Cheers
To anyone interested, I documented most of the journey Amps >
Radio Conversion : DIY Fever – Building my own guitars, amps and pedals
good catch, I'll re-check everything around Q2
Cheers
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You have noticed that you've got PNP transistors here, hence the positive terminal of the battery or PSU has to be grounded?
Best regards!
Best regards!
In post #1 Measures: Positive voltages ????
They should be negative as the -9V supply with PNP transistors.
Battery +7.7V ???
Battery reversed ?
They should be negative as the -9V supply with PNP transistors.
Battery +7.7V ???
Battery reversed ?
Ok.
That was my guess, I wanted to make sure.
Carefully check transformers ( out of circuit ), for isolation between windings and resistance of each coil.
On circuit, check for continuity to make sure about wiring mistakes, shorts, opens.
To check transistors ( out of circuit ), do as if they were two diodes. Diode emitor base and diode base collector.
That was my guess, I wanted to make sure.
Carefully check transformers ( out of circuit ), for isolation between windings and resistance of each coil.
On circuit, check for continuity to make sure about wiring mistakes, shorts, opens.
To check transistors ( out of circuit ), do as if they were two diodes. Diode emitor base and diode base collector.
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